Thursday, January 10, 2008


Ground effect is due to the interference of the surface with the flow pattern about the airplane in flight. Ground effect can be detected and measured up to an altitude equal to one wing span above the surface. However, ground effect is most significant when the airplane (especially the low-wing airplane) is maintaining a constant attitude at low airspeed and low altitude (for example, during landing flare before touchdown, and during takeoff when the airplane lifts off and accelerates to climb speed).

When the wing is under the influence of ground effect, there is a reduction in upwash, downwash, and tip vortices. As a result of the reduced tip vortices, induced drag is reduced. When the wing is at a height equal to one-fourth the span, the reduction in induced drag is about 25 percent, and when the wing is at a height equal to one-tenth the span, the reduction in induced drag is about 50 percent. At high speeds where parasite drag predominates, induced drag is a small part of the total drag. Consequently, the effects of ground effect are of greater concern during takeoff and landing.

Assuming that the airplane descends into ground effect maintaining a constant angle of attack and a constant airspeed, the following effects will take place.

Because of the reduction in drag, a smaller wing angle of attack will be required to produce the same lift coefficient or, if a constant-wing-angle of attack is maintained, the wing will experience an increase in lift coefficient.

As a result of the reduction in drag, the thrust required at low speeds will be reduced.

The reduction in down wash at the horizontal tail will reduce the effectiveness of the elevator. It may cause a pitch-down tendency, thus requiring greater up elevator to trim the airplane.

In the majority of cases, ground effect will cause an increase in pressure at the static source and produce a lower indication of airspeed and altitude.

During the landing flare when the airplane is brought into ground effect at a constant angle of attack, the airplane will experience an increase in lift coefficient. Thus, a "floating" sensation may be experienced.

Because of the reduced drag in ground effect, any excess speed at the point of landing flare may result in a considerable "float" distance. If a power approach is being made, the power setting should be reduced as the airplane descends into ground effect to avoid overshooting the desired touchdown point.

During takeoff, the airplane leaving ground effect encounters the reverse of entering ground effect. For example, an airplane leaving ground effect will:
  • require an increase in angle of attack to maintain the same lift coefficient,
  • experience an increase in induced drag and thrust required,
  • experience a pitch-up tendency requiring less elevator travel to trim the airplane because of the increase in down-wash at the horizontal tail, and
  • usually experience a reduction in static source pressure and an increase in indicated airspeed.

    Due to the reduced drag in ground effect, the airplane may seem able to take off below the recommended airspeed. However, as the airplane rises out of ground effect with an insufficient airspeed, initial climb performance may prove to be marginal because of the increased drag. Under extreme conditions such as high-density altitude, high temperature, and maximum gross weight, the airplane may be able to become airborne at an insufficient airspeed, but unable to fly out of ground effect. Consequently, the airplane may not be able to clear an obstruction, or may settle back on the runway.

    Under marginal conditions, it is important the airplane takes off at the recommended speed that will provide adequate initial climb performance. If the runway is long enough, or no obstacles exist, ground effect can be used to an advantage by using the reduced drag to improve initial acceleration. Ground effect is important to normal flight operations in the performance of soft and rough field takeoffs and landings. The procedure for takeoff from these surfaces is to transfer as much weight as possible to the wings during the ground run, and to lift off with the aid of ground effect before true flying speed is attained. It is then necessary to reduce the angle of attack gradually until normal airspeed is attained before attempting to climb away from the ground effect.

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