Editing Ground effect (aerodynamics) (section)

===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>