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{{Short description|Increased aircraft lift generated when close to fixed surface}}
'''Ground effect''' (or '''Wing In Ground''' effect) is a phenomenon of ] where the flow of air around part of an ] or a ] is interrupted by the ground.
For ], '''ground effect''' is the reduced ] that an aircraft's ]s generate when they are close to a fixed surface.<ref name="Irving Gleim">{{harvnb|Gleim|1982|p=94}}.</ref> During ], ground effect can cause the aircraft to "float" while below the recommended ]. The pilot can then fly just above the runway while the aircraft accelerates in ground effect until a safe ] is reached.<ref name="Flight theory and aerodynamics">{{harvnb|Dole|2000|p=70}}.</ref>


For ], ground effect results in less drag on the rotor during hovering close to the ground. At high weights this sometimes allows the rotorcraft to lift off while stationary in ground effect but does not allow it to transition to flight out of ground effect. Helicopter pilots are provided with performance charts which show the limitations for hovering their helicopter in ground effect (IGE) and out of ground effect (OGE). The charts show the added lift benefit produced by ground effect.<ref>{{cite book|url=https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/|title=Helicopter Flying Handbook|chapter=Chapter 7 - Helicopter Performance|chapter-url=https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch07.pdf|publisher=Federal Aviation Administration|year=2020}}</ref>
⚫ ==Ground effect in aircraft==


For fan and jet-powered ] (VTOL) aircraft, ground effect when hovering can cause suckdown and fountain lift on the airframe and loss in hovering thrust if the engine sucks in its own exhaust gas, which is known as hot gas ingestion (HGI).<ref name=isbn0-930403-51-7>{{cite book|last=Raymer|first=Daniel P.|url=https://soaneemrana.org/onewebmedia/AIRCRAFT%20DESIGN%20%3B%20A%20Conceptual%20Approach%20BY%20DANIEL%20P%20RAYMER.pdf|title=Aircraft Design: A Conceptual Approach|publisher=American Institute of Aeronautics and Astronautics, Inc.|edition=2|year=1992|isbn=0-930403-51-7|access-date=2019-12-26|archive-date=2019-07-04|archive-url=https://web.archive.org/web/20190704162411/https://soaneemrana.org/onewebmedia/AIRCRAFT%20DESIGN%20%3B%20A%20Conceptual%20Approach%20BY%20DANIEL%20P%20RAYMER.pdf|url-status=dead}} Section 20.6</ref><ref>{{cite journal|url=https://core.ac.uk/download/pdf/8005.pdf|title=An evaluation of the historical issues associated with achieving non-helicopter V/STOL capability and the search for the flying car|last1=Saeed|first1=B.|last2=Gratton|first2=G.B.|page=94|issue=February|year=2010}}</ref>
Aircraft obtain increased ] and therefore better efficiency by flying very close to the ground: on a fixed-wing ], about half the distance from a wingtip to the fuselage. Ground effect therefore affects most aircraft only at takeoff and landing. Ground effect also occurs over water.


{{TOC limit|limit=2}}
Most pilots, especially of small aircraft, will experience ground effects on landing; in fact the art of landing largely comes down to understanding when these effects need to be taken into account. As the aircraft descends towards the ], it will not be affected by ground effect, but as the aircraft flares and descends the last few feet, ground effect will cause a pronounced increase in lift. This can cause the aircraft to rise suddenly and significantly &mdash; an effect known as a "balloon". Left uncorrected, a balloon can lead to a dangerous situation where the aircraft is rising yet decelerating, a condition which can rapidly lead to a ], especially when it is considered that landing speeds are generally only a very small margin above the stall speed. A stall even from a few tens of feet above the ground can cause a major, possibly fatal, crash. A balloon may be corrected given sufficient runway remaining, but for novice pilots a better option is to ]. A good landing approach allows for ground effect such that the aircraft flares and is ''held off'' in ground effect until it gently descends onto the runway.


==Explanations==
For ] the ability to hover ''in-ground-effect'' or IGE is much improved, on the order of 1200 to 1500 m higher in altitude when compared to ''out-of-ground-effect'' or OGE hover capability.
===Fixed-wing aircraft===
When an aircraft flies at or below approximately half the length of the aircraft's ] above the ground or water there occurs an often-noticeable ''ground effect.'' The result is lower ] on the aircraft. This is caused primarily by the ground or water obstructing the creation of ] and interrupting ] behind the wing.<ref>''Aerodynamics for Naval Aviators.'' RAMESH TAAL, HOSUR, VIC. Australia: Aviation Theory Centre, 2005.</ref><ref name="Pilot's Encyclopedia of Aeronautical Knowledge">''Pilot's Encyclopedia of Aeronautical Knowledge'' 2007, pp. 3-7, 3-8.</ref>


A wing generates lift by deflecting the oncoming airmass (relative wind) downward.<ref>
The ] which describe ground effect are still very much under debate. A common belief is that ground effect is caused by a "cushion" of compressed air between the wing and the ground. However, wind tunnel testing and experiments have indicated that while a "cushion" effect is present, ground effect is almost solely due to the ground interrupting the formation of ]. Wingtip vortices destroy massive amounts of the lift generated by the wing, and eliminating thereby increases the wing's efficiency greatly.
{{Cite web|url=https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/right2.html |title=Beginner's Guide to Aerodynamics: Lift from Flow Turning |first=Tom|last=Benson|publisher = NASA Glenn Research Center |access-date=July 7, 2009}}</ref> The deflected or "turned" flow of air creates a resultant force on the wing in the opposite direction (Newton's 3rd law). The resultant force is identified as lift. Flying close to a surface increases air pressure on the lower wing surface, nicknamed the "ram" or "cushion" effect, and thereby improves the aircraft lift-to-drag ratio. The lower/nearer the wing is to the ground, the more pronounced the ground effect becomes. While in the ground effect, the wing requires a lower ] to produce the same amount of lift. In wind tunnel tests, in which the angle of attack and airspeed remain constant, an increase in the lift coefficient ensues,<ref name="Pilot pp.3-8">{{harvnb|Dole|2000|pp=3–8}}.</ref> which accounts for the "floating" effect. Ground effect also alters ] versus velocity, where reduced induced drag requires less thrust in order to maintain the same velocity.<ref name="Pilot pp.3-8"/>


] are more affected by ground effect than ] aircraft.<ref>Flight theory and aerodynamics, p. 70</ref> Due to the change in up-wash, down-wash, and wingtip vortices, there may be errors in the airspeed system while in ground effect due to changes in the local pressure at the ].<ref name="Pilot pp.3-8"/>
Some critics of ]' massive ] claim the famous ]'s first (and only) flight was due entirely to ground effect and the craft was incapable of sustaining flight above a very low altitude.


===Rotorcraft===
A '''ground effect vehicle''' (]) is an aircraft that always operates in the ground effect and cannot sustain flight more than a few feet above the ground. The ] is such an aircraft. Actually, most GEVs are intended to operate over water since suitable operational areas are rare over land. ] are often erroneously called ground effect vehicles.
When a hovering rotor is near the ground the downward flow of air through the rotor is reduced to zero at the ground. This condition is transferred up to the disc through pressure changes in the wake which decreases the inflow to the rotor for a given disc loading, which is rotor thrust for each square foot of its area. This gives a thrust increase for a particular blade pitch angle, or, alternatively, the power required for a thrust is reduced. For an overloaded helicopter that can only hover IGE it may be possible to climb away from the ground by translating to forward flight first while in ground effect.<ref>, Defense Technical Information Center (1974)</ref> The ground-effect benefit disappears rapidly with speed but the induced power decreases rapidly as well to allow a safe climb.<ref>{{cite web |url=https://www.abbottaerospace.com/downloads/agard-r-781/ |title=Aerodynamics of ROTOR CRAFT |website=ABBOTTAEROSPACE.COM |date=April 12, 2016 |pages=2–6}}</ref> Some early underpowered helicopters could only hover close to the ground.<ref>Basic Helicopter Aerodynamics, J. Seddon 1990, {{ISBN|0 632 02032 6}}, p.21</ref> Ground effect is at its maximum over a firm, smooth surface.<ref>{{cite book |url=https://rotorcraft.arc.nasa.gov/FAA-H-8083-21.pdf |title=Rotor raft Flying Handbook |year=2000 |publisher=Federal Aviation Administration |pages=3–4 |access-date=2021-11-03 |archive-date=2016-12-27 |archive-url=https://web.archive.org/web/20161227042500/https://rotorcraft.arc.nasa.gov/faa-h-8083-21.pdf |url-status=dead }}</ref>


===VTOL aircraft===
⚫ ==Ground effect in cars==
There are two effects inherent to VTOL aircraft operating at zero and low speeds in ground effect, suckdown and fountain lift. A third, hot gas ingestion, may also apply to fixed-wing aircraft on the ground in windy conditions or during thrust reverser operation. How well, in terms of weight lifted, a VTOL aircraft hovers IGE depends on suckdown on the air frame, fountain impingement on the underside of the fuselage and HGI into the engine causing inlet temperature rise (ITR). Suckdown works against the engine lift as a downward force on the airframe. Fountain flow works with the engine lift jets as an upwards force. The severity of the HGI problem becomes clear when the level of ITR is converted into engine thrust loss, three to four percent per 12.222&nbsp;°c inlet temperature rise.<ref>{{cite report |url= https://ntrs.nasa.gov/api/citations/19710022894/downloads/19710022894.pdf |title=MODEL TESTS OF CONCEPTS TO REDUCE HOT GAS INGESTION IN VTOL LIFT ENGINES(NASA CR-1863) |last=Hall |first=Gordon R. |year=1971 |publisher=Nasa |page=4}}</ref><ref>{{cite report |url=https://ntrs.nasa.gov/api/citations/19710022894/downloads/19710022894.pdf |title=AN ANALYSIS OF CORRELATING PARAMETERS RELATING TO HOT-GAS INGESTION CHARACTERISTICS OF JET VTOL AIRCRAFT |last=Krishnamoorthy |first=V. |year=1971 |publisher=NASA |page=8}}</ref>


Suckdown is the result of entrainment of air around aircraft by lift jets when hovering. It also occurs in free air (OGE) causing loss of lift by reducing pressures on the underside of the fuselage and wings. Enhanced entrainment occurs when close to the ground giving higher lift loss. Fountain lift occurs when an aircraft has two or more lift jets. The jets strike the ground and spread out. Where they meet under the fuselage they mix and can only move upwards striking the underside of the fuselage. {{sfn|Raymer|1992|pp=551,552}} How well their upward momentum is diverted sideways or downward determines the lift. Fountain flow follows a curved fuselage underbody and retains some momentum in an upward direction so less than full fountain lift is captured unless lift improvement devices are fitted.<ref>{{cite book |url=https://www.ntrs.nasa.gov/search.jsp?R=19870014977&qs=t%3D0%26N%3D4294955891%2B4294904888%2B4294965980 |title=Proceedings of the 1985 NASA Ames Research Center's Ground-Effects Workshop (NASA Conference Publication 2462) |last=Mitchell |first=Kerry |year=1987 |publisher=Nasa |page=4}}{{dead link |date=June 2021 |bot=medic}}{{cbignore |bot=medic}}</ref> HGI reduces engine thrust because the air entering the engine is hotter and less dense than cold air.
In racing cars, a designer's aim is not for increased lift but for increased ], allowing greater cornering speeds. (By the 1970s 'wings', or inverted ]s, were routinely used in the design of racing cars to increase downforce, but this is ''not'' ground effect.)


Early VTOL experimental aircraft operated from open grids to channel away the engine exhaust and prevent thrust loss from HGI.
However, substantial further downforce is available by understanding the ground to be part of the aerodynamic system in question. The basic idea is to create a volume of low ] underneath the car, so that the higher pressure above the car will apply a downward force. Naturally, to maximize the force one wants the maximal area at the minimal pressure. Racing car designers have achieved low pressure in two ways: first, by using a fan to push air out of the cavity; second, to design the underside of the car as an inverted aerofoil so that large amounts of incoming air are accelerated through a narrow slot between the car and the ground, lowering pressure by ]. Official regulations ] disallow ground effects in many types of racing, such as ].


The ], built to research early VTOL technology, was unable to hover until suckdown effects were reduced by raising the aircraft with longer landing gear legs.<ref>The X-Planes, Jay Miller1988, {{ISBN|0 517 56749 0}}, p.108</ref> It also had to operate from an elevated platform of perforated steel to reduce HGI.<ref>{{cite web |title=Application of Powered High Lift Systems to STOL Aircraft Design |first=Frederick Donald |last=Ameel |year=1979 |page=14 |s2cid=107781224}}</ref> The ] VTOL research aircraft only ever operated vertically from a grid which allowed engine exhaust to be channeled away from the aircraft to avoid suckdown and HGI effects.<ref>{{cite book |url=https://catalog.princeton.edu/catalog/5869200 |title=Addendum to AGARD report no. 710, Special Course on V/STOL Aerodynamics, an assessment of European jet lift aircraft |last=Williams |first=R.S. |website=catalog.princeton.edu |series=AGARD report; no. 710, addendum |year=1985 |page=4|isbn=9789283514893 }}</ref>
], the first car aerodynamicist to harness downforce, built ] cars to both these principles. His 1961 car attempted to use the shaped underside method but there were too many other aerodynamic problems with the car for it to work properly. His 1966 cars used a dramatic high wing for their downforce. His Chaparral 2J "sucker car" of 1970 was revolutionary. It had two fans at the rear of the car driven by a dedicated ] engine; it also had "skirts", which left only a minimal gap between car and ground, so as to seal the cavity from the atmosphere. Although it did not quite win a race, the competition lobbied for its ban, which came into place at the end of that year. Movable aerodynamic devices were banned from most branches of the sport.


Ventral ]s retroactively fitted to the P.1127 improved flow and increased pressure under the belly in low altitude hovering. Gun pods fitted in the same position on the production Harrier GR.1/GR.3 and the AV-8A Harrier did the same thing. Further lift improvement devices (LIDS) were developed for the AV-8B and Harrier II. To box in the belly region where the lift-enhancing fountains strike the aircraft, strakes were added to the underside of the gun pods and a hinged dam could be lowered to block the gap between the front ends of the strakes. This gave a 1200&nbsp;lb lift gain.<ref>Harrier Modern Combat Aircraft 13, Bill Gunston1981, {{ISBN|0 7110 1071 4}}, p.23,43,101</ref>
] in the late 1970s was the next setting for ground effect in racing cars. In 1977 ] brought out their "Wing Car", the ], designed by ], ], and ]. Its sidepods, bulky constructions between front and rear wheels, were shaped as inverted aerofoils and sealed with flexible "skirts" to the ground. The team won 5 races that year, and 2 in 1978 while they developed the much improved ]. The most notable contender in 1978 was the ] BT46B "fan car", designed by ]. Its fan, spinning on a horizontal, longitudinal axis at the back of the car, took its power from the main gearbox. The car avoided the sporting ban by claims that the fan's purpose was for engine cooling. It raced just once, with ] winning at the Swedish Grand Prix. The team though, led by ] who had recently become president of the ], withdrew the car before it had a chance to be banned. The Lotus 79, on the other hand, went on to win 6 races and the world championship for ]. In following years other teams copied and improved on the Lotus until, after a series of fatal accidents, flat undersides became mandatory from 1983 .


] weapons-bay inboard doors on the F-35B open to capture fountain flow created by the engine and fan lift jets and counter suckdown IGE.
⚫ == External links ==
<gallery widths="200" heights="150">
File:Bell X-14 colour ground.jpg|Bell X-14 showing lengthened landing gear legs to reduce suckdown
File:Dassault Mirage IIIV.jpg|Dassault Mirage IIIV hovering over open grid
File:Hawker P.1127 ‘XP831’ (19253036156).jpg|Underside view of the first prototype P.1127 showing small ventral strakes to increase fountain lift
File:BAe Harrier GR9 ZG502 landing arp.jpg|Harrier GR9 showing the lift improvement devices, large ventral strakes and a retractable dam behind nosewheel
File:RAF F-35B STOVL RIAT 2016.jpg|F-35B showing weapon's bay inboard doors open to capture rising fountain flow
</gallery>


===Wing stall in ground effect===
*
The stalling angle of attack is less in ground effect, by approximately 2–4 degrees, than in free air.<ref name=NTSB>"The NTSB’s John O’Callaghan, a national resource specialist in aircraft performance, noted that all aircraft stall at approximately 2-4 deg. lower AOA with the wheels on the ground." (from NTSB Accident Report concerning loss of a swept wing business-class jet airplane in April 2011) </ref><ref>{{Cite web|url=https://aviation-safety.net/database/record.php?id=19530303-1|title=ASN Aircraft accident de Havilland DH-106 Comet 1A CF-CUN Karachi-Mauripur RAF Station|first=Harro|last=Ranter|website=aviation-safety.net}}</ref> When the flow separates there is a large increase in drag. If the aircraft overrotates on take-off at too low a speed the increased drag can prevent the aircraft from leaving the ground. Two ]s overran the end of the runway after overrotating.<ref>Aerodynamic Design Of Transport Aircraft, Ed Obert 2009, {{ISBN|978 1 58603 970 7}}, pp.603–606</ref><ref>{{Cite web|url=https://www.flightsafetyaustralia.com/2019/10/reprise-night-of-the-comet/|title=Reprise: Night of the Comet &#124; Flight Safety Australia|author=Staff writers|date=October 25, 2019}}</ref> Loss of control may occur if one wing tip stalls in ground effect. During certification testing of the ] business jet the test aircraft rotated to an angle beyond the predicted IGE stalling angle. The over-rotation caused one wing-tip to stall and an uncommanded roll, which overpowered the lateral controls, leading to loss of the aircraft.<ref>{{Cite web|url=https://www.ntsb.gov/investigations/accidentreports/reports/aar1202.pdf|title=Crash During Experimental Test Flight Gulfstream Aerospace Corporation GVI (G650), N652GD Roswell, New Mexico April 2, 2011|website=www.ntsb.gov}}</ref><ref>From NTSB Accident Report: Flight test reports noted "post stall roll-off is abrupt and will saturate lateral control power." The catastrophic unrecoverable roll of the aircraft in the Roswell accident was due in part to the absence of warning before the stall in ground effect.</ref>
⚫ *
*
*
*
*


⚫ ==Ground-effect vehicle==
⚫ ]
{{Main|Ground-effect vehicle}}


A few vehicles have been designed to explore the performance advantages of flying in ground effect, mainly over water. The operational disadvantages of flying very close to the surface have discouraged widespread applications.<ref>Understanding Aerodynamics - Arguing From The Real Physics, Doug McLean 2013, {{ISBN|978 1 119 96751 4}}, p.401</ref>
]

]
==See also==
]
* ]
]
⚫ * ]
]
* ]
* ]

==References==
===Notes===
{{Reflist}}

===Bibliography===
{{Refbegin}}
* {{cite book |last=Dole |first= Charles Edward |title=Flight Theory and Aerodynamics |location=Hoboken, New Jersey |publisher=John Wiley & Sons, Inc |date= 2000 |isbn=978-0-471-37006-2 }}
* {{cite book |last=Gleim |first=Irving |title=Pilot Flight Maneuvers |location=Ottawa, Ontario, Canada |publisher= Aviation Publications |date=1982 |isbn= 0-917539-00-1}}
* ''Pilot's Encyclopedia of Aeronautical Knowledge'' (Federal Aviation Administration). New York: Skyhorse Publishing, 2007. {{ISBN|1-60239-034-7}}.
{{Refend}}

⚫ ==External links==
{{Commons category|Ground effect}}
⚫ * . SE-Technology ('dead' site)
* . Aerospaceweb.org
* {{cite web |url=http://www.abbottaerospace.com/download/reference_data/wing_in_ground_effect/DSTO-GD-0201WinginGroundEffectCraftReview.pdf |title=Wing in Ground Effect Craft Review |author1=M. Halloran |author2=S. O'Meara |publisher=The Sir ] Centre for Aerospace Design Technology, ] |date=February 1999 |format=PDF-9 MB |via=Abbott Aerospace }}{{dead link|date=October 2017 |bot=InternetArchiveBot |fix-attempted=yes }} DSTO-GD-0201. Sponsored by ] Aeronautical and Maritime Research Laboratory, Australian Government. ()
* . dynamicflight.com
* Tongji University Scientists in Shanghai announce design of a new vehicle, inventorspot.com, 14 July 2007
* . hanggliding.org
* (PDF<!-- 715 KB-->) ''Journal of Theoretical and Applied Mechanics'', '''45''', 2, pp.&nbsp;425–36, Warsaw 2007. ptmts.org.

⚫ ]
]

Latest revision as of 23:12, 31 December 2024

Increased aircraft lift generated when close to fixed surface

For fixed-wing aircraft, ground effect is the reduced aerodynamic drag that an aircraft's wings generate when they are close to a fixed surface. During takeoff, ground effect can cause the aircraft to "float" while below the recommended climb speed. The pilot can then fly just above the runway while the aircraft accelerates in ground effect until a safe climb speed is reached.

For rotorcraft, ground effect results in less drag on the rotor during hovering close to the ground. At high weights this sometimes allows the rotorcraft to lift off while stationary in ground effect but does not allow it to transition to flight out of ground effect. Helicopter pilots are provided with performance charts which show the limitations for hovering their helicopter in ground effect (IGE) and out of ground effect (OGE). The charts show the added lift benefit produced by ground effect.

For fan and jet-powered vertical take-off and landing (VTOL) aircraft, ground effect when hovering can cause suckdown and fountain lift on the airframe and loss in hovering thrust if the engine sucks in its own exhaust gas, which is known as hot gas ingestion (HGI).

Explanations

Fixed-wing aircraft

When an aircraft flies at or below approximately half the length of the aircraft's wingspan above the ground or water there occurs an often-noticeable ground effect. The result is lower induced drag on the aircraft. This is caused primarily by the ground or water obstructing the creation of wingtip vortices and interrupting downwash behind the wing.

A wing generates lift by deflecting the oncoming airmass (relative wind) downward. The deflected or "turned" flow of air creates a resultant force on the wing in the opposite direction (Newton's 3rd law). The resultant force is identified as lift. Flying close to a surface increases air pressure on the lower wing surface, nicknamed the "ram" or "cushion" effect, and thereby improves the aircraft lift-to-drag ratio. The lower/nearer the wing is to the ground, the more pronounced the ground effect becomes. While in the ground effect, the wing requires a lower angle of attack to produce the same amount of lift. In wind tunnel tests, in which the angle of attack and airspeed remain constant, an increase in the lift coefficient ensues, which accounts for the "floating" effect. Ground effect also alters thrust versus velocity, where reduced induced drag requires less thrust in order to maintain the same velocity.

Low winged aircraft are more affected by ground effect than high wing aircraft. Due to the change in up-wash, down-wash, and wingtip vortices, there may be errors in the airspeed system while in ground effect due to changes in the local pressure at the static source.

Rotorcraft

When a hovering rotor is near the ground the downward flow of air through the rotor is reduced to zero at the ground. This condition is transferred up to the disc through pressure changes in the wake which decreases the inflow to the rotor for a given disc loading, which is rotor thrust for each square foot of its area. This gives a thrust increase for a particular blade pitch angle, or, alternatively, the power required for a thrust is reduced. For an overloaded helicopter that can only hover IGE it may be possible to climb away from the ground by translating to forward flight first while in ground effect. The ground-effect benefit disappears rapidly with speed but the induced power decreases rapidly as well to allow a safe climb. Some early underpowered helicopters could only hover close to the ground. Ground effect is at its maximum over a firm, smooth surface.

VTOL aircraft

There are two effects inherent to VTOL aircraft operating at zero and low speeds in ground effect, suckdown and fountain lift. A third, hot gas ingestion, may also apply to fixed-wing aircraft on the ground in windy conditions or during thrust reverser operation. How well, in terms of weight lifted, a VTOL aircraft hovers IGE depends on suckdown on the air frame, fountain impingement on the underside of the fuselage and HGI into the engine causing inlet temperature rise (ITR). Suckdown works against the engine lift as a downward force on the airframe. Fountain flow works with the engine lift jets as an upwards force. The severity of the HGI problem becomes clear when the level of ITR is converted into engine thrust loss, three to four percent per 12.222 °c inlet temperature rise.

Suckdown is the result of entrainment of air around aircraft by lift jets when hovering. It also occurs in free air (OGE) causing loss of lift by reducing pressures on the underside of the fuselage and wings. Enhanced entrainment occurs when close to the ground giving higher lift loss. Fountain lift occurs when an aircraft has two or more lift jets. The jets strike the ground and spread out. Where they meet under the fuselage they mix and can only move upwards striking the underside of the fuselage. How well their upward momentum is diverted sideways or downward determines the lift. Fountain flow follows a curved fuselage underbody and retains some momentum in an upward direction so less than full fountain lift is captured unless lift improvement devices are fitted. HGI reduces engine thrust because the air entering the engine is hotter and less dense than cold air.

Early VTOL experimental aircraft operated from open grids to channel away the engine exhaust and prevent thrust loss from HGI.

The Bell X-14, built to research early VTOL technology, was unable to hover until suckdown effects were reduced by raising the aircraft with longer landing gear legs. It also had to operate from an elevated platform of perforated steel to reduce HGI. The Dassault Mirage IIIV VTOL research aircraft only ever operated vertically from a grid which allowed engine exhaust to be channeled away from the aircraft to avoid suckdown and HGI effects.

Ventral strakes retroactively fitted to the P.1127 improved flow and increased pressure under the belly in low altitude hovering. Gun pods fitted in the same position on the production Harrier GR.1/GR.3 and the AV-8A Harrier did the same thing. Further lift improvement devices (LIDS) were developed for the AV-8B and Harrier II. To box in the belly region where the lift-enhancing fountains strike the aircraft, strakes were added to the underside of the gun pods and a hinged dam could be lowered to block the gap between the front ends of the strakes. This gave a 1200 lb lift gain.

Lockheed Martin F-35 Lightning II weapons-bay inboard doors on the F-35B open to capture fountain flow created by the engine and fan lift jets and counter suckdown IGE.

  • Bell X-14 showing lengthened landing gear legs to reduce suckdown Bell X-14 showing lengthened landing gear legs to reduce suckdown
  • Dassault Mirage IIIV hovering over open grid Dassault Mirage IIIV hovering over open grid
  • Underside view of the first prototype P.1127 showing small ventral strakes to increase fountain lift Underside view of the first prototype P.1127 showing small ventral strakes to increase fountain lift
  • Harrier GR9 showing the lift improvement devices, large ventral strakes and a retractable dam behind nosewheel Harrier GR9 showing the lift improvement devices, large ventral strakes and a retractable dam behind nosewheel
  • F-35B showing weapon's bay inboard doors open to capture rising fountain flow F-35B showing weapon's bay inboard doors open to capture rising fountain flow

Wing stall in ground effect

The stalling angle of attack is less in ground effect, by approximately 2–4 degrees, than in free air. When the flow separates there is a large increase in drag. If the aircraft overrotates on take-off at too low a speed the increased drag can prevent the aircraft from leaving the ground. Two de Havilland Comets overran the end of the runway after overrotating. Loss of control may occur if one wing tip stalls in ground effect. During certification testing of the Gulfstream G650 business jet the test aircraft rotated to an angle beyond the predicted IGE stalling angle. The over-rotation caused one wing-tip to stall and an uncommanded roll, which overpowered the lateral controls, leading to loss of the aircraft.

Ground-effect vehicle

Main article: Ground-effect vehicle

A few vehicles have been designed to explore the performance advantages of flying in ground effect, mainly over water. The operational disadvantages of flying very close to the surface have discouraged widespread applications.

See also

References

Notes

  1. Gleim 1982, p. 94.
  2. Dole 2000, p. 70.
  3. "Chapter 7 - Helicopter Performance" (PDF). Helicopter Flying Handbook. Federal Aviation Administration. 2020.
  4. Raymer, Daniel P. (1992). Aircraft Design: A Conceptual Approach (PDF) (2 ed.). American Institute of Aeronautics and Astronautics, Inc. ISBN 0-930403-51-7. Archived from the original (PDF) on 2019-07-04. Retrieved 2019-12-26. Section 20.6
  5. Saeed, B.; Gratton, G.B. (2010). "An evaluation of the historical issues associated with achieving non-helicopter V/STOL capability and the search for the flying car" (PDF) (February): 94. {{cite journal}}: Cite journal requires |journal= (help)
  6. Aerodynamics for Naval Aviators. RAMESH TAAL, HOSUR, VIC. Australia: Aviation Theory Centre, 2005.
  7. Pilot's Encyclopedia of Aeronautical Knowledge 2007, pp. 3-7, 3-8.
  8. Benson, Tom. "Beginner's Guide to Aerodynamics: Lift from Flow Turning". NASA Glenn Research Center. Retrieved July 7, 2009.
  9. ^ Dole 2000, pp. 3–8.
  10. Flight theory and aerodynamics, p. 70
  11. HANDBOOKS, OPERATIONAL READINESS, MISSION PROFILES, PERFORMANCE (ENGINEERING), PROPULSION SYSTEMS, AERODYNAMICS, STRUCTURAL ENGINEERING, Defense Technical Information Center (1974)
  12. "Aerodynamics of ROTOR CRAFT". ABBOTTAEROSPACE.COM. April 12, 2016. pp. 2–6.
  13. Basic Helicopter Aerodynamics, J. Seddon 1990, ISBN 0 632 02032 6, p.21
  14. Rotor raft Flying Handbook (PDF). Federal Aviation Administration. 2000. pp. 3–4. Archived from the original (PDF) on 2016-12-27. Retrieved 2021-11-03.
  15. Hall, Gordon R. (1971). MODEL TESTS OF CONCEPTS TO REDUCE HOT GAS INGESTION IN VTOL LIFT ENGINES(NASA CR-1863) (PDF) (Report). Nasa. p. 4.
  16. Krishnamoorthy, V. (1971). AN ANALYSIS OF CORRELATING PARAMETERS RELATING TO HOT-GAS INGESTION CHARACTERISTICS OF JET VTOL AIRCRAFT (PDF) (Report). NASA. p. 8.
  17. Raymer 1992, pp. 551, 552.
  18. Mitchell, Kerry (1987). Proceedings of the 1985 NASA Ames Research Center's Ground-Effects Workshop (NASA Conference Publication 2462). Nasa. p. 4.
  19. The X-Planes, Jay Miller1988, ISBN 0 517 56749 0, p.108
  20. Ameel, Frederick Donald (1979). "Application of Powered High Lift Systems to STOL Aircraft Design". p. 14. S2CID 107781224. {{cite web}}: Missing or empty |url= (help)
  21. Williams, R.S. (1985). Addendum to AGARD report no. 710, Special Course on V/STOL Aerodynamics, an assessment of European jet lift aircraft. AGARD report; no. 710, addendum. p. 4. ISBN 9789283514893. {{cite book}}: |website= ignored (help)
  22. Harrier Modern Combat Aircraft 13, Bill Gunston1981, ISBN 0 7110 1071 4, p.23,43,101
  23. "The NTSB’s John O’Callaghan, a national resource specialist in aircraft performance, noted that all aircraft stall at approximately 2-4 deg. lower AOA with the wheels on the ground." (from NTSB Accident Report concerning loss of a swept wing business-class jet airplane in April 2011) Thin Margins in Wintry Takeoffs AWST, 24 December 2018
  24. Ranter, Harro. "ASN Aircraft accident de Havilland DH-106 Comet 1A CF-CUN Karachi-Mauripur RAF Station". aviation-safety.net.
  25. Aerodynamic Design Of Transport Aircraft, Ed Obert 2009, ISBN 978 1 58603 970 7, pp.603–606
  26. Staff writers (October 25, 2019). "Reprise: Night of the Comet | Flight Safety Australia".
  27. "Crash During Experimental Test Flight Gulfstream Aerospace Corporation GVI (G650), N652GD Roswell, New Mexico April 2, 2011" (PDF). www.ntsb.gov.
  28. From NTSB Accident Report: Flight test reports noted "post stall roll-off is abrupt and will saturate lateral control power." The catastrophic unrecoverable roll of the aircraft in the Roswell accident was due in part to the absence of warning before the stall in ground effect.
  29. Understanding Aerodynamics - Arguing From The Real Physics, Doug McLean 2013, ISBN 978 1 119 96751 4, p.401

Bibliography

  • Dole, Charles Edward (2000). Flight Theory and Aerodynamics. Hoboken, New Jersey: John Wiley & Sons, Inc. ISBN 978-0-471-37006-2.
  • Gleim, Irving (1982). Pilot Flight Maneuvers. Ottawa, Ontario, Canada: Aviation Publications. ISBN 0-917539-00-1.
  • Pilot's Encyclopedia of Aeronautical Knowledge (Federal Aviation Administration). New York: Skyhorse Publishing, 2007. ISBN 1-60239-034-7.

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