Dumb fixed-wing guy question. The absolute top-end speed of a TACAIR jet isn't a be-all and end-all performance metric, thanks to Col Boyd and E-M theory. I know helos are often valued by how much they can lift rather than how fast they can go unloaded. And that they have all the funky aero and physics issues incumbent in the spinny thing on top that limits their top end speed. What practical purpose is Bell designing for that happens to show up in a 200 knot helo?
The top end speed of a cargo airplane is a more appropriate analogy. As said above, it is speed from A to B - and the greater the speed, the greater the range. As the most important limitation on top speed is not drag but rather retreating blade stall, conventional helicopters are normally limited to below 200 knots (140 - 170 is normal). If you want to go substantially faster, there are 3 different ways:
1) Tilt rotor (MV-22, AW-609, V-280) - fastest, longest range but less hover performance (the ability to land at high altitude), less payload, and by far the most complicated / expensive. Not too many militaries or civilian operators can afford it.
2) Advancing blade concept (ABC) (contra-rotating main rotors with a pusher propeller). Sikorsky X-2 and Sikorsky S-97 - substantially faster than conventional helicopters but less than a tilt-rotor. Solid hover performance. Contra-rotating main rotors would seem to limit the upper end of size as the rotors have to be close together to limit drag but far enough apart for them to flap without hitting each other. Sikorsky seems to think they can make the rotor system stiff enough to get up to 30,000 lbs. I would bet this would be the future.
3) Compound helicopter (main rotor + wings + pusher or tractor propellers) - Eurocopter X3, Lockheed AH-56 Comanche - roughly same performance and capabilities as the ABC. Less complicated than a ABC.
All 3 are competing for the FVL - Future Vertical Lift - which is due out in 10 to 15 years.
I was asking a similar question earlier in the Osprey thread. Perhaps the way of matching a tilt-rotors range is to use a bigger helicopter - more lift thus more payload (fuel). The MH-53E with 22,000 of gas normally (about 7 hours) and the MH-47G with double the fuel of a regular Chinook seem to have close to the range of an Opsrey - the tradeoff is what the the helicopters lack in speed they can make up with a bigger payload downrange.