Performance Measurements on Rotors Hovering above Moving Surfaces

Hub load measurements were taken on a rotor in ground effect above a ground plane undergoing single degree-of-freedom motion. A two-bladed teetering hub rotor was operated in a hovering condition at multiple heights above a sinusoidal pitching or heaving ground plane. A six-axis load cell was used to measure the load generated by a rotor at multiple blade collective pitch angles, as well as ground plane amplitudes and frequencies. The magnitude and frequency of the ground plane motion was determined based off a ratio of rotor blade passage frequency (for a representative maritime helicopter) to ground motion frequency (for a DDG-51 class ship). A distinct and measurable fluctuation on the rotor pitching moment was observed in the presence of ground plane pitching motion, and variations in thrust as high as twenty percent were observed in the presence of ground plane heaving motion. An appropriate scaling parameter for the problem was investigated and determined to be the ratio of the rotor inflow velocity to the peak ground motion velocity. This parameter showed good agreement over the complete range of measurement conditions, including when the rotor was operated at or near zero thrust.