IFMU
Thanks for considering the question of one-speed versus multiple-speed rotors.
I can agree that a multiple speed rotor will allow a system to be 'tweaked' to improve its efficiency, slightly. This is the intent of Karem's patent US 6,007,298 'Optimum Speed Rotor'.
However, it appears that there is not any significant justification for two-speed rotors. A couple of statements by Nick, which are mentioned in your posting, may need clarification. Nick can correct me if the following is wrong.
Originally Posted by NickLappos
It pulled 2.5 G's at 25,000 feet and cruised at over 250 mph, as the thrust engines pushed it along in autorotation (it was an autogyro at high speed!).
I believe that the craft was not in autogyro mode, nor was it in helicopter mode. It was probably in a mid-mode where the two internal turbines were driving the rotors for lift and the two external turbines were propelling the craft forward.
Originally Posted by NickLappos
A turbine helo has way too much blade chord for hover (it throws away power) because it needs that chord for higher speed forward flight.
I believe that the above statement refers to a conventional rotor. In this situation the rotor is the sole provider of lift AND forward thrust. Therefore, the rotor must provide more thrust in high-speed cruise than in hover. To achieve this greater thrust the chord was increased, because the RRPM could not be increased.
These new ABC helicopters will have rotors and propulsors (propellers etc.). The propulsors will provide all or most of the forward thrust. The rotors will basically provide only the lift and this 'lift' component should not significantly vary between hover and high speed cruise.
If this lift does not vary (climb & manuvering excluded), then there is no reason why the RRPM or the blade area should be varied. The
Calculations re Constant Speed Slowed Rotors suggest the preference is single-speed rotors.
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And furthermore
If Sikorsky had been able to fix the RRPM of the earlier XH-59A ABC at its slower rotational tip speed of 450 ft/sec and then calculated the optimal solidity ratio, I believe that the craft would have experianced these attributes:
1/ Silent rotors in all flight modes.
2/ A 50% reduction in the very strong moments and forces, which are caused by the opposing gyroscopic precession of the two rotors.
Dave