A STUDY OF HELICOPTER ROTOR AERODYNAMICS IN GROUND-EFFECT AT LOW SPEEDS

An experimental and analytical study of helicopter rotor aerodynamics in ground effect at low speeds has been conducted. The experimental studies included flow visualization experiments and measurements of the mean induced velocity distribution near the rotor plane using the hot wire technique using a model rotor.;A simplified free-wake-rolled-up-wake flow model was developed to study the aerodynamic interaction between the rotor wake, the ground and the rolled-up wake and to calculate the induced velocity distribution at the rotor plane. A semi-empirical method based on experimental location of the ground vortex center has been developed to estimate the total circulation of the rolled-up wake. The results show that the rotor near-wake deforms significantly because of the flow field induced by the ground and the rolled-up wake. This deformation of the rotor wake causes large variations of the induced velocity distribution at the rotor plane, especially near the leading edge of the rotor. The variations of the induced velocity distribution at the rotor plane caused by the ground and the rolled-up wake through deforming the rotor near-wake is much larger than that induced directly by the ground and the rolled-up wake. When the vorticity in the rolled-up wake is concentrated, the semi-empirical method can be used to estimate the total circulation of the ground vortex. The free-wake-rolled-up-wake flow model can be used to predict the induced velocity distribution at the rotor plane with reasonable accuracy.;When a lifting rotor operates near the ground at low speeds, its wake will roll up after impinging on the ground because of the effects of the ground and the free stream air flow. The flow field induced by the rolled-up wake influences the rotor aerodynamic properties significantly. When the rolled-up wake moves near to and under the rotor plane, very large and irregular variations of the induce velocity distribution in the forward part of the rotor will occur, as a function of the rotor advance ratio. This causes irregular variations of the rotor forces and moments that have been observed in previous experiments.