Savoia,
XC-142 was really a Tilt-Wing rather than a Tilt-Rotor. While it was tested extensively and advanced concepts were proposed, it didn't go very far for a few reasons.
For one thing, in vertical mode, you were essentially pushing the largest airbrake in the world through the air, meaning less efficiency During transition, you're flying with a partially stalled wing and rotors that aren't at maximum efficiency, which limits the kinds of maneuvers you can do during that phase. Also, until transition is fairly far along, you're not getting much benefit from the wing. There's also a pretty high disc loading, which can lead to the autorotational characteristics of a brick. And, of course a tilting wing is heavy and complex.
Bell's concept stems from the realization that you don't really need to tilt the whole wing, just the rotors. The wing starts generating lift as soon as there is any forward motion, transition either way does not impose any extra maneuvering limitations, the whole thing weighs less, etc.
For JMR, the proof will be in the pudding...
Last edited by Commando Cody; 17th Apr 2013 at 19:38.
Reason: spelling