Originally Posted by
Petit-Lion
I for one think that the most obvious improvement to a conventional helicopter would be a swiveling tail rotor. I am surprised that only Karem ventured into that technology... to no avail so far. Why is it so difficult to implement, compared to seemingly much more complex designs?
Swiveling tail rotor as in, rotating the tail rotor from a horizontal thruster to a largely axial thruster as the aircraft accelerates so it becomes like a propeller on the back of the aircraft (like X-2 or AH-56)? Sikorsky actually built and flew this mechanism (they called it "Roto-Prop") on a modified S-61 used as a technology demonstrator (S-61F). Basic issue why you don't see more of this is for the prop is only more effective than the main rotor at providing forward thrust at fairly high speeds. To really utilize the prop to push the aircraft to speeds a good conventional helicopter cannot achieve requires a lot of power installed, just for the prop, and thus the tail of the aircraft becomes home to the high power transmission system as well as the swiveling mechanism. An edgewise flow main rotor will always have pretty terrible drag compared to a fixed wing aircraft with axial flow propellers. So the power required to hit, say, 250 knots, looks pretty ridiculous for the size of the aircraft.
In the end, you can achieve near 200 knots in a more conventional helicopter without a prop or swiveling prop. If you desire going much faster than that, a tilt rotor ends up being the better answer and really the only answer at around 250 knots and above. So there is a possible range of 200 to 250 knot target cruise speed where some sort of thrust compounded helicopter MIGHT be a better balanced design than the alternatives, but that's an awfully narrow range to spend a lot of development time/effort into. It also requires a lower drag main rotor/hub and aircraft than is typically achievable.
The Airbus Racer or X3 concept is interesting and can achieve high speeds, but the main rotor is significantly offloaded via wings (like other high speed conventional helicopters) and there is no swivel function as yaw control comes from differential blade pitch across two smaller props. This configuration might practically fill that 200 - 250 knot gap, maybe.
Short answer: wings are simpler and cheaper than props to get a helicopter to 200 knots... if you want more speed than that, the tilt rotor appears to be the ideal answer. Airbus is working a hybrid concept that might fill the gap between the two.