Having read Nick's et al. patent, would it be fair to comment that the collective servo intended for autorotation entry could benefit efficiency too?
My thinking is that the collective servo system would gently "resist" collective change until engine could respond to maintain required RRPM. This would be near instantaneous in a piston helicopter, but long enough in a tubine to keep safe TET. The original intention was as a safety feature, since the system automatically always maintains constant RRPM. A failing engine would automatically result in a lowerd collective in order to maintain RRPM.
Since this is part of the engine govenor system, it would be very easy to vary RRPM as well as torque with collective position. Naturally an "engine response model" would be required in the system, but it would be easy enough to include change of RRPM as part of that response. With some thought, i actually feel it could be done with relatively inexpensive analogue op-amp CR circuitry. This means such a system is not limited just to Comanche levels of sophistication.
Pilot would basically just feel a mild resistance to collective increase while RRPM gained a safe margin. Naturally the response/margin would need to be fast enough to avoid collective PIO in ground effect.
Mart