[justlurking]

I think you guys missed the important part: I couldn't see any rotor pitch control in this prototype. That's a huge mechanical complexity in helicopters and the V-22.

It was completely controlled with differential thrust: left/right sides control roll, front/back control pitch.

[NorthMountain]

It was completely controlled with differential thrust:

That's also how quadcopters work.

[Yo-Yo]

It was completely controlled with differential thrust: left/right sides control roll, front/back control pitch.

Something trivially easy to do in small scale (see the myriad of RC controlled quad copters) but is almost impossible with a full scale aircraft because of the required thrust and throttle response. Full scale helicopters use pitch control because it reacts faster than throttle control.

The thing that killed the XC-147 was the wing flex being incompatible with the cross shaft that connects all four engines together. The cross shaft is vitally important for an engine out situation.