.15 Scale Model Studies Of Main And Tail Rotor Interaction

This paper presents the results of a 0. 15 scale model main and tail rotor test conducted in an acoustically treated section of the Vought 7x10 ft. wind tunnel. The effect of wake interaction on tail rotor noise was investigated by changing the tail rotor hub position with respect to the main rotor, and by adjusting the lateral spacing between tail rotor and vertical fin. Simulated flight conditions included level flight, descent, and climb. Six tail rotor placements were tested, covering the region where most current tail rotors are located. The main rotor wake within this region was analytically predicted to serve as a guide in interpreting the measured noise data. With the tail rotor positioned outside the main rotor wake, individual tail rotor harmonics were reduced as much as 7 db; summated discrete frequency tail rotor noise was reduced as much as 4 db. Increasing the separation between the vertical fin and tail rotor reduced individual tail rotor harmonics by as much as 4 db, and reduced summated discrete frequency noise by 1 db. This effect was observed only during level flight. It is concluded that tail rotor placement and, to some extent, vertical fin/ tail rotor separation influence noise sufficiently to be significant design parameters. In addition, the noise reductions observed occur at tail rotor positions commonly used in today's helicopters.