XV-15 AEROACOUSTIC PROJECT
The tilt rotor aircraft holds great promise for improving air travel in the future. It's benefits include vertical take off and landing combined with airspeeds comparable to propeller driven aircraft. However, the noise from a tilt rotor during approach to a landing is potentially a significant barrier to widespread acceptance of these aircraft. This approach noise is primarily caused by Blade Vortex Interactions (BVI), which are created when the blade passes near or through the vortex trailed by preceding blades. The XV-15 Aeroacoustic test measured the noise from a tilt rotor during descent conditions and demonstrated several possible techniques to reduce the noise.
In 1997, the XV-15 Aeroacoustic test at NASA Ames Research Center measured acoustics and performance for a full-scale XV-15 rotor. A single XV-15 rotor was mounted on the Ames Rotor Test Apparatus (RTA) in the 80- by 120-Foot Wind Tunnel. The test was conducted in helicopter mode with forward flight speeds up to 100 knots and tip path plane angles up to ± 15 degrees. These operating conditions correspond to a wide range of tilt rotor descent and transition to forward flight cases.
Rotor performance measurements were made with the RTA rotor balance, while acoustic measurements were made using an acoustic traverse and four fixed microphones. The acoustic traverse provided limited directionality measurements on the advancing side of the rotor, where BVI noise is expected to be the highest. Baseline acoustics and performance measurements for the three-bladed rotor were obtained over the entire test envelope. Acoustic measurements were also obtained for correlation with the XV-15 aircraft In-flight Rotor Aeroacoustic Program (IRAP) recently conducted by Ames.
Several techniques were studied in an attempt to reduce the highest measured BVI noise conditions. The first of these techniques used sub-wings mounted on the blade tips. These subwings altered the size, strength, and location of the tip vortex, therefore changing the BVI acoustics of the rotor. The subwings are approximately 20% of the blade chord and increase the rotor radius by about 3 percent. Four different subwing configurations were tested, including square tipped subwings with different angles of incidence.
The ability of active controls to reduce BVI acoustics were assessed. The dynamic control system of the RTA were used to implement open- and closed-loop active control techniques, including individual blade control. Open-loop testing was conducted using a personal computer based, automated, real-time data acquisition system. This system features combined automated output of open loop control signals and automated data acquisition of the resulting test data.
A final technique to alter the noise of the rotor was examined. This involved changing the number of blades from three to four. A four-bladed rotor hub was fabricated on which the XV-15 blades was mounted. While the solidity of the rotor increased, much useful information was gained by examining the changes in the thrust and RPM with four blades.
Kitaplioglu, C., McCluer, M. and Acree, C. W., "Comparison of Full-Scale XV-15 Wind Tunnel and In-Flight Blade-Vortex Interaction Noise," Proceedings of the 53rd Annual Forum of the American Helicopter Society, April-May 1997.
Light, J. S., "Results From an XV-15 Rotor Test in the National Full-Scale Aerodynamics Complex," Proceedings of the 53rd Annual Forum of the American Helicopter Society, April-May 1997.
Light, J. S., Kitaplioglu, C., and Acree, C. W., "XV-15 Aeroacoustic Testing at NASA Ames," Proceedings of the Twenty-Third European Rotorcraft Forum, Dresden, Germany, September 1997.
Point of Contact:
NASA Ames Research Center
Moffett Field CA 94035-1000