FLIGHT MEASUREMENTS OF BLADE-VORTEX INTERACTION NOISE INCLUDING COMPARISONS WITH FULL-SCALE WIND TUNNEL DATA

Acoustic measurements of a Sikorsky S-76C helicopter in flight are presented. In addition, comparisons are made with acoustic measurements of a full-scale S-76 rotor tested in the 80- by 120-Foot Wind Tunnel at NASA Ames Research Center. The comparisons provide a preliminary determination of the validity of using the wind tunnel as an acoustic testing facility for full-scale rotor research and are the first direct comparison between in-flight and full-scale wind tunnel noise measurements. The investigation was designed to further the understanding of the blade vortex interaction noise generated by the S-76 rotor. The flight measurements were acquired using the NASA Ames YO-3A research aircraft fitted with acoustic instrumentation. Time-accurate separation distances between the S-76C and YO-3A were achieved using a portable laser rangefinder. Effects of tip-path-plane angle and advance ratio on the in-flight acoustics are presented for a limited number of conditions. Increasing tip-path-plane angle causes the BVI peaks to reach a maximum earlier in time; these observations are similar to those observed in two-bladed rotor flight test data and small-scale wind tunnel data. Flight and wind tunnel data are compared for three conditions. For the low and moderate advance ratio conditions presented, the BVI pulse widths and overall sound level of the flight and wind tunnel data are very similar. Comparisons between flight and wind tunnel data waveforms for the high advance ratio condition are poor.