COMPARISON OF XV-15 FULL-SCALE WIND TUNNEL AND IN-FLIGHT BLADE-VORTEX INTERACTION NOISE

A full-scale isolated XV-15 rotor was tested in the NASA Ames 80 - by 120-Foot Wind Tunnel to determine the baseline blade-vortex interaction (BVI) noise signature at typical descending flight conditions. Representative acoustic data at peak BVI noise conditions are presented. The sensitivity of the acoustic near-field to small changes in tip-path-plane angle is shown. The far-field, on the other hand, is shown to be relatively insensitive to similar small changes. In parallel, the XV-15 tiltrotor aircraft was flight tested at several operating conditions similar to those of the wind tunnel test for high BVI noise. The objective of this work was to document the extent to which the wind tunnel acoustic measurements of the isolated rotor correspond to the XV-15 flight data. Unlike many previous in-flight noise tests, direct measurement of aircraft angle-of-attack, rotor gimbal angle, and rotor torque allowed matching of rotor operating conditions in the wind tunnel. Nevertheless, questions remain as to the accuracy of the flight test parameters. Comparison of a limited number of cases indicates qualitative agreement of the main features of BVI noise. However some pressure peaks in the wind tunnel data exhibit values substantially higher than those observed in flight. Some factors that can lead to these differences are pointed out.