@Ken Ryan, supersonic rotors
"This could find broad application commercially in several respects - reduction of noise in helicopters (rotor tips are usually supersonic), the windmills noted above, turbine blades, and so on."
I believe this to be false - the major limiting factor in helicopter top speed is that as the speed of the copter + the speed of the leading rotor blade crosses the sound barrier, there is a catastrophic loss in lift generated. Couple this with retreating blade stall and you have a bird in trouble.
The characteristic "chop chop" sound of a helicopter is created by interacting tip vortices between the main rotor and tail rotor. This is why fancy NOTAR helicopters are used in cities, as they are quieter (and safer, with no tail rotor to worry about striking buildings).