Development and validation of a generalized ground effect model for lifting rotors

Helicopter flight simulation requires a generalized ground effect model with an appropriate combination of accuracy and computational efficiency. Existing ground effect models are either too simple to capture necessary physics or too involved computationally. This research aims at development of a finite-state ground effect model suitable for rotorcraft flight simulation of general in-ground-effect operations, which include not only the hovering and forward flight in normal ground effect but also the inclined, partial and dynamic ground effect cases. The new model is developed based on the pressure perturbation due to the ground, which has a source distribution and is determined by satisfying a pressure condition at the ground. The ground interference velocity at the rotor disk is expressed in terms of a Fourier series and a polynomial distribution. A ground influence coefficient matrix relates the ground interference velocity coefficients to the rotor pressure coefficients. The research has resulted in generalized expressions for the ground influence coefficient matrix. Using this ground influence coefficient matrix, not only the normal ground effect case but also the inclined, partial and dynamic ground effect cases have been modeled for both hovering and forward flight conditions. The new model has been applied to study the effect of the ground on the average inflow, inflow distribution at the rotor disk, the longitudinal inflow gradient, and the induced power required. The predicted results using the new model have been found to correlate well with available experimental data and with results of other models from literature. The new model is shown to capture the essential characteristics such as reduced average inflow/induced power with decreased rotor height and increased ground inclination, non-uniform inflow distribution for the cases of hovering above an inclined ground plane or partially above a ship deck, and reduced inflow gradient for the case of ground effect in low speed forward flight.