For
fixed-wing aircraft
A fixed-wing aircraft is a heavier-than-air flying machine, such as an airplane, which is capable of flight using wings that generate lift caused by the aircraft's forward airspeed and the shape of the wings. Fixed-wing aircraft are distinc ...
, ground effect is the reduced
aerodynamic drag
In fluid dynamics, drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) is a force acting opposite to the relative motion of any object moving with respect to a surrounding fl ...
that an aircraft's
wing
A wing is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is expres ...
s generate when they are close to a fixed surface.
[.] Reduced drag when in ground effect during
takeoff
Takeoff is the phase of flight in which an aerospace vehicle leaves the ground and becomes airborne. For aircraft traveling vertically, this is known as liftoff.
For aircraft that take off horizontally, this usually involves starting with a t ...
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
A rotorcraft or rotary-wing aircraft is a heavier-than-air aircraft with rotary wings or rotor blades, which generate lift by rotating around a vertical mast. Several rotor blades mounted on a single mast are referred to as a rotor. The Internati ...
, 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
A vertical take-off and landing (VTOL) aircraft is one that can take off and land vertically without relying on a runway. This classification can include a variety of types of aircraft including helicopters as well as thrust-vectoring fixed-win ...
(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).
[ Section 20.6]
Explanations
Fixed wing aircraft
When an aircraft flies at or below approximately half the length of the aircraft's
wingspan
The wingspan (or just span) of a bird or an airplane is the distance from one wingtip to the other wingtip. For example, the Boeing 777–200 has a wingspan of , and a wandering albatross (''Diomedea exulans'') caught in 1965 had a wingspan of ...
above the ground or water there occurs an often-noticeable ''ground effect.'' The result is lower
induced drag
In aerodynamics, lift-induced drag, induced drag, vortex drag, or sometimes drag due to lift, is an aerodynamic drag force that occurs whenever a moving object redirects the airflow coming at it. This drag force occurs in airplanes due to wings or ...
on the aircraft. This is caused primarily by the ground or water obstructing the creation of
wingtip vortices
Wingtip vortices are circular patterns of rotating air left behind a wing as it generates lift.Clancy, L.J., ''Aerodynamics'', section 5.14 One wingtip vortex trails from the tip of each wing. Wingtip vortices are sometimes named ''trailing'' ...
and interrupting
downwash
In aeronautics, downwash is the change in direction of air deflected by the aerodynamic action of an airfoil, wing, or helicopter rotor blade in motion, as part of the process of producing lift.Crane, Dale: ''Dictionary of Aeronautical Terms, thir ...
behind the wing.
[''Pilot's Encyclopedia of Aeronautical Knowledge'' 2007, pp. 3-7, 3-8.]
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 with regards to the ground, the more pronounced the ground effect becomes. While in the ground effect, the wing requires a lower
angle of attack
In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a reference line on a body (often the chord line of an airfoil) and the vector representing the relative motion between the body and the fluid through which it is m ...
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
Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that syst ...
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
A monoplane is a fixed-wing aircraft configuration with a single mainplane, in contrast to a biplane or other types of multiplanes, which have multiple planes.
A monoplane has inherently the highest efficiency and lowest drag of any wing confi ...
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 10 °F 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
The Bell X-14 (Bell Type 68) is an experimental VTOL aircraft flown in the United States in the 1950s. The main objective of the project was to demonstrate vectored thrust horizontal and vertical takeoff, hover, transition to forward flight, ...
, 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
The Dassault Mirage IIIV, also spelled Mirage III V, was a French vertical take-off and landing (VTOL) prototype fighter aircraft of the mid-1960s developed and produced by Dassault Aviation.
The Mirage IIIV was a VTOL derivative of an existin ...
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
strake
On a vessel's hull, a strake is a longitudinal course of planking or plating which runs from the boat's stempost (at the bows) to the sternpost or transom (at the rear). The garboard strakes are the two immediately adjacent to the keel on ...
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 lb lift gain.
Lockheed Martin F-35 Lightning II
The Lockheed Martin F-35 Lightning II is an American family of single-seat, single-engine, all-weather stealth multirole combat aircraft that is intended to perform both air superiority and strike missions. It is also able to provide ele ...
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.
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
Wing stall in ground effect
The stalling angle of attack is less in ground effect, by approximately 2–4 degrees, than in free air.
["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 ngle of attackwith 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
/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
The de Havilland DH.106 Comet was the world's first commercial jet airliner. Developed and manufactured by de Havilland in the United Kingdom, the Comet 1 prototype first flew in 1949. It featured an aerodynamically clean design with four d ...
s 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
The Gulfstream G650 is a large business jet produced by Gulfstream Aerospace. 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, led to loss of the aircraft.
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.[Understanding Aerodynamics - Arguing From The Real Physics, Doug McLean 2013, , p.401]
See also
* Coandă effect
The Coandă effect ( or ) is the tendency of a fluid jet to stay attached to a convex surface. ''Merriam-Webster'' describes it as "the tendency of a jet of fluid emerging from an orifice to follow an adjacent flat or curved surface and to ent ...
* Ground effect (cars)
Ground effect may refer to:
* Ground effect (aerodynamics), the increased lift and decreased aerodynamic drag of a wing close to a fixed surface
* Ground effect (cars), an effect that creates downforce, primarily in racing cars
* Ground effect veh ...
* Hovercraft
A hovercraft, also known as an air-cushion vehicle or ACV, is an amphibious Craft (vehicle), craft capable of travelling over land, water, mud, ice, and other surfaces.
Hovercraft use blowers to produce a large volume of air below the hull ...
* Vortex ring
A vortex ring, also called a toroidal vortex, is a torus-shaped vortex in a fluid; that is, a region where the fluid mostly spins around an imaginary axis line that forms a closed loop. The dominant flow in a vortex ring is said to be toroidal, ...
References
Notes
Bibliography
*
*
* ''Pilot's Encyclopedia of Aeronautical Knowledge'' (Federal Aviation Administration). New York: Skyhorse Publishing, 2007. .
External links
Engineering explanation
SE-Technology ('dead' site)
Ask Us – Ground Effect and WIG Vehicles
Aerospaceweb.org
* {{dead link, date=October 2017 , bot=InternetArchiveBot , fix-attempted=yes DSTO-GD-0201. Sponsored by DSTO Aeronautical and Maritime Research Laboratory, Australian Government.
WebArchive
Wing in Ground Effect and helicopters
dynamicflight.com
Plane Can Fly Inches Over Water
Tongji University Scientists in Shanghai announce design of a new vehicle, inventorspot.com, 14 July 2007
Ground-effect gliding
hanggliding.org
Numerical Analysis of Airfoil in Ground Proximity
''Journal of Theoretical and Applied Mechanics'', 45, 2, pp. 425–36, Warsaw 2007. ptmts.org. PDF-715 KB
Aerodynamics
Articles containing video clips