Active and Passive Helicopter Noise Reduction Using the AVINOR/HELINOIR Code Suite

A suite of computational tools capable of predicting in-plane low-frequency rotorcraft noise and its control using blade-tip geometry modifications is developed. The combined code, consisting of AVINOR, a comprehensive rotorcraft analysis code, and an acoustic code called HELINOIR, is first validated against wind-tunnel tests and subsequently verified by comparing with computational results. Three rotor configurations resembling the Messerschmitt-Bolkow-Blohm BO105 with a tip sweep, dihedral, and anhedral were simulated for level flight at a moderate advance ratio. The impact of passive blade geometry modification on in-plane noise and vibration is studied and compared to the in-plane noise reduction obtained using a single 20% chord active plain trailing-edge flap with a feedback microphone on the left boom. Active control, which is implemented using an adaptive higher harmonic control algorithm, reduces in-plane low-frequency sound pressure levels below the horizon by up to 6 dB, but there is an increase in vibratory loads. The tip dihedral reduces low-frequency sound pressure level by up to 2 dB without a vibratory load penalty, but there is an increase in the midfrequency sound pressure levels. The tip sweep and tip anhedral increase in-plane low-frequency sound pressure level below the horizon. There is a general tradeoff associated with in-plane low-frequency sound pressure level reduction, vibration performance, and midfrequency sound pressure level.