Shy and Dave,
Dave's premise is right, the single rotor has a fundamental limit at about 250 mph, and a practical limit at about 210 mph (practical means this is all the aircraft will do in commercial service).
The problem is not beating that number. It is maintaining commercial viability while beating that number. Fuel flow, payload, parts count all count in the real equation.
I think there are coaxial configurations that will serve, but they do not involve low offset dual systems where the down sweeping blades are free to flap. That is because the downsweeping blade will flop around and create real problems for the designer as it goes into stall/reverse flow. Wild flapping will cause major problems.
The rotodyne is very ineffecient in hover (with its tip jets running) and the jets create drag at high rotor tip speeds. If that drag can be reduced with better nozzle shapes, it might be possible to create a high speed vtol rotodyne with very brief hover capability. The rotodyne only flew at 165 knots, fast for its day, but no barn-burner!
Last edited by NickLappos; 9th April 2005 at 13:55.