The recent developments in "Quiet Rotor" technology are quite interesting. It is a little puzzling that it has taken this long for the manufacturers to tackle the issue of achieving lower noise emissions from rotorcraft, though.
I have been researching the history of the various prototypes and test ships of the H-6/Model 500 series of helicopters and just selecting that helicopter as a single example, quite a lot of research and testing has been carried out in the past in relation to "Quiet Rotor" technology and, indeed, other systems that can contribute to quieter helicopters. NASA and Hughes carried out testing on a H-6 in the late sixties and early seventies and achieved quite large noise reductions with their "Quiet One" prototype. Air intakes, MGB transmission and engine shrouded in sound-absorbent material, a tuned engine exhaust, shaped rotor tips, an increase in blade numbers (main and tail) and reduced rotor RPM all made the "Quiet One" exceptionally quiet, apparently.
Another system tested by Hughes and NASA was their "Higher Harmonic Control" system, which was primarily seeking to reduce vibrations emanating from the main rotor system. It was successful in reducing vibration but it was also possible to "tune" the noise coming from the main rotor. This system was tested in the period 1982 to 1985 using computers of the day. Given the power of today's computers, the HHC system could be worth exploring again for the various benefits it has to offer.
Eurocopter seem to have had some success with the "Staggered-Blade" design fenestron tail rotor as regards noise emissions. Perhaps if a similar fan was fitted to the NOTAR anti-torque system, it might lower noise levels even further.
Combine all three of these systems in one helicopter and the noise reductions could be notable. Perhaps the cost of R&D in this area has been prohibitive up to now but given the demand for eco-friendly transport these days, the next generation of helicopters could be very interesting indeed.
500 Fan.
Last edited by 500 Fan; 13th June 2010 at 18:11.