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.