Editing Ground effect (aerodynamics) (section)

==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.<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>
{{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 third 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 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,<ref name="Pilot pp.3-8">{{harvnb|Dole|2000|pp=3–8}}.</ref> 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.<ref name="Pilot pp.3-8"/>

[[Low wing|Low winged aircraft]] are more affected by ground effect than [[high wing]] 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 [[Pitot–static system#Static pressure|static source]].<ref name="Pilot pp.3-8"/>

===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.<ref>[https://archive.org/details/DTIC_ADA002007 HANDBOOKS, OPERATIONAL READINESS, MISSION PROFILES, PERFORMANCE (ENGINEERING), PROPULSION SYSTEMS, AERODYNAMICS, STRUCTURAL ENGINEERING], 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===
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.

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.<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 |url=https://www.semanticscholar.org/paper/Application-of-Powered-High-Lift-Systems-to-STOL-Ameel/d77cdbba3fea3a81678bb76f9070ac2ee546bd55}}</ref> 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.<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. |series=AGARD report; no. 710, addendum |year=1985 |page=4|isbn=9789283514893 }}</ref>

Ventral [[strake]]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>

[[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.
<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 [angle of attack] with the wheels on the ground." (from NTSB Accident Report concerning loss of a swept wing business-class jet airplane in April 2011) [http://aviationweek.com/business-aviation/thin-margins-wintry-takeoffs?NL=AW-05&Issue=AW-05_20190104_AW-05_763&sfvc4enews=42&cl=article_2&elq2=bb67d18ab5f24f3b940fbdcc9122aad6 ''Thin Margins in Wintry Takeoffs'' AWST, 24 December 2018]</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 [[de Havilland Comet]]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 [[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.<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>