Ground effect (aerodynamics)

In aircraft, the ground effect is the reduced aerodynamic drag that an aircraft's wings generate when they are close to a surface (land or water). Ground effect is relevant for fixed-wing aircraft, rotorcraft, VTOL/STOL, and ground vehicles. Ground effect reduces drag by 40–50%, improving aircraft lift-to-drag ratios to 20–30, compared to 15–20 for conventional aircraft.

The principal benefit of operating in ground effect is to reduce its lift-induced drag. The closer the wing operates to a surface such as the ground, when it is said to be in ground effect, the less drag it experiences. When an aircraft enters ground effect, the surface pushes back against the downwash, which reduces its drag.

During takeoff, ground effect can cause an aircraft to "float" while accelerating towards the climb speed, reducing friction.

For rotorcraft, ground effect results in less drag on the rotor while hovering near the ground. At high weights this may allow lift off while stationary in ground effect, but does not allow it to transition to flight while in ground effect. Helicopter pilots are provided with performance charts that 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).

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, reducing downwash behind the wing as well as the upwash in front of the wing. 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, concurrently the wing also experiences a reduction in drag. In wind tunnel tests, in which the angle of attack and airspeed remain constant, an increase in the lift coefficient ensues, which combined with the reduced drag accounts for the "floating" effect experienced by pilots.

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.

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.