Helicopter Rotor Noise Prediction and Control

This paper summarizes the results of Sikorsky Aircraft's research in the prediction and control of helicopter rotor noise. The work to he discussed was partly funded by the U.S. Army Aviation Material Laboratories under Contracts DA 44‐177‐AMC‐141(T) and DA 44‐177‐AMC‐448(T). An improved procedure has been developed for the prediction of main rotor vortex noise under conditions of uniform inflow for a single rotor helicopter. Both the overall sound pressure level and the spectrum shape of the vortex noise from square tipped blades can be calculated as a function of tip speed, blade area, and thrust. The geometry of the blade tip can alter both levels and spectrum shape appreciably. The USAAVLABS contracts also resulted in two computerized analyses for rotational noise prediction. Both analyses extend the work of Gutin to include the noise produced by harmonics of airload acting on the rotor blades. These analyses have demonstrated the importance of the higher harmonics of airload and the chordwise distribution of loading for accurate rotational noise prediction.