A Study of Dual-Rotor Interference and Ground Effect Using a Free-Vortex Wake Model

A free-vortex rotor wake model has been developed to examine the problems of rotor-on-rotor interference, and rotors operating in ground effect. Rotor performance was determined for dual-rotor systems under different combinations of vertical and horizontal separation (gap and stagger, respectively), and the predictions were compared to experimental measurements. A new momentum theory result for the performance of overlapping rotors with gap was obtained, and used as a reference solution. The free-vortex wake results indicated a beneficial aerodynamic interference effect for closely spaced staggered rotors, but not substantially overlapping rotors. Two methods of modeling ground effect were examined, the method of images and a surface singularity method. The predictions for the ground effect problem were also compared to experimental measurements, with generally good agreement.