Message posted by Griffon_314 on May 05, 2011 at 10:45:30 PST:

Helo noise suppression tackles the main sources of noise:

1. Main rotor
- more blades = smaller blades = less noise
- lower main rotor RPM = less noise (remember the WHOP-WHOP of a UH-1H in a descending right turn? That's the advancing blade tips going supersonic. Those blades were very large. Modern helos use 4+ blades instead of two, smaller blade chord, and tips remain subsonic)
- unfortunately, reducing MR blade size and RPM directly reduces MR momentum, and thus autorotation performance. Modern UH-1Y autorotation profiles are brick-like, compared to the glider-like profile of the old -1H.

2. Tail rotor
- same deal: more, smaller blades, and less RPM
- reduced interaction between main rotor downwash and tail rotor (MR downwash can reduce tail rotor authority, requiring more, bigger blades at higher speed, ergo more noise)
- you can see a five-bladed tail rotor in the photos of the downed aircraft

3. Engines
- measures to reduce engine acoustic signatures are often combined with IR suppression measures (baffles, ducting)
- these measures often are at odds with performance - which is important in hot/high locations (like, well, Abottabad!)

4. Gearbox
- mainly a factor with larger aircraft - gearbox signatures in scout-class helicopters (e.g. B206, H500 class) are minimal. Even an H-60 doesn't produce much gearbox noise.

See the attached file for more comprehensive info (warning - 190 pages, so it takes a while to load).

In Reply to: Re: Army Times article on stealthy MH-60 - mention of "a facility in Nevada" posted by OldThudMan~!!~ on May 05, 2011 at 10:18:22 PST:

Replies:

• Re: reducing rotor noise OldThudMan~!!~ 05/05/2011, 17:38:21 PST (0)