Predicted Noise for a Main Rotor with Modulated Blade Spacing

An analytical study has been undertaken to explore an innovative rotor design concept that has the potential of reducing helicopter main rotor noise significantly while at the same time changing the “sound quality” of the radiated noise. Following the successful demonstration of tail-rotor noise reduction through the use modulated (unequal) blade spacing, a quiet main-rotor design with modulated blade spacing has been evaluated for feasibility and acoustic effectiveness. The design considerations, performance tradeoffs and unique aspects of the noise prediction are presented in this paper. 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 five-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 significantly quieter than the baseline rotor. The rotor with modulated blade spacing offers considerable additional reduction of the A-weighted noise levels for some operating conditions and observer locations, but is less effective in some cases.