Dave, i would say there are three contenders for high speed commercial VTOL:
1. Compound Helicopters (Which the Reverse Velocity Rotorcraft ended up as)
2. Tiltrotors
3. ABC Helicopters.
Any good engineer will tell you that the key to efficient VTOL is low disk loading - which means as large a disk diameter as practical, thus a compact rotor layout. When you take into account the down thrust associated with wings, this only leaves the ABC as the best contender IMHO. The complexity of that coaxial rotor is just the price you have to pay for the increase in performance.
ShyTorque, that's an interesting point about larger helicopters needing all of the infrastructure of an airport anyway. What size from your experience means that fixed wing is the best option? I'm guessing S92 is about as large as a high speed commercial passenger helicopter would get to. For ABC there are rotor system mass advantages with size, since you need 6 or 8 blades in total anyway.
Last edited by Graviman; 31st Mar 2008 at 11:39.