Predicted Noise for a Main Rotor with Modulated Blade Spacing

摘要

An analytical study has been undertaken to explore an innovative rotor design concept that has the potential of reducing and changing the "sound quality" of helicopter main rotor noise. A quiet main-rotor design with modulated (unequal) blade spacing has been evaluated for feasibility and acoustic effectiveness. The design considerations, performance tradeoffs, and the unique aspects of the noise prediction of a modulated main rotor are addressed. Overall sound pressure level (OASPL) and A-weighted sound pressure level time histories are computed for a baseline four-bladed main rotor, a low-noise evenly spaced five-bladed main rotor and a low-noise flve-bladed main rotor with modulated blade spacing. Comparisons of the noise from each of the three rotors are presented for level flyover, takeoff and approach conditions. Both low-noise rotor designs are generally quieter than the baseline rotor, but the noise reductions are gained through a penalty in cruise airspeed, maximum airspeed, and payload. Modulated blade spacing dramatically changes the character of the sound but does not uniformly offer noise reductions as compared to the even blade spacing.