I don't know for sure, but I'd guess that helicopter rotors aren't
supersonic, especially in the days of the Huey. Guestimating the
Huey's rotor diameter at, say, 15 meters, the rotor tips travel about
50 meters for 1 revolution. To go supersonic (more than 330 m/s,
ignoring any velocity of the aircraft itself), the rotor would have to
spin 7 times or more in one second, which sounds a lot faster than it
actually does spin (2-3 rev/s, maybe?).
I think the distinctive whop-whop-whop of the Huey is from main-tail
rotor interaction. The main rotor produces tip vortices, just like
those you can occasionally see on a fixed-wing aircraft when conditions
are right. When the tail rotor slices through a vortex, it makes a
whop.
Newer helicopters are much quieter in this regard, so now you hear
mostly engine noise and tail rotor noise.
Mark
I have read that the distinctive beat is because the rotor tips are
exceeding the speed of sound so you get a sonic boom.
Giles
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