STUDY OF NOISE ON A SMALL-SCALE HOVERING TILT ROTOR

A study of the noise from a small-scale, semi-span test of a hovering tilt rotor is presented, The effect on noise of the tilt rotor fountai n and of several tilt rotor configuration changes is examined, Measure ments were made of an isolated rotor; rotor and wing; rotor and image plane; and rotor, wing and image plane. This last configuration models a tilt rotor, With a tilt rotor, the wing turns some of the rotor dow nwash inboard, When the flows meet, some of the flow is forced upward creating a fountain, Some of the fountain is ingested into the rotor d isc, Rotor fountain interactions produce very unsteady impulsive noise to the rear of the aircraft. Configuration changes to investigate dow nload reduction included increasing the rotor/wing separation, deflect ing the flap, adding the nacelle, adding vortex trapping plates and bl owing on the upper surface. These configuration changes produced littl e or no effect on the noise, although they changed the net download, I ncreasing the wing span, however, reduced the annoying content of this impulsive noise about 2 dB.