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Limits of current human knowledge

Looking over the article and all the archived discussions, one basic thought occurs to me: Humans do not fully and clearly understand why airfoils generate lift. There are various competing explanations, and there is no broad, general, and complete consensus among scientists as to why airfoils generate lift. This is an astounding fact. I think this aspect of current human understanding should be mentioned in the introduction to the article. It is remarkable with the wide-spread use of airfoils throughout history (age of sail, age of flight) that humans do not fully and clearly understand why the airfoil works. --Westwind273 (talk) 18:06, 20 March 2013 (UTC)

I rather strongly disagree with the assessment that "there is no broad, general, and complete consensus among scientists as to why airfoils generate lift." In fact I would say just the opposite, that the science and engineering of airfoils and lift is a mature subject that is well understood and "settled" science. If you have gotten the opposite impression from the article, I apologize.

Where there is disagreement is in how to explain these rather complicated and sophisticated ideas in layman's terms. This disagreement over pedagogy is quite distinct from any controversy over the underlying science.

I am now more inclined to agree with J Douglas McLean's statement upthread that "the quote by John D. Anderson gives the misleading impression that there are disagreements over the science itself, not just the qualitative explanations" and will remove that material. Mr. Swordfish (talk) 19:02, 20 March 2013 (UTC)

I agree that when read by a layman, the article certainly gives the impression that lift is not "settled" science. To the layman, it certainly reads as if there are various competing explanations for the underlying science. For example, this introductory section definitely gives the impression of various competing explanations of the underlying science: "There are several ways to explain how an airfoil generates lift. Some are more complicated or more mathematically rigorous than others; some have been shown to be incorrect. For example, there are explanations based directly on Newton’s laws of motion and explanations based on Bernoulli’s principle. Either can be used to explain lift." Does the Newton explanation conflict with the Bernoulli explanation? Newton and Bernoulli are not alternative explanations of lift; rather they are both in effect simultaneously to create lift, aren't they? Then why present them as two alternatives? It is relatively easy for the layman to understand the Bernoulli effect, but the key question that the article never explains clearly to a layman is: Why does the air on the top flow faster than the air on the bottom? This is the key point. Later the article says "Explaining lift while considering all of the principles involved is a complex task and is not easily simplified." To me, this is a cop out as far as Misplaced Pages is concerned. The article is basically saying "We're going to punt on any attempt to explain this to a layman, and instead divert into scientific mumbo-jumbo that you will never understand." Is it really so hard to explain why the air on top is flowing faster than the air on bottom? Overall I am quite disappointed in this Misplaced Pages article. --Westwind273 (talk) 19:29, 21 March 2013 (UTC)

I think the whole concept of "multiple explanations" is the wrong way to approach this topic. It leads to deep confusion in the layman. If it is settled science, there should be one explanation which can be presented in greater or lesser levels of depth. --Westwind273 (talk) 19:38, 21 March 2013 (UTC)

I think that the problem being described here is the one about the way science is taught (in some schools). First there are phenomena in the real world, then there are scientific theories and mathematical models that attempt to explain the phenomena. There's nothing after that - no point at which physical phenomena stop being what they were and start following the dictates of the mathematical models, rather than their own inscrutable processes. That does not mean that science is wrong or badly understood, but that the scientific models and equations we have, are all we have, apart from the mysterious wonders of nature itself. People try to use similar arguments to rubbish climate science - "If this is all just a theory, then let's wait until it's been sorted out", etc. Lift is settled science - jet airliners fly every day to within tolerances very close to those their designers intended - but there isn't one simple explanation. We just have to get used to that. --Nigelj (talk) 20:35, 21 March 2013 (UTC)

>... there should be one explanation which can be presented in greater or lesser levels of depth.

Alas, the world is not that simple. Almost every scientific principle admits multiple explanations. For instance, there are at least nine ways to prove the fundamental theorem of algebra, and this is an area that has been "settled" for centuries. As another example, many problems in Newtonian physics can be approached in multiple ways - conservation of energy, conservation of momentum, direct application of newton's laws, the principle of least action, etc. These are just different ways of looking at the same problem.

Much of the controversy surrounding lift is due to some individuals thinking that there is "one true explanation" that is correct and therefore all other explanations must be incorrect. Unfortunately, one of the most common explanations actually is incorrect; were this not the case I surmise that the differing explanations would simply sit quietly side-by-side as they do in most topics.

I agree that the sentence "Explaining lift while considering all of the principles involved is a complex task and is not easily simplified." adds little to the article, and we have contemplated removing it. Maybe its time has come? Opinions form other editors?

Finally, to answer your question "Why does the air on the top flow faster than the air on the bottom?" there's a very simple answer:

When air follows a path that is curved, the pressure is lower on the inside of the curve than on the outside. So, there's a region of lowered pressure on the top side of the wing. When the air flows from atmospheric pressure into this lower pressure region, there is more pressure behind than in front resulting in a net force on the air which speeds it up. (the last sentence is basically Bernoulli's principle in a nutshell)

Perhaps something like this should be added to the article. I've never considered "why does the air speed up" to be a "key point" since lift can be explained (albeit incompletely) without even mentioning the speed changes. But if our readers are coming here looking for the answer to that question, perhaps we should present it. Mr. Swordfish (talk) 20:42, 21 March 2013 (UTC)

I agree with Mr Swordfish, and I disagree with Westwind273 when he writes Humans do not fully and clearly understand why airfoils generate lift. I disagree for two reasons. Firstly, science does not address questions about why the universe operates the way it does. That is a question for theology. Science merely observes the universe and attempts to determine principles and laws which describe these things. As a simple example, scientists understand and apply Newton's first law of motion but science does not bother with the question of why a force is necessary to cause an object to accelerate. Theologians might explain that a force is necessary because God dictated that it would be so, but scientists find Newton's observations, and his laws of motion, to be entirely adequate. Similarly, Bernoulli observed the flow of fluids and the interchange of speed and pressure and described it all in his famous principle; but he didn't bother to include any speculation about why it is so. That would not be science.

Secondly, Westwind273 has grasped John Anderson's statement that scientists and engineers disagree over what is the most fundamental way to explain the phenomenon of aerodynamic lift. Westwind273 has misunderstood Anderson to be referring to some remarkable mystery about aerodynamic lift. If it is true that humans do not fully and clearly understand these things, then that is equally true of any scientific phenomenon. Westwind273 should also be writing that humans do not fully and clearly understand magnetism, electricity, meteorology, thermodynamics, and so on. There is nothing specially intractable about aerodynamic lift which is a splendid application of Bernoulli's principle, Kutta's observations, Newton's third law of motion, the principle of momentum and so on. Each person contemplating the phenomenon of aerodynamic lift, whether that person is a student pilot, college student, professional engineer or research aerodynamicist, must choose whichever of these scientific principles is most satisfactory for him. There is no one true explanation for any scientific phenomenon, and certainly not aerodynamic lift. Dolphin (t) 23:28, 21 March 2013 (UTC)

I think it's understandable that Westwind273 got the mistaken impression from the current article that there is a lack of consensus on the science of lift. And I largely agree with the responses by Mr. Swordfish, Nigelj, and Dolphin, but with some quibbles below. I have a suggested revised version of the article in my sandbox User:J_Doug_McLean/sandbox that I think would avoid the mistaken impression that Westwind273 got. It sets the record straight on the science and also answers the question of why the flow over the upper surface speeds up a little differently from the way Mr. Swordfish did. I'd appreciate it if Westwind273 would read my suggested version and provide feedback.

The answer by Mr. Swordfish to the question "Why does the air on the top flow faster than the air on the bottom?" is a good start, but it needs to be added that the cause-and-effect relationship between the pressure field and the velocity field is circular, as explained in my suggested revision. Both flow curvature and changes in flow speed are caused by differences in pressure, and the differences in pressure are sustained by the changes in flow direction and speed.

The issue of cause-and-effect and "why" has come up before, and I still don't entirely agree with the hard-line take on that topic by Dolphin. I agree that for some fundamentals like Newton's second law we don't understand the "why". But at other levels, I think it's perfectly reasonable to talk about causation and "why", as in saying that a force causes an acceleration. So "why" does something accelerate? Because of a force. "Why" are force and acceleration related the way they are? We don't know. J Doug McLean (talk) 02:07, 6 April 2013 (UTC)

My opinion is that this article has long suffered from the fact that we editors are sometimes too close to the subject matter and get caught up in meta-discussions about the material instead of just presenting the material in a srtraightforward manner. We've all read the great Bernoulli/Newton debates of the 90s, and while this is interesting to folks who already know quite a bit about the subject I do not think it is helpful to re-hash that debate here. Perhaps it belongs in its own wikipedia article, but the purpose of this article is to introduce the basic ideas in a manner understandable to the lay reader. It seems to me that when we write about how we're we're going to explain the actual subject, we've made the artcle harder to understand by encapsulating it in a second layer of meta-analysis.

The result is that some readers, such as Westwind, get the mistaken impression that there is a lack of scientific consensus. Doug has suggested that we add language to correct that impression, eg; The mathematical theories are scientifically rigorous, are supported by empirical observations, and have been agreed upon by the scientific and engineering communities since the early 20th century. My take is that this should go without saying, of course the theories are rigorous and well accepted - having to state it explicitly seems like we "doth protest too much".

To put a finer point on it, everything in science has multiple explanations; all science is rigorous (otherwise it wouldn't be science). There is no need to apologize for choosing one explanation over another, or to have to make assurances that the science is actually science.

So, I would advocate editing the article to downplay the Bernoulli/Newton "controversy", and to avoid giving the impression that these are two or more competing theories. As an example, take a look at the Bernoulli's_principle article itself - it states that BP can be derived from either conservation of momentum or by directly integrating newton's 2nd law. To my eyes, it does so without implying that one is correct and the other is wrong, or that there's any controversy over which derivation is correct. The subtext is that both are correct; somehow that subtext seems to be missing in this article. I do think that this article should present both explanations, but I think we can do so without making such a fuss over it. In fact, the more I think about the subject, the more I am persuaded that it's really one big explanation that fits together in harmony, rather than multiple competing theories.

By way of analogy, this subject reminds me of the blind men and the elephant. It's a great parable, but here we're writing an article describing elephants, not the arguments of the blind men. Statements like "some say the elephant is like a snake and some say it is like a wall" doesn't really advance the article.

In other matters, Doug has pointed out a definite shortcoming of the current article - the airfoil affects the flow over a wide area around it - and this fact along with the reasons why it occurs (ie the self re-enforcing interplay between pressure and fluid motion) should be incorporated into the article. I'll take a swat at this in coming days, most likely just stealing Doug's text. Mr. Swordfish (talk) 21:20, 8 April 2013 (UTC)

The blind men and the elephant is spot on. Avoid the arguments between the blind men, and ignore those who ask "Why is there an elephant in the room anyway?" This is an article about lift, and the main conceptual and mathematical models that enable pilots and engineers to understand and manipulate lift. People who want to know why, or how did we get here, are looking for different topics, for which there may or may not be articles at this time. --Nigelj (talk) 22:33, 8 April 2013 (UTC)

I understand the urge to simplify things by downplaying the "meta-analysis", but I think it would be a mistake.

If we were the only ones who had ever discussed alternate explanations of lift, and if the controversies were limited to our little talk-page circle, then I'd agree that those discussions would be out of place in the article. But that's not the situation. The public folklore on this topic is full of misconceptions and erroneous explanations. And the controversies have been out there in public view for decades. I'll bet many of the potential readers of this article have already read a Bernoulli explanation (likely based on longer path length) or deflection explanation of lift and have probably also read somewhere that one or the other is wrong. And they're also very likely to have already gotten the impression from somewhere that the science is unsettled.

So the fact that the science on this particular topic is in good shape doesn't speak for itself. It needs to be spelled out. And given what's out there in the popular culture, we'd be short-changing the reader if we left out discussion of the pros and cons of the alternate explanations (and the outright errors in some versions). Whether we like it or not, these issues are now part of the topic of lift. Just the straight facts will not be enough to enable a reader to see through the fog.

Mr. Swordfish (Mr. Swordfish), you suggest that we should present "both explanations" of lift in the same way that the Bernoulli's_principle article presents its two alternate derivations. I don't entirely agree. Both derivations of Bernoulli are actually correct and self-sufficient. Our two simpler explanations of lift are both correct to some extent but also have significant shortcomings. The shortcomings shouldn't be swept under the rug.

Mr. Swordfish, you also say you're persuaded that the two simpler explanations really amount to "one big explanation that fits together in harmony". If you follow that idea to its logical conclusion and try to write it out as an explanation of lift, including the ideas of flow influence over a wide area and mutual interaction between pressure and velocity, what you arrive at is my "more comprehensive explanation" in my proposed revised version in my sandbox User:J_Doug_McLean/sandbox. The hard work is already done, including integrating the "one big" explanation with the simpler ones in the pedagogically favored order.

My proposed revision also attempts to put the mathematical theories and the various qualitative explanations in perspective in the new sections "The understanding of lift as a physical phenomenon", "Popular physical explanations of lift" and "Shortcomings of the popular explanations". Not all the material in these sections is new, but I think organizing it in this way makes things clearer. It explains the science in a way that would prevent misunderstandings like the one expressed by Westwind273, and it provides the necessary "meta-analysis" of the qualitative explanations. J Doug McLean (talk) 22:33, 17 April 2013 (UTC)

I get the science vs theology thing. For example, we know a lot about how gravity works, but we don't actually know why it works. We feel confident that there are such things as photons, but are there such things as gravitons? We don't know. But I think the explanation of lift should be much closer to science than theology. I strongly agree with the statements above that the article's explanation bends strongly toward circular reasoning: Why does the air move faster on top? Because there is lower pressure. Why is there lower pressure? Because the air is moving faster. This circular reasoning really turns off the layman reader, and I think it is reasonable to ask science to avoid this kind of circular reasoning without resorting to theology. The air on top is being made to both go faster and become lower pressure, but why? The best I can figure is that it is some sort of combination of surface cohesion for the molecules closest to the top of the wing, and a whiplash effect for the molecules a bit further away from the wing. This surface cohesion and whiplash combo creates both the lower pressure and faster airflow on top. Is this correct?

I think the authors of this article need to realize how uniquely odd this article is in beginning the explanation with the "There are several ways to explain..." paragraph. I challenge you to find any other scientific article on Misplaced Pages that starts an explanation in this way. For example, look at the Misplaced Pages articles on nuclear fission or freezing. The explanation is straightforward and does not wade into this "several different ways" explanation. --Westwind273 (talk) 04:50, 30 January 2014 (UTC)

I was not saying that the current article "bends strongly toward circular reasoning", and I don't think it does. What I was saying is that the article should be revised so as to state explicitly that the cause-and-effect relationship between pressure and velocity in an airfoil flow is circular. Circular cause-and-effect is not the same thing as "circular reasoning", which generally refers to a false argument that purports to establish something that was assumed a priori and for which there is no support other than a circular argument. The circular cause-and-effect relationship between pressure and velocity in fluid flows isn't in the circular-reasoning category because it's the way the physics actually works.

Of course Newton's second law is the key physical principle here. In many applications of Newton's second law, it's appropriate to think of the force as the "input" and the motion as the "output", but that way of thinking misses part of the picture in continuum fluid flows. The motion of a local parcel of fluid does depend on the net force exerted on it by all the adjacent parcels in contact with it, consistent with Newton's second law. But that force depends on the motions of the adjacent parcels, which depend on the motions of the parcels adjacent to them, and so on. Because we're dealing with the motions of many parcels, all interacting with their adjacent neighbors, we effectively have a situation in which the motions depend on the forces, and the forces depend on the motions, i.e. circular cause-and-effect.

So of course science should avoid circular reasoning, but circular cause-and-effect between pressure and velocity is a fact of life in aerodynamic flows, and a good qualitative explanation of lift needs to make that clear.

One way to summarize the relationship between pressure and velocity is as follows: The pressure gradient causes a fluid parcel to accelerate (consistent with Newton's second law), and the combination of the parcel's inertia and acceleration causes the pressure gradient to be sustained. The necessity of acceleration to sustain the pressure gradient involves Newton's third law. When the pressure gradient is nonzero, a fluid parcel experiences a net pressure force exerted by it's neighbors. A net force on a fluid parcel can exist only if the parcel pushes back, consistent with Newton's third law. In the effectively inviscid flow outside the boundary layer, a fluid parcel can push back only through the combination of its inertia and acceleration.

I'll admit that the idea that the parcel's acceleration "sustains" the pressure gradient isn't that east to grasp intuitively. Does "sustain" mean the same thing as "cause" in this case? I think the answer is yes, but a bit indirectly. I found it helpful to think of a simple example from solid mechanics.

Think of a square block of wood, a couple of inches on a side, resting on a rigid table. Center your thumb on the top of the block and press downward. The forces exerted on the external surface (your thumb pushing down on the top and the table pushing up on the bottom) cause the stress field throughout the interior of the block to be altered. For one thing, there will be a non-uniform distribution of vertical compression stress, likely more concentrated directly under your thumb and more spread out at the bottom of the block where it presses on the table. This will be balanced by a non-uniform distribution of shear stress such that the net force on any parcel of material in the interior is zero, consistent with Newton's second law. At the local level at any point in the interior, the only cause we can identify for the compression-stress gradient is the shear-stress gradient. Locally, the two stresses are engaged in a mutual interaction, i.e. circular cause-and-effect. At the global level (the whole block), the cause of the whole non-uniform compression-stress field is the forces applied at the surface. There is circularity at this level as well because the distribution of compression stress applied by your thumb depends on the deformation of your thumb, which depends on the distribution of stress. This is not circular reasoning, just circular cause-and-effect associated with a mechanical interaction.

We can apply the same line of reasoning to an airfoil flowfield. Because the shear stress in most of the field is insignificant, the compression-stress gradient (i.e. the pressure gradient) acting on any fluid parcel must be balanced by fluid acceleration instead. At the local level, the interaction between the pressure gradient and the acceleration is mutual, or circular, just as it was with the two interacting stress gradients in the solid. At the global level, the cause of the non-uniform pressure field (and thus also of the non-uniform velocity field) is the force applied to the flow by the airfoil, acting at the airfoil surface. As in the case of the wood block, the pressure field in the flow is just a state of stress in the interior of the domain, resulting from the application of forces at the boundaries, and distributed in a manner consistent with the laws of motion throughout the field.

I think this answers the questions that crop up a little later in the exchange between Westwind and Swordfish: Why does the flow above the upper surface follow a curved path? And why is there a pressure gradient associated with that curvature? At the local level, the answer is in the mutual interaction between pressure and velocity that I've already described. At the global level, the ultimate cause of the non-uniform pressure and velocity fields is the force exerted on the flow by the airfoil. This sounds unsatisfyingly circular because that force is just the equal-and-opposite reaction to the lift force that we're trying to explain. But that's how the physics works. The force exerted by the airfoil makes the flow non-uniform, and the non-uniform flow exerts force on the airfoil. The cause-and-effect is circular even at this global level, but it's all tied together by the facts that the airfoil shape and angle of attack impose a boundary condition on the velocity at the surface and that Newton's second law applies throughout the field. This is circular cause-and-effect, but not circular reasoning.

On a related issue raised by Westwind273, the following of the curved surface by the flow has nothing to do with "surface adhesion". Air molecules don't adhere in significant numbers to solid surfaces, and air can't be put in tension. The background atmospheric pressure is high enough that the pressure at the airfoil upper surface, though lower than ambient, is still strongly positive in an absolute sense. So the flow follows a curved path and is able to follow the convex upper surface because it is pushed from above by higher pressure. There is no pulling from below. And the following of the curved surface has nothing to do with viscosity either. J Doug McLean (talk) 19:45, 3 April 2014 (UTC)

If readers see circular reasoning when they are trying to find a satisfying explanation for lift, it is most likely because the question "Why does the air move faster on top?" is not a particularly serious scientific question. (I think the best answer to this question would be "Because air observes the laws of physics"; but this is unlikely to satisfy many of the people who ask "Why does the air move faster on top?") It is a bit like Kepler's laws of planetary motion. Science is absolutely fascinated that the planets move in such a regular, repeatable and predictable manner that Kepler was able to postulate three laws that accurately summarise their motion; but science is not at all interested in the question "Why do the planets obey Kepler's laws of planetary motion?" If someone (presumably a layman) set about trying to give a fundamental explanation as to why the planets move in a regular, repeatable and predictable manner, it is my guess that he would end up presenting circular reasoning.

Similarly, the air moves around an airfoil in such a regular, repeatable and predictable manner that we can see its motion is consistent with Newton's laws of motion, the Kutta condition, Bernoulli's principle etc. If the air did not move faster across the top of an airfoil, it would demonstrate a flaw in these fundamental laws of physics.

The scientific approach to lift is firstly to select Newton's laws of motion or the Kutta condition or Bernoulli's principle, explain it in some detail and then present the phenomenon of lift on an airfoil as a practical example of Newton or Kutta or Bernoulli. If it is done in this way, the question "Why does the air move faster on top?" doesn't arise because the air moving faster on top is exactly what the Kutta condition predicts for airfoils and lots of other bodies with sharp edges.

If Lift (force) attempts to answer this question, or give the impression it is attempting to answer this question, there is grounds for amending the article to avoid that impression. Dolphin (t) 05:48, 30 January 2014 (UTC)

Dolphin, I'm going to disagree with you here. Why does the air move faster on top? is perfectly reasonable scientific question, and the article does explain it, although in a somewhat roundabout (but not circular) manner. It's not presented in this order in the article, but the pieces are there to fit together the following explanation:

Why does the air move faster on top?

Because there is a region of low pressure along the top of the wing. According to Bernoulli's principle when air flows into a region of low pressure it speeds up. This is because there is more pressure behind than in front which results in a net force on the air molecules. Consequently they accelerate to a higher speed.

Ok, so why is there a region of low pressure on top of the wing?

So, why does the air follow a path that is curved?

It is deflected by the wing, with the geometry of the flow path dependent on the shape of the wing and the angle of attack. It's obvious why it is deflected downward by the bottom the wing - the wing is solid and there is nowhere else for the air to go. Along the top, the air follows the surface of the foil, resulting in a curved path.

Why does the air follow the surface of the foil instead of just continuing on in a straight line?

At this point we're getting beyond the scope of the article, but this is usually explained using viscosity.

I don't think Why does the air move faster on top? is central to the topic of lift, so I wouldn't re-structure the article to answer it. But the answer is there if the reader is willing to hunt for it. And the reasoning is not circular.

Regarding Kepler's laws of planetary motion, they can be derived from Newton's laws (including the law of gravity), although since Kepler died a dozen years before Newton was born he obviously didn't derive them that way. So a reasonable non-circular explanation of Kepler's laws would be to start with Newton and proceed from there - Kepler's laws are true because they are a logical extension of Newton's laws. Of course, this begs the question of Why are Newton's laws true?, and the answer to that is that you've got to start somewhere and the reason we accept them (without a logical proof starting from more fundamental assumptions) is the millions of observations and experiments confirming them.

All this said, I do agree that the various physical phenomenon surrounding lift (a net force, pressure differences, speed differences, air changing direction etc.) can be explained starting from basic principles. And I think the present article does this. Mr. Swordfish (talk) 15:54, 30 January 2014 (UTC)

Mr Swordfish wrote: Because the air is following a path that is curved. Euler's equation, which is derived directly from Newton's laws says that whenever a fluid follows a curved path there are pressure differences, with lower pressure on the inside of the curve and higher pressure on the outside. The flow turning causes a region of low pressure along the top of the wing.

This is what I don't understand. Your explanation of the first and third questions here helped me a lot, but it is this middle second one that still seems unexplained to me. Specifically, why does air that follows a curved path have lower pressure on the outside? I know that Euler's equation says that it does, but why? Is it simply that the outside path is longer and therefore the molecules get spread out over a greater distance? This seems remarkably close to the equal-transit-time theory, which we know is false. Is the answer to this theology? I think it should not be. I read the Misplaced Pages article on Euler's equation for fluid dynamics, and it did not help me. --Westwind273 (talk) 20:37, 30 January 2014 (UTC)

Sorry, I was being a bit imprecise. When I said "Euler's equation" I meant the one referenced in this article, which is different than the one treated at Euler's_equations_(rigid_body_dynamics). So, it's unsurprising that that article didn't shed much light on it. Mathematically, the equation in question (dp/dr = rho*v^2/R) is derived by just writing an expression for centripetal force, applying F = ma and doing a bit of algebra. Babinsky's paper ( http://iopscience.iop.org/0031-9120/38/6/001/pdf/pe3_6_001.pdf ) has a concise derivation at the very end.

To get an intuitive notion of why it's true, ie why there is less pressure on the inside of the curve, imagine riding on a train between two cushions pressing equally on you from the sides - as the train goes around a curve, your body will press on the outside cushion and pull away from the inside cushion; less pressure on the inside and more pressure on the outside. The tighter the turn and the faster the train is moving, the greater these pressure differences become. The analogy only goes so far, but air following a curved path experiences the same forces. It's just a consequence of the centripetal force necessary to make the air follow a curve. Mr. Swordfish (talk) 22:28, 30 January 2014 (UTC)

Yes, this is what I was trying to understand. It is quite similar to what I was saying before. The overall airflow follows the curve of the wing on top because of the tendency of the air molecules to adhere to the wing, once they come in contact with it. But at the same time, there is a kind of whiplash centripetal force that is throwing the air molecules away from the top of the wing. The result is a powerful spreading out of the air molecules in the area above the wing, resulting in lower pressure there and the resulting lift. Does this sound right? Could the article be modified to give some kind of explanation like this to the layman? --Westwind273 (talk) 00:38, 31 January 2014 (UTC)

There is no force "throwing the air molecules away from the top of the wing". However, there is inertia which would make the air follow a straight line in the absence of any force. Since there is a force pulling the air down towards the wing surface, the air changes direction.

I'm not sure what you mean by "a powerful spreading out of the air molecules in the area above the wing", but if you mean the air becomes less dense (fewer air molecules per unit volume) that is incorrect. In a first approximation, the air is incompressible meaning the density remains constant throughout the airflow. This is not strictly true since air does compress at sufficiently high airspeeds, but trying to explain lift via changes in air density is barking up the wrong tree. Mr. Swordfish (talk) 14:58, 31 January 2014 (UTC)

I think the authors of this article need to realize how uniquely odd this article is in beginning the explanation with the "There are several ways to explain..." paragraph. I challenge you to find any other scientific article on Misplaced Pages that starts an explanation in this way.

Two that come to mind immediately are derivation of Bernoulli's principle which states: "The Bernoulli equation for incompressible fluids can be derived by either integrating Newton's second law of motion or by applying the law of conservation of energy" and the proofs of the fundamental theorem of algebra. Granted, the latter doesn't call a lot of attention to the fact that there are dozens of different lines of reasoning that can be used to arrive at the result, it just presents them. The phrases "yet another" and "still another" occur numerous times.

A quick search or two ( https://en.wikipedia.org/search/?title=Special%3ASearch&profile=default&search=several+ways+to+explain&fulltext=Search , https://en.wikipedia.org/search/?title=Special%3ASearch&profile=default&search=multiple+explanations&fulltext=Search ) turns up thousands of wikipedia articles that present multiple explanations. Which is unsurprising, since there are usually several ways to explain any phenomena. Insisting on only presenting one when the literature presents several would violate WP:NPOV Mr. Swordfish (talk) 16:25, 30 January 2014 (UTC)

Hi Mr Swordfish. Thank you for taking the time to write such detailed explanation on the subject. I don’t have any major objection to what you have written. I think my only objection is that your explanations, regardless of their correctness, are not answering the question “Why does the air move faster on top?” but that is not because there is anything wrong with your answer; it is because the question is not a good one.

Let me illustrate my thinking. Person A might ask “Why do ripe apples fall to the ground?” Person B might answer “Because of Newton’s law of universal gravitation.” Person A might be entirely satisfied with this answer, but I don’t find it a satisfying answer at all. Ripe apples were falling to the ground for many millions of years before Newton was born so Newton cannot possibly be part of the explanation of falling apples. (Science is fascinated that ripe apples, and all other unrestrained objects, always fall towards the center of mass of the Earth with predictable initial acceleration, but science has little or no interest in why.)

I would prefer the conversation go like this: “We observe that ripe apples always fall to the ground. Is Newton’s law of universal gravitation consistent with this observation?” To which the answer is “Yes, Newton’s law of universal gravitation appears to be consistent with this observation.” (Albert Einstein made some observations and found that Newtonian mechanics were not consistent with the observations, so Einstein developed a replacement system of mechanics that more closely match his observations. The universe does not change its ways in order to behave in accordance with our laws!)

In your explanation of why there is lower pressure on the inside of a curve than on the outside, you have strayed too close to suggesting it is because of Euler’s equation. The pressure gradient across curved streamlines existed for millions of years before Euler’s birth so Euler’s equation is not an explanation of why this pressure gradient has always existed. However, it is reasonable to say “We observe a pressure gradient across curved streamlines. Is there any scientific principle that matches this observation?” To which the answer is “Yes, Euler’s equation is consistent with these observations”. Euler’s equation has stood the test of time and we confidently use it to predict pressure gradients and the curvature of streamlines, but we should not imagine Euler or his equation is an answer to the question “Why is there a pressure gradient across curved streamlines?” That is not a sound question for a scientist. Dolphin (t) 06:23, 31 January 2014 (UTC)

My background is mathematiics, so I'm used to working with axiomatic systems. To me, Newton's laws of motion are like axioms, and if I can deduce that something logically follows from the axioms I'm satisfied - "why does X happen? because it's logical consequence of Newton's laws" - is a perfectly acceptable explanation of why in my book. I understand that not everyone will agree, and that I'll never get an answer to why the axioms (Newton's laws) are correct.

If someone asks me "why is the sky blue" or "why are there infinitely many prime numbers" or "why is a catamaran harder to tack than a monohull?" I'll try to give them an answer based on scientific principles and logic. I don't scold them for asking an unsound question, or say that we can never know why about anything. Agree that we can never really know why, but we can explain complex things in terms of simpler, easier to understand concepts. Anyway, we've kind of gotten away from discussing the article, so I'll let you have the last word if you care to. Mr. Swordfish (talk) 15:47, 31 January 2014 (UTC)

I have been grappling with the difficulty I see whenever Misplaced Pages pretends to answer scientific questions that begin with “Why” such as "Why does an airfoil generate lift?"

It is true that Misplaced Pages (and others) give answers in response to these questions but I don’t believe the question they are answering is the one beginning with “Why”. Superficially, it appears that an answer has been given to the Why question, but philosophically I doubt it.

I would prefer it if Misplaced Pages answered the question “How does an airfoil generate lift?” It is easy to give a genuine answer to this question by referring to the flow pattern:

When a symmetric airfoil is moving relative to the surrounding air but it isn’t generating lift, the flow patterns above and below the airfoil are identical. At each point in the atmosphere above the airfoil, the flow speed and air pressure are identical to the flow speed and air pressure at the corresponding point below the airfoil. The force of the air pressure acting on the top surface of the airfoil is equal to the force acting on the bottom surface. The two forces balance and no lift is generated. But when a symmetric airfoil is generating lift, the air is approaching the airfoil with a non-zero angle of attack and the flow pattern above the airfoil is significantly different to the flow pattern below the airfoil. At each point in the atmosphere above the airfoil, the flow speed and air pressure are different to the flow speed and air pressure at the corresponding point below the airfoil. In particular, the streamlines adjacent to the top surface are closer together than the streamlines adjacent to the bottom surface. The air is moving past the top surface faster than it is moving past the bottom surface and the force of the air pressure acting on the top surface is less than the force acting on the bottom surface. The two forces don’t balance and the resultant is called lift.

I think this kind of response is an answer to the question “How does an airfoil generate lift?” rather than the question “Why does an airfoil generate lift?” Everyone agrees on the answer; it is the question that still perplexes us! Dolphin (t) 06:31, 26 February 2014 (UTC)

++++ The OP stated that humans do not understand why airfoils generate lift.

Now, "Humans understand lift" could mean several different things. The answer depends upon the meaning assigned to the proposition.

But the common interpretation in the scientific community isn't at all that vague. It might be stated: "broadly accepted scientific theory (in this case, mainly the laws of Newtonian fluid dynamics) accounts for the all the salient observed facts concerning lift, without arbitrary ad hoc assumptions, as confirmed by repeatable experiments." If we were to accept this interpretation, then it would remain only to ask ourselves, do we agree or not? I think that all experts on fluid dynamics would agree that the proposition is true. If so, then the article should reflect that accepted dogma, until and unless it is superseded by a new paradigm.

It is understandable that some participants in the discussion are not experts: they don't know Napier's equation, they don't understand what it does and doesn't say about causality, they don't understand the relationship in science between theoretical models and experiments, etc. But the article should not reflect their ignorance...it should reflect humankind's current best understanding of the subject.

That's just my opinion. Do you agree? Mark.camp (talk) 00:33, 27 March 2014 (UTC)

I agree.

There is a temptation to imagine that to fully and clearly understand why airfoils generate lift, the explanation must be esoteric, complex, with lots of advanced math. There is no reasonable ground to imagine that. Different people will prefer different explanations of lift, and all those different explanations can be legitimate. For example, in one of John D. Anderson's books he goes looking for the simplest explanation of lift and concludes that it is based on the observation that the pressure on the upper surface of an airfoil is different to the pressure on the lower surface, resulting in an aerodynamic force; lift is the component of the aerodynamic force perpendicular to the vector representing the relative motion between the airfoil and the free stream of fluid moving past it. That is a simple explanation but it is entirely legitimate as an explanation of how (or why) airfoils generate lift. Many people, including those who embrace Anderson's simple explanation, are entitled to object to the OP's suggestion that Humans do not fully and clearly understand why airfoils generate lift. Dolphin (t) 05:41, 27 March 2014 (UTC)

+ + + + + + I agree that the science allows multiple formulations of explanation or presentation. This an inherent characteristic of Newtonian physics; it is a set of definitions, assumptions, and equations, and the equations can be transformed mathematically without changing their meaning. Often, for example, one can present the predictions of science, in a given case, in terms of forces, or alternatively in terms of energies. You can present Newton's original way of describing the evolution of a system, or you can use the form developed later, the law of least action, to describe the very same system evolution. As a final example, you can present Maxwell's laws in differential form or in integral form. Same science, same math, different presentation.

But that is an entirely different question from that of presenting the explanation of lift as the result of net force on the body, as Anderson does. This is a *stage* in the explanation, not a *form* of the explanation. I think we would all agree that it is a necessary introductory stage. But it is only the beginning. Any curious reader will be happy to understand at this simple level but will immediately want an explanation of the pressure field and velocity field itself: "OK, I understand that if that is the velocity field, then that is the pressure field, and the conclusion is that there is lift. But now I want to know WHY those are the fields". The next stage, I think all of us would perhaps agree, is to proceed to a very simple model (called ironically "complex" potential) that accounts for a particular solution for the two fields that is consistent with lift (and an infinite number of others consistent with the boundary condition but NOT consistent with the observed lift!). But again a persistent reader will say, "ok, I see how ONE of the infinite number of possibilities yields the observed lift, but you have not explained why nature always settles on JUST THAT ONE, the one that happens to produce the observed lift." The next stage, I think you will agree, is to allow a slightly more realistic model...still far from the truth, still no turbulence, no separation, no chaos...but one which can account for the solution (Kutta condition, other approximations concerning effective shape of the foil rather than its actual shape) which nature has been proven experimentally to choose, very approximately.

A good explanation for this article, I think, would allow the reader to pick his own level of advance. If he didn't understand the first stage about net force, he could understand just that much and be satisfied to stop. If he had the next obvious question, he could get a clear exposition of that. If he still were curious, he could pursue the next level, which allows for friction and thus the Kutta condition. If he were still curious, he could move on to boundary layer, and then to turbulence, and then to separation, and then to chaos....

Mark.camp (talk) 00:36, 29 March 2014 (UTC)

I agree that while this article is very informative, it does not really contain a clear and simple explanation of the obvious forces at work.

As for why air 'moves faster over the upper surface of the wing', there are some inbuilt misconceptions here. It only moves faster in relation to the lower surface air and not for the reason most people think.

The air does not 'speed up' over the upper surface - in fact relative to the static air, it slows down...

If we look at the Cambridge University Wind Tunnel video <http://www.cam.ac.uk/research/news/how-wings-really-work> (yes wind tunnels have some limitations) and watch the pulsed smoke (with the viewpoint of the wing moving and the air still) we can see that the air under the lower surface is pushed forwards (i.e. in the direction of the wing's travel) and downwards as the wing pushes into the static air. (This is a large component of lift hence the very significant convex on the underside of SC wings-particularly at the root)

The upper surface air is pushed up and over the leading edge by the pressure bubble/bow wave under the leading edge and as it turns the corner and changes direction (as it must for fluid continuity) the rapid change of direction and the rapid equalisation following the mechanical displacement effect of the wing, cause the pressure change. (This is why high speed wings (SC and others) have minimal upper surface camber-to minimize the pressure drop from angular change induced into the air and thus reduce standing shock waves.)

If you watch the video and the vertical columns of pulsed smoke you will see that the air streamlines near the wing are pulled forwards (i.e. slowed down) not sped up over the wing. Follow a single column of streamlines and watch how they are displaced by the wing.

In particular, look at the reference streamlines near the wind tunnel wall and compare them with their counterparts above and below the wing. You will see that being undisturbed, they have 'travelled further' than the streamlines near the wing. The 'speed differential between upper and lower is only a relative thing. The pressure differentials are caused by the wing pushing the air forwards and downwards under it and bending the upper air through an angular change relative to the AOA of the upper surface. F=Ma.

Pressure differentials are an effect of the angular change of the static air not the cause of lift. No angular change= no pressure diff.

There are pieces of evidence to support the idea that this is important for conceptual understanding:

First: view a flat plate wing in a wind tunnel or using NASA's FoilSim. Flat plates have a better L:D ratio than aerofoils and do not have any camber, so this simplifies things. Observe that flat plates generate lift in exactly the same way as an aerofoil but stall earlier. This demonstrates simple angular mechanical intervention. (Oddly unless I have misread it, this article claims that flat plates create more drag than streamlined aerofoils yet simple simulations like NASA's Foilsim show this to be incorrect as does basic common sense because flat plates have much less 'wetted' area means that for a start, they must produce less form drag. All that is left is induced drag and that is AOA/angular flow change dependent.

Second: Think of the air as a fluid (which it how it behaves subsonically) If you imagine any of the functions or effects of an aerofoil occurring under water, all of s sudden it is easier to conceive. We don't ask why the water follows the curve of the upper surface AOA because it is self evident. After all-what is the alternative?

It seems to me that unless you are an aerodynamicist and fiddle at the edges of the obscure, wings are not that complicated. Newton explains them just fine. I don't go into Bernoulli as it is derived from the 2nd law anyway and causes more confusion than clarity.

I have found the bigger problem in teaching undergraduate pilots is debunking all of the silly 'theories' like ETT that are still taught in flying schools!

Just my two cents Completeaerogeek (talk) 06:09, 22 October 2014 (UTC)

A major revision proposed to be installed soon

Some of us have been saying for some time that this article needs revisions, to clarify the relationship between the mathematical theories and the qualitative physical explanations, to make clear the incomplete nature of the popular explanations, to offer a better explanation that emphasizes the spread-out nature of the flowfield and the reciprocal nature of the interaction between pressure and velocity, and so on. Over a year ago Mr. Swordfish offered to take a "swat" at incorporating these last two points into the article, but it hasn't happened yet, so I'm proposing adoption of my own revisions, which do essentially the same thing Mr. Swordfish proposed, among other things.

A draft of my proposed revised version is in my sandbox User:J_Doug_McLean/sandbox. I've just made major revisions to it, and I think it's now worthy to replace the current article, but I hope others will suggest corrections and improvements, so I'll leave it for a few weeks before I install it in place of the current version.

The reasons revisions are needed have been discussed at length on this page, by me and several others (Zapletal recently made some very good points), so I'll touch on them here only in summary form.

The proposed new version is longer than the current one, but I would argue that the additional material is needed to clarify the issues and avoid confusion on the part of the reader. If you don't think this kind of clarification is needed, just read the discussion on this page. The typical reader of the article is likely to be puzzled by some of the same questions that puzzle the participants in this discussion. Questions such as why the flow speeds up over the upper surface, why the flow follows a curved path, and why the pressure changes in the ways that it does. My draft attempts to make the answers to such questions clearer than they are in the current article. Read my draft carefully. If you find it leaves you with questions unanswered, let me know, and I'll try to fix it.

The proposed new version cites my own book, but I think the citations are relevant and not excessive, and thus in keeping with Misplaced Pages guidelines.

The proposed revision preserves some later sections of the current article, with minor changes: "Pressure integration", "Lift coefficient". and "Lift forces on bluff bodies". Otherwise, the changes are major, and the headings are new.

The title is proposed to be changed to "Lift (aerodynamic force)" to be less ambiguous. The introductory section and a new section, "Lift is a result of pressure differences and depends on airfoil shape, angle of attack, air density, and airspeed", describe what lift is and its general behavior, i.e. the "what" of lift, but without explanation of "how" or "why".

Next, "The understanding of lift as a physical phenomenon" is an all-new section that tries to establish a key distinction that isn't clear in the current article, i.e. the settled status of the science compared with the less settled status of the qualitative physical explanations, to set the stage for going into the details in following sections.

"Popular physical explanations of lift, and their shortcomings" covers much of the same ground as the current article, but the organization and most of the words are new. It integrates the explanations and their shortcomings into a single section to make the shortcomings more prominent. The "blind-men-and-the-elephant" problem is part of the folklore of explaining lift and thus should not be swept under the rug. I think this section, combined with the discussion in "The understanding of lift as a physical phenomenon", puts it all in perspective and equips the reader with enough background to avoid the common misunderstandings.

"A more comprehensive physical explanation" is a shorter version of the explanation in my book, but with different graphics to avoid copyright issues. It does not incorporate Zapletal's suggestion to adopt Lanchester's "wave of sustenance" metaphor, because I don't regard the wave idea as helpful. To understand Lanchester's "wave", you have to go into the details of the interaction between the pressure and velocity fields, as my explanation already does. Once you've done that, I don't think the "wave" idea adds anything. The resemblance of an airfoil flow to a wave is superficial, not fundamental, it seems to me.

"Mathematical theories of lift" attempts to make the nature of the theories clearer without going into too much detail. It does not incorporate Zatletal's suggestion to get into the history of fluid mechanics. That might be a good topic for another article, but it would be out of place in this one. In this article I think it's appropriate to explain the physical principles involved, as currently understood, not the history of their discovery.

"Lift of three-dimensional wings" is an all-new section that tries to remedy the current article's lack of information on lift in 3D.

"Viscous effects: Profile drag and stalling" explains how viscosity produces profile drag and limits the lift curve (stalling) in more detail than the current version. "Coandă effect is not relevant to explaining lift" takes a firmer stand on the relevance of Coandă and debunks the idea of a role for viscosity in the flow's ability to follow the upper surface. It also debunks the idea that the flow "sticks" to the upper surface and is "pulled down" toward it.

The "Further reading" and "External links" sections have been shortened by the deletion of a few items that were less than helpful for one reason or another or already in the reference list.

I look forward to constructive feedback and to getting these revisions installed.

J Doug McLean (talk) 19:19, 23 May 2014 (UTC)

My apologies for not following through my promise to incorporate your proposed changes. I have not forgotten, but other matters have garnered my attention and this one went on the back burner.

I have not had a chance yet to review your latest revision. I will do so in the coming week and give my feedback. Thanks for contributing; I think it is great that we have someone with your knowledge and expertise on the edit team. I am optimistic that we can come to a consensus that improves the article. Mr. Swordfish (talk) 22:27, 24 May 2014 (UTC)

Doug,

I have now had a chance to read the proposed revision in some detail.

I think the revision is much improved from the one from last January and it is clear that you have put quite a bit of thought and effort into it. However, I still do not think it is an improvement on the current article, and would not support changing it out wholesale. Apologies for repeating myself, but the current article's structure is based on the work educators who have written about the pedagogy of explaining lift. As the American Association of Physics Teachers states:

"At least for an introductory course, lift on an airfoil should be explained simply in terms of Newton’s Third Law, with the thrust up being equal to the time rate of change of momentum of the air downwards."

The decision to present the material the way it is currently ordered was informed by the AAPT and other peer-reviewed articles addressing pedagogy. My strong preference is to state early and explicitly that "lift is a reaction force" and explain it in terms of Newton's 3rd law. Any revision that presents a more complicated explanation first will not have my support.

I do recognize that a pressure based approach is favored by aerodynamic engineers, and surely that approach should appear in the article. We just need to start with the simpler Physics 101 explanation such as provided in Halliday and Resnick.

You have written much that is of value that should be incorporated into the article. I have been remiss in not following through with my promised integration. I hope to have a first pass up in my sandbox in a week or so. Mr. Swordfish (talk) 21:45, 30 May 2014 (UTC)

I think the proposed change has lost the focus on explaining what lift is and why it happens - the proposed changes seem to focus more on giving a critique of various "simplified explanations". While there might be a case for having an article on that topic, putting it at the top of this article seems likely to add to the confusion of the typical reader of this article, rather than reducing it. Djr32 (talk) 22:43, 31 May 2014 (UTC)

First for the response from Mr. Swordfish (talk). Thank you for the encouraging words. But I don't see the basis for your objection that my draft "presents a more complicated explanation first". I don't think it does. In "Lift is a result of pressure differences and depends on airfoil shape, angle of attack, air density, and airspeed" I do say that lift is a result of differences in pressure, but that's just part of describing what lift is; it's not explaining lift in terms of the physical principles involved. The first explanation presented in my draft is "Flow deflection", the same one advocated by the AAPT.

In the quote from the AAPT, they advocate explaining lift "simply in terms of Newton's third law", but then they equate the force to a "time rate of change of momentum". This is the domain of the second law, not the third. When you use both laws you should explicitly mention both, as my draft and the current article do.

So it seems to me that we've both followed the AAPT recommendation in spirit, if not to the letter.

Regarding the structure of the article, I think it's important to present a sufficient description of "what" lift is before presenting explanations of "how" it works, i.e. how the principles of physics apply to it. The fact that lift is exerted in the form of differences in pressure is a key part of an adequate description, independent of any physics-based explanation as to how the pressure differences come about.

The current article muddles the distinction between description and explanation by presenting its explanations under headings that bill them as "description", i.e. "Description of lift on an airfoil" and "A more detailed physical description".

Pedagogically speaking, describing something adequately before you present the physics-based explanations is just common sense. I doubt that the educators at the AAPT would object to the way I've structured things.

As for explicitly stating that lift is a "reaction force", I don't think it's a good idea. It seems to me that the term is either misleading or practically meaningless, depending on how it's couched and understood. The first sentence of the current article's "Deflection" section, i.e. "Lift is a reaction force—an airfoil deflects the air as it passes the airfoil", is misleading because it implies that causing an acceleration is part of the definition of a "reaction force". After this misleading introduction, one has to study the subsequent sentences to get the straight story. Of course the correct definition of a reaction force has nothing to do with causing acceleration, as is clear in the article "Reaction (physics)" to which the reader is redirected in that first sentence. When "reaction force" is properly understood as simply a force that has an action/reaction partner in accordance with Newton's third law, the statement "Lift is a reaction force" is practically meaningless because the same can be said of any other force. If we were to keep this section of the current article, I would at least delete the first clause, "Lift is a reaction force". Then you'd have something close to my draft, but I think my draft is more clearly worded.

So it doesn't seem to me that your objection to my proposed structuring of the article is justified, or that you've suggested any real improvement to my draft, at least so far. I think my draft represents a marked improvement over the current article, in both structure and content. Unless you or someone else suggests improvements soon, I'll go ahead and make the replacement. BTW, I've changed the proposed title to "Lift (fluid-dynamic force)" to be consistent with identifying "aerodynamic force" as a special case, as is done in the current article.

I disagree with the response by Djr32 (talk). Just focusing on a straightforward explanation sounds appealing in its simplicity. But given the history of multiple, mostly flawed explanations that have been presented to the public over the years, just presenting yet another explanation, without discussion of the earlier ones, would cause confusion. Given the history of this topic, context-setting is essential if confusion is to be avoided. Again, just look at the confusion manifested in the discussion on this talk page. My proposed revisions are intended to reduce this kind of confusion by providing context.

J Doug McLean (talk) 22:35, 3 June 2014 (UTC)

J Doug McLean wrote: Unless you or someone else suggests improvements soon, I'll go ahead and make the replacement.

I'm sorry, Doug, but that is not the way wikipedia works. The objective is to arrive at a consensus that the proposed changes to the article are an improvement. Thus far, I have not seen any evidence of such a consensus here on the talk page, either with this iteration or the one you proposed last January. To issue an ultimatum that you will "go ahead and make the replacement" unless certain terms are met is at variance with the wikipedia processes.

I agree with Djr32's criticism above.

Moreover, the proposed revision has other problems at the moment:

• Major sections have no citations at all. (e.g. A more comprehensive physical explanation)
• Other sections have insufficient citations.
• Controversial assertions are made in a somewhat opinionated manner, with the only reference being to your book. This violates both WP:NPOV and WP:COI (e.g. Coandă effect & viscosity)
• In many places it reads more like a personal opinion than a neutral encyclopedia piece.

I think it is a good article, but I don't think it is a good wikipedia article. I would like to continue to work towards incorporating some of it, and I'm hoping that you, I, and the other editors can work together to improve the current article .

At this point I'd like to solicit the opinions of other editors. What's your take? Mr. Swordfish (talk) 14:44, 4 June 2014 (UTC)

Thanks for the response. I'm sorry about the ultimatum. I am appropriately chastised and won't let it happen again. Still, there's a lot about the current article that needs changing, and I'm eager to get things off top-dead-center. And I still intend to push for large parts of the article to be replaced, either by me or by others.

I can see now how I got on the wrong side of WP:NPOV regarding Coanda and viscosity, and possibly on the popular explanations of lift. I've reworked those parts to take a more neutral point of view in the writing, while still making clear that there are reliable sources whose position is that some arguments are incorrect. When there are two opposing views on something, there is by definition a "controversy". But it doesn't follow that both views should be labeled "controversial". When one view is supported by the science (and can be verified as such by reliable citations), and the other is not, the physically sound view isn't controversial, to my way of thinking. In the case of the Coanda effect, the primary issue is the questionable use of a term, so I've labeled it "controversial". In the case of the purported role of viscosity in flow turning, it's not just semantics. On this one the science is clear, so I've labeled the "pro" statement a "misconception" and provided a citation of Milton Van Dyke, a leading authority on fluid-flow theory, whose mathematical analysis convincingly supports the "con" statement, i.e. it shows that viscosity plays no significant role in supporting flow curvature in boundary layers. In contrast, the sources for the pro side of this argument (Anderson and Eberhardt, and Jeff Raskin) provide only arm-waving statements with no mathematical support. In a case like this, it seems to me that not all sources should be given equal weight.

I don't see how I've violated WP:COI. As I've said, I think the citations of my book are relevant and not excessive. Boeing, with which I now have no relationship, holds the copyright, and I have no financial interest in the book's sales. And my book is not the only source I cite for the argument that Coanda is not relevant to lift. There are three others. So I think the citations are OK there, but I have revised that section to take a neutral point of view.

You wrote "Major sections have no citations at all. (e.g. A more comprehensive physical explanation)". I'm sure there are places where more citations would be good, but I found no section that had none at all. "A more comprehensive physical explanation" cited my book in the opening paragraph. In one sense that should be sufficient, since everything in the section can be found in that reference. This explanation of lift is my original work, but it is not "original research" by Misplaced Pages's definition because it has a citable source. I know of no other source that has put all of these ideas together in one explanation, so if we're to include this explanation I think we have no choice but to cite my book. The constituent ideas (e.g. the spread-out effects of the airfoil on the flow, the reciprocal relationship between pressure and velocity, and the acceleration of fluid parcels by the pressure gradient), on the other hand, are all well established in fluid mechanics, and there are other sources that can be cited for them. I've added some and continue to look for more. If you see other places that could benefit from more citations, I'd appreciate it if you'd point them out specifically.

J Doug McLean (talk) 22:48, 5 June 2014 (UTC)

I now have a first pass at integrating the proposed changes into the current article. It's at https://en.wikipedia.org/User:Mr_swordfish/Lift

I don't claim that it is perfect, or even ready (yet) to replace the current article. Since it is a mash-up of two different articles the writing style varies a bit. It would be nice to address that. In particular, the cites from the current article are very "web friendly' with hyperlinks where available and brief quotations of the relevant supporting text. In the main article space the reader can mouse-over the footnote and see for him or herself the supporting material without having to go find a book in a library (unfortunately, the mouse-over feature doesn't work in a sandbox). It would be nice to flesh out the citations in the new material to take advantage of this feature.

Regarding WP:COI, I did not mean to accuse anyone of ethical lapses or attempting to profit off our efforts here. Merely that Misplaced Pages's COI policies are rather strict - an author citing his own work and unilaterally editing an article over the objections of other editors would be a problem. As long as it is a group effort with other editors involved we should be in the clear.

Not everything from both articles made it into the integration. Arguments in favor of "restoring" certain sections cheerfully accepted. But when combining two rather long articles it's inevitable that some material would be cut. Perhaps more should be cut or moved to a separate article.

Some material is repeated. Since we're not making a set of mathematical axioms or constructing a normalized database, repetition in itself is not a problem. But there are redundancies that probably should be removed.

So, I'm soliciting comment on this draft. Whether that happens here on this talk page or the talk page in my sandbox shouldn't matter. Since the proposed revision is a release candidate, edits in place are welcome. I'll continue to make edits over the next few days, then I'm on vacation for a while. I'm thinking maybe a target date of July 1 to get it in order and replace the current article, subject to reaching consensus here, of course. Mr. Swordfish (talk) 21:11, 10 June 2014 (UTC)

Integration of McLean's proposed changes and current article

The integration is up in my user space at https://en.wikipedia.org/User:Mr_swordfish/Lift. Please make general comments here, and please start a new section for each specific topic so that we can discuss unrelated issues one at a time. Mr. Swordfish (talk) 21:49, 10 June 2014 (UTC)

After spending only a few minutes looking at in-line citations I noticed that some merely nominate an author and a year; no page or section number, no title, no publisher etc. That standard does not meet Misplaced Pages's guidelines. For complex referencing tasks, I recommend Misplaced Pages:Harvard citation template examplesDolphin (t) 01:08, 11 June 2014 (UTC)

Note that the author/date citations in the "Notes" section refer to entries in the alphabetized "References" list that give the title and publisher. True, some of them need to have a page or section number added. This is a format I've seen in the article on Navier-Stokes equations and in the Journal of Fluid Mechanics. I think the alphabetized list makes some things much easier for the reader, such as answering questions like "does this article cite Lanchester?" The "Notes" section is now a mixture of this new format and the original format. In the original entries that include quotes out front, author information is deeply buried and thus not so easy to find in a quick visual scan. A change that would be helpful, but would take some work, would be to put all the notes in a name/date/section/quote format and put all the all the other information on the references in the "References" list. J Doug McLean (talk) 20:01, 20 June 2014 (UTC)

Proposed changes to early sections of the integration

This integration is a good start. With this as a starting point, I'm proposing some moves and revisions for the early sections. I've installed them in my sandbox User:J_Doug_McLean/sandbox for your review, and I'll explain my rationale here.

I propose moving my new section "The understanding of lift as a physical phenomenon" back to a prominent position, just after the overview. It doesn't fit with the rest of the material in the "A more comprehensive physical explanation" section and would be much less effective there. Discussions over the years on this talk page have convinced me that there's widespread misunderstanding of the status of the hard science and how the qualitative explanations relate to it, so I think this context-setting section should come before the explanations start. I also propose adding words to the effect that the qualitative explanations cannot provide quantitative information for engineering. That's a property of all qualitative explanations, so I think it belongs here rather than in its current place as a limitation of flow deflection.

The integration's "Description of lift on an airfoil" section seems to me to mix explanation and description in a way that's not coherently organized and thus not as easy to follow as it could be (e.g. several of the subheadings under "Newton's laws: lift and the deflection of the flow" don't fit there.). I think the whole section could benefit from reorganization.

So I'm proposing rearranging this as two main sections. The first is "Simplified physical explanations of lift on an airfoil", which gives the deflection explanation first, in keeping with the AAPT advice. I put the points covered by the current "Flow on both sides of the wing" in with the deflection explanation. I kept separate headings for limitations of deflection and Bernoulli, but I'm proposing changes to what's included in those sections. Failure to produce quantitative information is no longer a limitation of deflection, as it's been covered above. I also propose a change under the limitations of Bernoulli (see below).

I propose putting everything else in the current "Description..." section under "Basic attributes of lift", with the current "Summary" statement moved to the front, and keeping the subheadings other than "Flow on both sides of the wing", the main points of which are would now be covered under flow deflection. I'd really prefer to put this section between "Overview" and "The understanding of lift as a physical phenomenon", but putting it after the simplified explanations, so that flow deflection can come earlier, is a compromise I can live with.

I look forward to feedback on these proposed changes. J Doug McLean (talk) 20:01, 20 June 2014 (UTC)

Apologies for taking some time off from this article. We didn't make the July 1st deadline, but hopefully we can get things moving again in the next several weeks.

I think the section https://en.wikipedia.org/User:J_Doug_McLean/sandbox#The_understanding_of_lift_as_a_physical_phenomenon is a fine section and don't disagree with anything in it. However, I do not think it deserves to be the opening section fight after the overview. I recognize that opinions may vary about this, but my issue is that the section is rather meta- that is, it talks about the explanation rather than simply giving it. At some point some amount of meta-analysis about the explanation is in order, but I prefer to cut to the chase and go right in to the explanation. Mr. Swordfish (talk) 20:29, 22 July 2014 (UTC)

I have integrated this section into my draft later in the article. Mr. Swordfish (talk) 19:42, 23 July 2014 (UTC)

Proposed change to limitations of Bernoulli

Under limitations of Bernoulli, I propose deleting the third bullet item because it implies that the assumptions behind the steady incompressible Bernoulli equation are invalid for "real world airfoils" in general. Actually, for low-Mach-number steady flow outside the boundary layer in the reference frame of the airfoil, the Bernoulli equation is highly accurate, contrary to what this paragraph and its sources imply. Significant limitations to the Bernoulli equation apply only in circumstances different from those of the usual airfoil explanations (e.g. accelerating flow or high-speed flow). The arguments in the four cited sources are seriously flawed, and I can give you my detailed reasons if you wish to see them.

I look forward to feedback on this proposed change. J Doug McLean (talk) 20:01, 20 June 2014 (UTC)

I have no objections to this. I've implemented this proposed change in my version of the draft. https://en.wikipedia.org/User:Mr_swordfish/Lift#Limitations_of_explanations_based_on_Bernoulli.27s_principle Mr. Swordfish (talk)

Proposed changes to "Mathematical theories"

The integration makes some changes to the new section "Mathematical theories of lift". The added explanation of getting velocity vectors from CFD and then the pressure from Bernoulli isn't applicable to CFD in general, only to potential-flow methods. I've taken a crack at fixing that and at dealing with the repetition regarding the Kutta condition.

I look forward to feedback on these proposed changes. J Doug McLean (talk) 20:01, 20 June 2014 (UTC)

I'm ok with this. I've replaced this section in my draft with the version from your draft. Mr. Swordfish (talk) 20:42, 22 July 2014 (UTC)

Proposed deletion or substitution of two figures

The figure currently illustrating streamtube pinching, with the caption "Streamlines around an airfoil in a wind tunnel. ..." seems to me to be contradictory. The horizontal bars appear to be intended to represent wind-tunnel walls, but the streamlines don't appear to be constrained by the walls. And wind-tunnel walls are not relevant to the explanation anyway. I propose, as I show in my sandbox, replacing this figure with the flow animation, with a caption tailored to the streamtube pinching explanation.

The figure caption "Uniform flow plus vortex flow (circulation) gives the total flow below" is not technically accurate. To get the flow around the NACA 0012 you would need to add a particular distribution of vortex strength along the chord (not just simple circulation), as well as a distribution of sources and sinks to represent airfoil thickness. I propose just deleting this figure. J Doug McLean (talk) 06:13, 21 June 2014 (UTC)

I agree that the diagram depicting streamtube pinching leaves something to be desired, but it is what's available in the public domain. That said, I think it does a better job of depicting streamtube pinching than the animated picture (https://upload.wikimedia.org/wikipedia/commons/9/99/Karman_trefftz.gif). Ideally, we'd find a better diagram. I'll see what I can turn up.

Agree that the other diagrams are not ideal either. The idea was lifted from one of Anderson's books, but I did the graphics and I'm a terrible graphic artist. I've deleted them from my draft since they may give a too-simplified impression. I do think that a picture representation of the idea of vortex flow + steady flow == total flow helps with a layman's understanding of circulation. But maybe it doesn't need to be in this article.

In coming days I hope to take a good look at both versions of the opening sections and attempt further integration. What would be nice would be to get a third (or fourth or more) editor(s) to help with this. Mr. Swordfish (talk) 20:58, 22 July 2014 (UTC)

I found a better picture for the streamtube pinching.

I think this depicts it better than the animated diagram, and it doesn't have the issues you mention that the old/current picture have. Mr. Swordfish (talk) 14:14, 23 July 2014 (UTC)

Release candidate?

I've spent some time further integrating Doug's draft into my draft, adopting much of his organizational structure and copying entire sections. At this point I think we have a release candidate.

The immediate question is whether it is an improvement over the current article. If so, we should replace the current article now and move forward with further improvements in-place. No article is ever "done" on wikipedia; refinements, additions, and improvements will likely be made in coming months and years.

If the draft is not an improvement then let's discuss how to improve it so that it is better than the current article.

My own opinion is that it's a stronger article. Many thanks to Doug for his effort and patience.

The draft is at https://en.wikipedia.org/User:Mr_swordfish/Lift Please take a look and share your opinion here. Mr. Swordfish (talk) 14:02, 25 July 2014 (UTC)

Great work! I have had a quick look at the release candidate and left a couple of comments at User talk:Mr swordfish/Lift. I will try to do more in coming days. Dolphin (t) 06:38, 26 July 2014 (UTC)

I agree that it's a stronger article, and by that criterion it qualifies for release. But I'd still like to advocate for some further changes that I think would improve it further, if you'll bear with me.

The first has to do with the section "The understanding of lift as a physical phenomenon". We have agreement that it should be included, but not on where to put it. I still think it should precede the qualitative explanations because it seems to me that when things aren't put in perspective at the start, the article ends up giving a misleading impression. If you just "go right in to the explanation" as you prefer, the article gives the impression, intended or not, that that's how lift is understood, and that the rest, including the mathematical theory, is just filling in the details. I think that's a misleading picture of how we really understand lift. And I don't think that reading the meta-analysis later in the article, assuming the reader even gets that far, will be very effective in undoing the impression. Better, I think, not to give the impression in the first place.

The mathematical theory is the bedrock of human understanding of lift. The qualitative explanations are secondary, really just attempts to square the theory with our intuition. And that's a hard thing to do, given that the continuum flow in effect consists of innumerable little parcels of fluid moving in concert to get around the airfoil, each one obeying the 2nd law in a mutual interaction with its neighbors. The theory handles this complexity correctly by requiring the solution of a set of PDEs, but our intuition doesn't do so well when faced with an entire flowfield. For one thing, how the pressure field comes about in such a flow is very difficult to grasp intuitively. The popular explanations either avoid the question altogether (the deflection explanation) or do it badly (the Bernoulli explanations), and even my "more comprehensive explanation" is shaky on this point.

So I feel strongly that before we dive into the qualitative explanations, we owe the reader a heads-up on where such explanations stand in the overall scheme of things, and why. "The understanding of lift..." attempts to do this, but it may not be entirely satisfactory as it stands. It should perhaps be beefed up to make it clear that the difficulty of the problem dooms the qualitative explanations to fall short of being completely satisfying, not just that they don't produce numbers and that there's been disagreement on what to include in them. In my sandbox User:J_Doug_McLean/sandbox I've added a sentence to try to do that.

There are times when some meta-analysis up front makes things better for the reader, and I think this is one of those times.

And the section heading should remain "The understanding of lift as a physical phenomenon", not "Understanding lift as a physical phenomenon". The former implies we're talking about the understanding held by the community at large (which is what this section is doing), while the latter implies a concentration on changing the understanding held by the reader. I think the difference is significant.

Next are a couple more issues with headings:

The content in "Description of lift on an airfoil" isn't really description; it's explanation. I think "Simplified physical explanations of lift on an airfoil" is more consistent with the content.

I think the heading "Methods to determine lift on an airfoil" promises more than we deliver. To apply either "Lift coefficient" or "Pressure integration" you have to know something a priori that's tantamount to knowing the lift. So these are really just relationships for converting one form of knowledge about lift to another; they don't really "determine" lift. I propose deleting the heading "Methods to determine lift on an airfoil" and promoting "Lift coefficient" and "Pressure integration" to sections in their own right.

Most of the material in the section "Kutta-Joukowski theorem" is now covered in "Circulation and the Kutta-Joukowski theorem" in the "Mathematical theories of lift" section. I propose integrating some material from "Kutta-Joukowski theorem" into "Circulation and the Kutta-Joukowski theorem", moving the description of the Magnus effect to "Lift forces on bluff bodies", and deleting "Kutta-Joukowski theorem". I've taken a crack at this in my sandbox User:J_Doug_McLean/sandbox.

Then a few technical issues:

Under "Flow deflection and Newton's laws", the statement "The resulting force upwards is equal to the time rate of change of momentum of the air downwards" is problematic on two counts, in spite of the fact that it has a citable source.

The first problem with the statement is that for it to be true, the downward force exerted by the airfoil on the air surrounding it would have to be the only force being exerted on "the air". There are many possibilities for how we can define the body of air we're considering, and this condition (no other force but the lift) isn't met in general. The airfoil exerts a downward force on the inner boundary of the body of air surrounding it (at the airfoil surface), but the surrounding environment exerts unbalanced pressure forces on the outer boundary of the body of air. This problem cannot be eliminated just by increasing all the dimensions of the "box" of air we consider, even to the limit of infinity. As the box is made larger, the pressure disturbances at the outer boundary get weaker, but the area over which they act gets larger, and the integrated force remains comparable to the lift. How much of the lift is accounted for by this pressure force rather than by momentum change depends on the proportions of the box. For example, for a box that is very large horizontally compared to its vertical dimension, practically all of the lift is accounted for by pressure at the outer boundary, and practically none by momentum changes. Only in the limit as the vertical dimension of the box becomes large relative to the horizontal is it true that lift is accounted for by momentum changes.

The other problem is that most such analysis in fluid mechanics deals with boxes whose boundaries are fixed in space. The time rate of change of momentum in such a box is zero in steady flow, and momentum changes must be assessed in terms of fluxes in and out, not the time rate of change.

Thus the simple explanation in terms of flow deflection is correct only if it's couched in vague terms such as "for the airfoil to deflect the flow downward, it must exert a downward force on the air". The more specific statement "lift is equal to the time rate of change of momentum of the air" is not correct in general. I recommend deleting this sentence.

Under "Limitations of deflection/turning", the only limitation mentioned is that this explanation doesn't produce numbers. This limitation is a characteristic of all of the qualitative explanations and is now included in "The understanding of lift as a physical phenomenon". I recommend substituting the paragraph under "Limitations of the flow-deflection explanation" in my sandbox version User:J_Doug_McLean/sandbox, which discusses some limitations specific to deflection.

Under "Increased flow speed and Bernoulli's principle", the first paragraph needs to stipulate that Bernoulli's principle requires steady flow.

In that same section, I recommend deleting the second paragraph. The statement "Bernoulli's principle does not explain why the air flows faster over the top of the wing" isn't true. On the contrary, Bernoulli's principle tells us that if the air flows faster, it is because of the lower pressure. It's just that that didn't help the originators of the Bernoulli explanations in boot-strapping their way toward an explanation of where the low pressure comes from, and they had to find other reasons for the faster flow.

We're getting close, but I'd appreciate it if you'd consider the above changes. Thank you for your patience.

J Doug McLean (talk) 01:58, 27 July 2014 (UTC)

A lot to respond to at once, so I'll break it up into bullet points;

• Placement of the "The understanding of lift as a physical phenomenon" section - As I've stated before I think it s a good addition to the article, but leading with it seems off-putting to the general audience. And really, fundamentally I think our disagreement here stems from a different idea of who the intended audience is. For the layperson who knows little about the subject (i.e. the vast majority of wikipedia readers) the section would make little sense without first providing some context. I am open to moving it up in the article, say, between "Basic attributes of lift" and "A more comprehensive physical explanation."

• Section heading - I don't see a big difference in meaning between "The understanding of lift as a physical phenomenon" and "Understanding lift as a physical phenomenon" to my eyes, the former merely has two extraneous words. But I can see how you would parse it differently than I, so I've reverted the heading to your original.

• Section heading - "Simplified physical explanations of lift on an airfoil" is fine by me, I'll implement that change too.

• Section heading - propose deleting the heading "Methods to determine lift on an airfoil" and promoting "Lift coefficient" and "Pressure integration" to sections in their own right. that sounds reasonable. I'll do it and see how it looks.

• "The resulting force upwards is equal to the time rate of change of momentum of the air downwards" is problematic - I'm gong to punt on this one for now. Let me give it some thought and attention and I'll get back to you.

• "Limitations of deflection/turning" - I think you make a reasonable criticism, but I also think there's a bit of strawman there - the basic deflection explanation does not refer only to forces "exchanged at the airfoil surface, where the air and the airfoil are actually in contact". The diagrams clearly show air being deflected at some distance from the foil, not just at the surface.
  
  Agree that it doesn't explain why the air is deflected a distance away from the foil, or how the force manifests itself as a pressure difference, but my take is that it doesn't have to. For instance, it also doesn't explain why the air moves faster over the top or any of a hundred other related phenomena. It does explain where the lift force comes from and that's the point of the exercise.
  
  We have to be careful that we don't give the misleading impression that deflection is wrong or incorrect and I think the typical wikipedia user could get that impression from what you have written. I'll take a stab at addressing your concerns by adding some of these issues to the list of limitations.

• Redundancies in K-J theory section. Your proposal sounds fine. I'll take a look at integrating your changes into my version.

• Under "Increased flow speed and Bernoulli's principle", the first paragraph needs to stipulate that Bernoulli's principle requires steady flow. Ok, I'll add something to that effect.

• "Bernoulli's principle does not explain why the air flows faster over the top of the wing" isn't true. Yeah, that sentence has always bothered me too. Once you know that the pressure is reduced, BP tells you that the speed is faster. So it does explain why. Equal transit time doesn't explain why the air goes faster, and most explanations based on BP do not explain (correctly anyway) why the air goes faster. I'll look at changing that sentence.

It's now Friday, August first, and I think we are close enough that we can go live with the revision as-is. We can continue to discuss outstanding issues afterwards. I'll make the changes outlined above and unless I hear objections I'll replace the current article with the draft early next week.Mr. Swordfish (talk) 15:19, 1 August 2014 (UTC)

UPDATE: I've now completed the above edits. One issue I see is in response to I propose deleting the heading "Methods to determine lift on an airfoil" and promoting "Lift coefficient" and "Pressure integration" to sections in their own righ... I propose integrating some material from "Kutta-Joukowski theorem" into "Circulation and the Kutta-Joukowski theorem", moving the description of the Magnus effect to "Lift forces on bluff bodies", and deleting "Kutta-Joukowski theorem".

I've done this in my draft (with the exception of the treatment of the Magnus effect - will take a look at that next) A question: does it make sense for Lift Coefficient and Pressure Integration to have their own sections, or does it make more sense for them to be sub-sections under "Mathematical theories of lift"? I'm in favor of the latter, but could be convinced otherwise. I'm going to move them under the math section pending further discussion. Mr. Swordfish (talk) 18:29, 1 August 2014 (UTC)

Doug McLean wrote: ...the statement "The resulting force upwards is equal to the time rate of change of momentum of the air downwards" is problematic...

I have to say that I was surprised by this, so much so that I needed to take a few days to think about it before responding. And the reason for the surprise is that the statement is merely a combination of Newton's 2nd and 3rd laws with dp/dt replacing ma. This should be about as uncontroversial as it gets. In the simple model where all we consider is the air flow and the foil, it follows directly from Newton's laws. Of course, if the air is being accelerated by something other than its interaction with the foil then that additional acceleration will not contribute to the lift force, but it seems clear to me from the context that we're not talking about that scenario. I've added some language to clarify:

The resulting force upwards is equal to the time rate of change of momentum of the air deflected downwards by the foil.

Agree that this total momentum change is difficult to calculate or measure. But in theory at least it must be equal to the lift force.

BTW, there's not just one cite for it, three others are included elsewhere: https://en.wikipedia.org/User:Mr_swordfish/Lift#cite_note-7 https://en.wikipedia.org/User:Mr_swordfish/Lift#cite_note-23 https://en.wikipedia.org/User:Mr_swordfish/Lift#cite_note-32

Mr. Swordfish (talk) 18:33, 4 August 2014 (UTC)

I'm also surprised by Doug's comment. Perhaps he is alluding to the idea that the time rate of change of momentum is equal to the aerodynamic force and not just its vertical component, lift. Dolphin (t) 22:28, 4 August 2014 (UTC)

No. The problem has nothing to do with whether we consider the total force exerted by the foil or only the lift component. See my response to Mr. Swordfish above. J Doug McLean (talk) 00:19, 7 August 2014 (UTC)

Your reasoning regarding placement of "The understanding of lift as a physical phenomenon" puzzles me. You argue that leading with it would be "off-putting to the general audience" and that it would "make little sense without first providing some context". But providing context is what this section is intended to do. It seems to me that launching directly into the deflection explanation without the context provided by "The understanding of lift as a physical phenomenon" gives a mistaken impression to the reader, to be remedied only later in the article: "Oh, by the way, those explanations we gave you early on aren't the real story on how lift is understood."

As for making "little sense" to an audience without prior knowledge, I don't see it that way. The section is brief and to the point, and, I think, easy to understand. I assume the target audience of an article in a general encyclopedia is literate adults, not children. If I were the reader, I'd welcome the context-setting up front. This is an encyclopedia article, not a mystery story.

So how do we decide this? Let's try a little meta-analysis of the arguments pro and con. I've argued that logical exposition of the subject matter favors having "The understanding of lift as a physical phenomenon" precede the simplified explanations, so as to put them in context with general understanding of lift. You haven't offered a counter-argument to this but have instead brought up other issues: "It's -meta." "It's potentially difficult for an audience without prior knowledge to understand." I think I've offered effective rebuttals to these arguments.

Regarding my proposed passage in "Limitations of deflection/turning", I think your argument that there's a strawman there is unjustified. I do say that the only forces referred to are those "exchanged at the airfoil surface, where the air and the airfoil are actually in contact", which is true. I don't say that those forces are all that the explanation refers to. I think the passage should be included. It doesn't imply that deflection is incorrect, just that it leaves a gap in that it doesn't explain how a deeper swath of flow is deflected than is touched by the airfoil.

Regarding "The resulting force upwards is equal to the time rate of change of momentum of the air downwards", I thought I made it clear in my previous posting what the problem with this statement is, but your response indicates that you don't agree that the force exerted by the airfoil is not generally the only force exerted on "the air" as a result of the lift. Let's look at this further.

A crucial question raised by the statement is what is meant by "the air". Again, any body of air that you choose to define as "the air" surrounding the airfoil must have both an inner boundary where the airfoil contacts it and an outer boundary where the surrounding environment contacts it. As a result of the lift there is generally an unbalanced pressure force on the outer boundary, so that the force exerted by the airfoil on the inner boundary isn't the only force resulting from the lift. With some detailed bookkeeping this pressure force can be quantified. If we put the outer boundary far enough from the foil, the idealized model of a uniform flow plus a vortex suffices, and we can draw general conclusions. Please read my previous comments where I explain how the percentage of the airfoil's force that is offset by the pressure force on the outer boundary depends on the proportions of the outer-boundary box.

Anyway, for most ways of defining what is meant by "the air", the statement is untrue. For example, the reader might reasonably assume that "the air" refers to the whole atmosphere. Given this definition of "the air", the downward force exerted on the air by the airfoil is completely offset by a distribution of over-pressure on the ground (see the famous figure 150 in Prandtl and Tietjens for what this pressure footprint looks like in 3D), so that the net force exerted on the air as a result of the lift is zero. Then the time rate of change of the integrated vertical momentum of the air must be zero as well. It's just Newton's second law, as you say. So again, the problem with the statement in the current draft is that it's not generally true, because it doesn't account for all the forces.

Your proposed clarification, i.e. limiting the statement to "the air deflected downwards by the foil", doesn't fix the problem. The subset of the air that's actually experiencing downward acceleration is still a body of air that an outer boundary can be drawn around. That body will still in general have a net pressure force on its outer boundary, so that the downward force exerted by the foil will not be the only force acting on that body of air. Thus even your clarified version of the statement isn't generally true.

There is one way to define "the air" so that the statement is true, but I think it's too specialized and complicated to be appropriate for this article. Draw the outer boundary of the box so that the vertical dimension is much larger than the horizontal dimension. In the limit as the vertical dimension goes to infinity relative to the horizontal dimension, the net vertical pressure force on the outer boundary vanishes. Then all of the lift is accounted for by the momentum transfer and none by the outer-boundary pressure force. But we're not done yet. To observe the momentum transfer as a time rate of change, we have to take another special step. The outer boundaries of the box must be assumed to be moving with the flow so that the box is not gaining or losing fluid anywhere along the boundary (This is different from the usual approach to control-volume analysis, in which box boundaries are fixed in the reference frame of the body). Only for this very special definition of "the air" can we make the statement that the lift is equal to the time rate of change of the integrated downward momentum of "the air". Unless we're willing to add these specialized qualifications to the statement (along with an appropriate citation), I think we should delete it.

This isn't just a quibble about rigor. The statement L = dp/dt is actually untrue for many reasonable ways of defining what is meant by "the air", i.e. it's untrue for anything other than an infinitely tall vertical sliver with boundaries that move with the flow.

The three sources you mention all make an error that's easy to make, i.e. applying the second law to a body of air, but without adequately defining what is meant by "the air" and without identifying all the relevant forces. The idea that "the air" generally has lift-related forces acting on it other than that exerted by the foil seemingly didn't occur to them. J Doug McLean (talk) 00:19, 7 August 2014 (UTC)

There seem to be three remaining areas of disagreement;

• Placement of the "The understanding of lift as a physical phenomenon" section. At this point, I don't think either of us are going to be swayed by the others opinion. My sense is that we have a disagreement over the intended audience and how best to serve that audience. My view is that the section makes a lot of sense to those who are already familiar with the material, but that it's "inside baseball" for 99% of the audience. My editorial sense is that we shouldn't lead with it.
  This will have to be resolved by seeing what the other editors think or getting a third opinion.
  I do take exception to your characterization "Oh, by the way, those explanations we gave you early on aren't the real story on how lift is understood." The simplified explanations are every bit as "real" as the more thorough explanations. Replace real with full and I'll agree with you. But the article is quite upfront about the fact that the simplified explanations are not the "full story".

• "Limitations of deflection/turning" I've added a few lines reflecting the issues you bring up. Please take a look.

• "The resulting force upwards is equal to the time rate of change of momentum of the air downwards" I have to say that I am not able to follow your line of argumentation. And even if I could and agreed with it, it wouldn't matter for our purposes here as editors. Discussions on this page are not citable. Our job as editors is to reflect what is published in reliable sources. In reading literally hundreds of articles on this subject, I have never run across a single one refuting this assertion. Meanwhile, there are several reliable sources that support the statement. Perhaps if there was some disagreement in the literature we could present it as a controversy. But unless we have some reliable published source we are bound to present what's been published. See WP:TRUTH for more details.

Mr. Swordfish (talk) 18:33, 7 August 2014 (UTC)

My arguments on the "rate of change of momentum" statement are all supported by citable sources. A minimal but sufficient set of them will be quoted below. I didn't bring them up before because I was advocating for deleting the statement, and I didn't think that would require citations.

Because the statement doesn't specify what it means by "the air", a reader would and should expect it to be true for any reasonable assumption as to what "the air" encompasses. However, it is well established in the aerodynamics literature that the statement is false for most of the assumptions the reader might make, i.e. it is false if "the air" refers to the whole atmosphere or to any subset of it that isn't very tall compared to its width. If the statement failed only in exceptional circumstances I wouldn't press the issue. But it fails for the most obvious assumption the reader is likely to make, i.e. that "the air" refers to the whole atmosphere. So the problem is serious.

Because the statement has been shown by reliable sources to be contradicted in relevant situations, it has been effectively refuted, and letting it stand "as is" would be inaccurate and inconsistent with "what's been published". I think that leaves us two options:

1) Delete the statement and the citation. It isn't crucial to the deflection explanation, which is most often stated without the quantitative assertion " is equal to...." anyway. We have ample evidence from the mainstream aerodynamics literature that the statement is faulty, justifying our deleting it.

2) Keep the statement but add the clarification that's needed to make it clear when it's true and when it's not. Here's a rough draft of what I think that would have to look like:

In the text of the deflection explanation:

The resulting force upwards is equal to the time rate of change of momentum of the air downwards. This statement assumes that all of the lift can be accounted for by a momentum change in "the air", which is true only if "the air" refers to a region that is very tall relative to its width. For the atmosphere as a whole, or for a subset of it that is not tall compared to its width, part or all of the lift is accounted for by pressure differences on the top and bottom of the body of air in question, reducing the proportion accounted for by the momentum change.

In the notes section:

For the atmosphere as a whole, the integrated time rate of change of vertical momentum due to the lift on a wing is zero , and the lift is reacted entirely by a pattern of overpressure on the ground. For regions that are subsets of the atmosphere, the proportions of the lift that are accounted for by momentum change and by pressure differences depend on the size and shape (vertical dimension compared to horizontal dimension) of the region. Only if the vertical dimension is very large relative to the horizontal dimension are the pressure differences negligible, leaving the lift entirely accounted for by the change in momentum. (Section numbers and quotes would be added to these citations, and other citations could be added. I mention only the ones that come immediately to mind, but I think even just these would be sufficient.)

I expect you'll agree that the second option is too complicated and technical to be appropriate for this article. I'd argue that deleting the statement and the citation is the better option.

I wouldn't advocate presenting this as a "controversy" because I don't think it amounts to one. The "con" arguments are from the mainstream aerodynamics literature, where they are supported by rigorous math. The "pro" statements you've cited are not supported by rigorous analysis and are all from "The Physics Teacher", which is not a mainstream source of information on aerodynamics. The error made by the statement isn't something esoteric about which experts might disagree; it's basic: It is wrong to apply Newton's second law to just a subset of the forces exerted on a body. In addition to the force exerted by the foil, the air around an airfoil generally has unbalanced pressure forces acting on it. Not being aware of these pressure forces is understandable in this case. Authors of articles in "The Physics Teacher" are not typically mainstream experts on aerodynamics.

A relevant quote from WP:TRUTH: "To know where we have a dispute and where a simple mistake, consider whenever the author is really an expert on the topic (and not an expert on another topic, making a brief reference to something beyond his area of expertise)...." So we editors are not just cyphers. We are expected to exercise judgment as to the relative authoritativeness of our sources. Weighing what's been published in the mainstream aerodynamics literature against the statement in question, I think we'd be on firm ground deleting the statement and the citation.

Your proposed changes to "Limitations of deflection/turning move in the right direction, but not far enough, in my opinion. And the first and second sentences have a jarring relationship. The first sentence deals with the failure to produce quantitative results. The second begins with "In particular," implying it is about to home in on a particular aspect of that issue, but then deals only with the incompleteness of deflection as a qualitative explanation, which is a separate issue. I'd replace "In particular" with "Furthermore". The third sentence deals with issues that are treated further later in the article, so some tie-in would be good. Here's a shot at fixing the whole paragraph, with some rearranging to keep the quantitative and qualitative issues separate:

This simple explanation, while correct in as far as it goes, is not sufficiently detailed to support the precise calculations required for engineering. Quantitative predictions require a mathematical theory as described below under "Mathematical theories of lift."

Furthermore, this explanation does not explain pressure and velocity variations in the vicinity of the airfoil or how the airfoil can impart downward turning to a much deeper swath of the flow than it actually touches. "A more comprehensive physical explanation" given below attempts to address these issues in a qualitative way.

On an earlier question, I don't see "Pressure integration" and "Lift coefficient" as belonging in the mathematical-theory section. I think they would fit well in "Basic attributes of lift", with the material in "Pressure integration" merged into the current "Pressure differences", and the material in "Lift coefficient" merged into the current "Air speed and air density". I've tried this out in my sandbox User:J_Doug_McLean/sandbox, and I think it works well.

J Doug McLean (talk) 06:44, 10 August 2014 (UTC)

This discussion on change of momentum raises some interesting questions. It seems there's no controversy over the physics, but you disagree with the way some authors have concisely described the fulfilment of Newton's second and third laws. I appreciate that a quantitative integration of momentum change would require bounds to be defined carefully to avoid incorrect results, but our reader is not asked to do so. I agree this level of detail would be unnecessary for the article.

Does the reader need any concept of packets of air which are subject to changes in either momentum or pressure? Isn't all fluid pressure ultimately caused by the change in momentum of particles of fluid as they strike their container?

If the change of momentum of the air deflected downwards by the foil is understood to refer only to air which has its momentum changed downwards because of the movement of the foil, what other subset of 'the air' is included in the description which shouldn't be? Burninthruthesky (talk) 11:21, 10 August 2014 (UTC)

It's not just that I disagree with the concise statement. It's that the statement, taken at face value, has been refuted by authoritative sources. But you raise interesting questions.

Yes, all pressure in gases is caused by changes in momentum of molecules striking and rebounding from the surface. In liquids, it's more complicated because molecules are in constant contact with their neighbors and can transmit force and exert pressure on the surface without changes in momentum. So the answer to your question is yes, but only for gases, not fluids in general.

The molecular momentum change involved in gas pressure must be assessed very close to the surface. The only molecules that can be included are those between the last collision with another molecule before striking the surface and the first collision after rebounding. At sea level the region of interest would be about a micron (several mean free paths) thick, and only a fraction of the molecules in that region would qualify. That's a very limited subset of the air surrounding the airfoil.

We could make the AAPT statement true by limiting it to that small subset of the molecules surrounding the foil. But I see several serious drawbacks to casting the statement in that form:

1. Aerodynamics generally deals with fluid motion as if the fluid were a continuum rather than individual molecules because it is very difficult to gain understanding or make predictions at the flowfield level with the molecular approach.

2. The continuum description is more generally applicable. Gases and liquids behave very differently at the molecular level but practically identically at the continuum level (for low Mach number in the gas).

3. Lift is an aerodynamic (and hydrodynamic) phenomenon. In the continuum approach to aerodynamics, "momentum" refers to the bulk momentum of the flowing fluid. The explanation of pressure in terms of molecular momentum, as you're proposing, refers to the thermal momentum of the molecules, not the bulk momentum of the fluid. This kind of explanation doesn't distinguish between aerodynamic and aero-static situations. Even in still air we could say that the pressure force on a portion of a surface is equal to the time rate of change of thermal momentum of the air molecules near the surface (the right subset of them). But this is a situation in which the air has no bulk momentum in the conventional aerodynamic sense.

4. The statement in the AAPT article refers to bulk momentum in the conventional continuum aerodynamic sense and deals with momentum changes taking place over distances measured in airfoil chords, not molecular mean free paths. If we were to recast it in the limited molecular sense, we'd need a different citation.

The upshot: I don't think that interpreting the AAPT statement in terms of molecular momentum is a good solution to the questions it raises. I still think we should just delete it. J Doug McLean (talk) 20:34, 10 August 2014 (UTC)

Thank you for your detailed reply. I now understand the texts refer to momentum at the continuum level rather than the molecular level.

No doubt your argument is well supported by sources, but I don't see a refutation of the statement. On the contrary, I see a description of how to prove mathematically that the statement is true; identifying which subset of the air is subject to downwards momentum changes and avoiding errors such as applying Newton's second law to a subset of forces, or summing action and reaction in the calculation of a single force. We agree that that calculation is beyond the scope of the article.

I'm still not convinced that the cited non-specialist authors have made a mistake. I should point out that Chris Waltham's article does mention that, "to do this more correctly, we would box in the wing with a control volume of infinite vertical thickness". It happens all the time in science that a problem needs to be described and understood at the basic level before more rigorous treatement is attempted. Mr. Swordfish makes a good point that there's a risk of losing the reader by expecting too much knowledge too early in the article. Burninthruthesky (talk) 08:07, 12 August 2014 (UTC)

Also, Eastlake says:

In the interest of generalization, it is appropriate to recognize that the isolated wing is not the only type of flow-field geometry. When there are other surfaces nearby, such as walls in flow through ducts or the ground, those other surfaces can and do change the momentum of the flow as well.

Would it help the article to clarify that it refers to an isolated wing/foil? Burninthruthesky (talk) 16:33, 12 August 2014 (UTC)

My preference is to keep the section under discussion as brief and to the point as possible, without adding a lot of qualifying language that obscures the simple meaning. It's an introductory section aimed at the lay reader after all. I'd rather remove the sentence than replace it with a multi-paragraph treatment of how to compute the infinite integrals to make the statement true. I don't think the statement is essential to the presentation and if there is consensus to delete I will reluctantly go along.

That said, I still think its a fairly straghtforward re-statement of Newtons' 2nd and 3rd laws and should be uncontroversial. Granted, when attempting to precisely elaborate on it, what one means by "the air" can be thorny. In a very simple model where all that is present is a uniform infinite fluid flow and a single airfoil, it has to be true. Add other things to the model such as gravity or the ground or other phenomena that affect the air and one has to be much more careful to obtain that result via the calculations. But it seems clear to me from the context that we're talking about a simple model rather than a more complex one.

Adding the word "isolated" probably won't make the sentence any clearer to the lay reader, but I'll go along with that if necessary. I really don't want to add a paragraph of introductory language to set up the simple statement of F = dp/dt. Mr. Swordfish (talk) 15:52, 13 August 2014 (UTC)

I agree. The section describes a principle of physics, not how to model a real-world example. I suppose the article wouldn't suffer greatly from the removal of that sentence, but I don't agree that it is false at face value. Burninthruthesky (talk) 17:33, 13 August 2014 (UTC)

On that note, I think the context was clearer and flowed more naturally from, "Whenever airflow changes direction", before "by the foil" was . As Doug McLean said, it didn't help. Burninthruthesky (talk) 07:13, 14 August 2014 (UTC)

Agreed. I'll remove "by the foil" Mr. Swordfish (talk) 11:38, 14 August 2014 (UTC)

>Doug wrote:Here's a shot at fixing the whole paragraph , with some rearranging to keep the quantitative and qualitative issues separate:

I adopted this language in the draft. I will look at moving the "Pressure integration" and "Lift coefficient" as per your suggestions. I think we are closing in on a releasable article. Mr. Swordfish (talk) 15:57, 13 August 2014 (UTC)

I think the draft is an improvement and look forward to the release. Burninthruthesky (talk) 17:33, 13 August 2014 (UTC)

I've now moved the "Pressure integration" and "Lift coefficient" sections from the "Mathematical theories of lift" to "Basic attributes of lift". However I kept them as their own sub-sections - my take is that they are important enough on their own to merit their own sub-section. However, I did retain most of Doug's edits to the material. Mr. Swordfish (talk) 20:22, 13 August 2014 (UTC)

I'd like to take one more try at easing your reluctance to delete the L = dp/dt statement. The statement's problems really are more serious than either of you have given them credit for.

The conclusion that "In a very simple model where all that is present is a uniform infinite fluid flow and a single airfoil, it has to be true" isn't supported by the math unless you make stipulations about the shape of the region you're talking about. To evaluate either the pressure forces or the momentum fluxes in an infinite atmosphere, you have to evaluate the integrals over some finite box and then take the limit as the dimensions of the box go to infinity. Even in the case without a ground plane or any other surface, the L = dp/dt statement is true only if the ratio of the vertical height of the box to the horizontal width is infinite, a very specialized condition. It is untrue for any other shape of box. Thus simply adding "isolated" wouldn't by itself make the statement true in general.

I'll mention two examples with an infinite atmosphere, in which the statement fails, both from the mainstream literature. In both cases the results hold as the size of the box goes to infinity.

1. For a "pancake" box (infinite ratio of width to height) all of the lift shows up as integrated pressure differences between the top and bottom, whether there is a ground plane or not. The only difference is that with a ground plane all of the integrated force comes from the pressure excess on the bottom (the ground), while without a ground plane the force is equally divided between the pressure excess on the bottom of the box and a pressure deficit on the top.

2. For a square box centered on the foil, once the sides of the box are longer than about ten airfoil chords, effectively half the lift is accounted for by the pressure differences between the top and bottom of the box, and half by the change in momentum. As the dimensions go to infinity, "effectively half" converges to "exactly half". The same goes for a circular box.

I realize that this is counterintuitive. How can the pressure differences infinitely far from the foil account for half the lift? Well, for the small disturbances far from the foil the pressure perturbations are proportional to the velocity perturbations associated with the circulation, which die off as 1/r. The area over which these must be integrated is proportional to r. The upshot is that in the limit as the box size goes to infinity the integrated pressure force on the outer boundary of the box goes to a constant value equal to half the lift. So the force exerted by the foil is not the only force exerted on the air in the box. No matter how large the box is made, the air outside it exerts an unbalanced pressure force on the air inside.

So I still maintain that reliable sources have shown that the L = dp/dt statement is false at face value. There is a wide range of reasonable interpretations of what is meant by "the air" for which it fails, even in the simple "isolated" case. It isn't a "fairly straightforward re-statement of Newtons' 2nd and 3rd laws" because meeting the requirements for applying Newton's second law (the force used in the equation must be the resultant of all the forces, not just a subset) still requires a very special shape for the box.

I disagree with the statement "It happens all the time in science that a problem needs to be described and understood at the basic level before more rigorous treatment is attempted", at least when it comes to aerodynamics. Trying to understand aerodynamics at the basic level without support of rigorous analysis is too error-prone. J Doug McLean (talk) 19:29, 14 August 2014 (UTC)

Thank you for your time and patience in explaining your point. It's been an interesting discussion and I've learned a lot:

• In an infinite (square) universe, the forces between the airfoil, the air, and the environment can all be calculated by integration. Changing the bounds of integration changes the results, although the facts and forces in the situation remain the same.

• It can be shown that the airfoil exerts a force on the air which is equal to the time rate of change of momentum of the air downwards.

• It can also be shown that the environment exerts a force on the air which is equal to the time rate of change of momentum of the air upwards.

• In accordance with Newton's third law, the upwards and downwards forces on the air are equal and opposite. As a result, the net change in momentum of the air as a whole is zero.

• It is not correct to say, "half the lift is accounted for by the pressure differences between the top and bottom of the box, and half by the change in momentum." The lift is reacted entirely by a pattern of overpressure on the ground . If you had it both as an overpressure on the ground and as momentum in the air, that would be double bookkeeping ].

I still see no reason to remove the disputed statement but if I've misunderstood any of the above, I am happy to be corrected. Burninthruthesky (talk) 07:00, 15 August 2014 (UTC)

My take is that if the integration results are dependent on the relative dimensions of the box as the limit goes to infinity, then the results are not physical but rather an artifact of the model. We've seen this before: a well known rule of thumb for real world air foils is that the downwash angle is approximately equal to one half the angle of attack. This can be measured without recourse to taking integrals over an infinite area. But when you model it as 2-D airflow, in the limit as the span -> infinity the downwash angle becomes zero. This is an artifact of the model, not a reflection of the physics. I think that's what's going on here.

I agree with both Doug and Burninthruthesky that it's incorrect to say that half the lift comes from momentum change and half comes from pressure differences. Granted, it may be possible to integrate in such a way that two terms appear that can be interpreted as momentum change and pressure difference, and depending on the relative dimensions of the box it can be 50-50, 0-100, or 100-0. My understanding of the physics is that all of the lift force can be ascribed to momentum change, and that all of the lift force can be ascribed to pressure differences. This half-and-half nonsense appears in some popularizations (one variant says "the foil generates lift on the bottom by Newtons law and on the top by Bernoulli's principle").

I'm with Burninthruthesky in seeing no reason to remove the statement. Mr. Swordfish (talk) 19:43, 15 August 2014 (UTC)

Both of you ( Mr swordfish and Burninthruthesky) seem still to think that my arguments against the L = dp/dt statement don't really apply to an infinite atmosphere and can be dismissed. The counterarguments you offer, however, aren't supported by the math or by reliable sources, only intuition. Intuition is not a reliable guide on this issue, as I'll try to show. You're probably tired of my arguments on this issue, but I intend to persist as long as the arguments you present for dismissing them are erroneous.

The "artifact of the model" argument doesn't hold water for either example.

The example of the airfoil "rule of thumb" is comparing apples and oranges, I think. The downwash angle behind "real world airfoils" equals roughly half the angle of attack (relative to the zero-lift line), when evaluated at a location about a quarter chord behind the trailing edge. That close to the airfoil, the downwash isn't affected much by aspect ratio, even as it goes to infinity, so the rule of thumb should apply regardless of aspect ratio. In 2D airfoil theory, the predicted near-field flow agrees with the rule of thumb, and the predicted downwash angle goes to zero only far behind the airfoil. So I think you're comparing the rule of thumb for the downwash angle near the foil with what 2D theory predicts for the downwash angle far away, and unjustly blaming the discrepancy on the 2D theory. And on the centerline far behind a 3D wing, the downwash angle goes to zero as aspect ratio goes to infinity, both in theory and in the real world.

In the example of the integration of pressures and momentum changes in boxes surrounding an airfoil, the results are indeed "dependent on the relative dimensions of the box", even as the box size goes to infinity. But this is not because the flow model used is "not physical". It is because an infinite atmosphere is an artificiality. For boxes of finite size, no matter now large, the different results for different relative box dimensions reflect physical reality. The fact that the differences persist as the dimensions go to infinity isn't an "artifact" of the math. It reflects the fact that the momentum aspect of the physics is actually ill posed on an infinite domain. See comments below on Burninthruthesky's first bullet item.

The fact is that the partition of the force into pressure differences and momentum changes actually does depend on the shape of the box, no matter how large. So you haven't refuted my objections to the L = dp/dt statement.

To repeat and address the bullet points:

Burninthruthesky wrote * In an infinite (square) universe, the forces between the airfoil, the air, and the environment can all be calculated by integration. Changing the bounds of integration changes the results, although the facts and forces in the situation remain the same.

It's true that what's going on physically doesn't change depending on how we choose to model it. But to quantify the forces and momentum changes, we have no choice but to calculate them by integration, and to do that you have to specify a domain. Then if the integration is done correctly, the results reflect the "facts and forces in the situation" in that domain. As I've pointed out, how much of the lift is accounted for by pressure differences and how much by momentum changes depends on the shape of the domain, even as the dimensions of the domain go to infinity.

Our intuitions resist this, and we tend to assume that there must be a single correct answer for the entire infinite domain. But our intuitions about infinite domains are often wrong, and they're wrong in this case. The double (2D) or triple (3D) integral for the net vertical momentum due to a lifting airfoil or wing in an infinite atmosphere is non-convergent, which means that the vertical momentum is indeterminate. And you can't make quantitative statements about an indeterminate quantity. Still, one thing these integrals tell us about the "facts and forces in the situation" is true: The balance between pressure differences and momentum changes is different in domains of different shapes, no matter how large the domains are.

This non-convergence doesn't matter in the real world because there is always a ground plane. Even for a semi-infinite space above a ground plane, the integrals converge, and the integrated vertical momentum in the whole atmosphere is zero. For finite subsets of the atmosphere, the shape of the box has strong effects on the results, as I've argued all along. See comment on fourth bullet below for what happens when the height above the ground is large compared to the box size.

The non-convergence of the momentum integral in an infinite domain and convergence in a semi-infinite domain is not reported in the aerodynamics literature, other than in my book, as far as I know. It's based on a careful analysis of the far-field behavior of the integral by a colleague, a qualified mathematician.

Back to the infinite-atmosphere case: What you say about the effects of "Changing the bounds of integration" could benefit from some clarification. The results of the integration change drastically with the shape of the domain, but if the shape is held constant, the results cease to change with the size of the domain once it is large enough.

Burninthruthesky wrote * It can be shown that the airfoil exerts a force on the air which is equal to the time rate of change of momentum of the air downwards.

No, not in general. It can be shown only for a domain with an infinite ratio of height to width.

Burninthruthesky wrote * It can also be shown that the environment exerts a force on the air which is equal to the time rate of change of momentum of the air upwards.

This is not true for any domain that contains a lifting airfoil. Remember, a force is equal to a time rate of change in momentum only if it's the only force (or the vector sum of all the forces) acting on the object in question. When there's a lifting airfoil, the force exerted by the environment isn't the only force acting on the air in the domain.

Burninthruthesky wrote * In accordance with Newton's third law, the upwards and downwards forces on the air are equal and opposite. As a result, the net change in momentum of the air as a whole is zero.

Are you referring to the forces exerted on the air by the foil and by the surrounding environment? If so, what you're saying isn't a correct application of Newton's third law. The third law refers to the forces exchanged between two objects, not to the forces exerted by two objects (the foil and the environment) on a third (the air). There is no reason two separate forces acting on a third object must be equal and opposite.

With this bullet and the two previous, you've argued yourself into a contradiction: The net change of momentum of the air is zero. Thus according to the first of the three bullets, the lift is zero, which I don't think is what you were assuming.

Burninthruthesky wrote * It is not correct to say, "half the lift is accounted for by the pressure differences between the top and bottom of the box, and half by the change in momentum." The lift is reacted entirely by a pattern of overpressure on the ground . If you had it both as an overpressure on the ground and as momentum in the air, that would be double bookkeeping.

I think I made it clear that the statement you quote applies to the case of a square or circular box without a ground plane. And it is correct for that case. It's also correct if there is a ground plane, provided the distance to the ground plane is large compared to the dimensions of the box, so that the box and it's environs are effectively in free air. Of course it's not true if the box gets close to the ground plane, and especially if the bottom of the box is the ground plane. You're right to say that would be double bookkeeping.

No reason "to remove the disputed statement"? As I've said, the statement is inaccurate, to say the least, unless we add clarification. It's been shown to be false for the most obvious interpretation the reader is likely to make of what "the air" means. This is from reliable sources, and no one here has effectively rebutted it. We all agree that the required clarification would be inappropriate for the article. To me, this adds up to a compelling reason to delete it.

J Doug McLean (talk) 05:19, 16 August 2014 (UTC)

To my mind, if you calculate something twice using two different methods, you should get the same answer, otherwise there is a problem with the calculation. If a physical fact is mathematically proven as true, then it is a fact. The conclusion I draw from argument you have made on this page is that the statement is true. I have not seen any reliable source, nor any reference to one, which says it is not.

I think the physics between the three bodies is pretty simple. The air is given some momentum by the wing. That momentum reverses direction as it rebounds from the surface, causing an overpressure. Momentum is a vector quantity. Equal and opposite vectors add up to zero. Burninthruthesky (talk) 17:15, 16 August 2014 (UTC)

Most of the time you'd be right in saying that if you calculate something by two different methods you should get the same answer. But that's true only if the "something" you're trying to calculate has a definite value. A non-convergent integral, which is what we have in the case of the vertical-momentum integral in an infinite domain, doesn't have a definite value. A classic symptom of non-convergence (also called non-existence) in a double or triple integral on an infinite domain is that when you try to calculate it you get different values depending on the order or the relative rates with which you take the dimensions of the box to infinity. In a case like this, there is indeed a "problem with the calculation", but it's not what you're thinking. The problem is that no correct way to do the calculation exists when the thing you're trying to calculate doesn't have a definite value. This isn't just math. What it says about the physics is true. The vertical momentum due to a lifting foil in an infinite atmosphere is indeterminate. So I stand by my statement that the vertical-momentum aspect of the physics is ill-posed on an infinite domain. And this means that The Statement is problematic if the atmosphere is assumed to be infinite.

Your conclusion that The Statement is "true" is unfounded. You can't make a blanket statement that something is true if there are common situations in which it is untrue. The situations for which The Statement is untrue are well documented in the literature. In my entry of 10 August I cited a sufficient set of reliable sources. Do you disagree with these sources? If so, what are your specific objections?

I think it's clear from this discussion that my understanding of the quote you gave from Prandtl and Tietjens is quite different to yours. Burninthruthesky (talk) 09:50, 17 August 2014 (UTC)

I see a couple of problems with your discussion of the physics of three bodies. First, if the momentum imparted by the foil actually "reverses direction" in its interaction with the ground, the downward force on the ground would be twice the lift (Say the lift has a value of +1, for which the foil imparts downward momentum to the air with a value of -1. If the ground reversed the direction of that, it would go to +1, and the imparted change in momentum would be +2). Second, it's true that equal and opposite vectors add to zero, but in this case the vectors are equal and opposite only because you supposed them to be so. It isn't required by Newton's third law as you implied in the fourth bullet of your previous posting. J Doug McLean (talk) 05:31, 17 August 2014 (UTC)

I'm saying the air presses down on the ground with a total force equal to lift. The ground presses back with an equal and opposite force. That is Newton's Third law. It may be true that Newton's Third law alone is not sufficient to prove conservation of momentum, but that's besides the point. Burninthruthesky (talk) 08:34, 17 August 2014 (UTC)

A further thought: Even if I agreed with you that The Statement was true for an infinite atmosphere, I'd argue that the right decision would still be to delete it. We have an explanation (deflection) that works fine without it and is usually stated without it. Why add a statement that's true only in a fictional abstract situation (an infinite atmosphere) and not in the real world where lift actually takes place (the real atmosphere with a ground plane)? The infinite-atmosphere case plays a role in the mathematical theories, but that's no reason to add something that's true only in that case to an explanation for the general reader.

J Doug McLean (talk) 16:10, 16 August 2014 (UTC)

The onus is on you to obtain consensus for your proposal. Burninthruthesky (talk) 18:46, 16 August 2014 (UTC)

That's what I've been trying to do. But I'd say the onus isn't just on me. The Statement: "The resulting force upwards is equal to the time rate of change of momentum of the air deflected downwards" is a paraphrase of a quote from a cited source in one of the notes in the current article. It doesn't appear in the body of the current article. Putting it in the body of the article, as Mr swordfish proposes in his draft replacement is a change relative to the current article. Technically speaking, Mr swordfish may be more obliged to obtain consensus to add The Statement to the body of the article than I am to obtain consensus to omit it. But this shouldn't be a legalistic game. It's supposed to be a cooperative effort.

I've put some effort into trying to convince you and Mr swordfish that The Statement is technically sloppy and that without proper clarification it raises more problems than it's worth. I've offered detailed technical arguments backed by citable sources, and detailed rebuttals to your counterarguments. Neither of you has really attempted to rebut my arguments in any detail. Instead, you offer general observations on how you think the physics ought to work. When I've rebutted these, you've either tried another tack or simply restated your previous general assertion, but you haven't pointed out specific faults in my arguments. I'm open to being corrected, but I feel like one of the reasons this discussion is at a stalemate is that you're not really engaging with me on the technical details.

I've just argued that even if The Statement were true in a fictional abstract situation, it is still untrue in the real world, and that's enough to justify omitting it. What, specifically, do you disagree with in that argument? If this is to be a cooperative effort, the onus is on you as well. J Doug McLean (talk) 05:31, 17 August 2014 (UTC)

I now understand the statement can be proven true even in a square flowfield, and at least to a good approximation in most real-world situations. You argue that in a square atmosphere, the ground reaction only supports half the lift. This cannot be true; if it were, the sky would fall. That is more than a sufficient rebuttal of your argument, in addition to the others I've given. Personally I don't have a strong opinion on the inclusion of the sentence, but I am quite satisfied from this discussion that it is true. Please WP:LISTEN to Mr. Swordfish. Burninthruthesky (talk) 08:34, 17 August 2014 (UTC); edited 11:51, 20 August 2014 (UTC)

Having had some more time to consider, I begin to understand how some of the misconceptions in this discussion relate to the citations provided.

For the atmosphere as a whole, the integrated time rate of change of vertical momentum due to the lift on a wing is zero.

When the momentum of a parcel changes from mv to -mv, the change in momentum is mv - -mv = 2.

The total momentum is mv + -mv = 0. There is no change.

Therefore, there is no contradiction between, "For the atmosphere as a whole, the integrated time rate of change of vertical momentum due to the lift on a wing is zero", and, "The resulting force upwards is equal to the time rate of change of momentum of the air downwards".

This citation does not imply that changes in momentum do not happen in the atmosphere, or that the AAPT statement can sometimes be false.

the lift is reacted entirely by a pattern of overpressure on the ground

Reacted does not mean produced. The airfoil is fully supported by the air. The air is fully supported by the ground. Therefore the air's reaction to the airfoil and the ground's reaction to the air are both equal to lift.

The overpressure is caused by the ground's reaction to the momentum imparted to the air by the airfoil. It cannot exist in isolation, that would breach Newton's Third law.

For regions that are subsets of the atmosphere, the proportions of the lift that are accounted for by momentum change and by pressure differences depend on the size and shape (vertical dimension compared to horizontal dimension) of the region. Only if the vertical dimension is very large relative to the horizontal dimension are the pressure differences negligible, leaving the lift entirely accounted for by the change in momentum

I can see why calculating this would create challenges, although I have not seen the details. As agreed, it is wrong to apply Newton's second law to a subset of forces, which is why any calculation would have to account only for the force relevant to that calculation.

It makes sense that a region with large vertical dimensions relative to the horizontal would account mainly for momentum changes due to the airfoil because that region contains the entire airfoil and a relatively small proportion of the ground. For similar reasons, a horizontal box would account mainly for ground reaction.

Enlarging the region to a square box would not achieve the isolation described above. As a result, the downwards forces on the air will enter the calculation as well as the upwards forces. This is incorrect. I would expect such a calculation to give a result of zero, since the net change of momentum in the atmosphere as a whole is zero.

This citation does not imply that changing the bounds of a calculation has any effect on forces in the physical world.

I understand the objection that external pressure differences will slow down the air as it travels downwards and prevent it from keeping all of the momentum it has been given by the wing. However, a slowing down of the air is still a negative change of momentum. It is not correct to say that the statement does not apply when there are other forces involved. It does apply, it's just more difficult to quantify.

By the way, I have found an additional which is more specifically related to aerodynamics. Hope this helps. Burninthruthesky (talk) 11:51, 20 August 2014 (UTC)

As Mr. Swordfish says, there is not yet a clear consensus on this issue. Unfortunately I am not able to engage in all the technical details since I am not a subject expert. My ideas may be naïve but you are of course welcome to discuss them. I am sorry if I have created the impression otherwise. Burninthruthesky (talk) 07:42, 26 August 2014 (UTC)

I am disappointed to see another week of embarrassing silence. The issue should be resolved, not as an exercise in schadenfreude, but in order to progress the volume of knowledge which we have all worked towards.

I just found the following quote: lift is accounted for either by pressure or by momentum flux, depending on the proportions of the control volume.

This seems clearer than the final citation given above, confirming that the control volume can account entirely for the force exerted on the air by the airfoil or for the reaction from the environment, not 'proportions of the lift'. I hope this settles any remaining doubt or confusion.

I don't think anyone disputes that it is possible to define a region of 'the air' which only accounts for the downward change in momentum caused by the foil, and also a different region which only accounts for overpressure caused by the reaction from the environment. The fact remains that the environment only touches the outer boundary of the air, it does not contact the foil and cannot contribute to the lift force.

The question is whether the wording, "The resulting force upwards is equal to the time rate of change of momentum of the air downwards" is sufficient to identify the region of air referred to, without necessarily describing it in detail. I say it does, because there is only one correct way to calculate 'rate of change of momentum of the air downwards'.

Do we now have consensus for the proposed addition? Burninthruthesky (talk) 09:03, 4 September 2014 (UTC); edited 10:56, 4 September 2014 (UTC)

I don't see any evidence that anyone has changed their mind. And I have little expectation that further discussion will alter that fact. A couple of weeks have gone by without hearing from the lone dissenter, so the answer is a qualified "maybe". I'm not going to add the sentence, but I won't object if someone else does. Mr. Swordfish (talk) 16:16, 4 September 2014 (UTC)

It does shake your confidence a little to be told you're wrong, repeatedly by someone who is in a position to know, doesn't it? On the other hand, we know that an argument from authority is fallacious. Even experts can make mistakes, sometimes very serious ones.

I think what would help us most now would be the considered opinion of an aerodynamics expert. Burninthruthesky (talk) 16:41, 4 September 2014 (UTC)

Mr. Swordfish wrote: if there is consensus to delete I will reluctantly go along... In the interest of moving forward, I have removed it

Thank you for demonstrating your willingness to compromise by removing it, and for your patience awaiting further discussion. I agree that now looks unlikely to happen. I was going to wait until after my holiday to post this, but I think we've waited long enough for a resolution to this dispute.

I see a quality argument that the statement is in keeping with guidance from the AAPT. I see no legitimate concern in opposition, so I am re-adding it. Burninthruthesky (talk) 05:10, 7 September 2014 (UTC)

L=-dp/dt

In the discussion above we have failed to reach consensus (yet) about the inclusion of the sentence "The resulting force upwards is equal to the time rate of change of momentum of the air deflected downwards."

In the interest of moving forward, I have removed it from the draft and would like to proceed with publishing the draft in it's current state. We can continue to discuss the issue, but I don't want to hold up publication pending the resolution of what I think is a fairly minor issue in the greater scheme of things. Have we reached consensus on publishing the article as-is? If so, I'll make the switch. Mr. Swordfish (talk) 19:56, 19 August 2014 (UTC)

I agree. I'm in favor of you making the switch. Well done! Dolphin (t) 06:12, 20 August 2014 (UTC)

It is clear how much time and effort you (J Doug McLean and Mr. Swordfish) have both put into improving the clarity, coverage, and accuracy of the article. Thank you. I haven't been able to review the draft in as much detail as you, but I think it would be a shame to let it go stale because of a disagreement over one or two sentences. It makes sense to move into the article space and iron out any remaining details in the normal course of Misplaced Pages editing. Burninthruthesky (talk) 08:28, 20 August 2014 (UTC)

It's now been a week, and seeing no opposition I'm making the draft live. We can continue to discuss and improve that article moving forward. Mr. Swordfish (talk) 14:49, 26 August 2014 (UTC)

Lift does NOT only apply to airfoils - they are a special case

This is my first ever contribution to Misplaced Pages - so I apologise in advance if I breach any guidelines.

My concern is that this whole article suffers from a major distortion in its emphasis. Lift is defined as being the force perpendicular to the motion of an object relative to a fluid. The object does not need to be an airfoil (wing shape). But this whole article is utterly preoccupied with airfoils. And as a consequence with Bernoulli's principle (and its various possible explanations). Therefore the article largely duplicates the existing Bernoulli Principle article. They should be merged together.

The article also - as a consequence of the above - completely ignores other mechanisms whereby Lift is generated, and all the real-world examples. EG the rudder of a boat; a kite; the tail of a fish or dolphin, a scuba diver's fins, a weathervane etc etc. There are plenty of examples of flat objects generating lift purely because of their angle of attack, nothing to do with Bernoulli and airfoils. These should be the first topic in an article on Lift. Airfoils should be mentioned as a special case with a link to the existing articles on Airfoil and Bernoulli. To ignore the most simple and straightforward source of Lift is bizarre. — Preceding unsigned comment added by 94.11.120.238 (talk) 00:39, 4 September 2014 (UTC)

Welcome to Misplaced Pages, and thanks for your contribution. The only deficiency with your first edit is that you forgot to sign your "name" by typing four tildes - see WP:SIGN. This should be done at the end of all contributions to a Talk page. (You will notice a Bot has signed your "name" for you.)

• But this whole article is utterly preoccupied with airfoils. I disagree. The word "airfoil" is not used until the seventeenth sentence. Many textbooks concede that even a flat plate can experience lift when it is behaving like an airfoil. This article is about lift and how it is generated. It is reasonable to clarify the meaning of lift by describing it as a force experienced by airfoils and other objects when they are behaving like airfoils.
• The article also completely ignores other mechanisms whereby lift is generated, ... I disagree. The seventh sentence in this article states "Lift is also exploited in the animal world, and even in the plant world by the seeds of certain trees." (Or are you alluding to the Magnus effect?)
• To ignore the most simple and straightforward source of lift is bizarre. You forgot to state what it is you believe is the "most simple and straightforward source of lift." Please clarify.

Please read the article carefully and if you find a sentence or paragraph that states or implies lift is produced only by airfoils, or predominantly by airfoils, please let us know by replying here on this Talk page. Dolphin (t) 06:38, 4 September 2014 (UTC)

The Statement (L = -dp/dt) should be deleted

I've been away from internet access for several weeks and thus not able to participate in the discussion. I see that in the interim Mr swordfish installed the revised article, which I support, and that Burninthruthesky has added to it The Statement, "The resulting force upwards is equal to the time rate of change of momentum of the air deflected downwards", which I don't support.

Two general lines of argument have been put forward in support of The Statement, an earlier one by Mr swordfish and a more recent one by Burninthruthesky. Both of these lines of argument are contradicted by what actually happens in lifting flows, as I'll show below. Burninthruthesky has put forward several rebuttals of my arguments against The Statement, but none of these rebuttals is consistent with the physics, as I'll also show.

Thus in the interests of technical accuracy and consistency with the published specialist sources The Statement should either be deleted, or the required clarification should be added, as I've discussed in earlier posts. I may be the "lone dissenter", but my "con" position is supported by the physics as described in the mainstream aerodynamics literature, while the "pro" position is supported only by the statement itself in an article in a journal for physics teachers, and by intuitive and insufficiently rigorous arguments on this page, which I believe to be erroneous and for which I know of no citable source.

Let's look at the failings of the "pro" arguments.

Limiting integration to "the air deflected downwards"

Some time ago Mr swordfish argued, and Burninthruthesky later agreed, that The Statement must be true if the rate of change of momentum is integrated only over the air that is being "deflected downwards", i.e. that is undergoing downward acceleration. But this idea has no support in the physics or in the literature.

In a steady flowfield around a lifting airfoil, consider the air that is currently undergoing downward acceleration and thus contributing to the integrated rate of change. This is a body of air around which a boundary can be drawn and for which we could in principle calculate the integrated rate of change of vertical momentum. But we can arrive at a qualitative assessment of Mr swordfish's idea more easily by looking at the forces exerted on this body of air. For The Statement to be true, the total force exerted on "the air deflected downwards" would have to be equal to the force exerted on it by the airfoil, and the net force exerted on it by its other surroundings would have to be zero.

For simplicity, assume the inner boundary of this body of air is everywhere in contact with the airfoil surface, though that may not always be true. Assume further that at this inner boundary the airfoil exerts a downward force on the air equal to the lift. What we need to evaluate now is whether the force exerted on the air by its surroundings at the outer boundaries is zero as required if The Statement is to be true. What do these outer boundaries look like? In general they will take the form of two curves fanning forward from near the leading edge, one upward and one downward, and two curves spreading aft from the trailing edge, one upward and one downward. In the far field, these curves will approach +-45-degree lines configured like a letter X (see fig 7.3.23 in my book). The air being accelerated downward is thus contained within two generally fan-shaped regions, one above the airfoil, and one below, with the combined regions forming a general hourglass shape with the airfoil spanning the waist. For our purposes, the key characteristic of these regions is that the boundaries have extensive projected horizontal area on which the pressure differences in the field will act and thus exert an integrated vertical pressure force. The theory gives us no reason to expect that the integrated vertical pressure force on this entire outer boundary is zero, and in fact it isn't. The boundaries of the upper region are immersed in lower-than-ambient pressure, and the boundaries of the lower region are immersed in higher-than-ambient pressure, resulting in an unbalanced pressure force. Thus the total force exerted on "the air deflected downwards" is not equal just to the downward force exerted by the airfoil.

So I stand by my earlier argument that The Statement is true only for a box that is very tall compared to its width, and is not generally true for "the air deflected downwards". J Doug McLean (talk) 01:19, 17 September 2014 (UTC)

I accept your point that 'the air deflected downwards' includes more air than just a slim column. However, for the reasons you have given, there is only one correct way to calculate 'the rate of change of momentum of the air deflected downwards'. The resulting force is always equal to lift, regardless of what other incorrect calculations could be made. Burninthruthesky (talk) 15:18, 17 September 2014 (UTC)

If we take 'the air deflected downwards' to be a small box, then the assumption that its surroundings exert no force on it is false: in accelerating downwards, it experiences an increase in lift from the box above, which in turn experiences an increase in downforce. But the counterargument to "The Statement" actually asks us to; "consider the air that is currently undergoing downward acceleration and thus contributing to the integrated rate of change. This is a body of air around which a boundary can be drawn" This boundary, around the total volume undergoing acceleration, is in fact arbitrarily tall. The assumption is true for an arbitrarily tall volume, and consequently the counterargument is not valid. — Cheers, Steelpillow (Talk) 13:28, 22 September 2014 (UTC)

To cite a source on this. According to McLean, "...we define a control volume ... with vertical outer boundaries at the front and back, extending to infinity vertically. Conservation of momentum requires that the lift on the airfoil be balanced by the forces and momentum fluxes at these outer boundaries. Because the boundaries are vertical, there is no net vertical force contribution by the pressure, and the lift must be accounted for by the net flux of vertical momentum into the control volume." This is saying the same thing: when we consider the total lift, the pressure component disappears and we are left with just the momentum change. — Cheers, Steelpillow (Talk) 12:46, 23 September 2014 (UTC)

Actually, I think we are all agreed on the physics, the issue seems more of what aspect to present where. "The Statement" was made as part of an introductory discussion and I can see no issue with placing it there. It is only later, when the air under consideration is in a finite box, that we need to qualify it, and this article has not gone there. — Cheers, Steelpillow (Talk) 12:54, 23 September 2014 (UTC)

Lift manifested as a rate of change of momentum in the neighborhood of the airfoil, with that momentum being removed elsewhere by interaction with the ground

Burninthruthesky has argued that the airfoil imparts downward momentum to air in some region surrounding it at a rate equal to the lift and that that momentum is then canceled in the far field in its interaction with the ground, resulting in the overpressure on the ground. This idea is intuitively appealing, but it isn't consistent with the detailed physics of the flow around an airfoil. See, for example, my argument above regarding the fan-shaped regions of "the air deflected downwards", and the fact that even in the near field that body of air will generally have unbalanced pressure forces acting on it in addition to the force exerted on it by the airfoil.

Or consider the air in a square box surrounding the airfoil, where the box is at least several chords in size but small compared to the distance from the ground, so that the flow in the box is as if the airfoil were in free air. In this case only half the lift is manifested as a rate of change of momentum, and the other half as pressure differences on the top and bottom of the box (Burninthruthesky took issue with this result for a square box, but that was the result of a misunderstanding on his part, as I discuss below).

A further counterargument: Removing downward momentum from the air requires upward acceleration. If the overpressure on the ground were a result of downward momentum being removed from the air in the neighborhood of the ground, there should be a region of upward acceleration of the air overlying the area of overpressure on the ground. But this isn't what we see. For an airfoil many chords above the ground, the overpressure on the ground is centered directly under the airfoil (The 2D version of the overpressure distribution is qualitatively just a 2D version of the 3D drawing in Prandtl and Tietjens). The dominant central portion of this region of overpressure, where the overpressure is strongest, is overlain by air that is accelerating downward, not upward. Only the weaker parts of the overpressure distribution, well ahead of the airfoil and behind, are overlain by air that is accelerating upward (This can be shown based on the model of the flow as a uniform flow with a lifting vortex and the image of the lifting vortex under the ground superimposed, which would be valid for large height compared to the chord. For smaller heights, some details would differ, but not the overall conclusion). Thus Burninthruthesky's simple momentum explanation for the overpressure is not consistent with the real pressure and velocity fields. J Doug McLean (talk) 01:19, 17 September 2014 (UTC)

I never specified the nature of the region of air accelerating upwards. I simply inferred its existence from the facts that the airfoil accelerates air downwards and the net rate of change of momentum of the atmosphere as a whole is zero. I see no evidence that the momentum explanation I gave is inconsistent with yours. Burninthruthesky (talk) 15:18, 17 September 2014 (UTC)

What happens in the far field is wholly irrelevant to the way in which forces are generated and momentum is changed locally. This sub-topic is equally irrelevant to the main discussion. — Cheers, Steelpillow (Talk) 13:36, 22 September 2014 (UTC)

Rebutting the rebuttals of the "con" arguments

Burninthruthesky has put forward several rebuttals of the "con" arguments, but none of these rebuttals is consistent with the physics. I'll address some of the main points here.

Burninthruthesky wrote: *I now understand the statement can be proven true even in a square flowfield, and at least to a good approximation in most real-world situations.

It can't be proven for a square box because it isn't true for a square box. You haven't proven it, and you haven't offered a citable source that proves it. J Doug McLean (talk) 01:19, 17 September 2014 (UTC)

If it's possible to perform a calculation on a subset of the atmosphere, it is likewise possible to perform a calculation on a subset of a square box. That is my interpretation of the citations you have given. Let's not forget as well that The Statement itself is already cited. Burninthruthesky (talk) 15:18, 17 September 2014 (UTC)

Burninthruthesky wrote: *You argue that in a square atmosphere, the ground reaction only supports half the lift. This cannot be true; if it were, the sky would fall.

This is a misinterpretation of my argument. As I wrote originally and later reiterated, for a square box, equal partition of the effect of lift between momentum changes and pressure differences is the result for the case where there is no ground plane, or the ground plane is far away compared to the size of the box. With equal partition, the half of the lift accounted for by pressure differences is equally split between the pressure excess at the bottom of the box and the pressure deficit at the top, i.e. one quarter each.

I never said that the partition is equal when the bottom of the box is the ground plane. If the airfoil is centered in a square box that is large compared to the airfoil chord, and the bottom of the box is the ground plane, it can be shown that half the lift is accounted for by the pressure excess on the part of the ground plane that forms the bottom of the box, about 14% is accounted for by the pressure deficit on the top, and about 36% is accounted for by the change in momentum. To find all of the lift accounted for by the overpressure on the ground, you must integrate over the entire ground plane, not just the part inside the square box. J Doug McLean (talk) 01:19, 17 September 2014 (UTC)

So, we agree that this calculation does not fully account for either of the two forces on the air. Burninthruthesky (talk) 15:18, 17 September 2014 (UTC)

Burninthruthesky wrote: *Therefore, there is no contradiction between, "For the atmosphere as a whole, the integrated time rate of change of vertical momentum due to the lift on a wing is zero", and, "The resulting force upwards is equal to the time rate of change of momentum of the air downwards".

If you simply assume, as you did, that the airfoil imparts downward momentum at a rate equal to the lift, and that the ground takes it away at the same rate, then the total rate adds up to zero as it should, and there is no contradiction. But the fact that the assumed rates add up to the right sum doesn't prove that the assumed rates were correct to start with. In a real airfoil flow, the rate of momentum change is equal to the lift only for a very restricted definition of "the air" (a tall "sliver" of a box). For any other definition of "the air", including for "the air deflected downwards", the rate of momentum change is not equal to the lift, and there is a contradiction. See my arguments above. J Doug McLean (talk) 01:19, 17 September 2014 (UTC)

As I said, the environment only touches the outer boundary of the air, it does not contact the foil. You seem to be implying that the rigidity of the ground actually contributes to the lift, rather than simply reacting to it. That is not what is said in the citation you gave from Prandtl and Tietjens. If the ground reaction cannot contribute to lift, the foil must be entirely supported by momentum changes. So I believe it is absolutely true that 'the airfoil imparts downward momentum to the air at a rate equal to the lift.' I disagree with your opinion that it would be 'reasonable' for anyone attempting a calculation of the rate to assume that air subject to other forces should be included. Burninthruthesky (talk) 15:18, 17 September 2014 (UTC); edited 07:13, 20 September 2014 (UTC)

Burninthruthesky wrote: *I don't think anyone disputes that it is possible to define a region of 'the air' which only accounts for the downward change in momentum caused by the foil, and also a different region which only accounts for overpressure caused by the reaction from the environment.

If you mean one region surrounding the airfoil in which the force exerted by the airfoil is reflected only in momentum changes, and another region farther away in which the environmental pressures act on the imparted momentum, then I dispute it. In general, there is no support in the physics or in the literature for the idea that these effects can be separated into different regions in that way. You haven't shown us any example of a region that meets your requirement that it "only accounts for the downward change in momentum caused by the foil". J Doug McLean (talk) 01:19, 17 September 2014 (UTC)

You have already given an example: "Only in the limit as the vertical dimension of the box becomes large relative to the horizontal is it true that lift is accounted for by momentum changes." Burninthruthesky (talk) 15:18, 17 September 2014 (UTC)

I've shown that any rectangular box that isn't very tall compared to its width doesn't satisfy the requirement, and that the body of "the air deflected downwards" doesn't either, because unbalanced pressure forces on the outer boundaries of "the air deflected downwards" are acting even in the near-field of the airfoil. Again, the only way to see lift manifested only as a change in vertical momentum is to confine your view to a box that is very tall compared to its width.

The bottom line

Those are the technical issues as I see them. I think it's clear from what's written in the mainstream literature that The Statement isn't true unless it is explicitly stated that "the air" refers to a region of air in the form of a very tall "sliver". And because this level of detail isn't appropriate for the article, it would be better just to delete The Statement. As for restricting it to the air undergoing downward acceleration (i.e. by changing "momentum of the air downwards" to "momentum of the air deflected downwards"), that doesn't fix the problem either.

However, in one way the "air deflected downwards" issue is beside the point. Even if adding "deflected" did fix the problem, it would be an unwarranted extrapolation from the source material and thus constitute original research. The AAPT article says nothing about integration at all, let alone about the idea of restricting the integration to the air undergoing downward acceleration.

But the bottom line is that no viable defense of either version of The Statement has been given. The Statement in either form is inconsistent with what's known from the mainstream literature, and it should be deleted.

If you still disagree with me, I'd suggest you seek a second specialist opinion. I'm not going to recommend a particular expert to you because I'd be open to the accusation of seeking a friendly witness. The arguments I've made here are pretty basic aerodynamics, based on the standard control-volume framework for analyzing momentum balance in fluid flows, and using a well-accepted model for 2D lifting flow (uniform flow plus a vortex, plus an image vortex if there is a ground plane). If another aerodynamics specialist agrees to look at this, I'd expect him/her to look at the same sources, apply the same models, and reach the same conclusions that I have. J Doug McLean (talk) 01:19, 17 September 2014 (UTC)

I appreciate a reply with some clarification. However, I see no significant new material or progress towards resolving the issues raised.

I see no response to the problem I pointed out, with a fresh citation which clarifies that the proportion of momentum and pressure accounted for by the control volume is dependent on its aspect ratio. My understanding is that the force on the foil is caused entirely by momentum changes, and the force on the ground is caused entirely by pressure, and both forces are always equal in magnitude to lift. A control volume which accounts for a mixture of these effects will therefore account for a mixture of the equal and opposite forces on the air. This would not be a calculation of the lift force. As you say in your book, "You can apply the standard procedures for evaluating integrals and, without making any procedural error, obtain a wrong answer."

You are now requesting a 'specialist opinion', knowing I am not a specialist. I would welcome an opinion from an alternative specialist, but unfortunately I don't know how, or whether, that can be arranged. More importantly, I agree with your earlier statement, that it "isn't something esoteric about which experts might disagree; it's basic: It is wrong to apply Newton's second law to just a subset of the forces exerted on a body." I have studied physics at degree level and consider myself qualified to comment on the Newtonian mechanics between three bodies. You have described in detail how it is possible to choose a region which mathematically eliminates the reaction force in order to prove that the force on the airfoil is equal to the rate of change of momentum, yet you claim this statement is not generally true. The sources you have offered do not appear to support your assertion. Burninthruthesky (talk) 15:18, 17 September 2014 (UTC)

I prefer to understand lift as a reaction to the force accelerating the air downwards: F=ma. This is dimensionally equivalent to the rate of momentum change over time, dp/dt, with both expressions having dimension MLT (Mass x Length / Time squared). The statement given in the article is perfectly valid - no amount of sophistry is going to overcome such basic physics. — Cheers, Steelpillow (Talk) 13:50, 22 September 2014 (UTC)

22 Sept 2014 - I've remained silent for the past week to give others a chance to express their opinions. Welcome back, Doug. I am pleased to see that your absence was only temporary. My take on the current situation:

• We still haven't reached consensus on the inclusion of The Statement. I do not think a temporary absence by one of the main editors should be interpreted as achieving consensus, so in the interest of fairness and following the wikipedia protocols I am (reluctantly) removing The Statement until we reach consensus to add it. Whether this is temporary or permanent remains to be seen.

• I have posted requests for assistance at the parent project pages to solicit wider opinion. I see that Steelpillow has already responded. (welcome!) Hopefully, we'll get some other views. The next step would be filing an RFC, but let's see what the folks from the project pages have to say.

Mr. Swordfish (talk) 14:30, 22 September 2014 (UTC)

• I have only looked at the extensive walls of text here in a very cursory way, but my initial impression is that this wording is actually a bit confusing, independent of the question of whether it is rigorously true. Why is it being described as a rate of change in momentum rather than a force? The more intuitive wording would be something like, "The force applied upward is equal to the ". The current wording requires that you convert "change in momentum over time" to "mass times acceleration" in your head just to get the units correct.0x0077BE 16:49, 22 September 2014 (UTC)

Yeah, it's quite a wall, isn't it? Anyway, The American Association of Physics Teachers has this pedagogical recommendation:

"...lift on an airfoil should be explained simply in terms of Newton’s Third Law, with the thrust up being equal to the time rate of change of momentum of the air downwards." (https://en.wikipedia.org/Lift_%28force%29#cite_note-7)

So, that's where the language comes from. It seems pretty clear to me, but maybe I'm too close to the issue. Mr. Swordfish (talk) 17:17, 22 September 2014 (UTC)

I suppose that a strict derivation from Newton's Third Law would require one to identify the net force on the air as, say, F. One then points out that by said law, F and L are equal and opposite. Both F = ma and F = dp/dt are then valid derivations of F. I guess which of them one uses will depend on whether momentum or acceleration is more to the fore in the subsequent treatment. Here, I did not notice any subsequent treatment, so I'd suggest that we describe both relationships, making it clear that they are equivalent. — Cheers, Steelpillow (Talk) 17:57, 22 September 2014 (UTC)

Yes, I would agree with this - I am seeing this entirely out of context, and it's not obvious to me why we're comparing a force to a change in momentum over time. I guess it's because the statement is conveying two ideas simultaneously - the fact that the two forces (the force downward on the air and the force upward on the wing) are equal, and the fact that forces are changes in momentum over time. I think it would work fine if it were broken out more clearly: "The force applied upward is equal to the - or, equivalently, the change in momentum over time - ." Of course, the question is still open, in my opinion, as to why we're bringing momentum into this at all. If there was some reasoning for this particular choice in the original source, does it still apply here if, Steelpillow says, we're not actually discussing the momentum specifically? 0x0077BE 18:40, 22 September 2014 (UTC)

• I came here from the physics project. Walls of text and sometimes missing signatures and/or inconsistent indents mean that I've only skimmed the controversy. To help resolve the problem of whether the statement, or something like it, should be in the article, we should appeal one of the pillars of Misplaced Pages, verifiability. If mainstream reliable sources assert that the statement is true, then the statement is verified and with due weight that explanation deserves a place in the article, along with citations to said sources. If there are reliable source that claim the statement isn't true, then that controversy should be reported with due weight and cited sources. It doesn't matter whether I or any other editor think the statement is true or false or incomplete, only the reliable sources matter. Indeed, our personal takes on the subject could become original research if we stray too far from the sources.

Given the pillar of verifiability, perhaps we can resolve this controversy by giving an accounting of the reliable sources for and against. Then inclusion of the statement would be based on judging the quality and weight of the sources, not our personal takes on the subject. I confess to not parsing all the verbiage to extract those sources. Could they be summarized here? --Mark viking (talk) 18:38, 22 September 2014 (UTC)

Yes, I would also appreciate a summary of the sources, and if someone who understands it better could clarify in a succinct way the particular nature of the conflict, that would be appreciated. I'm starting to parse out the particular objections, but it'd be much easier if this were done RfC style, with a simple summary of the conflict.0x0077BE 18:49, 22 September 2014 (UTC)

I've re-signed some text which I broke up using inline replies. I hope that helps a little. Burninthruthesky (talk) 07:44, 23 September 2014 (UTC)

The suggestion of presenting a controversy was discussed, then rejected, 'I wouldn't advocate presenting this as a "controversy" because I don't think it amounts to one' on 10 August. Burninthruthesky (talk) 13:35, 23 September 2014 (UTC)

Thank you for adding signatures--it does help in following the conversation--and thank for the pointer to the controversy discussion. --Mark viking (talk) 17:45, 23 September 2014 (UTC)

Summary of the nature of the conflict:

As part of the recent extensive rewrite the following statement (which came to be known as "The Statement") was added :

The resulting force upwards is equal to the time rate of change of momentum of the air deflected downwards.

The Statement is supported by several RS cites (see below).

One of the contributors (a well respected expert in the field) described it as "problematic":

"..the statement "The resulting force upwards is equal to the time rate of change of momentum of the air downwards" is problematic on two counts, in spite of the fact that it has a citable source.

The first problem with the statement is that for it to be true, the downward force exerted by the airfoil on the air surrounding it would have to be the only force being exerted on "the air". There are many possibilities for how we can define the body of air we're considering, and this condition (no other force but the lift) isn't met in general. The airfoil exerts a downward force on the inner boundary of the body of air surrounding it (at the airfoil surface), but the surrounding environment exerts unbalanced pressure forces on the outer boundary of the body of air. This problem cannot be eliminated just by increasing all the dimensions of the "box" of air we consider, even to the limit of infinity. As the box is made larger, the pressure disturbances at the outer boundary get weaker, but the area over which they act gets larger, and the integrated force remains comparable to the lift. How much of the lift is accounted for by this pressure force rather than by momentum change depends on the proportions of the box. For example, for a box that is very large horizontally compared to its vertical dimension, practically all of the lift is accounted for by pressure at the outer boundary, and practically none by momentum changes. Only in the limit as the vertical dimension of the box becomes large relative to the horizontal is it true that lift is accounted for by momentum changes.

The other problem is that most such analysis in fluid mechanics deals with boxes whose boundaries are fixed in space. The time rate of change of momentum in such a box is zero in steady flow, and momentum changes must be assessed in terms of fluxes in and out, not the time rate of change."

Much more has been written, but that's the gist. On the other side, it is asserted that L=dp/dt is simply a re-statement of Newton's 2nd and 3rd laws.

Summary of sources supporting The Statement:

• "Most of the texts present the Bernoulli formula without derivation, but also with very little explanation. When applied to the lift of an airfoil, the explanation and diagrams are almost always wrong. At least for an introductory course, lift on an airfoil should be explained simply in terms of Newton’s Third Law, with the thrust up being equal to the time rate of change of momentum of the air downwards." Cliff Swartz et al. Quibbles, Misunderstandings, and Egregious Mistakes - Survey of High-School Physics Texts THE PHYSICS TEACHER Vol. 37, May 1999 pg 300 http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PHTEAH000037000005000297000001&idtype=cvips&doi=10.1119/1.880292&prog=normal

• "Birds and aircraft fly because they are constantly pushing air downwards: L = dp/dt Here L is the lift force and dp/dt is the rate at which downward momentum is imparted to the airflow." Flight without Bernoulli Chris Waltham THE PHYSICS TEACHER Vol. 36, Nov. 1998 http://www.df.uba.ar/users/sgil/physics_paper_doc/papers_phys/fluids/fly_no_bernoulli.pdf

• "Now let’s move on to conservation of momentum: the force exerted on a fluid equals the time rate of change (derivative with respect to time) of its linear momentum. If you exert a force on something, you change its momentum. If you don’t exert a force on something, its momentum stays unchanged or is conserved. This is Newton’s laws, if you choose to call it that. When an airfoil is producing lift, that force does in fact change the vertical component of the airflow’s linear momentum, and the drag force changes the horizontal component of the airflow’s linear momentum. ...Measuring lift by measuring the increase in downward vertical velocity in the flow coming off the trailing edge of the airfoil is conceptually possible. This downward velocity is definitely there and is known as downwash. I have never heard of anyone actually measuring it with sufficient precision to calculate lift, not because it is physically unsound but because it is not a practical experiment." Charles N. Eastlake An Aerodynamicist’s View of Lift, Bernoulli, and Newton THE PHYSICS TEACHER Vol. 40, March 2002 http://www.df.uba.ar/users/sgil/physics_paper_doc/papers_phys/fluids/Bernoulli_Newton_lift.pdf

• "The Rate of change of momentum of a body is equal to the resultant force acting on the body, and takes place in the direction of the force." An Introduction To Fluid Mechanics : CIVE1400 SCHOOL OF CIVIL ENGINEERING http://www.efm.leeds.ac.uk/CIVE/FluidsLevel1/Unit03/T5.html

I am unaware of any cites directly refuting The Statement. Perhaps Doug or someone can provide some specific references. Mr. Swordfish (talk) 19:45, 22 September 2014 (UTC)

Can we just prepend "Where no other force but the lift is imposed (which is rare), ..."? I can see merit in the statement, at least as a theoretical exercise. Gryllida (talk) 03:44, 23 September 2014 (UTC)

It's a bit cheeky of me, but I think it may help focus on the real problem here, so I would offer this quote (which I also posted above) from McLean in support of The Statement: "...we define a control volume ... with vertical outer boundaries at the front and back, extending to infinity vertically. Conservation of momentum requires that the lift on the airfoil be balanced by the forces and momentum fluxes at these outer boundaries. Because the boundaries are vertical, there is no net vertical force contribution by the pressure, and the lift must be accounted for by the net flux of vertical momentum into the control volume." This is saying the same thing: when we consider the total lift, the pressure component disappears and we are left with just the momentum change. So I think we are all agreed on the physics, the issue seems more of what aspect to present where. "The Statement" was made as part of a "Simplified physical explanations of lift on an airfoil" and I can see no issue with placing it there. It is only later, when the air under consideration is in a finite box (e.g. ground effect), that we need to qualify it, and this article has not gone there. — Cheers, Steelpillow (Talk) 12:54, 23 September 2014 (UTC)

I also keep thinking we're all agreed on the physics, only to find myself contradicted. I think the issue of the shape of 'the air' would only occur to someone with an in-depth knowledge of aerodynamics. They are not the target audience of this section.

Even still, I think an expert reader should understand what the article means and take a reasonable interpretation. It has been said above that, 'Not being aware of these pressure forces is understandable in this case. Authors of articles in "The Physics Teacher" are not typically mainstream experts on aerodynamics.' This was refuted with evidence that the cited authors are perfectly well aware of the need to remove the effects of other forces from any mathematical treatment. (See the discussion on 12 August.) Burninthruthesky (talk) 13:25, 23 September 2014 (UTC)

Regarding the physics of the situation, I'm unclear about this point regarding "other forces" on the wing. The example of gravity, in particular, seems strange; generally in a physics textbook (or other educational material), you would talk about the contribution from each component independently unless they have some sort of non-linear dependence on one another, so if I were to throw a ball up in the air, I would say that I've impelled it with a certain amount of force (F), and I, in turn, am absorbing an equal force in the opposite direction (-F). The actual velocity of the ball can't be calculated without taking into account other forces acting on the ball (e.g. wind resistance, gravity), but that's independent of the force imparted to it its interaction with me. I'm wondering if we can solve this problem by being slightly more specific (or deciding that the wording already contains sufficiently specific terms of art) with the phrasing, to indicate that when we say "lift" we're only talking about the force on the wing created by the deflection of air, and that when we say "momentum of the air", we're specifically talking about the change in momentum as a result of the deflection. If I'm not mis-understanding the objection here, I'd say we're at least 90% of the way towards having the appropriate specificity anyway.0x0077BE 18:04, 23 September 2014 (UTC)

To clarify the point about 'other forces', I think the main objection is that as the wing pushes air downwards, that air experiences a push upwards in the form of pressure from the environment. The air spreads out as it goes, so the reaction force from the environment is generally exerted over a wider area than the wing. As I understand it, this is why a calculation over a wide box of air mainly represents the pressure from the environment, and a tall box mainly represents the force on the foil.

I think there has been some confusion in this discussion caused by use of the term 'lift' to refer to the force on the foil, the reaction from the environment, and even a combination of the two. I absolutely agree that when we say "lift" in this context we're only talking about the force on the wing created by the deflection of air and I think the wording in the article is quite clear. Burninthruthesky (talk) 07:47, 24 September 2014 (UTC)

Glad to know that we seem to be on the same page about the lift portion. I still don't quite understand the objection about the volume of air. Clearly the wing is imparting some net downward force on something, and the only thing around it is the air. If the air which has been deflected by the wing then loses that momentum from interactions with the environment, that doesn't change anything about the balance of forces in the wing-air system (which sounds like what we're talking about in The Statement). Unless there's something that's being deflected that's not accounted for in The Statement, this seems like a simple restatement of Newton's laws, and as such I see no reason to believe that the sources are being inaccurate when they make this claim. 0x0077BE 14:04, 24 September 2014 (UTC)

I agree. To clarify the objection, if you measured the rate of change of momentum of a wide volume of air, your calculation would account for more environmental pressure than momentum. So you would get a rate of change which is not equal to the force on the foil. I would say that's because this isn't a calculation of the rate of change of momentum deflected downwards, and I think that's the crux of the disagreement. Burninthruthesky (talk) 14:27, 24 September 2014 (UTC); edited 14:31, 24 September 2014 (UTC)

The issue remains that no simple rewording will overcome the objection, "in the interests of technical accuracy and consistency with the published specialist sources The Statement should either be deleted, or the required clarification should be added". (I understand the 'required clarification' is that listed under 'In the notes' on 10 August. Nobody supports this.) I fully agree that if The Statement is inaccurate, it should be deleted. But we need evidence. Burninthruthesky (talk) 13:30, 24 September 2014 (UTC); edited 13:51, 24 September 2014 (UTC)

Agree that if The Statement is inaccurate it should be deleted. Also agree that we need evidence, and unfortunately I haven't been able to follow the con arguments. I also haven't seen any reliable source backing the con argument, only a lot of WP:OR posted to the talk page. From a verifiability standpoint all the evidence is in favor of The Statement. Mr. Swordfish (talk) 17:07, 24 September 2014 (UTC)

The evidence against The Statement

I thank Mr swordfish for reverting the addition of The Statement until this is resolved.

I think there is ample citable evidence against The Statement, and I'll list some formal citations here. But I don't expect that to be enough. I've already described this evidence several times in my own words. Though I didn't include formal citations, I thought I made it clear that these arguments were based on published sources and were not just my own work. Still, the other participants, principally Mr. Swordfish, Burninthruthesky, and Steelpillow, have not been convinced that this evidence means what I think it does. So in addition to listing citable sources, I'll try again to convince you that this published evidence actually does contradict The Statement. First, to summarize the basic line of argument against:

The Statement in either of its forms is not "simply a re-statement of Newton's 2nd and 3rd laws" as asserted in Mr. Swordfish's summary of the "pro" evidence. This is clear from the published flowfield analyses I'll cite below and from the basic statement of the 2nd law cited by Mr. Swordfish: "The Rate of change of momentum of a body is equal to the resultant force acting on the body, and takes place in the direction of the force." (An Introduction To Fluid Mechanics : CIVE1400 SCHOOL OF CIVIL ENGINEERING http://www.efm.leeds.ac.uk/CIVE/FluidsLevel1/Unit03/T5.html) Note that in this basic physics context "resultant force" means "net force" or "vector sum of all the forces". Thus the L = dp/dt statement, where L is the total lift force exerted on the foil, is a correct application of the 2nd law only if L is actually the magnitude of the "resultant force" acting on "the air", which can be so only if the integrated force exerted on the air by its other surroundings is zero. The evidence shows that this is true only if "the air" is defined as a region that is infinitely tall compared to its width, and is untrue for every other shape of region that has been looked at, large or small. Thus The Statement is not true in general, but only true in one special case. Analyzing all of the forces, to be sure we have properly identified the "resultant force", isn't "sophistry", as Steelpillow would have it. It's basic physics.

In looking at the evidence it will be important to distinguish between The Original Statement from the AAPT papers, and The Revised Statement proposed for inclusion in the article. The Original Statement is not specific as to what body of air is to be integrated over to determine the time rate of change of momentum, referring to it simply as "the air" in Swartz's version and "the airflow" in Waltham's version. The Revised Statement attempts to be more specific, adding the word "deflected", specifying that the integration is to be limited to the region of the air that is experiencing downward acceleration at the current instant.

All of the mainstream published work I'm aware of that analyzes the momentum balance in the flow around a lifting airfoil predates the publication of AAPT papers and therefore does not specifically address The Original Statement per se. However, much of this work arrives at conclusions that contradict The Original Statement and thus effectively refute it.

What do I mean by "contradict"? We have a very specific statement, L = dp/dt, where L is the lift force exerted on the foil by the air, and dp/dt is the rate of change of momentum downward, integrated over some volume of "the air" surrounding the foil. The problem is that "the air" is not specifically defined, implying that you don't have to be very specific in choosing the body of air to find an integrated dp/dt that's equal to L. However, the analyses cited below find only one very specific choice for the region of "the air" that gives the stated result, while all of the other choices give values of dp/dt that are not equal to L. Thus, as I've argued before, we have a statement that claims to be true in a very general way, but that is actually true only for one special case. The Original Statement is thus inaccurate unless it is modified to be more specific.

Here are the citable sources I know of:

On basic control-volume analysis of the rate of change of momentum in a moving fluid:

• Shapiro, A. H. 1953. The Dynamics and Thermodynamics of Compressible Fluid Flow. New York: The Ronald Press Company. Section 1.5

This analysis shows that for a steady flow the integrated time rate of change of momentum of fluid parcels passing through the interior of a control volume is equal to the integrated (net) flux of momentum through the boundary. This is a basic ingredient in the other analyses cited below.

For the atmosphere as a whole, including a ground plane:

• Prandtl, L., and O. G. Tietjens. 1934. Applied Hydro- and Aeromechanics. New York: Dover Publications. Derivation in connection with figure 150.

I don't have a copy at hand, so I can't provide a quote, but this is the classic analysis showing that the pressure pattern on the ground constitutes a downward force on the ground, and thus an upward force on the atmosphere, equal to L. The net force on the atmosphere due to the lift, (i.e. the vector sum of the forces exerted by the wing and the ground) is therefore zero, so that the integrated rate of change of vertical momentum for the atmosphere as a whole must be zero. Thus for the most obvious assumption a reader is likely to make regarding what is meant by "the air" (i.e. the atmosphere as a whole), The Original Statement is false.

The Prandtl and Tietjens analysis is for the 3D case. It is easy to show that the same overall conclusion applies in 2D. A citable source for the 2D analysis probably exists, but I don't know of one offhand.

For a circular region centered on the airfoil:

• Durand, W. F., ed. 1932. Aerodynamic Theory, vol. 1. New York: Dover Publications. Sections B. V. 6 and B. V. 7.

This is a control-volume analysis of the flow around a 2D lifting body of arbitrary cross-section in an infinite atmosphere, using a circle of large radius as the outer boundary of the volume. It shows that in the far field the flow is independent of the details of the body, and that significant contributions to the pressure and the momentum fluxes at the outer boundary come only from the combination of the uniform flow and the bound vortex. It arrives at a derivation of the Kutta-Joukowski theorem in equation 7.3. Equation 5.6 shows that the flux of vertical momentum across the outer boundary, and thus the time rate of change of vertical momentum in the air in the interior, is equal to only half the lift. Equation 6.6 shows that the integrated vertical pressure force on the outer boundary is upward and equal to half the lift. The net force on the air due to the lift is therefore downward and equal to half the lift, and Newton's second law is satisfied. It is explicitly stated that this result holds regardless of how large the radius of the circle is made. Thus a large circle is another example of a region of "the air" for which a reader might reasonably expect The Original Statement to apply, but for which it is in fact false.

For rectangular control volumes:

• Lissaman, P. B. S. 1996. The facts of lift. AIAA 1996-161. Section titled "Lift in thin slices: the two dimensional case".

Lissaman assumes an infinite atmosphere with no ground plane and summarizes the results of his analysis as follows: "For a large rectangular control surface, part of the lift is attributable to pressure and part to momentum, depending on the aspect ratio of the surface. For a square control surface the contributions on the surface due to momentum and pressure are equal; for a tall, long vertical surface the contributions are mainly momentum, while for a streamwise, long, flat, horizontal surface the lift is primarily due to pressure. This illustrates that it doesn't make much sense to attribute the lift on an airfoil to either pressure or momentum effect, unless one takes a control surface on the actual airfoil surface, when the lift is indisputably due only to pressure."

According to Lissaman's results, if "the air" is taken to be the air in a rectangular box surrounding the airfoil, The Original Statement isn't even close to being true unless the box is a tall, slender sliver, and even then it isn't strictly true until the vertical dimension of the box is taken to infinity. Steelpillow quotes the section of my book that describes the result for the infinitely tall, slender sliver, the only control-volume shape for which The Original Statement has been shown to be true, and interprets it as being "in support of The Statement". A balanced recounting of what my book says would also quote the discussion in connection with figure 8.5.4, which deals with other control-volume shapes for which The Original Statement isn't true.

To me, the evidence cited above makes it clear that The Original Statement is inaccurate as it stands.

The Revised Statement was an attempt to fix this, the idea being, as I understand it, that the integrated dp/dt would be equal to the lift if the integration were limited to the region of air undergoing downward acceleration. However, no citable source has been put forward for this revision, so The Revised Statement isn't a viable candidate for inclusion. Even so, the idea is intriguing, and I looked at it in some detail. I plotted the far-field pressure distributions along the boundaries of this hourglass-shaped region of "the air" (the +-45-degree lines in the far field and the arbitrary horizontal lines at the top and bottom of the hourglass) and convinced myself that the integrated pressure force on the complete hourglass boundary cannot be zero, even if the height of the hourglass is taken to infinity, and thus that the rate of change of momentum of "the air deflected downward" cannot be equal to the lift. Of course this is just my own work. So the proposed revision has no citable evidence either for or against.

Now to respond to some of the recent arguments put forward on the "pro" side:

Burninthruthesky and Steelpillow have made several statements aimed at countering the general control-volume line of argument. But the gist of what they say, if I understand it correctly, isn't consistent with how the momentum balance in a control volume works.

The basic principle is that if you integrate the pressure and the momentum fluxes over the entire boundary of a control volume, the sum must be zero for a steady flow, provided a consistent sign convention is followed. This is a vector relationship, and in the examples I've discussed it has been applied to the vertical component of the pressure force combined with the flux of vertical momentum. Consider a control volume that completely surrounds a 2D airfoil. The airfoil surface forms the inner boundary of the control volume. The lift is exerted in the form of pressure on the airfoil surface, and thus can be calculated by integrating the vertical component of the pressure force over the inner boundary. And because the airfoil is impermeable, the momentum flux integrated over the inner boundary is zero. Because the sum of the integrals over the entire boundary (inner and outer) must be zero, the lift can also be calculated from the appropriately summed integrals of the vertical component of the pressure force and the flux of vertical momentum over the outer boundary of the volume.

This kind of analysis is valid for any control volume, large or small, and of any shape, as long as it completely surrounds the airfoil, and the boundary is piecewise smooth. As the cited evidence shows, the two outer-boundary integrals (pressure and momentum-flux) "account for" different portions of the lift depending on the shape of the control volume and on its disposition relative the ground plane, if there is one. This is not contradictory. It does not imply that the overall process of lift production is different depending on what control volume you choose, which would of course be incorrect. Neither does it imply that we're somehow subdividing the actual physical lift force. All that's being subdivided is the manifestations of the lift in the surrounding air. This subdivision merely reflects the fact that, even in the same flow, different regions see different balances between pressure and momentum flux. And regardless of the relative magnitudes of the two outer-boundary integrals, their sum always equals the lift. So Steelpillow is mistaken when he says "when we consider the total lift, the pressure component disappears and we are left with just the momentum change." Likewise Burninthruthesky is mistaken when he says "A control volume which accounts for a mixture of these effects will therefore account for a mixture of the equal and opposite forces on the air. This would not be a calculation of the lift force." On the contrary, the sum of the outer-boundary integrals is always equal to the total lift force.

In another argument aimed at countering the control-volume analyses Burninthruthesky quotes my book as saying "You can apply the standard procedures for evaluating integrals and, without making any procedural error, obtain a wrong answer". This takes the words out of context and reaches a wrong conclusion. That passage refers to integrals that are non-convergent on an infinite domain. The control-volume analyses I've been discussing here all involve integrals that converge, and the results correctly reflect physical reality.

Another idea put forward by Burninthruthesky and seconded by 0x0077BE goes as follows (paraphrased as I understood it):

Given that the foil and the ground are not in contact with each other, but communicate only through their contact with the air, the foil can transmit a force to the ground only by imparting momentum to the air at a rate equal to the force, and the force is transmitted to the ground when the air gives up that momentum in an interaction with the ground.

This would be a physically realistic possibility if the air were a projectile that flew between the foil and the ground, such that momentum was the only way for the air to "remember" the effect of the foil after leaving contact with the foil. But the air is not a projectile flying between the foil and the ground. It is an extended mass that moves as if it were a continuous material, and it is in constant, simultaneous contact with both the foil and the ground. Taking on momentum and giving up momentum is not the only way the air can transmit a force. It also transmits force through the pressure field.

As an illustration of this, consider replacing the air with a brick resting on the ground, and replacing the foil with your hand pressing downward on the top of the brick with a force F. The brick transmits F to the ground through its distribution of internal stresses, and the brick remains at rest with zero rate of change of momentum. True, the air around an airfoil is unlike the brick in the sense that the air has some freedom to move, and some change of momentum takes place, at different rates depending on what region of the air you look at. But the air is similar to the brick in the sense that it carries an internal distribution of stress (the pressure field). Because the non-uniform pressure field pervades the entire airspace and can transmit force between different portions of the air, Newton's second law does not require the integrated rate of change of momentum of the air, or any portion of the air, to be equal to the force applied by the foil, any more than it requires the rate of change of momentum of the brick to be equal to F.

To summarize: Arguments and rebuttals put forward on the "pro" side are not convincing. Citable sources in the mainstream aerodynamics literature provide sufficient evidence that The Original Statement is inaccurate and should not be added to the article. J Doug McLean (talk) 22:31, 27 September 2014 (UTC)

I'm sorry, but I am fully convinced that the statement is accurate. I think your objections lie in a practical analysis of specific control volumes of air, but I'm still failing to see how this well-cited statement (which comes from proper secondary sources, mind you) is anything but a simple restatement of conservation of momentum. It is implicit in these types of statement that you are talking about only the relevant quantities during the relevant time period, e.g. the momentum imparted on the foil and the momentum imparted on the air. These two quantities will definitely balance out, whether the force imparted on the air is dissipated by compressing the fluid, increasing the total pressure, or by heating due to friction, the instantaneous change in momentum in the air-foil system balances. My point about the air being the only thing around wasn't about how the pressure is transmitted to the ground, in fact the exact opposite, I was suggesting that the ground and the environment can't possibly matter when we're talking about the instantaneous balance of forces.

It sounds like what you are saying is that the measured change in momentum of a given volume of air will not necessarily match the lift on the foil because the energy can be dissipated in other ways. I'm suggesting that it doesn't matter where that momentum goes, just that it is initially transferred between the air and the foil. The standard way of teaching or explaining the physics of a system is to simplify it it to its most basic form, so with no further clarification you should assume that L = -dp/dt refers to only the relevant (interacting) parts of the system. The later consequences of the momentum imparted into the air may be relevant to airfoil design, but I don't think they invalidate this statement.0x0077BE 14:33, 29 September 2014 (UTC)

I agree with 0x0077BE. If The Statement is inherently wrong, it should not require such an immense wall of text to rebut it. In its context within the article, it is perfectly satisfactory. That is to say, the one "special case" in which it is wholly accurate happens to be the one special case that the article is introducing at that point. The furthest one might go at this stage in the article would be to add a caveat that it applies strictly only to unbounded flow, but really, the discussion is not even at that level of sophistication at this stage. — Cheers, Steelpillow (Talk) 16:05, 29 September 2014 (UTC)

The Statement is mathematically true when the correct control volume is considered, and I agree with 0x0077BE and Steelpillow that this is the only one which applies in the context under discussion. The truth of The Statement is unaffected by the fact it is possible to 'look at' other regions of the air and consider the same physics in terms of pressure. I think it's been well established that other analyses have no relevance here. Burninthruthesky (talk) 10:43, 30 September 2014 (UTC)

0x0077BE asserts that "the ground and the environment can't possibly matter when we're talking about the instantaneous balance of forces." If the exchange of equal-and-opposite forces between the foil and the air were all we were talking about, I'd agree. But it isn't, and I don't. The Statement also deals with the rate of change of momentum of the air, which can be defined only by a definite integral over some spatial domain, i.e. a "control volume". The foil exerts its force on the air at the surface, but the resulting change of momentum of the air is spread over a substantial area above and below the foil. Thus it isn't consistent with the physics of a continuum flow to think of the momentum as being "initially transferred between the air and the foil" and then "going" elsewhere. At any given instant, the imparting of downward momentum isn't just taking place at the surface. It takes place over a wide area. And integration over a small area close to the surface isn't going to see enough of it to make The Statement true. The "correct control volume", as Burninthruthesky refers to it, must, at a minimum, be fairly large and extend some distance above and below the foil.

The trouble is that any control volume large enough to contain a rate of change of momentum equal to the lift is also large enough to have substantial pressure forces imposed on it by its surroundings. So even in free air, "local pressure variations" come into play, and the "environment" matters. For The Statement to be true, a "correct control volume" must be large, but the air in it must behave as if the force exerted on it by the foil were the only force acting on it, i.e. the integrated pressure force on its outer boundary must be zero. Only one such control volume has been identified in the literature: the infinitely tall sliver analyzed by Lissaman (1996). If this is the control volume that 0x0077BE and Burninthruthesky are referring to, then maybe we've reached some level of agreement on the physics. But I would still disagree with the contention that the context within the article makes this clear. If The Statement is true only for the air in an infinitely tall sliver, then that needs to be spelled out.

If 0x0077BE and Burninthruthesky think The Statement is true for some control volume other than the infinitely tall sliver, they need to tell us specifically what control volume that is and provide citable sources for their assertion. I don't think the "air deflected downward" qualifies because there's no citable source for it that I know of, and it doesn't make The Statement true anyway, as far as I can tell, because its outer-boundary pressure integral isn't going to be zero (See my earlier discussion of the hourglass shape of this region of air).

Now I see that Steelpillow has added to the article a new version of The Statement, with the added stipulation that it applies to "free flow conditions", and not "in a restricted space". This fix is deficient on two counts.

First, the restriction to "free flow conditions" is not sufficient to make The Statement true. The only situation for which it has been found to be true is the infinitely tall sliver (Lissaman, 1996). For regions of any other shape, there is always an unbalanced pressure force on the outer boundary, even as the size of the region is taken to infinity. Note that the analyses for a circular control volume (Durand) and rectangular control volumes (Lissaman) assumed an infinite atmosphere (unbounded flow). The evidence is clear that "local pressure variations" come into play in both the near and far fields, even in an unbounded flow. The only proven way to eliminate outer-boundary pressure forces is to limit integration to the infinitely tall sliver. A control volume that is infinite in both directions doesn't do it, because the outer-boundary pressure contribution doesn't vanish in that case.

Second, the assertion that "free flow conditions" is a sufficient restriction isn't supported by any citable source. Lissaman doesn't support this assertion because he makes it clear that if the aspect ratio of the control volume isn't infinite, the outer-boundary pressure will account for part of the lift.

This addition to the article is both inaccurate and unsubstantiated, and should be reverted.

To summarize the situation as I see it:

The Original Statement referring simply to "the air" has a reliable source in the AAPT papers. But other sources in the mainstream literature find that for most of the assumptions a reader would be likely to make as to what is meant by "the air", The Original Statement is contradicted. The Original Statement is therefore inaccurate unless it is made more specific.

Neither "free flow conditions" nor "the air deflected downward" is acceptable as a fix for The Original Statement. The only acceptable fix is a stipulation that it is true only for Lissaman's infinitely tall sliver control volume, including a discussion of how that is the only way to make the outer-boundary pressure contribution negligible. This is the only fix that is both true and verifiable.

But the context here is the simplest explanation (flow deflection) in a section titled "Simplified physical explanations of lift". The flow-deflection explanation works just fine without The Statement. It is supposed to be qualitative and shouldn't need to include a quantitative mathematical statement. And consider the audience. The AAPT papers are advising teachers on how to explain lift to physics students. Our audience is the general public, not physics students. As I've said before, the best option is simply to leave The Statement out. J Doug McLean (talk) 19:38, 4 October 2014 (UTC)

I think your first sentence clearly concedes the point we were trying to make: "If the exchange of equal-and-opposite forces between the foil and the air were all we were talking about, I'd agree. But it isn't, and I don't." - that is exactly what we're talking about. The presumption in physics is that you are talking about the basic, ideal case; in this case, we're talking about the foil and the air with which it is interacting. The momentum in the system is conserved. I think there's general consensus at this point for inclusion of the statement (given that it is well-cited, and I see no citations from you explicitly stating that this commonly-taught concept is not true, just statements where you need to be an expert to see how it applies, and there's no support here for the idea that they are being applied appropriately).

I get the impression that you are arguing based on either a more complicated model of the system than the one being presented here (i.e. one where momentum gained by the air can be damped from the environment) or you have a different definition of momentum than the rest of us. By Newton's 3rd law, whatever the foil is interacting with must have a force applied to it equal and opposite to the one applied to the foil, and change in momentum over time is equivalent to force. Regardless of the appropriate "control volume" (which only you have brought up, and the rest of us feel is irrelevant to this very simple statement), what exactly is having a downward force applied to it (instantaneously), if not air? And if it is air, is it not valid to say that whatever air is having the downward force applied to it, its instantaneous change in momentum as a function of time (which is what dp/dt is anyway) is equal to the force applied upwards on the lift? 0x0077BE 20:39, 4 October 2014 (UTC)

I am concerned that "indefinitely large" and "infinite" heights appear to lead to mathematically conflicting solutions: to me, that suggests a flaw in someone's model somewhere. 0x0077BE also makes an excellent point: The Statement is directly verifiable in cited sources while its repudiation is not - that requires synthesis (WP:SYNTH) by the editor, which Misplaced Pages abhors. As such we are bound to present The Statement as current encyclopaedic knowledge. I concede that my phrase "in free flow conditions" may conceivably be bettered, but the statement it qualifies seems to be unavoidable in one form or another. We have a three-to-one consensus on that, I personally think it's time to accept that and move on. — Cheers, Steelpillow (Talk) 22:17, 4 October 2014 (UTC)

A vote is not a consensus. I realize that unanimity isn't required, but satisfying all editors' legitimate concerns is supposed to be a goal. I've cited ample evidence that my concerns about The Statement are legitimate. To simply add The Statement without some detailed qualification, as three of you advocate, would be to ignore these concerns and would be inconsistent with the published evidence.

The current majority view of the evidence assigns all the weight to the AAPT papers and none to the mainstream sources that address the same question of how much momentum is imparted to "the air" by a lifting foil. This is backwards. A quality-of-the-argument analysis of this body of evidence would assign the weighting the other way around.

The Statement in the AAPT papers simply says dp/dt = L, where dp/dt is the rate of change of the vertical momentum of "the air", and implies that this is a straightforward application of Newton's second law. But what is meant by "the air" is not defined, and no supporting analysis is given to establish whether or under what conditions this meets the requirements of a proper application of the second law, i.e. no evidence is given that L actually constitutes the resultant of all the forces acting on "the air". Thus the AAPT statement is both ambiguous and technically sloppy.

In contrast, the mainstream sources I've cited all define precisely what they mean by "the air", and they all apply the established, rigorous method for applying the second law in fluid flows, i.e. control-volume analysis. All of the relevant forces acting on the air are included in these analyses. The results of these rigorous analyses should be regarded as more reliable than the sloppy statement in the AAPT papers.

For the examples of circular and square control volumes, the cited analyses arrive at the result dp/dt = 0.5L, in direct contradiction to The Statement. I'd say a statement in the form of a mathematical equation constitutes "explicit" refutation of The Statement for these particular definitions of "the air". And comparing what different sources say about precisely the same question is not "synthesis".

0x0077BE feels that control-volume analysis is "irrelevant", and insists that we're just talking about "the air with which the foil is interacting". But this idea isn't consistent the physics or with the mainstream published sources. How can "the air with which the foil is interacting" be defined in any other way than as the air in some defined volume? It can't just be the air in direct contact with the surface. That would be a layer of zero thickness and thus zero mass and zero momentum. You have to consider a non-zero volume just to find a non-zero momentum for any body of air. And once you do that you have to consider the other forces acting on that volume besides the force exerted by the foil. 0x0077BE says the "change in momentum over time is equivalent to force", but that's true only if "force" refers to the resultant of all the forces acting on the body. And the mainstream sources have found that the force exerted by the foil on the air constitutes the resultant force only for the air in an infinitely tall sliver.

An extended control volume is a "more complicated model" than you'd apparently like to consider, but it's the minimum level of complication that makes physical sense for an extended substance like "the air". The mainstream sources I cited use this model because it's the appropriate way to apply the second law in such calculations. This is what the minimum "basic, ideal case" looks like in fluid flows.

Whether you have trouble seeing "how it applies", or are unsure whether it is "being applied appropriately", or have concerns about "a flaw in someone's model somewhere" is beside the point. These analyses are the most reliable published evidence we have available to us.

The published evidence taken as a whole supports my position that The Original Statement is not sufficiently specific and is contradicted by several citable examples. To be true, it needs to be qualified, and as I've said before, there is only one qualifying statement that could be added to it that is verifiable. If we're "bound" to include The Statement at all, then I think we're also bound to include the qualifier that it has been found to be true only for the air in an infinitely tall sliver, and that for regions of other shapes momentum accounts for only part of the lift, with outer-boundary pressure accounting for the rest.

J Doug McLean (talk) 00:10, 8 October 2014 (UTC)

As I pointed out on 12 August, one of the cited authors refers explicitly to a control volume later on in his discussion, but still sees fit to introduce The Statement without qualification. He references Bradley Jones. Elements of Practical Aerodynamics. This online edition introduces a Newtonian explanation on page 12.

A control volume analysis is a far more advanced concept than Newton's second law, which is what is being described at this point in the article structure. Even so, an analysis which eliminates reaction forces and accounts only for momentum confirms that Newton's second law is fulfilled.

To find all of the lift accounted for by the overpressure on the ground, you must integrate over the entire ground plane, not just the part inside the square box.
— User:J Doug McLean

Likewise, to find all of the momentum which accounts for lift, you must integrate over an infinitely tall sliver, not any other region. The citations which have been provided for other analyses are therefore irrelevant.

The Statement is cited and I see no legitimate concern to prevent its inclusion. Burninthruthesky (talk) 08:00, 8 October 2014 (UTC); edited 11:14, 11 November 2014 (UTC)

The bottom line? "Consensus on Misplaced Pages does not mean unanimity" - see WP:CONSENSUS. We have exhaustively discussed a minority view and given it due weight but have nevertheless established a clear majority consensus. The process between us here is now exhausted. Anybody who still wants to make an issue of it may seek wider support through the mechanisms mentioned on WP:CONSENSUS. — Cheers, Steelpillow (Talk) 11:01, 8 October 2014 (UTC)

Moving forward...

A lot or bits have been spilled about the statement L = -dp/dt and I don't want to re-hash it all here. But as a summary and to (perhaps) draw the matter to a close I offer the following observations:

• The Statement is correct and supportable, but only with certain limiting assumptions that are not generally applicable.
• Adding language to indicate the limited circumstances where it is true would distract from the discussion at this point in the article - this introductory section should be fairly simple.
• The article reads fine without it, and a good argument can be made that this qualitative intro section is not the place for a quantative statement.
• I don't think we have received consensus to include it, even though there's really only one holdout. Wiki policy is to leave out material unless there's consensus to include it, so I think we should follow that.
• At this juncture, I don't think that further discussion among the current active participants on the talk page will get us to consensus.
• If we are going to take it further, I think the right approach is to do a "request for Comment" or a "request for close".
• My own preference is to just leave it out and move on. That seems to be the de-facto result at this point and I'm fine with that.

All that said, let's return to the article as it currently appears. I'm not really happy with the following paragraph:

Newton's second law, F=ma, tells us that the lift force exerted on the air is equal to its mass times its downward acceleration. This is often more conveniently expressed as the rate of momentum change over time. This analysis is accurate for an airfoil in free flow conditions, but in a restricted space (such as when flying in ground effect) local pressure variations can also come into play.

I don't really know what we're trying to express with this paragraph, other than talking about the rate of momentum change while carefully avoiding saying The Statement. I think we either need to make it say something more direct/concrete or simply remove it. As it stands, I don't think it adds much to a laypersons understanding. Other thoughts? Mr. Swordfish (talk) 17:15, 10 November 2014 (UTC)

First of all, WP:CONSENSUS does not require unanimity, nor necessarily a majority in any discussion or vote, see also WP:CLOSE on closing discussions.

Secondly, there is a big difference between an approximation which is sometimes useful and a fundamental principle which is sometimes a sufficiently close approximation. This introductory section needs to be introducing the principles, not discussing their validity as approximations. The concluding apologia about restricted spaces was introduced in an attempt to address concerns expressed by one editor, based on struggles with the approximation. My own view is that the apologia is out of place in introducing the principle and should be deleted. (furthermore, contrary to the first bullet point above, the principle is indeed an adequate approximation for most purposes: it is mathematically equivalent to F=ma and nobody is challenging that).

Thirdly, the first part of the existing paragraph, introducing F=ma, is saying something very important. If the mathematically equivalent rate of momentum change is common in textbook discussions then it is useful to introduce it here, otherwise not. If it is to stay, it needs appropriate citation - I do not find Richard Feynman (currently cited) an adequate source in this context, as he was not an aerodynamicist.

Finally, if momentum change is to be introduced here, do we need to give the equation F=-dp/dt or are the existing words sufficient? my own view is that, unlike F=ma, the equation is not sufficiently well-known to be recognisable by the layman and should be left out.

— Cheers, Steelpillow (Talk) 19:15, 10 November 2014 (UTC)

I agree it seems unlikely this discussion will achieve unanimity, and also that unanimity is not a requirement. WP:CONSENSUS says, "The quality of an argument is more important than whether it represents a minority or a majority view". I believe articles should not be allowed to suffer through an argument to moderation, which is a risk when editors try to avoid a version of an article just because it provoked an argument, or leave a compromise which pleases nobody. My view on how WP:CONSENSUS should apply to this discussion hasn't materially changed (see 7 September).

Just to clarify, the 7 September revision of the article only cites Feynman to support a generic statement of Newton's second law (which is all it does support), and provides further citations to support more specific statements relevant to lift. As far as I'm aware, the equation F=-dp/dt has not at any time been mentioned in the article body (only the notes, which I think is appropriate.) Burninthruthesky (talk) 11:14, 11 November 2014 (UTC)

Correct, the equation F=-dp/dt has never been in the article (at least in recent memory - maybe it was a long time ago before I started participating), but it was expressed in English as "The resulting force upwards is equal to the time rate of change of momentum of the air deflected downwards" which is the same thing. I don't think anyone is advocating using the formula in the article. I'm not, although I have no problem using it here on the talk page as shorthand.

I think Steelpillow has a good point in saying " If the mathematically equivalent rate of momentum change is common in textbook discussions then it is useful to introduce it here, otherwise not." To answer that, there are several articles that indicate that rate of momentum change is fundamental to the understanding of lift, the most prominent is the AAPT's pedagogical advice "...lift on an airfoil should be explained simply in terms of Newton’s Third Law, with the thrust up being equal to the time rate of change of momentum of the air downwards." (https://en.wikipedia.org/Lift_%28force%29#cite_note-7) There are other cites, included elsewhere on this page. Physicists often use dp/dt and ma interchangeably, although I'm not sure how commmon it is among engineers. Looking at aerodynamic textbooks my impression is the idea of rate of momentum change is not explored very thoroughly, if at all. The thing is, trying to expand rate of momentum change into useful engineering is basically a dead end. Aerodynamics textbook writers are eager to get to the useful engineering bits so they don't do much with this idea. It's the sort of thing that appeals to a physicist taking a broad view of the physics, not someone who is trying to solve a particular problem.

So, do we treat rate of momentum change? The AAPT thinks we should. One editor found it "confusing". I can go either way - leave it out, or present a simple short declarative non-apologetic statement. Mr. Swordfish (talk) 14:51, 11 November 2014 (UTC)

If you're asking my opinion, yes, I would put The Statement back in (actually I did, on 7 September). Nobody is convinced that it is in fact, "contradicted by several citable examples" and I think the argument that, "The AAPT thinks we should" far outweighs any other.

I agree with Steelpillow there is a "clear majority consensus" here for including it in the article. Misplaced Pages policy says we should be WP:BOLD in updating the encyclopedia.

If you still think a more formal method of dispute resolution is more appropriate at this point, you are entitled to start one. Personally, I think the time and effort spent on this discussion became excessive some time ago, which is why I am reluctant to ask yet more editors to become involved. Burninthruthesky (talk) 07:47, 12 November 2014 (UTC); edited 11:04, 12 November 2014 (UTC)

I have posted a note at Misplaced Pages talk:WikiProject Aviation#Lift (force), asking for an uninvolved editor to close this whole discussion. — Cheers, Steelpillow (Talk) 10:43, 12 November 2014 (UTC)

Closing this discussion would be premature. Mr. Swordfish has proposed two options, either of which would involve changes from the current version of the article, i.e. to leave The Statement out, or to include it in "non-apologetic" form. His question hasn't really been answered.

I agree with Mr. Swordfish's third bullet item, i.e. that "this qualitative intro section is not the place for a quantitative statement". This by itself is a sufficient reason to leave The Statement out. Yes, Newton's second law is "something very important", but the version of this explanation of lift that was in place before The Statement was added already explained the importance of the second law in a qualitative way.

The other compelling reason is that The Statement in non-apologetic form misrepresents the momentum balance as being simpler than it really is. Mr. Swordfish has it right in his first bullet item: The Statement dp/dt = -L is true only "with certain limiting assumptions" (i.e. that dp/dt is integrated over a region that is very tall compared to its width). For integration over a region of any other shape in free air, dp/dt ranges from zero to -L, depending on the shape of the region. And for the atmosphere as a whole with a ground plane, dp/dt = 0. Details and citations from the mainstream literature are in my post of 27 September.

Note that all but one of these analyses assumes an unbounded atmosphere (Only the domains of integration are finite). So Steelpillow's "concluding apologia", which implies that The Statement's only problem is with "restricted spaces", doesn't address the problem. Steelpillow is also mistaken in characterizing the problem as one of "approximation", unless one regards dp/dt = 0 or dp/dt = -0.5L as "a sufficiently close approximation" for dp/dt = -L.

The question addressed by The Statement can be expressed symbolically as "dp/dt = ?" The AAPT papers give the answer as "-L", without specifying any qualifying assumptions or providing any supporting analysis (Mentioning a "control volume" is not the same as actually presenting a control-volume analysis). The mainstream sources I've cited state their assumptions explicitly, explain their analyses in detail, and find answers of "0", "-0.5L", and "-L", depending on the shape of the domain of integration. This is more complicated than the AAPT answer, and it seems to offend the physical intuition of a majority of this group, but this is what mainstream aerodynamics sources say dp/dt is for the air surrounding a lifting foil. The majority's speculations to the contrary (and their protestations that the mainstream results are somehow not "relevant") are not supported by specific analysis in any citable source.

The majority advocates relying solely on the AAPT papers and ignoring what the mainstream sources say on the same question ("dp/dp = ?"). To me, this seems like a case of Truth by consensus.

The momentum balance associated with lift is complicated. The fact that it's caused such confusion on this page indicates to me that the article should include some discussion of it. But the "Simplified physical explanations" section isn't the right place. I think the right approach would be to leave The Statement out of that section and add a new subsection, titled "Momentum balance in lifting flows", that briefly explains the findings of the control-volume analyses and mentions how the AAPT statement fits in with those findings. This might fit well after "Pressure integration" or after "Circulation and the Kutta-Joukowski theorem". I'd be willing to draft it if there's support for it.

J Doug McLean (talk) 23:52, 13 November 2014 (UTC)

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