riff-raff,
A variable speed drivetrain is much easier to do than a variable diameter rotor.
Totally agree.
IMO, the engine should be operating near its optimal RPM at all times. During hover the rotors should be receiving 100% of the allocated power, and, this power allocation should smoothly transition, so that the
Xrotors areX propeller is receiving 80-90% of the allocated power during cruise.
A major limitation is that existing variable power transmission devices, which would control this allocation, can consume a meaningful percentage of the engine's power.
This web page is the conceptualization a potential variable controller for an ABC helicopter. Theoretically, it will not consume power during hover and during cruise. However, it will consume approximately 3% of the power at the mid-range between hover and cruise. It also has some detail concerns. It was openly presented 5-years ago so that anyone can freely copy it, or improve on it.
One might hope that Sikorsky would openly or secretly develop a variable controller. One might also assume that this 'unexciting device' would be developed and tested before completing the envelope to wrap around it.
Dave