Model Helicopter Blade Slap at Low Tip Speeds: Theoretical and Experimental Correlations

An instrumented model rotor blade equipped with differential pressure transducers and hot film sensors has been tested in the M.I.T. 1.52 m x 2.3 m (5 x 7.5 ft) anechoic wind tunnel to investigate the blade slap caused by blade-vortex interaction at tip Mach number less than 0.2. For the operating conditions tested, blade slap is caused by the vortex induced unsteady pressure fluctuations at the leading edge of a rotor blade during the blade- vortex interaction. The intensity of the resulting noise has a very profound dependancy on the interaction geometry. The far field sound pressure level decreases as the interaction angle and/or the separation distance between the blade and the vortex filament increases. The measured acoustic signals have been compared with the predictions provided by the Widnall and Wolf Theory. Good correlations have been shown for the blade-vortex interactions occurring in a region outside the vortex core. The skew interaction effect is not modelled by the theory and is the cause of poor correlation with the experiment.