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I was wondering whether having a quad tiltrotor would allow for the use of turbofan engine rather than turbo prop. I read on another question that the V-22 uses turbo prop as it is easier to balance, but if there were 4 turbofans then it could surely balance and with the engines closer. Could a turbofan tilt rotor be possible for supersonic speeds and VTOL? And would tilt rotor or tilt wing be better in this application? Also wouldn't turbo fan mean there isn't the stream to worry about between sets of motors?

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    $\begingroup$ I don't think its a limitation of turboprop vs turbofan, but rather redundancy. The V-22 can still fly if one engine fails because it has a gearbox that connects the two engines, so the good engine can power the bad prop. This would be a little difficult with a turbofan engine since (most) of the thrust is generated via combustion gasses rather than the propeller. $\endgroup$ – Ron Beyer Jan 19 '17 at 2:07
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The main reason V-22 uses propellers is efficiency. To get high thrust for power, you need to affect a lot of air and that requires a large propeller. The V-22 has huge propellers, closer to helicopter rotors than typical aircraft propellers. This gives it the static thrust-to-weight ratio of more than 2, so even with one engine out it still has more than 1 and can keep hovering.

At higher speeds, propellers loose their advantage as they don't work well at high speeds, so for transsonic turbofans become better and as the speed increases further, you need to reduce the bypass ratio. But since jets have relatively low static thrust, a jet VTOL needs a really powerful engine.

Now, they can be made. F-35B is a supersonic VTOL and does it with just 1 engine. The engine needs to be really huge for the aircraft (which is the case for all fighters, but not other aircraft classes), but it still gives a static thrust-to-weight ratio of around 1 (F-35B can't do pure vertical take-off at MTOW). There is no way to install two such engines on the plane to get hover ability with one engine inoperative. And the fuel consumption is huge, which is acceptable for fighter, but would not be for transport aircraft.

Last, 4 engines would be actually much harder to balance than 1 that splits the air over multiple jets. This is because jet engines have significant lag in thrust command response and can't be controlled all that precisely. On quadcopters the electric motors respond immediately to control input and in large propeller/rotor aircraft the propeller/blade pitch is used which also gives immediate response, but jet engines need to match their rotational speed to power and that takes too much time. It is easier to split the flow from one engine using variable guide vanes.

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In a way, this has been done already half a century ago.

When NATO decided to try a VTOL strategy, the Harrier was not the only result. There was even a supersonic VTOL fighter, and it used swivelling engine pods at its wingtips, much like the V-22 does today. It was the VJ-101C, developed in Germany and first flown in 1963.

VJ-101C in flight
VJ-101C in flight. Note the opened intake door aft of the cockpit where two additional lift engines were complementing the thrust from the wingtip engines (picture source)

While the VJ-101C used six Rolls-Royce RB145 turbojets, a later production version was planned to use turbofan engines. NATO changed it's mind before the turbojet-powered version was built, however.

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    $\begingroup$ 6 engines? In a fighter? I wonder why this didn't make it to production... /sarcasm. So, I see one at each wingtip, and two in the hatch behind the pilot. Where are the other 2 hidden? Are there actually 2 at each wingtip? $\endgroup$ – FreeMan Jan 19 '17 at 21:09
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    $\begingroup$ @FreeMan: Yes, the engines were spread in pairs over three points. How many legs must a stool at least have so it does not topple over? Right, three! And pairs were chosen for redundancy. Six was actually the smallest sensible number here! $\endgroup$ – Peter Kämpf Jan 19 '17 at 21:48
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    $\begingroup$ Was the redundancy good enough to keep it flying if an engine failed on vertical take-off? The thing had insane amount of thrust, but according to the listed performance, one engine would still be a bit short of being able to hold its third of the weight and in hover the other engines can't help, because the thrust distribution is important. $\endgroup$ – Jan Hudec Jan 20 '17 at 9:12
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    $\begingroup$ @JanHudec: Not for a fully loaded plane, but for one at the end of its mission. An engine failure right after vertical liftoff with mass close to the limit would had resulted in an ejection and a wrecked plane. $\endgroup$ – Peter Kämpf Jan 20 '17 at 20:07
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    $\begingroup$ @Peter kämpf if 3 is needed like a stool then how does the v22 do it? $\endgroup$ – SRawes Jan 21 '17 at 0:26

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