safety that allows wake turbulence dissipation.  [Figure 5-15]  For more information on wake turbulence, see Advisory  Circular (AC) 90-23, Aircraft Wake Turbulence.  Ever since the beginning of manned flight, pilots realized  that just before touchdown it would suddenly feel like the  aircraft did not want to go lower, and it would just want to go  on and on. This is due to the air that is trapped between the  wing and the landing surface, as if there were an air cushion.  This phenomenon is called ground effect.  When an aircraft in flight comes within several feet of the  surface, ground or water, 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 5-16]  Ground  effect, then, is due to the interference of the ground (or water)  surface with the airflow patterns about the aircraft in flight.  While the aerodynamic characteristics of the tail surfaces  and the fuselage are altered by ground effect, the principal  effects due to proximity of the ground are the changes in  the aerodynamic characteristics of the wing. As the wing  encounters ground effect and is maintained at a constant  AOA, there is consequent reduction in the upwash,  downwash, and wingtip vortices.  Induced drag is a result of the airfoil’s work of sustaining  the aircraft, and a wing or rotor lifts the aircraft simply by  accelerating a mass of air downward. It is true that reduced  pressure on top of an airfoil is essential to lift, but that is  only one of the things contributing to the overall effect of  pushing an air mass downward. The more downwash there  is, the harder the wing pushes the mass of air down. At high  angles of attack, the amount of induced drag is high; since this  corresponds to lower airspeeds in actual flight, it can be said  that induced drag predominates at low speed. However, the  reduction of the wingtip vortices due to ground effect alters  In order for ground effect to be of significant magnitude, the  wing must be quite close to the ground. One of the direct  results of ground effect is the variation of induced drag with  wing height above the ground at a constant C$_{L}$. When the  wing is at a height equal to its span, the reduction in induced  drag is only 1.4 percent. However, when the wing is at a  height equal to one-fourth its span, the reduction in induced  drag is 23.5 percent and, when the wing is at a height equal  to one-tenth its span, the reduction in induced drag is 47.6  percent. Thus, a large reduction in induced drag takes place  only when the wing is very close to the ground. Because of  this variation, ground effect is most usually recognized during  the liftoff for takeoff or just prior to touchdown when landing.  Ground effect also alters the thrust required versus velocity.  Since induced drag predominates at low speeds, the reduction  of induced drag due to ground effect will cause a significant  reduction of thrust required (parasite plus induced drag) at low  speeds. Due to the change in upwash, downwash, and wingtip  vortices, there may be a change in position (installation) error  of the airspeed system associated with ground effect. In the  majority of cases, ground effect causes an increase in the local  pressure at the static source and produces a lower indication  of airspeed and altitude. Thus, an aircraft may be airborne at  an indicated airspeed less than that normally required.  the spanwise lift distribution and reduces the induced AOA  and induced drag. Therefore, the wing will require a lower  AOA in ground effect to produce the same C$_{L}$. If a constant  AOA is maintained, an increase in C$_{L}$ results.  [Figure 5-17]  Figure 5-16.  Ground effect changes airflow.  Ground Effect  Figure 5-17.  Ground effect changes drag and lift.  5-11  14  Thrust required  Velocity  Lift coefficient C L Angle of attack  Out of ground effect  In ground effect  Out of ground effect  In ground effect