Dynamic inflow modeling for simulation of a helicopter operating in ground effect

Adequate modeling of the ground effect phenomenon is important for rotorcraft flight dynamics analysis and flight simulation. A new approach has been developed for ground effect analysis based on the Peters-He finite-state dynamic inflow theory. The influence of the ground plane is represented as a sourcelike pressure perturbation in the flow field. The total pressure perturbation, which determines the induced inflow at rotor disk, is obtained as the superposition of contributions due to the rotor and the ground. Following this approach, a ground influence coefficients matrix [G] is obtained relating the ground interference velocity with the rotor pressure distribution. The method is applied to both hover and forward flight conditions. The elements of [G] matrix are obtained as functions of the normalized height, the tip-path-plane angle-of-attack, and the rotor wake skew angle. The structure of the [G] matrix indicates coupling among the radial shape modes and azimuthal harmonics of the ground interference velocity, implying that the ground plane has an effect on the inflow distribution at rotor disk. Correlation of predicted results with experimental data indicate good over all agreement. The new model performs as well as, or better than classical image methods. However, unlike an image rotor model, the present finite-state model can easily be implemented into a flight simulation program. Results indicate that the induced power of a rotor in ground effect increases, rather than decreases, as the forward speed is increased initially from hover. Results also indicate that the fore-to-aft inflow gradient is reduced by operation close to the ground.