Aerodynamics of Flight

deriuqer tsurhT Velocity C tneiciffeoc tfiL L In ground effect Out of ground effect Out of ground effect In ground effect Angle of attack Figure 5-17. Ground effect changes drag and lift. 41 safety that allows wake turbulence dissipation. [Figure 5-15] For more information on wake turbulence, see Advisory Circular (AC) 90-23, Aircraft Wake Turbulence. Ground Effect 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 Figure 5-16. Ground effect changes airflow. 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. the spanwise lift distribution and reduces the induced AOA This phenomenon is called ground effect. and induced drag. Therefore, the wing will require a lower AOA in ground effect to produce the same C . If a constant When an aircraft in flight comes within several feet of the L AOA is maintained, an increase in C results. [Figure 5-17] surface, ground or water, a change occurs in the three- L dimensional flow pattern around the aircraft because the Ground effect also alters the thrust required versus velocity. vertical component of the airflow around the wing is Since induced drag predominates at low speeds, the reduction restricted by the surface. This alters the wing’s upwash, of induced drag due to ground effect will cause a significant downwash, and wingtip vortices. [Figure 5-16] Ground reduction of thrust required (parasite plus induced drag) at low effect, then, is due to the interference of the ground (or water) speeds. Due to the change in upwash, downwash, and wingtip surface with the airflow patterns about the aircraft in flight. vortices, there may be a change in position (installation) error While the aerodynamic characteristics of the tail surfaces of the airspeed system associated with ground effect. In the and the fuselage are altered by ground effect, the principal majority of cases, ground effect causes an increase in the local effects due to proximity of the ground are the changes in pressure at the static source and produces a lower indication the aerodynamic characteristics of the wing. As the wing of airspeed and altitude. Thus, an aircraft may be airborne at encounters ground effect and is maintained at a constant an indicated airspeed less than that normally required. AOA, there is consequent reduction in the upwash, downwash, and wingtip vortices. In order for ground effect to be of significant magnitude, the wing must be quite close to the ground. One of the direct Induced drag is a result of the airfoil’s work of sustaining results of ground effect is the variation of induced drag with the aircraft, and a wing or rotor lifts the aircraft simply by wing height above the ground at a constant C . When the accelerating a mass of air downward. It is true that reduced L wing is at a height equal to its span, the reduction in induced pressure on top of an airfoil is essential to lift, but that is drag is only 1.4 percent. However, when the wing is at a only one of the things contributing to the overall effect of height equal to one-fourth its span, the reduction in induced pushing an air mass downward. The more downwash there drag is 23.5 percent and, when the wing is at a height equal is, the harder the wing pushes the mass of air down. At high to one-tenth its span, the reduction in induced drag is 47.6 angles of attack, the amount of induced drag is high; since this percent. Thus, a large reduction in induced drag takes place corresponds to lower airspeeds in actual flight, it can be said only when the wing is very close to the ground. Because of that induced drag predominates at low speed. However, the this variation, ground effect is most usually recognized during reduction of the wingtip vortices due to ground effect alters the liftoff for takeoff or just prior to touchdown when landing.