Ground Effect

Yer effect is defined as the change in flight characteristics of aircraft or flying organisms when they fly close to the surface. This condition reduces drag while increasing lift. This force is typically felt up to a height equal to one wingspan.

During takeoff, ground effect provides additional lift to aircraft due to air compressed beneath the wings. This reduces the required takeoff distance.

Ground effect facilitates aircraft control during turns, especially at low altitudes. The additional lift allows the aircraft to complete turns with less energy expenditure. At the same time, by increasing the aircraft’s critical angle of attack, ground effect prevents “stall” during low-speed maneuvers.

At high altitudes, the amount of air compressed beneath the wings decreases. This prevents aircraft from benefiting from ground effect at elevated altitudes.

During landing, when the aircraft enters ground effect, it encounters a phenomenon known as “float.” To counteract this lift force and ensure a smooth touchdown, pilots set the aircraft engines to “idle” position.

Ground effect fundamentally restricts the formation of wingtip vortices. Near the ground, wingtip vortices cannot grow as large as they do in free air. This reduces the downwash effect. The image below shows the size of wingtip vortices at different altitudes. As seen in the image, the wingtip vortices of the aircraft close to the ground have not fully developed.

^{Size of wingtip vortices at different altitudes -} Engineer Element

Air flowing over the wing and the wingtip vortices exit the wing at a downward angle. This causes the airflow over the wing to be directed downward. Since lift acts perpendicular to the direction of the airflow over the wing, the lift vector begins to tilt rearward. This results in a force known as induced drag. The restriction of wingtip vortices limits these effects.

^{Demonstration of downwash effects on a Cirrus SR-22 at high altitude -} Boldmethod

^{Demonstration of downwash effects on a Cirrus SR-22 at low altitude -} Boldmethod

Ground effect has different impacts on low-wing and high-wing aircraft. This can be better understood by examining two examples:

Cessna 172 and Piper Warrior.

When parked, the wing of the Cessna 172 is approximately two meters above the ground, which corresponds to about 20 percent of its wingspan. This results in the induced drag just before landing being approximately 60 percent of the induced drag experienced at high altitude.

The wing of the Piper Warrior, when parked, is approximately one meter above the ground. This equals about 10 percent of its wingspan. This causes the induced drag just before landing to be approximately 40 percent of the induced drag at high altitude. This difference allows the Piper Warrior to benefit more from ground effect.

^{Comparison of ground effect between Cessna 172 and Piper Warrior -} Boldmethod