Tiltrotor Acoustic Flight Test: Terminal Area Operations

This paper provides a comprehensive description of an acoustic flight test of the XV15 Tiltrotor Aircraft with Advanced Technology Blades (ATB) conducted in August and September 1991 at Crows Landing, California. The purpose of this cooperative research effort of the NASA Langley and Ames Research Centers was to obtain a preliminary, high quality database of far-field acoustics for terminal area operations of the XV-15 at a takeoff gross weight of approximately 14,000 lbs for various glide slopes, airspeeds, rotor tip speeds, and nacelle tilt angles. The test also was used to assess the suitability of the Crows Landing complex for full scale far-field acoustic testing. This was the first acoustic flight test of the XV-15 aircraft equipped with ATB involving approach and level flyover operations. The test involved coordination of numerous personnel, facilities and equipment. Considerable effort was made to minimize potential extraneous noise sources unique to the region during the test. Acoustic data from the level flyovers were analyzed, then compared with data from a previous test of the XV-15 equipped with Standard Metal Blades (SMB). The data comparison revealed no conclusive acoustic advantage of the ATB over the SMB although some differences were noted in the 3000 4500 Hz range. The spectral density in that range from the SMB was higher than that from the ATB. Level flyovers at 250 ft altitude and a rotor speed of 677 ft/sec, the maximum Overall Sound Pressure Levels (OASPL) from both sets of blades are approximately 103 dB, although the ATB maximum OASPL impacts a larger area on the ground than the SMB.