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 were co mpared 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 fli ght measurements were acquired using the NASA Ames YO-3A research airc raft fitted with acoustic instrumentation. Flight and wind tunnel data were compared for three conditions, For the low and moderate advance ratio conditions, the BVI pulse widths of the flight and wind tunnel d ata were very similar, indicating the conditions were well-matched, Co mparisons between the flight and wind tunnel data waveforms for the hi gh advance ratio case (mu approximate to 0.25) were poor, For this con dition, the wind tunnel data showed greater blade-to-blade and revolut ion-to-revolution variability than the flight data. Effects of tip-pat h-plane angle and advance ratio on the BVI flight data were also analy zed, The flight data showed the BVI peaks increased with increasing ti p-path-plane angle until reaching a maximum at an angle which varied w ith flight condition, Further increases in the tip-path plane angle re sulted in BVI peak reduction. Increasing advance ratio increased the m agnitude of the BVI noise for the flight conditions tested.