groundeffect

Ground effect is the aerodynamic phenomenon in which a wing experiences increased lift and reduced induced drag when operating close to a ground or other large surface. The wing’s wake and downwash are altered by the nearby boundary, which increases the wing’s effective lift coefficient at a given angle of attack.

The effect arises because the ground plane interferes with the formation of wingtip vortices and confines the vertical flow beneath the wing. This changes the pressure distribution around the wing, producing higher lift and lower induced drag than would occur in free air at the same altitude and speed.

Ground effect is most pronounced when the distance between the wing and the surface is on the order of a wing span (and to a lesser extent up to about two wingspans). As height increases, the wing behaves more like it is in open air and the lift increase and drag reduction diminish, becoming negligible beyond several wingspans. In practice, ground effect improves takeoff and landing performance, enabling shorter takeoff runs and lower descent speeds, while also affecting stability and handling near the surface.

The concept also applies to rotorcraft and watercraft near surfaces. For helicopters, hovering close to the ground enhances lift and reduces power requirements. In addition, ground effect underpins wing-in-ground-effect vehicles, or ekranoplans, which skim above water by exploiting the trapped air cushion between wing, hull, and surface.

See also: wing-in-ground-effect craft.