Test and Evaluation of a Quiet Helicopter Configuration HH-43B

A series of noise control modifications was made to the HH-43B helicopter. Each modification was evaluated by direct comparison of acoustic signatures of modified and unmodified configurations. Noise control modifications to the aircraft engine, drive and rotor systems were used and are evaluated. Testing was performed on ten aircraft configurations. The noise control modifications resulted in substantial reductions in flyover noise. All octave bands of interest, i.e., 63 Hz to 4000 Hz, were significantly reduced. The rotor system was the dominant noise source, in level flight, dominating each octave band in the modified aircrafts audible spectrum, i.e., with center frequencies from 31.5 Hz to 8000 Hz. This noise source was reduced through changes in rotor blade geometry and reduction in blade tip speed. All program noise signature reduction goals were met or exceeded.