Experimental Investigation of Half-Ground Effects on Hovering Rotor Aerodynamics

An experimental investigation was conducted to characterize the effects of partial-ground on the aerodynamics of a hovering rotor. A model-scale rotor was tested at a range of heights above ground and under partial-ground coverage, and rotor hub forces and moments were measured using a six-axis force/torque transducer during constant-power operation. The measurements were used to develop a semi-empirical thrust ratio model that accurately captures trends from out-of-ground effect to full-ground effect conditions. This model predicts realistic thrust behavior at low ground-coverage conditions, exhibiting high adjusted R2 and minimal root mean square error. Time-resolved particle image velocimetry was conducted for selected cases to examine induced flow features and to qualitatively assess changes in the downwash and edge-driven crossflow associated with partial-ground interactions. A geometric rotor-ground interaction area based on a circular-segment formulation was correlated to the thrust coefficient over the interacting region. Results show that thrust increases with ground coverage and decreases with increasing out-of-ground height, whereas moment coefficients increase with decreasing height. The increases in thrust and moment can be attributed to a reduction in the induced velocity above ground, as observed in the measured flow field, thereby increasing the effective ground-induced pressure beneath the rotor.