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Momentum transfer at microscopic level

Angle of attack

For an asymmetric foil, "angle of attack" can only be defined by arbitrary convention. (The concept is physically meaningless. It is physically meaningful only with respect to the trailing stagnation point, which is justifiably considered the aft point of the chord). The forward stagnation point is not completely defined by the geometry of the foil, but rather by the flow under specified conditions. The point defined, arbitrarily, as the forward point of the chord, has no physical meaning, only an intuitively simple significance.)

I edited two sentences accordingly. Each of these implicitly treated "angle of attack", incorrectly, as a physically meaningful concept in the general case, irrespective of symmetry.75.185.66.0 (talk) 03:07, 10 February 2015 (UTC)

Except, the first source I opened disagrees. It defines the "chord line" through from the centres of curvature of the leading and trailing edges, and uses this as the baseline for the angle of attack. Yes it is arbitrary in a theoretical sense, but not in an engineering sense. Also, your agonies of pedantry are not a style of writing that sits well with readability. It is easier to start again, so I am undoing your edits. — Cheers, Steelpillow (Talk) 09:55, 10 February 2015 (UTC)

Different sources define the chord line differently, and so the angle of attack will also vary depending on how one defines the chord line. Is this variance in the strict definition of "chord line" important? I don't think so, especially at this point in the article where it would be premature to go into too much technical detail. Clancy's definition is fine, but it's not the only one and I'm not sure that we are fairly representing the sources by using it without mentioning the others. Since I don't think the reader would be served by going onto a tangent about different definitions of "chord line", my preference is to simply link to the article Chord_(aeronautics) and remove Clancy's definition. I think the diagram to the right is sufficient to get the idea across. I've made the edit - let me know what you think. Mr. Swordfish (talk) 18:19, 10 February 2015 (UTC)

Yes, thank you, that is a definite improvement. — Cheers, Steelpillow (Talk) 20:32, 10 February 2015 (UTC)

75.185.66.0, I agree that "angle of attack" can only be defined by arbitrary convention and that different authors use different definitions. But the place to present that information is not in an introductory section aimed at lay readers in an article about aerodynamic lift. The minor differences among the differing definitions would be a distraction from the discussion and would not foster better understanding by the intended audience. There are articles on the angle of attack and chord line - either would be a better place for this level of detail. Mr. Swordfish (talk) 18:41, 10 February 2015 (UTC) Mr. Swordfish (talk) 18:41, 10 February 2015 (UTC)

I did not explain my concern very well.

Please be assured that I understand that chord has a conventional definition, or more than one, which is/are very useful in engineering, even though they only apply to common commercial configurations (not a Flettner rotor, for example, or a blob, or a foil with a sharp edge somewhat aft but not aftmost). My problem with it is NOT that it's not practically useful, say in an engineering handbook or product catalog. But this article is trying to explain lift, conceptually. Introducing it in the introduction is not just harmless but irrelevant, but I think highly misleading, because it amplifies a common, highly intuitive tendency to misunderstand lift. It reinforces the view that lift is like a billiard ball striking another off-center. In the billiards case, it is correct to believe that the forward-most point of the struck ball is a physically meaningful chord end-point.

We know that there is a common misconception that lift results from air striking the "bottom" of a "tilted" plane--with the "leading edge" being the farthest point forward and the trailing edge being the farthest aft, just as in the case of a billiard ball or a thin planar surface. The intuitive belief is that the airflow must split at this point, and that all points aft of this on the bottom are obstructing the flow, and thus creating high pressure, and all points aft of this on the top being in a condition of vacuum.

We know that this conception is completely without theoretical justification, and is factually incorrect (in the case of a barn door, even if we use the simplistic complex analysis model, the airflow splits well aft of the end of the "chord"!)--it is the misconception that we are trying to replace with a correct account of the actual pressure and velocity fields, and a correct account of why they occur. The aft stagnation point indeed happens to be the aftmost point on an ordinary commercially available wing, but the intuitive conception that it is the stagnation point BECAUSE it is the aftmost point is completely incorrect; in fact, the Kutta condition is the explanation, and the sharpness of the point, and not the fact that it is aftmost, is the relevant fact. And the idea that the foremost point is a stagnation point is not only based on completely spurious reasoning, it also happens to be completely incorrect experimentally and also inconsistent with every physical model from complex analysis up to and including the most complete differential equation.

My concern is that presenting an arbitrary engineering convention about how to define "tilted" in the intro, as if it were somehow related to the subject of lift, strongly encourages this misconception. ~~ — Preceding unsigned comment added by Mark.camp (talk • contribs) 02:51, 13 February 2015 (UTC)

"it is the misconception that we are trying to replace with a correct account..." - stop right there. No we are not. This is one of the commonest misconceptions by PoV editors as to what Misplaced Pages is about. We are trying to build and encyclopedia according to the WP:FIVEPILLARS and other goodly policies and guidelines. Most relevant here, we seek what is often called "verifiablility not truth". Where are the reliable sources (WP:RS) that claim this is a common misconception that must be addressed, and that the standard introductory texts do in fact explicitly debunk it? — Cheers, Steelpillow (Talk) 15:38, 13 February 2015 (UTC)

I'm not quite sure how this discussion sprung from the description of "angle of attack" in the article. It's a simple geometric definition, not a description of the physics.

From a pilot's perspective, AOA is very important to the subject of lift. In Stick and Rudder, Langeweische spends most of the book talking about it. The Lift coefficient is dependent on AOA. If it gets too high, the aircraft stalls.

When you say "tilted", are you referring to, "when an aircraft is climbing, descending, or banking in a turn the lift is tilted with respect to the vertical"? That sentence has nothing to do with angle of attack, but arises from the fact that lift is, by definition, perpendicular to the relative airflow over the wing. Burninthruthesky (talk) 16:07, 13 February 2015 (UTC)

I think what Mark is trying to say is a)it is not necessary to introduce the notion of chord line at this point in the article, and b) doing so is misleading because it may re-enforce by implication some common misconceptions.

I agree with point a) to some degree. The article used to say the angle of attack is the angle between the foil and the oncoming air; recently the definition was made more precise by introducing the notion of chord line and linking to that article. My take is that it was fine before, but that it's a bit stronger with the more precise definition.

As for point b), I suppose one could imply all those incorrect notions of which Mark speaks from our addition of four little words (" the chord line of"), but it would take quite a bit of imagination. I think all of the misconceptions mentioned by Mark are either directly addressed and corrected by the article, or at least not re-enforced. Perhaps material could be added elsewhere to address Mark's concerns? for instance, we don't cover the "skipping stone" misconception - perhaps we should. Mr. Swordfish (talk) 16:45, 13 February 2015 (UTC)

Mr Swordfish, I am sorry to say, expresses what I was trying to say just a wee bit better than I did. So I will start by taking his cue and asking this: what is the purpose of introducing the notion of chord line (which is useful only in establishing an arbitrary point of reference for purposes of communication, for example in operator's manuals and marketing materials) at this point of the article, which is not intended to be an operator's guide for some commercial product, but a general explanation on lift ? Mark.camp (talk) 04:40, 14 February 2015 (UTC)

I agree that for a cambered airfoil, the concept of chord line is arbitrary. In many analytical situations we are interested in the change in angle of attack rather than the absolute value, and in these situations it doesn't matter how the absolute values are determined. In analytical situations where we want to avoid the arbitrariness of a chord line it is customary to measure the angle of attack relative to the zero-lift line. Dolphin (t) 05:37, 14 February 2015 (UTC)

The main article on angle of attack covers these various reference lines in more detail. If it's felt that mentioning the "chord line" is confusing, I've no objection to a simpler description in this article. In that case perhaps an explicit "main article" link would be helpful. Burninthruthesky (talk) 08:23, 14 February 2015 (UTC)

So, let's see how reliable sources treat this subject. Here are the three elementary treatments that I have handy:

1. Kermode; Mechanics of flight, 1972 edition, pp 75-76: Titles a section "Chord line and angle of attack", then discusses the chord line before turning to the angle of attack.
2. Clancy; Aerodynamics, 1975, p 56: introduces the chord line and then, lower in the same section, defines the angle of attack in terms of the chord line.
3. Simons; Model Aircraft Aerodynamics, 1978, p 10: Introduces the angle of attack, then defines it with respect to the chord line, then defines the chord line - all within a single paragraph.

That list is uncensored - I didn't pull any books that disagreed with a PoV or anything.

It is clear to me that if we do not know what the angle of attack is, then any statement about the angle of attack of a cambered aerofoil is well nigh meaningless. A Misplaced Pages article is expected to provide, as a minimum, the information to make some kind of sense of its narrative, and to link to more detailed explanations where appropriate. As an example of the conceptual semantics involved here, the accompanying illustration finds it necessary to depict the chord line in order to give adequate realisation to the angle of attack. The text needs to do the same. As you can see, the reliable sources I found all support that view. None is followed immediately by addressing any confusion over the stagnation points, though Kermode discusses the more direct confusion over the difference between angle of attack and angle of incidence. Unless anybody can find sufficiently weighty sources to the contrary, these sources show that we need to introduce both topics in close association and not get distracted by indirect confusions over other topics. Therefore, rather than deleting all mention of the chord line, we should actually reintroduce its definition. — Cheers, Steelpillow (Talk) 11:58, 14 February 2015 (UTC)

The talk page is not a forum for general discussion

I would like to remind the participants of relevant wikipedia policy:

• Discussion forums. Please try to stay on the task of creating an encyclopedia. You can chat with people about Misplaced Pages-related topics on their user talk pages, and should resolve problems with articles on the relevant talk pages, but please do not take discussion into articles. In addition, bear in mind that talk pages exist for the purpose of discussing how to improve articles. Talk pages are not for general discussion about the subject of the article, nor are they a help desk for obtaining instructions or technical assistance. Material unsuitable for talk pages may be subject to removal per the talk page guidelines. If you wish to ask a specific question on a topic, Misplaced Pages has a Reference desk, and questions should be asked there rather than on talk pages.

See WP:FORUM. I would ask that participants restrict discussion to that related to editing the article's content and that lengthy discussion outside the context of how to improve the article should be moved to user talk pages. Mr. Swordfish (talk) 21:26, 17 February 2015 (UTC)

I would heartily endorse Mr. Swordfish. Please can users confine off-topic chat either to their own user talk pages or entirely off-wiki. — Cheers, Steelpillow (Talk) 21:39, 17 February 2015 (UTC)

For me this section "Angle of Attack" shows not as a separate article but as a continuation of another section, "Momentum transfer at microscopic level". Anyone else have this problem? I checked the syntax and it seems that the same tokens, leading and trailing instances of "==", are the same. Mark.camp (talk) 01:39, 18 February 2015 (UTC)

Mark, are all your latest questions aimed at improving this article or just for your own satisfaction? If they are not aimed at improving the aticle then it is time to move the convesation elsewhere. — Cheers, Steelpillow (Talk) 09:36, 18 February 2015 (UTC)

Suggested text on momentum transfer

Re:

"Some of the air passing the airfoil has downward momentum imparted to it at a rate equal to the lift."

The text complicates the discussion by dividing the air arbitrarily into two bodies, each of which must then be defined and analyzed. The subject gets extremely complex and difficult to explain.

Why not just directly apply Newton's second law, expressed in terms of rate of momentum change between two bodies, to the two bodies we are discussing, the foil and the air? I suggest this text.

"There is an exchange of vertical momentum between the foil and the air."

This raises some issues.

First, it is a controversial assertion here at the moment, and the controversy would first need to be resolved. I attempted to prove the assertion in a subsequently collapsed Section. I was waiting for a response correcting or accepting my proof. (I assume that that thread can still be responded to. Otherwise, I will duplicate the proof in this Section.)

Second, it would need authoritative references.

Third, assuming the assertion is correct, the article would need to address, somewhere, a very difficult and subtle issue: why does the continuum theory, which is critically important in the settled science and the article, and otherwise correct, imply that the momentum exchange is zero? I attempt to show the source of the incorrect result in detail in the collapsed post.

Fourth, the remainder of the article would need to be made consistent with the sentence.

Fifth, the momentum balances of the foil or craft, air and earth would need to be addressed in terms consistent with the above.

Mark.camp (talk) 23:16, 18 February 2015 (UTC)

I'm not gone, yet at least. Back to Mark's question.

I assume Mark originally brought his question up not just as a matter of general interest, but because he thought it might impact what should be included in the article. So it seems to me to have been a bit arrogant to judge the whole "Momentum transfer at microscopic level" section to be "unrelated to improving the article".

That said, Mark's apparent contradiction between the microscopic and continuum views isn't really a contradiction. Both ways of describing the flow are correct and consistent with physical reality. We have the appearance of a contradiction only if we use the word "momentum" indiscriminately. It is resolved when we note that "momentum" doesn't generally refer to the same thing in the continuum description as it does in the molecular description.

In the microscopic view, "momentum" is associated with the motions of molecules, including the random thermal part of the motion. The random thermal motion has no preferred direction, and in gas flows at low Mach number, the thermal motion predominates, with the "flow" looking like a relatively small directional "drift" superimposed. At any stationary solid surface, the average (continuum) velocity of the gas goes to practically zero, so sufficiently close to the surface of an airfoil the only molecular motion we see is random thermal motion. The bouncing of individual molecules against the surface is diffuse, not specular, and individual bounces are not elastic (an individual molecule can either lose energy to the surface or gain energy from it), so that incoming and outgoing perpendicular momentum are not usually equal-and-opposite for any one molecule. But on average over many molecules, incoming and outgoing perpendicular momentum are equal-and-opposite, provided the surface and the gas are in thermal equilibrium. So the surface pressure is the result of many molecules per unit time having the perpendicular component of their thermal momentum reversed, on average, in their collisions with the surface, and the thermal momentum fluxes of incoming molecules and outgoing molecules account for half of the pressure each.

"Momentum" in the continuum description is based on the continuum velocity of the fluid. As noted before, all components of the continuum velocity near the surface are practically zero, and there is no change or "exchange" of continuum momentum taking place as a result of exerting pressure on the surface. The pressure in the continuum description is just a force per unit area, and nothing needs to be said about the details of how molecules produce the force. Whether pressure consists of forces transmitted between molecules in direct contact with their neighbors, as in a liquid, or is produced by bounces of isolated molecules, as in a gas, doesn't matter in the continuum description. The representation of the pressure as a force per unit area in the continuum momentum and energy equations is the same regardless of whether the fluid is a liquid or a gas.

If we take the molecular view of upward lift on an airfoil, the lower surface is imparting a downward change to the thermal momentum of molecules at a greater average rate than the upper surface is imparting an upward change, so there is a net downward imparting of thermal momentum (Mark's "10,000 N") to molecules impacting the surface. On the other hand, nothing is happening to the vertical continuum momentum locally at either surface, so the net rate of change of vertical continuum momentum is practically zero for the fluid very close to the upper and lower surfaces (Mark's "0 N"). In the actual flow field, vertical momentum is imparted to the flow over an extended region around the foil, and practically none of it is associated with molecules that acquired it directly through collisions with the surface. So Mark's "10,000 N" and ""0 N" refer to two different things, and are both correct. There is no contradiction.

Further points worth noting:

• For atmospheric flight at ordinary scales, the molecular mean free path is extremely short, and the continuum theory is highly accurate. The molecular description offers practically no advantage in fundamental accuracy.

• If you know the density and temperature locally, the molecular theory can tell you the pressure. But the molecular theory is practically useless for predicting the density or pressure variations in ordinary aerodynamic flow fields. The continuum theory is the only choice for actual predictions. And for gasses at low Mach numbers, or for liquids, the continuum theory can predict the pressure differences without having to deal explicitly with the density differences.

• The continuum approach is also the only workable choice for qualitative explanations at the flow-field level. So if Mark was thinking that the microscopic description should perhaps play a role in explaining lift in the article, I think the correct response would be "No, it shouldn't". The molecular description is practically useless for understanding why ordinary flow fields behave as they do. This makes sense when you realize that at a given Reynolds number the flow field described in dimensionless terms is the same regardless of whether the fluid is a gas at low Mach number or a liquid. At the flow-field level, the only properties of the fluid that matter are the density and viscosity, which are macroscopic quantities. The details of the molecular motions have no effect on the global motion and thus provide no help in understanding it.

I agree with Mark that the current quantitative statement unnecessarily "complicates the discussion". But his suggested fix misses on two counts: 1) There is no vertical momentum imparted to the foil in level flight, and 2) To be correct in general, his version of the statement has to be referring to the microscopic version of momentum, which for the reasons I discussed above isn't how aerodynamics analyses and explanations are constructed. Citable sources that do it that way, and do it correctly, are nonexistent.

So no wholesale reworking of the article is in order. But Mark's problem with The Statement would be alleviated, and the article would be improved (as I've said repeatedly) if the current quantitative version were replaced by a qualitative version, as in Langewiesche. I don't think we're done with that discussion. More later.

J Doug McLean (talk) 02:36, 19 February 2015 (UTC)

Thanks much, Doug. I will need to study your latest to understand it, but unfortunately will not be replying as I'm excusing myself from the discussion. This community has become a very unpleasant place for conversation, and I admire your persistence in remaining engaged in the hopes of improving the article for its users.

Mark.camp (talk) 23:00, 19 February 2015 (UTC)

To reiterate a point made many times: Misplaced Pages does NOT document what its editors believe to be true. Misplaced Pages documents what reliable sources tell us is true (see WP:VERIFICATION and WP:RS). This conversation is paying no attention whatever to sources and as such it is mere off-topic chat masquerading as an edit discussion. If folks don't shut up I'll take this to ANI in my own way. — Cheers, Steelpillow (Talk) 07:41, 19 February 2015 (UTC)

@Steelpillow, I did not alter the article to agree with what I believe to be true, so you are preaching to the choir. This conversation is based on what I as an ordinary (non-expert) consumer find to be confusing about the current text, and until that is clarified by the experts here, and the text improved if that is deemed necessary, it is perfectly legitimate. Misplaced Pages policy doesn't require a person who finds article text confusing to provide any sources in the context of a Talk page...otherwise, no user would ever be permitted to raise such an issue--who would ever have written a paper about the fact that some Misplaced Pages text is confusing to one individual? This discussion is very much on topic, and my intention is not to masquerade about anything.

However, since you and others seem to find my attempts to help offensive, and you question my motives rather than just my words, I will not pursue either of the two discussions I started any further.

Mark.camp (talk) 23:00, 19 February 2015 (UTC)

Mark, if I misunderstood your motives, then I must of course apologise. But it makes no practical difference. The interminable edit discussion on the Newtonian model was closed. It should not be reopened gratuitously unless new sources are found to support revisiting the consensus finally reached. WP:DISRUPTION explains that "Editors may be accidentally disruptive because ... they lack the social skills or competence necessary to work collaboratively. The fact that the disruption occurs in good faith does not change the fact that it is harmful to Misplaced Pages." Also, "In some cases, editors have perpetuated disputes by sticking to an allegation or viewpoint long after the consensus of the community has decided that moving on to other topics would be more productive. Such behavior is disruptive to Misplaced Pages." This is where we are now on the matter of Newtonian lift. You are of course welcome to pursue these discussions on your own user talk pages, such as User talk:Mark.camp or User talk:J Doug McLean, and to invite other editors to contribute. Unless Zapletal creates themself a user account, they will have to make do with someone else's, as their IP address is not static (Even there, excessive abuse of other editors may lead to more formal sanctions such as IP blocking). If anybody would like some help on using Misplaced Pages's user pages more effectively, I will be glad to do what I can. — Cheers, Steelpillow (Talk) 10:37, 20 February 2015 (UTC)

J Doug McLean,

The reason I collapsed the discussion is that I did not see anything concrete regarding how the article might be changed. I'm sure you've been to meetings where discussion goes off on a tangent - at some point someone needs to bring it back on-topic. In structured meetings, it's required that some specific motion be on the table before any discussion takes place. I don't think we need that level of structure here, but long posts that don't relate to actual proposed edits don't get us anywhere - they are fine on user talk pages, but let's try to stay focused here.

You suggest replacing the current quantitative version with a qualitative version as in Langewiesche. Can you provide sample text?

Mark.camp,

I do not take offense at your participation, and think your input is valuable. In particular, if you find the text of the article confusing we should take that criticism seriously. It is not enough that the article simply be technically correct, it needs to be written in a way that ordinary non-experts can understand.

My apologies if you took my recent attempts to focus the discussion the wrong way.

As a housekeeping note, I'll be away from the internet for a while, so don't take it the wrong way if I don't respond immediately. Mr. Swordfish (talk) 20:40, 20 February 2015 (UTC)

Mr. Swordfish and Mark.camp:

A qualitative version of the flow-deflection explanation has long been part of the candidate text in my sandbox User:J_Doug_McLean/sandbox. I think this kind of explanation avoids the problem identified by Mark.camp, but I'd be interested to hear his opinion.

Steelpillow's assertion that "This conversation is paying no attention whatever to sources" is unfounded. Finding that one set of sources is not consistent with other more authoritative sources is a perfectly valid line of argument. Along that line, Zapletal's take on the physics is spot on, as is his position that The Statement and the firehose model, as presented by the AAPT authors and Clancy, represent a small-minority view that is at odds with the mainstream understanding. He's also right that giving this view the prominent place it has in the current article is misleading. In my opinion it's also inconsistent with Misplaced Pages policy. See in particular Due and undue weight.

Carrying this further, I would also cite Fringe theories. Among the sources supporting The Statement, only Waltham and Clancy present actual analyses, and those analyses are based on the firehose model. Is the firehose model a fringe theory? Steelpillow has described it as "a model that is still in the textbooks", implying it has wide support in the literature, but Clancy's and Waltham's works are the only places I've seen it. At most, the percentage of books and papers on aerodynamics that it appears in is very small. The other standard to apply is that of scientific rigor. As described in Fringe theories, fringe theories "characteristically fail to adhere to scientific standards and methods." In general, flow models used in aerodynamics should be consistent with the equations of motion locally (the potential equation, at least), and simplifications should be based on rigorous asymptotic analysis. The classical control-volume analyses of lifting flows meet these standards, but the firehose model does not. Thus I think the firehose model satisfies Misplaced Pages's definition of a fringe theory.

Regarding the idea that this issue has been "settled by consensus", I know I gave in and agreed to the new version of The Statement. I did so only because I thought it was the least-bad option I could get. I still think it degrades the article. The new wording ("some of the air") was intended to avoid saying anything that is outright false, and it succeeds in a legalistic, mathematical sense, at least in the body of the article. But when the average reader reads that "some of the air has downward momentum imparted to it at a rate equal to the lift", he's going to infer that all of that downward momentum actually shows up downstream of the foil, when, in fact, only half of it does. Thus it is still misleading unless the reader follows the link to the new section "Momentum balance in lifting flows", and reads it carefully. And if the reader follows footnotes 26, 28, and 29, The Statement in unqualified form (no "some") is there in a prominent place, in violation of Due and undue weight.

I have long maintained that a qualitative version of the deflection explanation would be more appropriate in this section of the article. And now it seems to me the "consensus" insisting on including a quantitative dp/dt = -L has eroded. Mr. Swordfish was originally in the pro camp, but in December he was finding The Statement "problematic". And now we have Mark.camp, who finds The Statement unduly complicated. And we have Zapletal, who has ruffled some feathers but is technically knowledgeable and still has a right to be heard. Even Burninthruthesky said in December that he could live with a qualitative version, as in Langewiesche. I'd like to see a fresh straw poll on the issue of whether we should keep the current quantitative statement or replace it with a qualitative version like the one in my sandbox.

J Doug McLean (talk) 02:58, 5 March 2015 (UTC)

No new source has been put forward in the above. In all the endless discussions that have gone on, not once has an independent and reliable source been produced that explicitly refutes the Newtonian model. To those of you who remain confused about the whole thing, I would ask you to think hard about why, in all this time, two highly motivated and knowledgeable people - one of whom has himself published the only known rebuttal - have been utterly unable to produce any independent corroboration of their attack.

@Mark.camp:, since you opened this discussion, do you have a view on where it should be going, bearing in mind that WP:CONSENSUS requires us to "Limit article talk page discussions to discussion of sources, article focus, and policy"? — Cheers, Steelpillow (Talk) 12:10, 5 March 2015 (UTC)

The firehose model is clearly not realistic, but it's only supposed to be a simple scientific model. As the linked article says, all models are simplified reflections of reality, but, despite their inherent falsity, they are nevertheless extremely useful. That's somewhat different from pseudoscience.

We agreed in January that the language in the current article is supported by Lissaman's classical analysis. Taken out of context, it is also consistent with the firehose model, but the remaining text and diagrams are clearly not. An argument against that model presented as an argument against the article is a form of straw man argument, therefore it fails. Burninthruthesky (talk) 15:45, 5 March 2015 (UTC)

Steelpillow again mischaracterizes the situation when he claims that opponents of The Statement have been "unable to produce any independent corroboration of their attack". I've already cited the mainstream sources for control-volume analyses that contradict The Statement, and they are all now cited in the article as well (more on this below re Berriman). The idea that some "new source" must now be put forward makes no sense. And his assertion that I have "published the only known rebuttal" is absurd. Besides, even if my work were the only known rebuttal, the fact remains that it's a citable source under Misplaced Pages policy.

Burninthruthesky: I agree that it's permissible for a scientific model to take liberties with reality. But a model is "useful" only if it 1) makes useful quantitative predictions, or 2) aids in arriving at a correct understanding of the phenomenon in question. The firehose model makes no prediction (It contains a free parameter, the depth of the assumed affected stream, which is in effect adjusted to produce the desired downwash angle), and the understanding it provides is, as Zapletal said, an unbalanced half-view of the true behaviour. I stand by my assessment of it as a fringe theory that falls short of accepted scientific standards.

Burninthruthesky has made the "straw man" accusation before, and it still doesn't hold water. The current version of The Statement is consistent with Lissaman in a legalistic way, but it is still misleading for the reasons Zapletal and I have explained at length. And the article still cites Clancy and Waltham in support of the statement, and thus criticizing the model on which they based their support of The Statement is not a "straw man" argument. J Doug McLean (talk) 00:36, 7 March 2015 (UTC)

(Zapletal Writes->) Doug, your qualitative version of the "Flow-deflection..." section is much better than the current, live, version, so I support making the change. However, I would actually prefer to go much further with these changes. Given the huge disruptions currently going on, this will likely be a long-winded process. So here are just some rough draft ideas that you might like to consider.

1. Delete the whole "2. Simplified.." section. The article is already very long and the "4. A more comprehensive explanation" covers it all anyway (and in a simple enough way). The "2..." section also gives the impression that there are two competing explanations, namely Newton vs Bernoulli, which IMO reinforces a longstanding, but WRONG, misunderstanding. I think this has been covered before, but I wanted to give my support here for a major deletion. Any "Newton XOR Bernoulli" debates can be covered under "misconceptions" (ie. "Bernoulli explanation" is simply "Newton" integrated).

2. The "2.1 ... Newtonian ..." section alone could perhaps be completely reworked so that it is given in the reference frame of the bulk-fluid, with particle-pathlines used instead of streamlines. There are abundant RSs giving these pathlines, including gold-standard RSs like Lanchester and Lamb. These pathlines, which typically look like roller-coaster "inverted-loops", give a good picture of how the fluid particles are first accelerated upwards, then downwards (at top of loop), then upwards again at the end of their motion. This approach also gives the "Newtonian" explanation in terms of a Lagrangian model, and the "Bernoulli" explanation in terms of a Eulerian model, which I think is a helpful introduction and distinction to give to students of the subject.

3. Two small points. I am particularly troubled by the all too widespread, though very wrong, view of "... horizontal onset flow in front of wing, then downwash behind wing". So...

3.1 - Rather than the flow being described as "curved downward", I think "concave down" might be better. I think "curved downward" allows the "horizontal flow, then downwash" view to perpetuate, because the average reader naturally assumes an undeflected, hence horizontal, flow in front of wing (ie. because no explicit denial is given of this wrong view). "Concave down", perhaps also with some emphasis on the UPWARDLY deflected upwash in front of the wing, may help dispel this misconception.

3.2 - There are several Figures (unnumbered) in the article showing an aerofoil cross-section with a horizontal arrow, labelled "flow direction" or similar, pointing rightward. I think this again misleadingly reinforces the above misconception of horizontal onset flow with no upwash. I think it would be better for the arrow to point leftward, and be labelled "direction of aerofoil movement" (or "direction of wing motion relative to the bulk fluid", or similar).

I have many more detailed suggestions on how to improve the article, but sadly too much time spent trying to cope with the disruptive editors! (End Zapletal, for now) 101.170.255.225 (talk) 03:50, 9 March 2015 (UTC)

(Zapletal Writes->) Further to above. The very last line of the article, under "External links", has this link - "One Minute Physics How Does a Wing actually work? You Tube video" . And, yet again, this gives the "Horizontal-onset-flow/Wing/Ever-downwardly-departing-flow" picture in all its absurd glory, supposedly as an accurate account of how real Lift works (ie. "because of Newton/TS"). This very wrong, but very ubiquitous, picture will need a great deal of explicit refutal before wider society can grasp the essence of Lift. (End Zapletal) 101.170.255.225 (talk) 04:54, 9 March 2015 (UTC)

Berriman

I just added a cite to Berriman's 1913 book on Aviation on the matter of Newtonian lift. In the quotation I cite, "Thus, the wing in flight continually accelerates a stratum of air downwards, and must derive a lift therefrom", the italicisation of "must" is his own. Berriman was a respected authority of his day, and learned theoretical writings of his published in Flight through 1912-13 make it abundantly clear that he was well aware of Lanchester's work. His book also received positive reviews. I have found not the slightest hint or suggestion of any historical controversy over the quantification of Newtonian lift at this time, least of all between Berriman and Lanchester. — Cheers, Steelpillow (Talk) 16:55, 2 March 2015 (UTC)

(Zapletal Writes ->) Steelpillow, you say "I have found not the slightest hint or suggestion of any historical controversy ... between Berriman and Lanchester."

As I have already pointed out (and RSed), the Cambridge School, including such authors as Berriman, did NOT accept the CToL until very late in the 1920s. So, once again and briefly, Berriman's Flat-Earth theories of Lift were exactly what Lanchester had to spend three decades battling against, at least in the UK. On the other hand, the Germans grasped Lanchester's ideas and ran with them almost immediately, from early 1900s. Hence "Kutta-Zhoukowski" theory, etc. You should think deeply about why you are so insistent on reintroducing those long discredited Flat-Earth theories. (End Zapletal) 101.171.213.65 (talk) 02:14, 5 March 2015 (UTC)

I see more than a "hint or suggestion" of inconsistency between Berriman and Lanchester. My dictionary defines stratum as "a horizontal layer of any material, especially one of several parallel layers arranged one on top of another". So Berriman's quote implies that one horizontal layer of air is being accelerated downward and that layers above and below it are not. And later in the same passage he quantifies the relationship as equivalent to dp/dt = -L. Berriman's implied model is thus similar to the firehose model, and identifying the affected air as a stratum puts him at variance with Lanchester, who makes it clear that to find dp/dt = -L in a real lifting flow field the control volume must be a column, not a stratum. Furthermore, we know from later sources such as Lissaman that a stratum that is wide compared to its vertical depth (a pancake control volume) contains just as much upward acceleration as downward acceleration, and a total dp/dt = 0.

Berriman's book may have been a fine book for its time, but this particular passage is not consistent with other more authoritative sources and should not be given weight. The quoted passage is misleading and negatively impacts the article. I have reverted the change. J Doug McLean (talk) 03:08, 5 March 2015 (UTC)

Doug, my key point is all about citing the sources. I have gone to the trouble of explicitly quoting and referencing the material that I find. You really do need to do the same - vague assertions to the contrary are not more good enough now than they ever were. Where are these citations of sources that explicitly refute Berriman's statement of the Newtonian principle? (And I don't mean just provide a different analysis using a different control volume, I mean explicitly refute the principle he states.) I am prepared to be educated about Berriman vs. Lanchester - many of my leads (mostly for work unrelated to this discussion) come from the pages of Flight, at a time that Berriman was its technical editor, so it is possible that I would pick up a selective view of his relationship with Lanchester. Meanwhile, one cannot remove a citation to a reliable source simply because one disagrees with it. Worse for you, it would break the terms of editing where there is a conflict of interest and also continue your pattern of disruptive editing. I would caution you not to remove the citation. This page is indeed the correct place for an editor with a conflict of interest to agree such details. — Cheers, Steelpillow (Talk) 10:29, 5 March 2015 (UTC)

Zapletal, this is a final caution about your abusive posts such as this. Stop the abuse or face sanctions. Rather than delete your rant on this occasion I sanitised it in order to make the point that my summary deletions are about abusive language, not about the technical arguments. If you cannot grasp even this simple point about civilised conversation, then you are not welcome on Misplaced Pages. I shall not sanitise any more, but shall return to wholesale deletion of abusive IP posts and if that fails to stop the abuse I shall seek page protection. — Cheers, Steelpillow (Talk) 10:29, 5 March 2015 (UTC)

Two sources are Lissaman (1996) and my own book, which paraphrases Lissaman's results, with attribution, of course. Berriman asserts that dp/dt = -L for "a stratum of air", which is contradicted by Lissaman's finding of dp/dt = 0 for a flat, horizontal control volume. It's not a matter of a "different analysis" or a "different control volume"; Lissaman's is an authoritative quantitative result for the same control volume, and it directly contradicts what Berriman says. And Lanchester finds dp/dt = -L only for the case of a tall column, which, in a way, is also in contradiction to Berriman.

Lissaman does not explicitly say "This result contradicts Berriman's explanation of 1913", but a direct quantitative contradiction should be enough. If you disagree, please point out specifically where Misplaced Pages policy requires "explicit refutation" for a source to be given reduced weight or no weight.

I'm not arguing that the citation should be removed "simply I disagree with it". I've made my case based on how this source stands relative to the more-authoritative sources, and I've pointed out in detail why those other sources should be regarded as more authoritative. As to whether I can remove the citation or not, I'd say that by adding the citation, you "bolded" a proposed change to the article. I have now reverted that change, which is an acceptable thing to do (and not "disruptive"), if I read the policy correctly. Before you reinstate your proposed change, the onus is on you to seek consensus in support of it.

Regarding my having a "conflict of interest", you've made this accusation before, and it is unfounded. I have said before that I receive no royalties or other financial reward for the publication of my book, nor does any friend or relative. And Misplaced Pages's definition of conflict of interest offers no other possibility that applies. So please stop making unfounded accusations. J Doug McLean (talk) 00:41, 7 March 2015 (UTC)

The section introduces the Newtonian theory of lift. Berriman's treatment is a particularly clear and unabmiguous introduction. He was also an acknowledged authority who around that time was technical editor of the Royal Aero Club's official journal Flight. This makes him an excellent source to quote in this section.

There is no contradiction, as you suggest there is. Lissaman does not contradict Newton. Both he and Berriman are consistent with the view that Newton's laws apply. Both are explicit that the force on the airfoil itself derives from pressure. Lissaman points out that different control volumes yield different distributions. Berriman does not define a control volume, so cherry-picking one of Lissaman's control volumes for comparison is wholly invidious. WP:FRINGE states that "For a fringe view to be discussed in an article about a mainstream idea, reliable sources must discuss the relationship of the two as a serious and substantial matter." There can be no doubt that the Newtonian theory of lift is a mainstream idea. On the other hand, the suggestion that there is some contradiction here is not mainstream: any discussion of such a relationship between the two positions is wholly absent from all of the many mainstream sources that have ever been cited in this year-long debate. You argue for such an interpretation of Lissaman's work, but the discussion is not there in Lissaman. You wrote that, "Lissaman does not explicitly say 'This result contradicts Berriman's explanation of 1913', but a direct quantitative contradiction should be enough." The above quotation from WP:FRINGE debunks that opinion of yours.

A conflict of interest is not about money. WP:CONFLICT is clear: "Any external relationship (any secondary role) may undermine that primary role, and when it does undermine it, or could reasonably be said to undermine it, that person has a conflict of interest." Your authorship of a book, whose unverifiable claim you promote to exhaustion on this talk page, is clearly within scope of that definition.

You have had enough policies thrown at you over the months to have figured this much for yourself, you may well have been told directly anyway. Your WP:WIKILAWYERING in an attempt to return to the fray is unacceptable. If you revert my addition of a perfectly appropriate citation once more, or take any other contentious action on this topic, I will deem the time ripe to take your behaviour to WP:ANI. @J Doug McLean: THIS IS YOUR FINAL WARNING. — Cheers, Steelpillow (Talk) 11:17, 7 March 2015 (UTC)

Your arguments here are not consistent with the physics, with the sources, or with Misplaced Pages policy:

Berriman says that dp/dt = -L for "a stratum of air". Lissaman analyzed three rectangular control volumes: a tall vertical column, a square, and a flat horizontal slab. I chose the flat horizontal slab because it matches the dictionary definition of a "stratum" and is thus the right one to compare with Berriman. Calling this "cherry-picking" is illogical. It's also illogical to say there's no contradiction when Berriman says dp/dt = -L, and Lissman finds dp/dt =0.

Regarding conflict of interest, I don't think my having written a book some years ago constitutes an "external relationship" or a "secondary role" that in any way compromises my role as an editor. You are misreading the letter and intent of this policy. J Doug McLean (talk) 20:03, 13 March 2015 (UTC)

At the top of this section, regarding Berriman's PoV, Steelpillow says "I have found not the slightest hint or suggestion of any historical controversy...".

No controversy? In Bloor's book, which I referenced in these Talk pages back in Dec 2014, the eminent academic G. H. Bryan is quoted giving this response to Berriman's view.

"... there is no such thing as "sweep" except in Newton's ideal medium of non-interacting particles satisfying the sine squared law. ... Mr. Berriman's "sweep" is, physically speaking, an IMPOSSIBILITY. If, however, "sweep" is defined as the depth of a hypothetical column of air, the change of momentum in which would represent the pressure on the plane, then the introduction of this new quantity is ONLY A USELESS AND UNNECESSARY COMPLICATION...".

(My added emphasis. "Sweep" = vertical thickness of "firehose-stream". This quote from G. H. Bryan's 1912 review of "The Dynamics of Flight" by A. G. Greenhill, Aeronautical Journal 16:264-67.)

So Bryan describes Berriman's version of TS as an "impossibility". This is the same view that Lanchester had a decade and a half earlier. This "impossibility" is nowadays so obvious that the majority of knowledgeable authors don't even bother mentioning it.

Conclusion: TS is a "fringe theory". (Zapletal) 101.170.42.154 (talk) 00:46, 18 March 2015 (UTC)

Lest others be misled: so here we have Zapletal's CAPITALISED SPIN on Bloor's report of Bryan's comment on Greenhill's report of one aspect of Berriman's analysis. And we are expected to believe that context has been preserved? Google Books shows the quote in question: go here and search for sweep. In my search, it was the second extract returned. Bloor first writes, "The basic formula is Force = Mass x Acceleration, but how is this formula to be applied? What is the mass of air that is involved? The original Newtonian picture must have underestimated this mass, hence the underestimation of the lift that can be generated. Berriman suggests that ... the wing exerts an influence on 'all molecules within an indefinite proximity to the plane; in other words a stratum of indefinite depth'"

The effecive depth for practical purposes is the "sweep" of the wing. Bryan was a mathematician and theorist. He pointed out only that the idea of sweep had no theoretical utility, he was not saying that it was wrong as such.

Thus, the discussion Zapletal quotes is essentially about how to identify "the air deflected downwards" (F = ma = -dp/dt), not how to confirm or deny it. Berriman's populist book Aviation, which began this thread, does not dig deep enough to mention "sweep" (as far as I can tell) but it does acknowledge "a cyclic or eddying disturbance around the leading edge of a wing in flight".(Page 101). It was that question of cyclic vs. eddying which lay at the heart of the controversy that Bloor was leading up to, not the TS bickering perpetrated above.

Bloor later recounts how Lanchester's theory effectively merged Berriman's "sweep" with the cyclic motion just mentioned, to develop his early circulation model. No controversy here, just a steady evolution of ideas.

Strip away the spin, and what we have left is an excellent source explicitly confirming TS and setting it in context. — Cheers, Steelpillow (Talk) 10:59, 18 March 2015 (UTC)

No. What is left is a hypothesis (= TS) which is "physically speaking, an impossibility", and is practically speaking "useless and unnecessary". Lanchester most definitely DID NOT "merge" Newton's idea of sweep (which, incidentally, is valid, but ONLY valid, in the Newtonian Medium) with the entirely different idea of cyclic hydrodynamic motion. Instead, he started by thoroughly refuting the ideas of "sweep" and "air deflected downward" with his Principle of No Momentun. He then developed the entirely different Circulation Theory of Lift, which to this day gives the most accurate description of Lifting flows. The two theories are as incompatible as Flat-Earth and Round-Earth theories. (Zapletal) 101.171.85.62 (talk) 00:56, 19 March 2015 (UTC)

RFC - Should we keep the current quantitative statement on momentum transfer in the second section or replace it with a qualitative version?

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Should we keep the current quantitative statement on momentum transfer in the second section or replace it with a qualitative version?

Mr. Swordfish (talk) 15:40, 9 March 2015 (UTC)

Background:

J Doug McLean wrote: I'd like to see a fresh straw poll on the issue of whether we should keep the current quantitative statement or replace it with a qualitative version like the one in my sandbox.

I think the time is ripe for an RFC. Please reply with keep or replace and whatever summary you'd like to include for your reasoning. Please keep the the discussion civil and on-topic. See WP:RFC for details on this process.

For reference, here's the relevant excerpt from Doug's sandbox:

The air flow changes direction as it passes the airfoil and follows a path that is curved downward. According to Newton's second law, this change in flow direction requires a downward force applied to the air by the airfoil. Then, according to Newton's third law, the air must exert an upward force on the airfoil. The overall result is that a reaction force is generated opposite to the directional change.

Here's what the current version of the article says:

Some of the air passing the airfoil has downward momentum imparted to it at a rate equal to the lift. (See "Momentum balance in lifting flows" for details) This is consistent with Newton's second law of motion which states that the rate of change of momentum is equal to the resultant force.

Mr. Swordfish (talk) 15:40, 9 March 2015 (UTC)

I thank Mr. Swordfish for taking this action. Since editors new to this topic have been invited to comment, I think providing them background in addition to the candidate texts would be good. Previous discussion on the talk page has covered many pages, and most of it has been archived. It would be difficult for a newcomer to discern the key points of the arguments. I think we should post brief summaries of the pro and con arguments. I'm willing to draft my side, if that would be appropriate.

J Doug McLean (talk) 17:22, 9 March 2015 (UTC)

If this was in "the intro section," I would strongly state that it should be qualitative not quantitative. However, it is not, and seems to be mis-characterised here. It is in the first actual explanation section, the subsection called 'Flow deflection and Newton's laws.' This comes after the introductory section to the main section called 'Simplified physical explanations of lift on an airfoil,' which itself comes after an introductory section called 'Overview', all of which come after the actual 'intro section' - normally called the lead - which appears above the contents. How can you call this 'the intro section'??! --Nigelj (talk) 18:05, 9 March 2015 (UTC)

I have re-cast the RFC to say "second section" rather than "intro section". Hope it's not too late. Apologies for the confusion. Mr. Swordfish (talk) 19:36, 9 March 2015 (UTC)

>Since editors new to this topic have been invited to comment, I think providing them background in addition to the candidate texts would be good.

Agreed. Herewith, a brief synopsis:

As part of a major re-org last August, a quantitative statement about momentum transfer was added to the article. At the time it stated: The resulting force upwards is equal to the time rate of change of momentum of the air deflected downwards. This impetus for adding it was an article published by the American Association of Physics teachers which said: "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." (http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PHTEAH000037000005000297000001&idtype=cvips&doi=10.1119/1.880292&prog=normal)

This addition provoked a rather lengthy and sometimes acrimonious discussion, of which a main focus was what is meant by "the air" in the statement. Eventually, the editors reached consensus/compromise on the current wording. Consensus can change, hence this RFC.

I'm trying to keep this synopsis as neutral as possible, and I'm not sure I can provide further details while maintaining neutrality. I'll have more to say by way of background in my response. Disclosure: I authored both the original statement from August and the compromise/current statement. Mr. Swordfish (talk) 20:04, 9 March 2015 (UTC)

• Keep The case for the quantitative statement is simple: it is widely prevalent in reliable sources, several of which are cited in the article. It does not matter what we editors believe, Misplaced Pages expects us to present the mainstream view, and in this it puts verifiability and due weight above what we believe to be true. The case against the quantitative statement is, by contrast, hollow. The debate itself has been running solidly for well over a year (I got bored trawling the archives), and in all that time nobody has ever cited a single reliable source discussing any controversy over the quantitative statement, as required by WP:FRINGE. One source has been offered, but it was written by the sole registered wiki editor arguing against the quantitative statement, and thus he has a clear conflict of interest in this debate. The only other voice against it has been an IP editor, who will be unable to join us for a week since this page has been semi-protected by an Admin who determined that this editor has been overly abusive. All remaining sources proffered against the quantitative statement have turned out to be the editor's interpretation and not the words of the source itself. Of course, if new sources can be brought forward that might change things, but for now at least there is simply no encyclopedic case for the quantitative statement's removal. None. — Cheers, Steelpillow (Talk) 21:00, 9 March 2015 (UTC)

Partial rebuttal of "Keep" The above misrepresents the case that has been made in favor of the change. First, it misrepresents the situation regarding the published sources. The proposed replacement does not discuss any fringe theory or any controversy, and so the WP:FRINGE requirement that the sources explicitly discuss a controversy does not apply. The sources quoted in the current version of this section make a quantitative statement (i.e. that downward momentum is imparted to "the air" at a rate equal to the lift) that is supported only in one special case and is otherwise contradicted by the more rigorous analyses in the sources from the mainstream aerodynamics literature cited in the later section "Momentum balance in lifting flows". Finding a contradiction between two groups of sources, even though no controversy is explicitly mentioned in any of them, and assessing the relative authoritativeness of the sources based on their scientific rigor, is permissible, in keeping with Due and undue weight. And the assertion that my own book is somehow central to the case that has been made is unfounded. Finally, the assertion that only two voices have been raised against the quantitative statement is untrue. In December one other editor posted a long discussion finding The Statement to be "problematic", and in February another found the statement confusing (with good reason) and suggested a replacement that was qualitative. Substantive objections to the current passage itself are given below. J Doug McLean (talk) 03:52, 10 March 2015 (UTC)

Different groups of sources present different bases for analysis, they reach different conclusions yet there is no evidence of controversy. The conclusion must be that overall they illuminate a consistent theory. Let me restate, whether any editor sometimes gets confused or has problems understanding the debate is quite beside the point. The sources speak for themselves, and we must not stand in their way. — Cheers, Steelpillow (Talk) 11:39, 10 March 2015 (UTC)

Steelpillow's arguments here flout simple logic and Misplaced Pages policy. True, there is no mention of a "controversy" in any of these sources, but no one has advocated that it be presented as one in the article. There is clearly a contradiction between the two groups of sources, and thus the conclusion cannot be that "overall they illuminate a consistent theory". The statement "The sources speak for themselves" is meaningless. All published sources speak for themselves, simply by being published. But that doesn't mean we should give them all equal prominence. Per Due and undue weight, this is a situation in which we shouldn't. J Doug McLean (talk) 19:40, 13 March 2015 (UTC)

• Replace The case for replacing the statement is based on article structure and a correct understanding of the physics as found in the preponderance of published sources.

The statement in question is not in an "intro" section, but it is in a prominent place near the top of the article (There are only five short paragraphs ahead of it), in a subsection of the major section titled "Simplified physical explanations of lift on an airfoil". Given its placement in the article structure, a qualitative version is more appropriate than a quantitative one, especially given that the oversimplified quantitative version is problematic, as argued below.

By referring to "some of the flow" instead of just "the flow", the current wording, in a legalistic way, avoids being outright wrong, but it still gives a misleading impression. The reader is likely to infer, and reasonably so, that the downward momentum that is imparted all shows up as downward momentum in the flow downstream of the foil. But in fact, half goes to canceling upward momentum that was imparted ahead of the foil, and only half shows up downstream. To have the wrong impression corrected, the reader would have to read the rather technical later section "Momentum balance in lifting flows". But why introduce a wrong impression at all in an early section that is supposed to be simplifying things?

Of the sources cited in support of the current version, only two present any supporting analysis, and that analysis is based on an idealized flow model (I call it the "firehose model") that is at variance with the actual physics and with the methods used in the mainstream literature, in ways that have been discussed at length on this talk page. In contrast, the mainstream analyses cited in "Momentum balance in lifting flows" use a model that they rigorously show is physically correct. Thus the current version gives undue weight to a view of the physics that is not mainstream, and, in fact, satisfies the definition of a fringe theory. J Doug McLean (talk) 03:52, 10 March 2015 (UTC)

The encyclopedic case for getting rid of the oversimplified quantitative statement is clear. This is not a case where the sources are such as to force us to choose verifiability over truth. This is a case where appropriately weighting the different sources allows us to have both. J Doug McLean (talk) 16:10, 10 March 2015 (UTC)

These arguments make the case for keeping very well. They corroborate the points that different sources using different bases for analysis reach different conclusions, and with no evidence of controversy. Undue weight suggests that all be presented faithfully in the article. Several are bundled together in the discussion of "Momentum balance in lifting flows", one is of a rather different character and so presented in a summary of "Flow deflection and Newton's laws." However the argument for removal is then suggesting that WP:UNDUE endorses the burying of one particular approach, a clear fallacy. — Cheers, Steelpillow (Talk) 11:39, 10 March 2015 (UTC)

I was just looking at Electron#Atomic_theory to see how the article handles the difference between shell-theory and quantum theory. Both models are presented in separate sections right next to each other, without any discussion of one refuting the other. Can we learn anything from the approach taken there? Burninthruthesky (talk) 11:00, 11 March 2015 (UTC)

Is this the "firehose model" in Lanchester §160? Burninthruthesky (talk) 22:28, 10 March 2015 (UTC)

A problem with drawing an analogy between Electron#Atomic_theory and our article is that while the two theories in Electron#Atomic_theory differ in their detailed modeling, their major predictions don't contradict or refute each other, in contrast to our case, in which one group of sources claims dp/dt = -L for "the air" without qualification, which is contradicted by the results from two of the three ways of defining "the air" examined by Lissaman. So I'd say that giving two models equal billing is more appropriate in the case of Electron#Atomic_theory than it is in our case.

Yes, Lanchester's §160 discusses a model that is essentially equivalent to the "firehose" model we've been discussing. But in §160 Lanchester is not so much advocating for the model as showcasing its shortcomings, concluding in the final sentence of §160 that "the hypothesis is insufficient". Then in §161 he emphasizes the importance of including the effect of the upwash ahead of the foil, something that is completely ignored in our other sources' versions of the "firehose". J Doug McLean (talk) 19:43, 13 March 2015 (UTC)

• Comment The proposed change would remove material supported by five citations. That may not be immediately obvious from the plain-text quotes above. Burninthruthesky (talk) 10:48, 10 March 2015 (UTC)

Actually, six of the sources cited in this section mention the quantitative statement dp/dt = -L in one way or another:

• Clancy: Here the analysis presented in support of the statement is based on the "firehose model", a model that doesn't actually substantiate the statement, but simply assumes it a priori. The "firehose model" also assumes, without justification, that the only force acting on "the air" is the force exerted on it by the foil.
• Three papers from one non-specialist journal (AAPT): Waltham also bases his analysis on the "firehose model". Swartz cites Waltham. Smith couches dp/dt in terms of m delta v/delta t, without saying how single values of m and v can be assigned to an infinite, non-uniform flowfield, unless what he has in mind is the "firehose model". All of these assume, without justification, that the only force acting on "the air" is the force exerted on it by the foil.
• Berriman: This citation is in dispute (see talk page section immediately above this one) because it makes an erroneous version of the statement, ascribing dp/dt = -L to "a stratum of air", a body of air for which Lissaman showed that dp/dt = 0.
• Lissaman: This is the only one of the six whose analysis meets the standards of mainstream aerodynamics analysis, by assuming a realistic form for the flowfield and properly taking into account the pressure field as exerting one of the forces acting on the air.

What Lissaman shows is that the rate at which momentum is imparted to the flow is affected by the pressure field and is different for different subsets of the air. The picture presented by the other five sources is thus misleading and not representative of mainstream aerodynamics.

Actually, Lissaman is cited in this section only because the qualifying wording "some of the air" would have no support without him. J Doug McLean (talk) 19:47, 13 March 2015 (UTC)

• Combine The current contains the relevant quantitative, cited section, the proposed is better written. Incorporating the quantitative rate into the proposed would leave the article clearer with the same accurate information. SPACKlick (talk) 12:04, 10 March 2015 (UTC)

• Replace and move quantitative statement to later in the article. First, I'd like to say that reasonable people can disagree on this. There are definitely two sides to this argument and I would encourage everyone to try to understand both.

In favor of keeping the quantitative statement, there is the American Association of Physics Teachers (AAPT) who recommend "...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.". This pedagogical recommendation is what persuaded me to include the statement in the article when doing the major re-draft last August. Moreover, there are several other sources that can be cited to back the assertion L = dp/dt.

So, including the quantative statement seemed like a slam-dunk. L=dp/dt is easily derivable from Newton's second law F=ma, (all you need to do is substitute L for F and rewrite ma as dp/dt), AAPT recommended it, and the statement has numerous cites to back it up, what could go wrong?

Well, the problem lies in what is meant by "the air". Clearly, it can't mean all of it since the net momentum change for a plane in straight and level flight is zero and therefore the momentum change for the air must also be zero to comply with conservation of momentum. I was unaware of control volume analysis when I included the statement last August, but upon further research it turns out that dp/dt depends on the shape of the region - that is, if you take a square region, integrate the momentum change over that region, and take the limit as the size of the square goes to infinity, then dp/dt = L/2, or one half of the lift. If you take a long flat horizontal rectangular region instead of square you get dp/dt=0. It's only when you employ a tall thin vertical region that you obtain dp/dt = L.

In short, the quantitative statement is only true for a very specific region of "the air" and not true for most sections. In particular, it is not true for most of the reasonable interpretations of "the air" that the readers might assume.

The upshot after all this is the carefully worded intentionally vague current verbiage: Some of the air passing the airfoil has downward momentum imparted to it at a rate equal to the lift.

This statement is technically true (or at least verifiable) in a legalistic sense and is well supported by the cites, so there is nothing in Wiki policy that would require its removal. But verifiability is a necessary but not sufficient criteria for inclusion. We editors must consider the experience of the reader and present an article that is logically well-organized and presents the material in a straightforward easily understandable manner. Ultimately, this is what determines my opinion here - what makes for a better article from the readers' perspective? It's not about being "right" or "wrong" or one side "winning".

My take is that the current quantitative statement is a bit awkward and intentionally vague due to its having to "write around" some complex details to be technically correct. The vagueness begs the question of what is meant by "some of the air". I could see someone putting the equivalent to a tag next to it. By contrast, the suggested replacement text is straight-forward and easily understandable. Since this is a very early section of the article that is aimed at the lay-reader, I don't think a quantitative statement is necessary. I don't have a particular antipathy to including a quantitative statement in this section, but thus far we have been unable to craft one that is clear and straightforward enough for my liking. To reiterate, this is an editorial judgment call and I respect the opinions on the other side.

One "tipping point" for me was this thread where one of the editors (Mark.camp) discusses "...what I as an ordinary (non-expert) consumer find to be confusing about the current text,...". My take is that if editors are confused about the material then the readers will also be confused. Much earlier, another editor (0x0077BE ) questioned "...why we're bringing momentum into this at all." (https://en.wikipedia.org/search/?title=Talk%3ALift_%28force%29&diff=626659376&oldid=626659076)

All that said I will not be unhappy if keep is the outcome upon closure. But my editorial judgement is that the material is clearer and more easily digestible to the intended audience if the quantitative statement is removed from the current section and integrated into the momentum transfer section. Mr. Swordfish (talk) 21:18, 11 March 2015 (UTC)

1. Clancy, L.J.; Aerodynamics, Pitman 1975, page 76: "Thus the lift of the wing is equal to the rate of transport of downward momentum of this air."
2. "...if the air is to produce an upward force on the wing, the wing must produce a downward force on the air. Because under these circumstances air cannot sustain a force, it is deflected, or accelerated, downward. Newton's second law gives us the means for quantifying the lift force: F_lift = m∆v/∆t = ∆(mv)/∆t. The lift force is equal to the time rate of change of momentum of the air." Norman F. Smith "Bernoulli and Newton in Fluid Mechanics" The Physics Teacher 10, 451 (1972); doi: 10.1119/1.2352317 http://dx.doi.org/10.1119/1.2352317
3. Berriman, A.E.; Aviation, Methuen 1913, Page 303: "Thus, the wing in flight continually accelerates a stratum of air downwards, and must derive a lift therefrom."
4. http://arc.aiaa.org/doi/pdf/10.2514/6.1996-161

I'd like to suggest that this post is an excellent demonstration of why the only way to navigate this minefield of editorial opinion is to stick to what the reliable sources actually say and not to try and interpret them (this is of course enshrined in the WP:NOR policy, especially WP:SYNTH). It is not for us to opine on what "the air" might be. mark.camp (talk · contribs) described himself as "an ordinary (non-expert) consumer", yet this post from some time ago is consistent with a slightly more knowledgeable status. So what the ordinary non-expert might see may or may not be what has been suggested. For my part I would suggest that the immediate question, "which air?" needs a great deal of analysis to answer and it may or may not spring a surprise. But it does not negate Newton's laws of motion as applied to lift, it just complicates things. This is just another reason to roll with the judgement of the many reliable authorities who have published on this topic - and that means keeping the quantitative aspect, although I do like the suggestion of SPACKlick (talk · contribs) to borrow the more readable phrasing. — Cheers, Steelpillow (Talk) 22:27, 11 March 2015 (UTC)

Steelpillow's take on the question "which air?" reaches the opposite of a logical conclusion. The deliberately vague "some of the air" is bound to raise the question in the minds of some readers. Whether they're non-experts or experts is beside the point. And how our readers are likely to take something is legitimately our concern as authors and editors. The question itself, i.e. what body of air has downward momentum imparted to it at the rate -L, is not a question that "needs a great deal of analysis to answer" or that has any surprises in store. We have a citable answer from Lissaman: dp/dt = -L for any tall rectangular column, of any horizontal width, as long as the horizontal width is a small fraction of the vertical height. Note that the shape is specific, but the size is non-unique. So the answer is a bit technical, and the current wording misleads by giving the impression that it's simpler than it really is. The suggested replacement solves this problem by avoiding the question all together, and it is just as well supported by sources as the current wording is. J Doug McLean (talk) 19:58, 13 March 2015 (UTC)

• Keep in some form We must put the most understandable explanations WP:UPFRONT, and make the article accessible to a wide WP:AUDIENCE. To that end Mr. Swordfish brought the suggestion that we follow the educational guidance from the AAPT, and I haven't seen a stronger argument than that.

There is a valid question as to whether the current description of "some of the air" could be confusing. I actually think this is too specific about leaving something explicitly undefined. If this were a fresh discussion I might have suggested removing "some of", but that isn't moving towards agreement. I'm happy to go with whatever consensus is determined. Burninthruthesky (talk) 09:08, 12 March 2015 (UTC)

REPLACE - At stake here is whether Misplaced Pages can ever be considered a genuine Encyclopaedia, or merely a random collection of citations and PoVs. The Lift article currently presents two very different and incompatible explanations of Fluid Dynamic Lift, with no explanation given for this incompatibility.

1. On the one hand there is "The Statement", which is presented both near the very top of the article, and also several times in the bottommost External Links. That is, TS is in the most read sections of the article. TS presents a picture of a horizontal onset flow in front of the aerofoil, and then an ever descending "downwash" behind the aerofoil. It is indisputable that TS presents this picture, because it is exactly this picture that accompanies so many presentations of TS (eg. see last External Link "One Minute Physics...").

2. On the other hand there is the "Circulation Theory of Lift", which is presented in the middle, more technical, parts of the article. CToL presents a picture of horizontal flow at large distances from the aerofoil, with only the local flow rising in a symmetric "hump" near the aerofoil. This picture is well depicted by the animation next to the section - "The airfoil affects the flow...".

Importantly, the TS-picture is utterly unrealistic. Real Lifting flows do not look anything like it. This was pointed out by Lanchester 120 years ago, and by many others since (eg. see Bryan's quote in the Berriman section above, which describes TS flow as "physically speaking, an impossibility"). Conversely, the CToL picture of Lifting flows has been shown to be accurate to within all experimental tolerances (when adjusted for details such as viscosity, turbulence in the onset flow, etc.).

Put simply, TS is analogous to a "Flat-Earth-at-the-Centre-of-Universe" theory, with CToL being the "Round-Earth-Orbiting-the-Sun" theory. It is worth noting that Flat-Earth theory can be considered to be a "good enough" approximation at small scales, perhaps when laying out the foundations of a house or drawing a street map of a small town. But NO good Encyclopaedia presents its Geology or Cosmology sections with a Flat-Earth-at-Centre-of-Universe introduction. At least not without some qualification of just how wrong such a view is. With regard to Lift, TS-theory is not accurate at any scale, so can never be said to be a "good enough" approximation. It does nothing but give an entirely misleading picture of the essence of Lift.

If TS is kept in this Lift article, then Misplaced Pages can abandon all pretence at being a genuine Encyclopedia. It will be more of a Wackypedia, where any and all wacky fringe theories can be paraded as encyclopediac knowledge, just as long as some editors can find a few obscure citations to support their PoVs, and are obstinate enough to force those PoVs through.

Note, of course, that this would just be a case of history repeating itself. The Ancient Greeks held the Round-Earth heliocentric view during their Golden Age, until it was eventually subverted by the Flat-Earth geocentric religious views. A thousand years of Dark Ages followed, and then a long climb out of that hole. Any editors with a sense of responsibility are reminded that future generations will reap what you now sow. (Zapletal) 101.170.42.154 (talk) 01:06, 18 March 2015 (UTC)

• Replace. I'm not saying this because of policy or anything. It's just that Doug's version simply explains how lift works better. In fact based only on that explanation I already understand in general terms how lift works, and don't feel the need to read the 'momentum balance in lift flows' section (which I'll say takes a lot more brainpower to understand). The paragraph as it is right now doesn't even mention Newton's third law, which is undoubtedly crucial for lift to be generated, either.

Frankly this actually seems so clear-cut obvious that it's probable I'm missing something; if so, please point it out. Banedon (talk) 04:19, 25 March 2015 (UTC)

The excerpts quoted above are not a complete list of changes - have a look at the full versions. The controversy behind this RFC is the proposal to remove the material from the current second paragraph of that section, along with its citations. Hope this clarifies. Burninthruthesky (talk) 08:01, 25 March 2015 (UTC)

I didn't compare the entire article in the sandbox, although I did note differences in the rest of the paragraph. They don't change what I perceive though: Doug's version simply explains things better, and the difference is clear-cut enough that I don't understand the reason for the disagreement. Banedon (talk) 01:36, 27 March 2015 (UTC)

• Comment. It seems to me that different editors have effectively been commenting on two different proposals. The RfC itself proposes to replace a quantitative statement with a qualitative one (that happens to be better written). Several responses are proposing that the badly written statement be replaced with a better-written one (that happens to be qualitative). For example if the current quantitative version were expressed like this:

"The air flow changes direction as it passes the airfoil and follows a path that is curved downward. According to Newton's second law, this change in flow direction requires a downward force applied to the air by the airfoil. Then, according to Newton's third law, the air must exert an upward force on the airfoil. The overall result is that a reaction force, the lift, is generated opposite to the directional change."

would all those "replace" votes be the same?

Given that this has happened, I would suggest that the RfC issue cannot sensibly be resolved until both of the existing and proposed texts are equally well written. And given that the bad writing of the present version is the end result of a long and painful controversy about the quantitative aspect, I would suggest that the controversy needs resolving first. In other words, I'd suggest that this RfC be abandoned or closed, in order to allow the mediation process on that controversy to run its course. — Cheers, Steelpillow (Talk) 10:23, 26 March 2015 (UTC)

It looks to me like your hypothetical change would express the "current quantitative version" in completely qualitative terms. In fact, it looks word-for-word the same as the "replace" proposal. Do I understand correctly? J Doug McLean (talk) 21:34, 26 March 2015 (UTC)

Related question: all these different wordings don't use numbers, yet one is called 'quantitative' and the other 'qualitative'. Why, and what's the difference? Banedon (talk) 01:36, 27 March 2015 (UTC)

The difference is essentially whether all of the lift is said to be derived in this way. The specific remark that "downward momentum imparted to it at a rate equal to the lift" is quantitative because of the stated fact of equality. The originator of this RfC is one of a number of editors involved in a very long-running discussion over how to treat this, and this RfC is an attempt to resolve the dispute. Hence my suggested addition of the phrase, "the lift," to Doug's better-written version. Hope this helps. — Cheers, Steelpillow (Talk) 10:36, 27 March 2015 (UTC)

Newtonian lift and the circulation theory

I just came across this:

"At the outset it may be set down that any defect in the theory is due, not to any want of exactitude in the fundamental theory - this rests definitely on the third law of motion and is absolute - but rather to the difficulty and uncertainty as to its manner of application in real fluids."

— Lanchester; Aerodynamics (1907), Page 5:

This very much bears out what I have been saying above, that the Newtonian model is not so much wrong as difficult to analyse (to identify "the air" deflected downwards). Lanchester goes on to expand his Newtonian flow model (that we have been likening to a firehose) to include some germ of the circulation theory of lift (see his Sections 160 and 161 in the above link). This expansion is key to the way the discussions here have gone: Lanchester does not treat the firehose as wrong, just as in need of much elaboration. A later controversy did erupt, between a British establishment elaboration based on perturbations (i.e. ignoring Lanchester from here on) and a German establishment elaboration based on circulation. Lanchester's words suggest to me that this controversy was not over the validity of the Newtonian model, but rather over its elaboration. As it happens, the perturbation model lost out and the circulation model survived into modern times - along with its Newtonian underpinning, as is well cited in the article. The controversy is documented in: Bloor, D; The Enigma of the Aerofoil: Rival Theories In Aerodynamics, 1909-1930, University of Chicago (2011). Sadly I do not have time to read it all, as apart from the first chapter (Haldane and the ACA for those who care) it is off my current line of work. Perhaps it may contradict what I suggest above, I don't know. Meanwhile, I think that all this might provide useful food for thought into whether the structuring of this article could be improved and whether we are putting the right emphasis in the right places. — Cheers, Steelpillow (Talk) 20:41, 12 March 2015 (UTC)

Checking back, I am not sure how far Lanchester goes towards a recognisably modern circulation theory, but it does not affect the rest of what I have said.

Bloor (referenced above) tells an interesting story. In an idle moment I found extracts at Google Books, here. In discussing Berriman, he writes; "The basic formula is Force = Mass x Acceleration, but how is this formula to be applied? What is the mass of air that is involved? The original Newtonian picture must have underestimated this mass, hence the underestimation of the lift that can be generated. Berriman suggests that..." and goes on to describe the "sweep" model (our "firehose"). Later, he describes how Lanchester took this as one of the key starting points from which he evolved his circulation theory of lift. — Cheers, Steelpillow (Talk) 11:15, 18 March 2015 (UTC)

Yes, and the key to this "starting point" for Lanchester (in the early 1890s!) was when he realised how utterly IMPOSSIBLE is the "sweep + downwash" model. It was only when Lanchester completely abandoned that line of thinking that he made progress and proposed CToL. (Zapletal) 101.171.85.77 (talk) 01:11, 19 March 2015 (UTC)

I'm glad to see that there's now some question in Steelpillow's mind whether "we are putting the right emphasis in the right places".

That said, what he says here still accords the "Newtonian model" too much importance in the overall scheme of things. The CToL won out because it makes accurate predictions of how upwash and downwash are distributed in the field, and how the imparting of downward momentum is different for different subsets of the air, things about which the firehose is just plain wrong. CToL and the firehose share a Newtonian "underpinning" in the sense that they both make use of NII. A key difference is that CToL applies NII correctly (taking the pressure field into account as one of the forces acting on "the air"), and the firehose does not. Getting from the firehose to CToL goes far beyond any reasonable meaning of the word "elaboration". It requires throwing out key assumptions of the firehose, i.e. that only a finite stream is affected by the foil, and that there is no direction change upstream of the foil. Zapletal is right that the "sweep" model wasn't really a "starting point" for CToL. J Doug McLean (talk) 23:34, 20 March 2015 (UTC)

Revisions to "Momentum balance in lifting flows"

Some time ago I proposed revisions to "Momentum balance in lifting flows" and posted the proposed new version in my sandbox. These changes simplify and clarify the section and align it better with the cited sources. Having heard no objection to these changes, I have installed them. J Doug McLean (talk) 21:29, 18 March 2015 (UTC)

Doug, just one tiny quibble. The very last words of that section are "...as was first noted by Lanchester in 1907." My understanding is that Lanchester first publicly presented his ideas in 1894, in a talk given to the Birmingham Natural History and Philosophical Society. He then expanded these ideas into a paper that he sent to two (?) scientific journals, including The Physical Society of London (1897), but was rejected. Meanwhile, the German school, including Kutta, Zhoukowsky (a Russian, but writing in German journals), and Prandtl, apparently got a whiff of these ideas and starting publishing many papers based on CToL in the very early years of 1900s. Many of these papers were in-school (Gottingen) memos, so exact dates are hard to establish. Anyway, the notion that, in Lanchester's words, "the fluid ... does not gain or lose momentum any more that does a cast-iron pillar supporting a load" was probably well understood quite a bit earlier than 1907. So I would suggest just dropping the word "first" from the above quote.

Also of historic interest is that "vortex/circulation lift" was used by Rayleigh in his 1877 paper "On the Irregular Flight of a Tennis Ball". Bizarrely, this idea was NOT taken seriously at the time, being considered only a flippant speculation regarding a trivial game. It is difficult to understand (beyond belief to me!) why it took the Cambridge school another 50 years to finally accept CToL. (Well, they were ideologically wedded to the idea of "Stokes flows", where viscosity is essential...) But then again, there are still people today who resist CToL. (Zapletal) 101.171.85.57 (talk) 02:17, 19 March 2015 (UTC)

Good point. Thanks. I made the change. J Doug McLean (talk) 17:34, 19 March 2015 (UTC)

Wait a minute. I think that it would have been more collaborative if you had discussed the changes 'before installing them. Robert McClenon (talk) 13:51, 20 March 2015 (UTC)

In particular, the momentum theorem had been tagged for discussion. Deleting the tag without reaching consensus here is not acceptable. That "Having heard no objection" does not stack up.

I am also unhappy about the addition of this paragraph:

Thus it is found that the change in momentum flux from upstream to downstream accounts for the entire downward force ${\displaystyle -L'}$ exerted by the airfoil only in the case of the tall, slender rectangular control volume. For control volumes of other shapes, the integrated pressure difference between the top and bottom offsets some or all of the ${\displaystyle -L'}$ exerted by the airfoil, and the change in momentum flux is between ${\displaystyle -L'}$ and zero.

I seem to recall from earlier discussions that if the firehose is cut off at a certain point about a chord's length behind the foil, the momentum change across the boundary equals the lift, so that "only in the case of the tall, slender rectangular control volume" is untrue. However the paragraph - whether corrected or no - adds nothing to the reader's understanding. Nor has any citation for it been provided. What I do recognise it as, is essentially an extract from the long-running spin against the Newtonian statement. I am now seriously considering whether this trips the wire and I should take J Doug McLean (talk · contribs) to WP:ANI and, if need be, WP:ARBCOM for his long-term WP:DISRUPTION. — Cheers, Steelpillow (Talk) 19:38, 20 March 2015 (UTC)

Robert McClenon must be new to this discussion. As noted in the first sentence of this talk section, these changes were proposed some time ago. That was in the context of a lengthy discussion on this talk page (in January of this year) that was archived by one of the other editors. In that discussion, valid questions were raised about the the article section in question, which was authored by me and was new at the time. The technical issues were discussed at length. I suggested revisions and posted them in my sandbox. No opposition to the proposed changes was expressed by any of the participants.

Steelpillow is not new to the discussion, but seems to have forgotten its history. The article's explanation of the momentum theorem was tagged for discussion, and it was subsequently discussed at length. I went to the trouble of drafting and explaining proposed changes to address the issues that were raised. Steelpillow and everyone else had ample opportunity to comment on the proposal, but no one did. And someone took it on himself to archive that section of the talk page before a resolution was reached. I think I exercised due diligence by participating in the discussion, making a proposal, and asking for feedback. Consensus and collaboration are supposed to be a two-way street. I finally took it on myself to make a bold edit. I think my revisions are technically correct and are supported by the sources cited.

The later paragraph that Steelpillow now is unhappy with is intended as a summary of the rather technical discussion in the paragraph preceding it, which describes the results of the published and cited control-volume analyses that used the uniform-flow-plus-vortex flow model, and as it stands it accurately reflects what the cited sources say. It is clear from the context that this paragraph refers only to the findings arrived at using the generally accepted uniform-flow-plus-vortex flow model. That a counterexample can be deduced from the firehose model is beside the point and is irrelevant to this section of the article. Besides, the firehose model was intended only to reflect the total integrated change in momentum flux. Even its originators would not have claimed that it has anything correct to say about the spatial distribution of the flux. The cited classical analyses, on the other hand, do accurately reflect the spatial distribution as well as the total.

I see now that Steelpillow has reverted my changes and has asked below for comments on the part dealing with the momentum theorem. We'll see what the response is this time around. So far, I'm the only one who has made a viable, constructive suggestion as to how to fix the passage in question. Regarding the paragraph he's unhappy with, I'd argue that it's a helpful summary and should be reinstated, but that leaving it out only harms the article a small amount. His contention that it is "untrue" is unfounded, as is his accusation of WP:DISRUPTION.

J Doug McLean (talk) 23:11, 20 March 2015 (UTC)

Momentum theorem

Here is the version proposed by J Doug McLean (talk · contribs) for the text tagged for discussion:

For an extended region, Newton's second law takes the form of the momentum theorem for a control volume, where a control volume can be any region of the flow chosen for analysis. In the analyses described below, the flow is assumed to be steady, and the effects of gravity on the flow are assumed to be negligible. Under these assumptions, the momentum theorem states that the integrated force exerted at the boundaries of the control volume is equal to the integrated flux of momentum through the boundary.

Is this an improvement? — Cheers, Steelpillow (Talk) 19:57, 20 March 2015 (UTC)

1. Emanuel (2000), Section 14.2
2. Shapiro (1953), Section 1.5
3. Emanuel (2000), Section 14.2. See equation 14.4 with the unsteady term (first term on the RHS) and the gravity term (first term on the LHS) set to zero.

Tie-in between "Momentum balance in lifting flows" and "some of the air"

Under "Flow deflection and Newton's laws", the passage "Some of the air passing the airfoil has downward momentum imparted to it at a rate equal to the lift. (See 'Momentum balance in lifting flows' for details)" implicitly promises that the issue of "what air" is so affected will be clarified in the later section. In the current version of "Momentum balance in lifting flows", the reader would have to be very technically astute to make the connection. The paragraph I proposed adding:

Thus it is found that the change in momentum flux from upstream to downstream accounts for the entire downward force ${\displaystyle -L'}$ exerted by the airfoil only in the case of the tall, slender rectangular control volume. For control volumes of other shapes, the integrated pressure difference between the top and bottom offsets some or all of the ${\displaystyle -L'}$ exerted by the airfoil, and the change in momentum flux is between ${\displaystyle -L'}$ and zero.

would help the reader make the connection. But now I think that directly referring back to the earlier section would be even better:

The simplified explanation of lift given in "Flow deflection and Newton's laws" states that some of the air passing the airfoil has downward momentum imparted to it at a rate equal to the lift. The analyses described above show that the region of air for which this is true is the tall, slender rectangular control volume. For control volumes of other shapes, the integrated pressure difference between the top and bottom offsets some or all of the ${\displaystyle -L'}$ exerted by the airfoil, and the change in momentum flux (the rate at which momentum is imparted )is between ${\displaystyle -L'}$ and zero.

I have posted this proposed wording in my User:J_Doug_McLean/sandbox, where it can be viewed in context. I hereby propose that this paragraph be added to the article. J Doug McLean (talk) 01:42, 24 March 2015 (UTC)

Observations and a Suggestion on How to Go Forward

I have not been a regular participant in the discussions on this talk page, but I have been here from time to time. I first came here to close an RFC on The Statement, and I see that another RFC is in process concerning whether to provide a quantitative or a qualitative statement. I have taken another look and have a few observations. On the one hand, this is a purely technical topic, and so should not result in as much controversy as, for instance, American politics. On the other hand, this is a purely technical topic with a considerable amount of controversy, both because of its complexity, and because there are alternate technical formulations, and academic issues about which formulation to use (or both). The controversy about this article spilled over to WP:ANI in a very poorly handled thread. The editors here are normally civil, but are frequently divided about content.

I would suggest that the editors all read or re-read the dispute resolution policy. It points out that disputes include content issues and conduct issues. The issues here are, fortunately, primarily content issues. (ANI wasn't, in my opinion, necessary or appropriate.) I would then suggest that, for this article, the appropriate way forward might be formal mediation by a member of the Mediation Committee. Formal mediation is a slow process, but the issues about this article have gone on for months and will go on for months, so why not spend the months with a mediator working to help the editors communicate to improve the article?

Robert McClenon (talk) 16:56, 22 March 2015 (UTC)

Thank you for the suggestion, Robert. I had thought about it. My concern is that this is not solely a content issue, 99% of it is just being disguised as one. I joined this discussion when one editor came to the Aviation WikiProject already feeling thoroughly jaded and asked for a fresh pair of eyes. I did find behavioural issues at first and had to quell the more abusive aspects. It soon became crystal clear that the discussion was being kept alive by a lone editor who is trying to present his own version of the truth and not the picture to be found in the reliable literature. His main weapons are vast screeds of techno-verbiage and tenacious persistence. He swamps every discussion (e.g. the RfC above) and it gets hard for visitors to these discussions - dare I say it, such as your good self - to see past it. It appears on the face of it a perfect case of long-term WP:DISRUPTION through WP:RUNAWAY to the talk page. His attitude to myself and others has been, "fine, as long as you agree with me," soon followed by, "If you don't agree with me then you are one of the enemy." I am under no illusion that he will listen to a formal mediator any more than he has listened to the rest of us, mediation will simply be seamlessly integrated into part of the drip-fed disruption. It will only be when this editor is removed from the discussion and can no longer drown it out that the content issue can be properly addressed. However, if you still feel that an experienced mediator can resolve this situation, I would be willing to give it a try. — Cheers, Steelpillow (Talk) 18:20, 22 March 2015 (UTC)

Herein is the root cause of the problem. Steelpillow says "It soon became crystal clear that the discussion was being kept alive by a lone editor who is trying to present his own version of the truth and not the picture to be found in the reliable literature." This statement, as with many others like it from Steelpillow, is UTTERLY FALSE. This is easily confirmed by the most fleeting scan of the last three Talk pages. These show that there are at least TWO people here who are strongly opposed to the TS-PoV. At least three, if the article's long time contributor Mr Swordfish is also included, and who was recently described by Steelpillow as "...newly befuddled".

It is worth noting that the people most strongly opposed to TS are those with the deepest understanding of the technical issues (ie. of "the picture found in the reliable literature"). In contrast, the two, and only two, people who strongly support TS have themselves acknowledged that they have negligible understanding of these technical matters. They have also shown that they are only aware of a very small fraction of the many RSs covering this field.

Steelpillow has a track record of "edit-warring". It is he who has been most persistent with his particular brand of WP:DISRUPTION, which includes the frequent use of the sort of falsehoods given above. Mediation may help him see this. (Zapletal) 101.171.255.242 (talk) 00:46, 23 March 2015 (UTC)

Steelpillow's description of how the discussion was "at first" being "kept alive" is quite reasonable. In fact he joined the discussion on 22 September. Zapletal didn't subsequently comment until 27 November. Since then, Zapletal has reignited it on occasion as well. Attacks on editors' failing memories do not work on Misplaced Pages. Burninthruthesky (talk) 10:38, 23 March 2015 (UTC)

I disagree with his history, especially the part about me being "thoroughly jaded" when I asked for assistance at the aviation project. Here's the diff: https://en.wikipedia.org/search/?title=Wikipedia_talk:WikiProject_Aviation&diff=prev&oldid=626611902 - As can be easily seen, it's a very neutral request for more eyes on the issue. I think it is correct that at one point Doug was the lone holdout, but that is no longer the case. It took me several months and many re-readings to finally understand his point, but once I took the trouble to understand it, it's a very reasonable point. Mr. Swordfish (talk) 15:33, 25 March 2015 (UTC)

I'm sorry Mr. Swordfish. I have no dispute with you, but I am having trouble understanding.

Any revision that presents a more complicated explanation first will not have my support. 21:45, 30 May 2014 (UTC)

My own preference is to just leave it out and move on. 17:15, 10 November 2014

So, do we treat rate of momentum change? The AAPT thinks we should. 14:51, 11 November 2014

Can you explain the reason for your changes of heart? Burninthruthesky (talk) 17:16, 25 March 2015 (UTC)

I eventually figured out what Doug was on about. I just didn't understand it at first.

BTW, I don't see any contradictions among the three quotes above. Mr. Swordfish (talk) 19:17, 25 March 2015 (UTC)

When you continued to emphasize your original argument for following the AAPT advice, I got the impression the only reason for your preference to leave it out was that you wanted some peace and quiet (I know I did, but I also wanted a fair outcome). Thank you for clarifying your position. Burninthruthesky (talk) 07:30, 26 March 2015 (UTC)

@Mr swordfish: My apologies if I have misrepresented you. It was a long time ago. I do recall at least one editor being thoroughly jaded at the time I came in, but it seems I mixed two of you up in my memory. I confess that I had not noticed you previously either say that you fully accepted Doug's point or argue unequivocally in its favour - you had not done so at first. Again, I am happy to stand enlightened. — Cheers, Steelpillow (Talk) 19:16, 25 March 2015 (UTC)

For me, conduct is the primary issue. A large part of the discussion seems devoted to constructing a false narrative about the opponents of poorly-justified change, by misrepresenting their views, followed by WP:IDHT towards any attempt at clarification. Once again I am alluded to as, "people who strongly support TS". I don't. I do, however, strongly support Misplaced Pages policies such as WP:CONSENSUS and WP:CIVILITY.

I regret that the previous ANI discussion was derailed by miscommunication. I still don't know whether those not deeply involved in this discussion don't agree that misrepresenting the views of others is against policy, or they choose to WP:IAR in order to keep a published author on board, or they cannot penetrate this discussion to see where the misrepresentations can be found. I appreciate Robert's constructive suggestion, although I too am unsure whether mediation would help with this. One thing I do know is that I don't have time for a full-scale repetition of the previous discussion. Burninthruthesky (talk) 10:38, 23 March 2015 (UTC)

What we have here includes a very knowledgeable editor, User:J Doug McLean, who is an authority in the field. However, it appears that there are two different technical ways of describing lift that are significantly different, and the expert uses one of them, and the other one is more widely used. If that is not the case, please clarify. We have at least two active editors who prefer the other approach. We also have an active but unregistered editor whose constructive contributions to this article are negligible, but who engages in personal attacks against critics of the expert. Robert McClenon (talk) 15:23, 23 March 2015 (UTC)

If that is not the case, please clarify. I think you basically have it right, but allow me to elaborate anyway: there's a simple way to explain lift that is understandable by almost anyone after reading a few paragraphs (or less) : air is deflected one way, the wing experiences a force in the opposite direction according to Newton's laws. There is also a much more precise mathematical treatment that requires understanding of partial differential equations, vector fields, conservation of mass, energy, and momentum, the principle of superposition, the Kuta Condition etc. This rigorous presentation is what you find in engineering texts with perhaps a brief mention of the simple explanation. The two are quite consistent with one another as they are just different ways of looking at the same problem. The current consensus (as evidenced by the state of the article for the last five years or more) is to present the simpler non-quantitative explanation first and proceed to give the more elaborate explanation. I don't think that's in dispute at the moment, other than including a quantitative statement as part of the simple explanation (i.e. the topic of the current RFC) Mr. Swordfish (talk) 16:02, 23 March 2015 (UTC)

Right. The debate isn't about the qualitative explanations versus the quantitative science. It's about what to include in the qualitative early section of the article. I have been opposed to including the quantitative statement (dp/dt = -L, in shorthand) along with the qualitative explanation because it is inappropriate in that section of the article and because the issues involved are complicated such that the oversimplified version is misleading. The debate is also about what weight to give to one body of sources (mostly non-specialist) versus specialist sources from the mainstream of aerodynamics. J Doug McLean (talk) 02:49, 24 March 2015 (UTC)

I would make some small clarification to the content issue. Both explanations are widely used, sometimes together, to introduce the subject. Having done so, the explanation espoused by our expert is the one more relevant to deeper technical analysis. Our expert believes that the other explanation at best overstates its case and in its barest form is just plain wrong. The rest of us, whatever we may think of it, accept that if it is reliably sourced then we should include it in the form, and with the due weight, that we find in the sources. — Cheers, Steelpillow (Talk) 18:24, 23 March 2015 (UTC)

Robert, your claim that I am someone "whose constructive contributions to this article are negligible" is deeply offensive. For the record I have brought a great deal to the article, including numerous RSs of the highest standard, and much technical rigour. The RSs include Lanchester (the originator of CToL and the Principle of No Momentum), Bryan (who directly refuted Berriman's version of TS), and many others such as Stokes, Kelvin, Helmholtz, Rayleigh, Lamb, Klein, Betz, Glauert, Durand, Bloor, etc., that some of the other editors were apparently not aware of, but who have made major contributions to this field. Technically I have clarified a great deal, although I accept that much of that has fallen on deaf ears. Please refrain from making any more WP:PERSONAL attacks. Please strike-through your above offensive remark. (Zapletal) 101.170.85.69 (talk) 01:33, 25 March 2015 (UTC)

"...two different technical ways of describing lift ... If that is not the case, please clarify." (<- McClenon). "The two are quite consistent with one another..." (<- Mr Swordfish). No. The pictures presented by CToL and TS are utterly incompatible. CToL presents the picture shown in the animation used in the article, which is remarkably similar to real flows, as proven by countless experiments. TS presents the picture shown in the "One Minute Physics..." cartoon at the very end of the article, which is as Bryan described it "physically speaking, an impossibility". The two pictures are as incompatible as Heliocentric-Round-Earth and Geocentric-Flat-Earth theories. (Zapletal) 101.170.85.69 (talk) 01:33, 25 March 2015 (UTC)

Zapletal: I don't care if you are a professional mind-reader and therefore you know for a fact that Steelpillow and Burninthetruthsky are maliciously telling lies. Even if you know that as an absolute fact, Misplaced Pages policy says that stating that other editors are telling untruths is a personal attack. Cease and desist from the personal attacks, or you can be blocked or banned from editing. Robert McClenon (talk) 15:23, 23 March 2015 (UTC)

Robert, can you please clarify your statement that "...Misplaced Pages policy says that stating that other editors are telling untruths is a personal attack". Can you please refer me to the appropriate policy page, and also comment on whether there is any inconsistency there? Specifically, can you show me where it says that it is perfectly acceptable for an editor to claim that there is only one other editor supporting a particular edit, when in fact there at least two, but it is considered totally unacceptable to bring such an arithmetical error to light? If this is in fact so, then what is the correct protocol for addressing such erroneous claims? (Zapletal) 101.170.85.69 (talk) 01:33, 25 March 2015 (UTC)

Zapletal, I came here because of a request to have this talk page protected. While not spelt out as such at Misplaced Pages:No personal attacks it has long been considered that accusing others of lying is a personal attack. Even when you can provide differences that would seem to show you are correct. If you think someone has made a statement that you think is incorrect is to Misplaced Pages:Assume good faith and just take the stance that they might have just made a simple error. Provide the necessary differences and just state that you think they are in error. I would prefer not to protect the page, but given that you change IPs I will if you continue attacking others. CambridgeBayWeather, Uqaqtuq (talk), Sunasuttuq 09:09, 27 March 2015 (UTC)

CBW, you say "Provide the necessary differences and just state that you think they are in error." That is exactly what I did (given that, IMO, "false" is an acceptably polite synonym for "in error"). The fact that you are now threatening to prevent me posting here, even though I have done nothing wrong as "spelt out" in Misplaced Pages policy, and have instead been doing my best to correct the numerous "errors" here, suggests that this whole Wiki system is heading down a very slippery slope. As a good example of the hypocrisy and double standards we are dealing with, please look at the recent edits in the "Rotary Wings" section below. Steelpillow is repeatedly deleted parts of my post (something he has done a great deal of in the past) on the basis of "personal attack", even though said text being deleted is a direct quote taken from one of Steelpillow's own personal attacks on another editor! This censorship of facts under the feigned guise of maintaining "Civility" will only lead to low quality articles. Is that what you want? Please read about WP:RANDY. (Zapletal) 101.170.85.65 (talk) 01:14, 28 March 2015 (UTC)

(PS. The discussion about the Moon being made of WP:GREENCHEESE sounds awfully familiar. PPS. I note that Robert has still not made right his personal attack on me... (Zapletal)) 101.170.85.82 (talk) 02:13, 28 March 2015 (UTC)

My main question is, again, whether the various editors are willing to try something else, namely, formal mediation. We have seen that discussions at this talk page get nowhere. Do we want to try mediation, or do we want to continue to have discussions that get nowhere? It doesn't appear that the two other active editors are about to go away. They have been here for months and continue to complain about being ignored. Mediation won't take any longer than the current conflict has been going on. Robert McClenon (talk) 15:23, 23 March 2015 (UTC)

I have no objection to formal mediation, although I have to say I don't really know what is involved. I mooted the RFC in an effort to reach closure on the Statement, but I think we (ie the editors on this page) have bigger issues than that and that the problems with civility and communication will continue after the RFC is settled. At this point it appears that some editors have let their emotions get the better of them. Agree that this is almost entirely a content dispute and that if we can all keep our cool we will work our way through it. Thanks for your assistance. Mr. Swordfish (talk) 16:08, 23 March 2015 (UTC)

I would welcome formal mediation provided a mediator can be found who has a background in the field and is thus in a position to understand what the various sources say about the theoretical issues in question. J Doug McLean (talk) 02:49, 24 March 2015 (UTC)

Made the formal request

I have filed the formal request for mediation at Requests for Mediation. Robert McClenon (talk) 03:10, 24 March 2015 (UTC) It is my understanding that the next step, within 24 hours, is that a bot will notify us that I have filed the request. Robert McClenon (talk) 03:11, 24 March 2015 (UTC)

Thanks Robert. I've accepted at Misplaced Pages:Requests for mediation/Lift (force). I fixed a red link to User:Mr swordfish. I hope the bot will pick up the change in due course. Burninthruthesky (talk) 08:31, 24 March 2015 (UTC)

I've accepted, but my participation may be constrained by my being away from internet access for most of April.

I've read the "policy" and "guide" pages on mediation, and it's not clear to me how this is supposed to work. I see that, so far, Robert McClenon and Steelpillow have described the dispute only in very brief terms on the request page. Question: Is this the level of detail on which the Mediation Committee will make its accept/reject decision? And another question: If the committee accepts the case, will we be expected to present our arguments on the project talk page, or is the committee expected to read our arguments on the article talk page?

I assume that other editors wishing to participate in the mediation may register on the request page at Misplaced Pages:Requests for mediation/Lift (force). J Doug McLean (talk) 18:45, 25 March 2015 (UTC)

All of the editors whom I identified have agreed to mediation. There are a few other conditions, such as that there must have been discussion on the article talk page, which there has been. Based on previous experience, further discussion will be on the mediation project page, and then the mediator will provide instructions as to how the discussion should be conducted. It is my understanding that other editors who read the article talk page may join in the discussion. I think that any further instructions on how the mediation proceeds will be provided by the mediator. Robert McClenon (talk) 18:52, 25 March 2015 (UTC)

I understand mediation cannot proceed whilst the RFC is open. See Misplaced Pages:Requests_for_mediation/Christian_terrorism#Decision_of_the_Mediation_Committee for a previous example. Burninthruthesky (talk) 07:33, 26 March 2015 (UTC)

I am aware that informal mediation cannot proceed while an RFC is open. If formal mediation cannot proceed, then I will let the mediator request that the originator of the RFC withdraw the RFC in favor of mediation, since mediation is another way to address long-standing content issues. Robert McClenon (talk) 21:34, 26 March 2015 (UTC)

Since User:Mr swordfish originated the RFC in order to address the long-standing content issue, I will request that he request closure of the RFC if it is a block to mediation. Robert McClenon (talk) 21:36, 26 March 2015 (UTC)

Rotary wings

Thinking about helicopters, it occurs to me that "the air deflected downwards" is unusually easy to identify. A rotary wing is just an ordinary wing moving in a circle. Most noticeably when hovering, a helicopter's rotor creates a strong downdraught in the form of a cylindrical column of air. The effect is readily visible as a characteristic texturing of waves when over water. This downwash evidently carries a good deal of momentum, which is constantly replenished by the rotary wings. But of course, the no momentum theorem cannot be violated. How can this be? The downwash splats against the ground and spreads out - low over water, spume is often thrown up to show this flow. The air then rises again, creating a toroidal vortex kept spinning by the rotor. The net momentum of the vortex is zero, but to argue that the rotor is not supporting itself by imparting momentum to the downwash appears absurd. Considering a short span of a single rotary blade or wing, we can treat it like any other short section of any other wing. We are forced to conclude that it is contributing to the downwash. Therefore, a normal wing also contributes to a downwash. Furthermore, in neither case is the no momentum theorem violated: the difference lies only in the geometry of the induced vortex. Is there a flaw in this picture? — Cheers, Steelpillow (Talk) 21:40, 25 March 2015 (UTC)

"Is there a flaw in this picture?" Yes, several flaws. The biggest is when you say "We are forced to conclude that is contributing to the downwash.". There is nothing "forcing" you to this conclusion other than your belief that it must be true, combined with a lack of rigour in your initial assumptions and development of the argument. I could explain all the Fluid Dynamic details of rotary wings at length here (I have covered some of it before), perhaps by mentioned that the "downwash" is a direct consequence of the inefficiencies that result from the "wing-tip vortices" and is not central to understanding the Lift generated by rotary wings, or that the more simplified "actuator disc theory" requires a consideration of both the momentum AND ENERGY in the flow before it can produce any useful results. But, previously when Doug tried such thoughtful and patient explanations you described his efforts as "... vast screeds of techno-verbiage and tenacious persistence swamp every discussion ... a perfect case of long-term WP:DISRUPTION", and "It will only be when this editor is removed from the discussion and can no longer drown it out that the content issue can be properly addressed.". So I won't bother spelling out these details here. Instead, if you want to properly address the content issues, then I suggest you study more deeply the RSs yourself, which for this subject begin in early 1800s. In doing so, it is worth considering a rotary wing in a sealed duct. (Zapletal) 101.170.255.232 (talk) 03:19, 26 March 2015 (UTC)

Without going into a lot of detail, I'd just say the situation is similar to what we've been discussing for a stationary foil with air flowing past. The simple actuator-disc theory for a rotor or propeller treats the flow in a time-averaged sense (averaging out the effects of individual blade passages) and ignores viscosity and compressibility. The rotor disc itself doesn't directly impart any change in vertical momentum because that would require a jump in vertical velocity, which would violate conservation of mass. Thus in the immediate vicinity of the disc the lift is manifested entirely as a pressure difference between the air below the disc and the air above. There is vertical momentum in the air passing through the disc, but it was imparted by the extended pressure field above the disc. The pressure jump imparted by the disc is what is needed to make the whole flowfield consistent with the equations of motion. To see the lift accounted for entirely by a change in momentum flux requires a control volume for which the integrated vertical pressure force on the outer boundary vanishes. which essentially requires looking at the equivalent of the tall, slender control volume we've been discussing in connection with airfoils. I think the upshot is that this example doesn't tell us anything we didn't already know. J Doug McLean (talk) 22:12, 26 March 2015 (UTC)

I take your point that the imparting of downward momentum is indirect. Nevertheless it is imparted, and tertiary sources in particular give a lot of weight to the momentum model (I just found another, from Sikorsky ca. 1960s, but I won't clutter this discussion unless asked). The problem has always been to follow that momentum, to identify "the air deflected downwards". It just seems to me that the cylindrical downwash of a helicopter disc is a rare case of easy visualisation. I'd add that the classic disc model imparts half the momentum above the disc and half below. I'd suggest that this helps picture how the momentum model works and consequently also to see its limitations. — Cheers, Steelpillow (Talk) 18:32, 27 March 2015 (UTC)

Readers wondering about good theoretical models for explaining "Rotary wings" might want to think about this particular case. Consider a helicopter in a very tall circular room, such that the blade-tips are spinning very close to, but not quite touching, the walls. It should be apparent that this helicopter can, indeed, hover above the ground (ie. it can sustain Lift, and thus fly). Big questions - Where is "the downwash"? Where is the air that has "downward momentum imparted" to it, in order to generate the required Lift, per "TS"? And what happens to this supposed downward moving air, given that the bottom of the room is sealed (ie. it has walls and a floor, but no open doors, etc.!)? This hypothetical problem is, of course, no different to the extremely common "fan in a duct". That is, it is VERY REAL. Any good model of "Lift" should, at the very least, be able to explain this simple and very real case. (BTW, the main difference between this case and the "helicopter in free air" is that one has wing-tip-trailing-vortices and the other does not.) (Zapletal) 101.170.85.82 (talk) 01:51, 28 March 2015 (UTC)

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