The problems which have been experienced by the V-22 Osprey are a
reminder of one very successful alternative: the Fairey Rotodyne. See:
http://www.groenbros.com/tech/FaireyRotodyne.htm
This was a large, passenger gyrodyne which had a separate lift rotor
and two turboprops mounted on a short wing. For take-off and landing,
gas from the turboprops was diverted to jets at the tip of the lifting
rotor, providing the thrust to spin it. For level flight, the
turboprops drove conventional propellers with the autorotating rotor
providing about half the lift.

Disadvantages compared with the V-22? Separate systems for vertical
and level flight.

Advantages compared with the V-22? The lift rotor and propellers were
designed to be optimal for their tasks, instead of being a compromise.
The autorotating ability of the big lift rotor provided a safety
margin. And the whole thing was technically simple and trouble-free.
It WORKED - decades ago! Its only really problem was noise from the
tip-jets, but that would be far less of an issue for a military plane
and they were working on that anyway. It was only cancelled due to
political/industrial reasons.

The company whose website contains the info listed above is proposing
developing new gyrodynes by converting ewxisting fixed-wing planes -
notably, the C-130 Hercules! This seems like a much lower-risk
approach than tilt-rotors.

Tony Williams
Military gun and ammunition website: http://www.quarry.nildram.co.uk
Discussion forum at: http://forums.delphiforums.com/autogun/messages/