Predictions of Rotor Performance in Ground Effect Using a Free‐Vortex Wake Model

A vortex‐singularity based model was developed to examine the performance problem of helicopter rotors operating in ground effect. Both relaxation and time‐marching free‐vortex wake approaches were examined. Two methods of modeling ground effect were studied, namely the method of images and a surface singularity method. The suitability of each method for the ground effect problem was examined both in hover and in forward flight. Results pertaining to both the predictive performance and the numerical behavior of each solution method are discussed. The vortex wake predictions confirm the basic features of experimental observations that the rotor wake is considerably distorted by the ground, this distortion being a strong function of ground proximity and forward speed. The rotor wake distortion near the ground affects rotor loads and performance, but primarily affects rotor power requirements. Some differences found between the relaxation and time‐marching free‐vortex wake results suggest that the rotor flow in ground effect was inherently aperiodic. Quantitative predictions of the ground effect problem were compared to experimental measurements, with generally good agreement.