Ground effect is a condition of improved performance encountered when the aircraft is operating very close to the ground. Ground effect can be detected and measured up to an altitude of about one wingspan above the surface. [Figure 7-9] However, ground effect is most significant when the WSC aircraft is maintaining a constant attitude at low airspeed and low altitude. Examples are during takeoff when the aircraft lifts off and accelerates to climb speed, and also during the landing flare before touchdown. When the wing is under the influence of ground effect, there is a reduction in upwash, downwash, and wingtip vortices.
Since the WSC wing is a high wing aircraft, the effects are not as pronounced as a low wing airplane, but during rotation, the reduction in induced drag is about 25 percent and decreases as the WSC aircraft climbs. At high speeds where parasite drag dominates, induced drag is a small part of the total drag. Consequently, the effects of ground effect are of greater concern during takeoff and landing.
On takeoff, the takeoff roll, lift-off, and the beginning of the initial climb are accomplished in the ground effect area. As the aircraft lifts off and climbs out of the ground effect area the following occurs.
- The WSC aircraft requires an increase in angle of attack to maintain the same lift coefficient.
- The WSC aircraft experiences an increase in induced drag and thrust required.
Due to the reduced drag in ground effect, the aircraft may seem capable of taking off below the recommended airspeed with less thrust. However, as the aircraft rises out of ground effect with an insufficient airspeed, initial climb performance may prove to be marginal due to increased drag. Under conditions of high density altitude, high temperature, and/or maximum gross weight, the aircraft may become airborne at an insufficient airspeed but unable to climb out of ground effect. Consequently, the aircraft may not be able to clear obstructions or may settle back on the runway. The point to remember is that additional power is required to compensate for increases in drag that occur as an aircraft leaves ground effect. But during an initial climb, the engine is already developing maximum power. The only alternative is to lower pitch attitude to gain additional airspeed, which results in inevitable altitude loss. Therefore, under marginal conditions, it is important that the aircraft take off at the recommended speed that provides adequate initial climb performance.
Ground effect is important to normal flight operations. If the runway is long enough, or if no obstacles exist, ground effect can be used to advantage by utilizing the reduced drag to improve initial acceleration. Additionally, the procedure for takeoff from unsatisfactory surfaces is to take as much weight on the wings as possible 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 to attain normal airspeed before attempting to fly away from the ground effect area.