[visionist]

As everybody knows, helicopters need a tail rotor to offset the torque of the main rotor, otherwise the body would spin beneath the blades (which would look hilarious) instead of vice-versa. Tail rotors are a major source of a helicopter's distinctive- read loud- aural signature, and research into silent helicopters often involves eliminating the rotor as a matter of course.

One method of doing this is to go coaxial, with one rotor atop another, contra-rotating. My question is, could the upper & lower blades effectively be tuned to cancel each other out acoustically? Like the Infinitis with speakers that fire counter waves in response to intrusive sounds from outside?

[sivispacem]

Put simply, no, because in order to cancel each other out acousticly (is that a word?) they would need to be "pushing" in opposite directions rather than the same one, which would cause it to stall.

[leik oh em jeez!]

Hold on a second. How exactly does replacing a five foot blade with a thirty foot blade equate to less sound?

Besides, wouldn't offset rotors perform far better than stacked rotors?

I'm also doubting anything could be done with the blades to cancel out the sound. The fact that it's a kind of chopping sound rather than constant whine is where the problem lies in my mind. Then you get into trying to keep it tuned throughout the entire throttle range...