The dynamic induced velocity field of a model rotor in hover conditions

Abstract Results obtained from measurements of the dynamic induced velocity field beneath a model rotor are presented. The collective and cyclic pitch of a four bladed rotor of 1·54 m diameter were excited at frequencies up to 1·5 times rotor shaft speed. Flow measurements were taken using hot wire anemometry probes and a laser doppler anemometer. A range of radial and vertical positions near the rotor disc were investigated. Analysis of the dynamic induced flow response is conducted in the frequency domain and it is found that there are significant radial and azimuthal variations which depend on the frequency of excitation. It is also observed that a change in the character of the inflow response occurs near and above the shaft rotational frequency and that vertical measuring distance from the rotor significantly affects the measured responses. Some results for the case of cyclic excitation are given. These show, that contrary to momentum theory predictions, the highest induced velocities in the dynamic case do not occur over the area of the disc where the blade pitch is at its maximum. Overall, the results show that the dynamic induced velocity field is highly complex and heavily influenced by the distribution of time varying shed vorticity within the wake.