Vertical Drag Recovery

When determining in ground effect (IGE) hover capability of the helicopter, the vertical fuselage drag must be corrected to account for a favorable effect that occurs in this regime. This is referred to as vertical drag recovery or airframe download recovery and offsets the download obtained on the fuselage from the rotor when the helicopter is hovering very near to the ground. This behavior results from a change in static pressure in the wake below the airframe. Measured results documenting this phenomenon are given

Fuselage height above ground, Hf /R

by Fradenburgh (1972) and Stepniewski & Keys (1984). The decrease in effective fuselage vertical drag is found to be significant for hovering heights of less than one rotor diameter. The results of measurements are shown in Fig. 6.27 in terms of the download ratio in ground effect to that out of ground effect (OGE). It is apparent that the behavior can be modeled approximately by

Rhge. = kg^i_ 1.22 exp {-Hf/R) for Hf/R > 0.25. (6.36)