Finite state in-ground effect inflow models for lifting rotors

Helicopter behavior is significantly affected when flying near the ground due to interactions between rotor wake and the ground plane. Existing ground effect simulation models use a simple factor to modify the out-of-ground effect inflow to get in-ground effect inflow, which is found to be rather approximate. A new ground effect model, which is based on the generalized wake theory is proposed in this study. The inflow at the rotor disc including the effect of ground is represented as a series of normalized Legendre functions of radial station and a series of harmonics of azimuth angle. The magnitude of each term is determined from first-order differential equations in time domain. The coefficients of the differential equations depend on the rotor height above the ground plane, rotor inclination and flight speed. The forcing functions are user-supplied, radial integrals of the blade loading. Initial validation results demonstrate the validity of the proposed model.