When an airplane in flight nears the ground (or water) surface, a change occurs in the three dimensional flow pattern because the local airflow cannot have a vertical component at the ground plane. Thus, the ground plane will furnish a restriction to the flow and alter the wing upwash, downwash, and tip vortices. These general effects due to the presence of the ground plane are referred to as “ground effect.”
AERODYNAMIC INFLUENCE OF GROUND EFFECT. While the aerodynamic characteristics of the tail and fuselage are altered by ground effects, the principal effects due to proximity of the ground plane are the 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 the upwash, downwash, and the tip vortices. These effects are illustrated by the sketches of figure 6.9. As a result of the reduced tip vortices, the wing in the presence of ground effect will behave as if it were of a greater aspect ratio. In other words, the induced velocities due to the tip (or trailing) vortices will be reduced and the wing will incur smaller values of induced drag coefficient, Cu., and induced angle of attack, on, for any specific lift coefficient, CL.
In order for ground effect to be of a significant magnitude, the wing must be quite close to the ground plane. Figure 6.9 illustrates one of the direct results of ground effect by the variation of induced drag coefficient with wing height above the ground plane for a representative unswept wing at constant lift coefficient. Notice that the wing must be quite close to the ground for a noticeable reduction in induced dfag. When the wing is at a height equal to the span (hjb=1.0), the reduction in induced drag is only 1.4 percent. However, when the wing is at a height equal to one-fourth the span (hjb = 0.25), the reduction in induced drag is 23.5
percent and, when the wing is at a height equal to one-tenth the span (hjb=0.1), the reduction in induced drag is 47.6 percent. Thus, a large reduction in induced drag will take place only when the wing is very close to the ground. Because of this variation, ground effect is most usually recognized during the liftoff of takeoff or prior to touchdown on landing.
The reduction of the tip or trailing vortices due to ground effect alters the spanwise lift distribution and reduces the induced angle of attack. In this case, the wing will require a lower angle of attack in ground effect to produce the same lift coefficient. This effect is illustrated by the lift curves of figure 6.9 which show that the airplane in ground effect will develop a greater slope of the lift curve. For the wing in ground effect, a lower angle of attack is necessary to produce the same lift coefficient or, if a constant angle of attack is maintained, an increase in lift coefficient will result.
Figure 6.9 illustrates the manner in which ground effect will alter the curve of thrust required versus velocity. Since induced drag predominates at low speeds, the reduction of induced drag due to ground effect will cause the most significant reduction of thrust required (parasite plus induced drag) only at low speeds. At high speeds where parasite drag predominates, the induced drag is but a small part of the total drag and ground effect causes no significant change in thrust required. Because ground effect involves the induced effects of airplane when in close proximity to the ground, its effects are of greatest concern during the takeoff and landing. Ordinarily, these are the only phases of flight in which the airplane would be in close proximity to the ground.
GROUND EFFECT ON SPECIFIC FLIGHT CONDITIONS. The overall influence of ground effect is best realized by assuming that the airplane descends into ground effect while maintaining a constant lift coefficient and, thus, a constant dynamic pressure and equivalent airspeed. As the airplane descends into ground effect, the following, effects will take place:
(1) Because of the reduced induced angle of attack and change in lift distribution, a smaller wing angle of attack will be required to produce the same lift coefficient. If a constant pitch attitude is maintained as ground effect is encountered, an increase in lift coefficient will be incurred.
(2) The reduction in induced flow due to ground effect causes a significant reduction in induced drag but causes no direct effect on parasite drag. As a result of the reduction in induced drag, the thrust required at low speeds will be reduced.
(3) The reduction in downwash due to ground effect will produce a change in longitudinal stability and trim. Generally, the reduction in downwash at the horizontal tail increases the contribution to static longitudinal stability. In addition, the reduction of downwash at the tail usually requires a greater up elevator to trim the airplane at a specific lift coefficient. For the conventional airplane configuration, encountering ground effect will produce a nose-down change in pitching moment. Of course, the increase in stability and trim change associated with ground effect provide a critical requirement of adequate longitudinal control power for landing and takeoff.
(4) Due to the change in upwash, down – wash, and tip vortices, there will be a change in position error of the airspeed system, associated with ground effect. In the majority of cases, ground effect will cause an increase in the local pressure at the static source and produce a lower indication of airspeed and altitude.
During the landing phase of flight, the effect of proximity to the ground plane must be understood and appreciated. If the airplane is brought into ground effect with a constant angle of attack, the airplane will experience
an increase in lift coefficient and reduction in thrust required. Hence, a “floating” sensation may be experienced. Because of the reduced drag and power-off deceleration in ground effect, any excess speed at the point of flare may incur a considerable “float” distance. As the airplane nears the point of touchdown on the approach, ground effect will be most realized at altitudes less than the wing span. An exact appreciation of the ground effect may be obtained during a field approach with the mirror landing system furnishing an exact reference of the flight path. During the final phases of the field approach as the airplane nears the ground plane, a reduced power setting is necessary or the reduced thrust required would allow the airplane to climb above the desired glide path. During shipboard operations, ground effect will be delayed until the airplane passes the edge of the deck and the reduction in power setting that is common to field operations should not be encountered. Thus, a habit pattern should not be formed during field landings which would prove dangerous during carrier operations.
An additional factor to consider is the aerodynamic drag of the airplane during the landing roll. Because of the reduced induced drag when in ground effect, aerodynamic braking will be of greatest significance only when partial stalling of the wing can be accomplished. The reduced drag when in ground effect accounts for the fact that the brakes are the most effective source of deceleration for the majority of airplane configurations.
During the takeoff phase of flight ground effect produces some important relationships. Of course, the airplane leaving ground effect encounters just the reverse of the airplane entering ground effect, i. e., the airplane leaving ground effect will (1) require an increase in angle of attack to maintain the same lift coefficient, (2) experience an increase in induced drag and thrust required, (3) experience a decrease in stability and a nose-цр change in moment, and (4) usually a reduction in static source pressure and increase in indicated airspeed. These general effects should point out the possible danger in attempting takeoff prior to achieving the recommended takeoff speed. Due to the reduced drag in ground effect the airplane may seem capable of takeoff below the recommended speed. However, as the airplane rises out of ground effect with a deficiency of speed, the greater induced drag may produce marginal initial climb performance. In the extreme conditions such as high gross weight, high density altitude, and high temperature, a deficiency of airspeed at takeoff may permit the airplane to become airborne but be incapable of flying out of ground effect. In this case, the airplane may become airborne initially with a deficiency of speed, but later settle back to the runway. It is imperative that no attempt be made to force the airplane to become airborne with a deficiency of speed; the recommended takeoff speed is necessary to provide adequate initial climb performance. In fact, ground effect can be used to advantage if no obstacles exist by using the reduced drag to improve initial acceleration.
The results of the airplane leaving ground effect can be most easily realized during the deck launch of a heavily loaded airplane. As the airplane moves forward and passes over the edge of the deck, whatever ground effect exists will be lost immediately. Thus, proper rotation of the airplane will be necessary to maintain the same lift coefficient and the increase in induced drag must be expected.
The rotor of the helicopter experiences a similar restraint of induced flow when in proximity to the ground plane. Since the induced rotor power required will predominate at low flight speeds, ground effect will produce a considerable effect on the power required at low speeds. During hovering and flight at low speeds, the elevation of the rotor above the ground plane will be an important factor determining the power required for flight.
The range of the reciprocating powered airplane can be augmented by the use of ground effect. When the airplane is close to the ground or water surface the reduction of induced drag increases the maximum lift-drag ratio and causes a corresponding increase in range. Of course, the airplane must be quite close to the surface to obtain a noticeable increase in (LfU)max and range. The difficulty in holding the airplane at the precise altitude without contacting the ground or water will preclude the use of ground effect during ordinary flying operations. The use of ground effect to extend range should be reserved as a final measure in case of emergency. Because of the very detrimental effect of low altitude on the range of the turbojet, ground effect will not be of a particular advantage in an attempt to augment range.
The most outstanding examples of the use of ground effect are shown in the cases of multiengine airplanes with some engines inoperative. When the power loss is quite severe, the airplane may not be capable of sustaining altitude and will descend. As ground effect is encountered, the reduced power required may allow the airplane to sustain flight at extremely low altitude with the remaining powerplants functioning. In ground effect, the reciprocating powered airplane will encounter a greater (_L/D’)max which occurs at a lower airspeed and power required and the increase in range may be quite important during emergency conditions.