A HIGHER HARMONIC CONTROL TEST IN THE DNW TO REDUCE IMPULSIVE BVI NOISE

A model rotor acoustic test was performed to examine the benefit of hi gher harmonic control (HHC) of blade pitch to reduce blade-vortex inte raction (BVI) impulsive noise. A 40-percent dynamically scaled, four-b laded model of a BO-105 main rotor was tested in the German-Dutch Wind Tunnel (DNW). Acoustic measurements were made in a large plane undern eath the rotor employing a traversing in-flow microphone array in the anechoic environment of the open test section. Noise characteristics a nd noise directivity patterns as well as vibratory loads were measured and used to demonstrate the changes when different HHC schedules (dif ferent modes, amplitudes, phases) were applied. Dramatic changes of th e acoustic signatures and the noise radiation directivity with HHC pha se variations are found. Compared to the baseline conditions (without HHC), significant mid-frequency noise reductions of as much as 6 dB ar e obtained for low speed descent conditions where BVI is most intense, For other rotor operating conditions with less intense BVI there is l ess or no benefit from the use of HHC. Low frequency loading noise and vibratory loads, especially at optimum noise reduction control settin gs, are found to increase.