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 compared 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 flight measurements were acquired using the NASA Ames YO-3A research aircraft 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 data were very similar, indicating the conditions were well-matched, Comparisons between the flight and wind tunnel data waveforms for the high advance ratio case (mu approximate to 0.25) were poor, For this condition, the wind tunnel data showed greater blade-to-blade and revolution-to-revolution variability than the flight data. Effects of tip-path-plane angle and advance ratio on the BVI flight data were also analyzed, The flight data showed the BVI peaks increased with increasing tip-path-plane angle until reaching a maximum at an angle which varied with flight condition, Further increases in the tip-path plane angle resulted in BVI peak reduction. Increasing advance ratio increased the magnitude of the BVI noise for the flight conditions tested.