Full-scale demonstration of higher harmonic control for noise and vibration reduction on the XV-15 rotor

A higher harmonic control (HHC) investigation was conducted on a full-scale , isolated XV-15 rotor in helicopter mode in the NASA Ames 80- by 120-Foot Wind Tunnel to independently control noise and vibration. The higher harmon ic blade pitch was generated using swashplate oscillations. The radiated bl ade-vortex interaction (BVI) noise footprint was measured on a plane beneat h the rotor with an acoustic traverse. Test results showed that HHC is high ly effective in reducing BVI noise, achieving a 12 dB reduction in peak noi se level within the noise footprint. Noise reduction with HHC remains effec tive even with perturbations in rotor trim and flight conditions. Blade pre ssure feedback was demonstrated to be a viable method for closed-loop noise control. Some noise reduction was achieved with no increase in vibratory h ub loads. Increases in control loads due to HHC generally limited further n oise reduction. The vibration controller achieved about 50 percent reductio n in vibratory hub loads with control loads limiting the HHC amplitude.