Hughes Helicopters carried out a lot of work on their OH-6A/Hughes 500 many years ago to produce a quiet helicopter.
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Apparently, the noise reduction was as high as 20 decibels in certain phases of flight. The main blade tips were trapezoidal in shape and the four-blade tail rotor turned at a lower rpm. Those who worked on it swear that it was quieter than any of the quietest of today's helicopters. It must have been very quiet indeed.
Would the "Quiet One" fitted with a NOTAR system be even quieter, especially if the NOTAR fan blades were "shaped" to produce even less sound?
500 Fan.