Directivity and prediction of low frequency rotor noise

Acoustic data obtained over a large horizontal plane under the model rotor and digitally filtered in order to determine the low-frequency content near the blade passage frequency is analyzed. Focus is placed on the directivity of low-frequency noise, and the changes in directivity as a function of the descent glide slope angle and advance ratio are presented and compared with predicted directivity results. The differences between the data and prediction are discussed for two observer positions, one below and on the rotor axis, and the other 60 degrees down from the horizontal. It is demonstrated that for the latter position, blade-vortex interaction noise is strong when it occurs, and the loading at the low frequencies is significantly affected during blade-vortex interactions.