HOVER ACOUSTIC CHARACTERISTICS OF THE XV-15 WITH ADVANCED TECHNOLOGY BLADES

An experiment has been performed to investigate the far-field hover ac oustic characteristics of the XV-15 aircraft with advanced technology blades (ATB). An extensive, high-quality, far-field acoustics data bas e was obtained for a rotor tip speed range of 645-771 ft/s. A 12-micro phone, 500-ft radius semicircular array combined with two aircraft hea dings provided acoustic data over the full 360-deg azimuth about the a ircraft with a resolution of 15 deg. Altitude variations provided data from near in-plane to 45 deg below the rotor tip path plane. Acoustic directivity characteristics in the lower hemisphere are explored thro ugh pressure time histories, narrow-band spectra, and contour plots. D irectivity patterns were found to vary greatly with azimuth angle, esp ecially in the forward quadrants. Sharp positive pressure pulses typic al of blade-vortex interactions were found to propagate aft of the air craft and were most intense at 45 deg below the rotor plane. Modest ov erall sound pressure levels were measured near in-plane indicating tha t thickness noise is not a major problem for this aircraft when operat ing in the hover mode with ATB. Rotor tip speed reductions reduced the average overall sound pressure level [dB (0.0002 dyne/cm2)] by nearly 8 dB in-plane, and by almost 5 dB at 12.6 deg below the rotor plane.