It is possible to fly an aircraft just clear of the ground (or water) at a slightly slower airspeed than that required to sustain level flight at higher altitudes.

When an aircraft in flight comes within several feet of the surface, a change occurs in the three-dimensional flow pattern around the aircraft because the vertical component of the airflow around the wing is restricted by the surface. This alters the wing’s upwash, downwash, and wingtip vortices. [Figure 4-13]

Ground effect is due to the interference of the ground (or water) surface with the airflow patterns about the aircraft in flight. The principal effects are changes in the aerodynamic characteristics of the wing: as the wing encounters ground effect and is maintained at a constant lift coefficient, there is a reduction in upwash, downwash, and wingtip vortices. The reduction of wingtip vortices alters the spanwise lift distribution and reduces the induced angle of attack and induced drag; therefore, the wing requires a lower AOA in ground effect to produce the same lift coefficient (CL). If a constant AOA is maintained, an increase in CL results. [Figure 4-14] Since induced drag predominates at low speeds, the reduction of induced drag due to ground effect causes the most significant reduction of thrust required at low speeds. Due to the change in upwash, downwash, and wingtip vortices, there may also be a change in the position (installation) error of the airspeed system: ground effect usually increases the local pressure at the static source, producing a lower indication of airspeed and altitude, so an aircraft may be airborne at an indicated airspeed less than that normally required.

For ground effect to be significant, the wing must be quite close to the ground. When the wing is at a height equal to its span, the reduction in induced drag is only 1.4 percent; at one-fourth its span, 23.5 percent; and at one-tenth its span, 47.6 percent. Thus a large reduction in induced drag takes place only when the wing is very close to the ground, which is why ground effect is most usually recognized during liftoff for takeoff or just prior to touchdown when landing. An aircraft leaving ground effect after takeoff encounters the reverse of an aircraft entering ground effect during landing; i.e., the aircraft leaving ground effect will:

• Require an increase in AOA to maintain the same lift coefficient.
• Experience an increase in induced drag and thrust required.
• Experience a decrease in stability and a nose-up change in moment.
• Experience a reduction in static source pressure and an increase in indicated airspeed.

Ground effect must be considered during takeoffs and landings. If a pilot fails to understand the relationship between the aircraft and ground effect during takeoff, a hazardous situation is possible because the recommended takeoff speed may not be achieved. Due to the reduced drag in ground effect, the aircraft may seem capable of takeoff well below the recommended speed; but as it rises out of ground effect with a deficiency of speed, the greater induced drag may result in marginal initial climb performance. In extreme conditions (high gross weight, high density altitude, high temperature), the aircraft may become airborne initially but be incapable of sustaining flight out of ground effect, settling back to the runway. A pilot should not force an aircraft to become airborne with a deficiency of speed; the manufacturer’s recommended takeoff speed is necessary for adequate initial climb performance, and a definite climb should be established before retracting the landing gear or flaps. During landing, if the aircraft is brought into ground effect with a constant AOA, it experiences an increase in CL and a reduction in thrust required, and a “floating” effect may occur; any excess speed at the flare may incur a considerable “float” distance.