Aero Academic
Aero Academic
The reduction in induced drag and change in airflow when a wing is within about one wingspan of the surface, which can make an airplane float on landing and lift off below normal speed.
Ground effect is the change in the three-dimensional flow pattern around an aircraft when it comes within several feet of the ground or water. The vertical component of the airflow around the wing is restricted by the surface, which alters the wing's upwash, downwash and wingtip vortices and reduces induced drag. The Pilot's Handbook of Aeronautical Knowledge describes it as the interference of the surface with the airflow patterns about the aircraft in flight.
The effect is only significant when the wing is quite close to the ground. At a height equal to the wingspan, the handbook gives the reduction in induced drag as 1.4 percent; at one-fourth of the span it is 23.5 percent, and at one-tenth of the span it is 47.6 percent. Because induced drag predominates at low speeds, thrust required falls noticeably at low speeds in ground effect.
Two practical consequences are tested. On landing, the airplane seems to float because less thrust is needed to maintain speed. On takeoff, an airplane can become airborne at an indicated airspeed below that normally required, but it may not be able to climb out of ground effect, so the pilot should accelerate to the proper climb speed before leaving it.