Originally Posted by
Ian Corrigible
the difference here is that the aircraft is using the same engines to mechanically drive both the rotor and the propulsors, in a similar vein to the Piasecki 16H/VTDP, Cheyenne and X2/S-97/SB>1. The XH-51A (and Bell 533, NH-3A, S-72, XH-59A & YUH-2, etc.) used auxilliary engines which imposed a weight/fuel burn penalty,
Good point - I'd overlooked that.
while the Rotordyne (and SO.1310, XV-1 & XV-9, etc.) used tip jets, with the associated acoustic issues.
True, but (as I alluded to previously) only when hovering or transitioning. In transient flight the rotor autorotated, so the penalty was the small additional weight of a fuel lifting pump & valves, pipes in the blades and the tip air-kerosene rockets themselves (which were tiny) plus some additional rotor drag from the non-operating tip motors. I would guess that this would be substantially less weight than the transmission systems for the single-engine alternative. The compressed air supply to the tip rockets was tapped from the compressors of the turboprop "thrust" motors, so what constitutes an "auxiliary engine" is a matter of definitions.
But as you say, the tip-rocket noise at the hover was completely unacceptable for something that was hoped to find a niche in ferrying between inner-city locations and airport hubs, and that's what killed it!
Of course the Fairey Gyrodyne (the original concept demonstrator) used a single engine to power both the rotor and the "thrust" propeller, while the Jet Gyrodyne that followed it used a single engine to drive the twin thrust propellers and the compressors (three Merlin superchargers, I believe!) that supplied the air feed to the tip-mounted air/kerosene rocket motors, so arguably both of these also used a single engine rather than a separate auxiliary engine. I believe the jet gyrodyne also used differential pitch on the thrust propellers for yaw control, but I could be wrong on that.
PDR