Ground Effect in Forward Flight

In forward flight the rotor obtains a beneficial effect from flying near the ground, just as it does in hover. The ground effect may be expressed in terms of the ratio of the induced velocity in and out of ground effect. This ratio has been determined for a rotor from the work of reference 3.10 and is plotted in Figure 3.10. Also shown is the same ratio for a wing from reference 3.11 and for a hovering rotor from Figure 1.41 of Chapter 1. Note the similarity of all three curves. The induced power required is proportional to the induced velocity ratio and thus is reduced for flight near the ground. If the ratio applied at all flight conditions, it could be concluded that the induced power would always decrease as the helicopter went from hover to forward flight near the ground as well as away from the ground.

Source for rotor in forward flight: Heyson, “Ground Effect for Lifting Rotors in Forward Flight," NASA TND-234,1960; for rotor in hover: Figure 1.41; for wing: Hoak, "USAF Stability and Control DATCOM,” 1960.

Test experience, however, shows that during transition from hover to forward flight at heights less than about half the rotor diameter, the power may actually increase rather than decrease. Pilots speak of this as "running off the ground cushion.” Figure 3.11 shows the effect as measured in flight and reported in reference 3.12 and in a wind tunnel from reference 3.13. The reversal of ground effect is due to the helicopter overrunning the ground vortex, as illustrated in Figure 3-12, which is based on the wind tunnel observations of reference 3.13. As the leading edge of rotor approaches the ground vortex, the inflow is increased just as if part of the rotor were in a climb, thus increasing the power required. The recovery to a more normal inflow pattern occurs suddenly as the vortex passes under the rotor. The effect of the ground vortex on tail rotor performance in sideward and rearward flight is discussed in references 3.1 and 3.14.