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Why do almost all civilian VTOL aircraft existing today use propellers (or rotors) for take off and landing and not jet engines (as the Harrier does)? Is there a technical reason for this?

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  • $\begingroup$ The V22 Osprey is a good example of a VTOL propeller aircraft.I guess you might also be able to include helicopters too ;-) $\endgroup$ – os1 May 3 '16 at 10:31
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    $\begingroup$ the V-22 Osprey would spring to mind @kepler22b, that and the Convair XFY-1 Pogo $\endgroup$ – GdD May 3 '16 at 10:32
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    $\begingroup$ @M.Amin you should edit your question to clarify you're asking about civilian aircraft. Also, why are you talking about T/O and landing? Are you asking for VTOL only (like helicopters) or STOVL / VSTOL planes? $\endgroup$ – Marco Sanfilippo May 3 '16 at 10:50
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    $\begingroup$ @MarcoSanfilippo - so we have a sample size of 1 (with rotors) and we're asking why there are no jets? Answer: None have been designed/developed yet. $\endgroup$ – Jamiec May 3 '16 at 10:59
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    $\begingroup$ Why was this question held as "unclear", it's perfectly clear, and has been answered exactly... Do I miss something? $\endgroup$ – mins May 3 '16 at 17:48
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Can?

Of course they can.

Does it make sense?

No, it does not.

A VTOL needs thrust at least equal to its weight. That is much more than a fixed-wing aircraft (HTOL) needs. Airliners have lift-to-drag ratio around 18–20. That is in clean configuration; in landing configuration it might be about half (~10) and we need twice as much thrust in case an engine fails, so around ¼ weight. So a VTOL would need 4 times as much thrust as a HTOL.

Likely more, because the craft would still have to be capable of reasonably controlled landing in case of engine failure. And while rotorcraft can do that by autorotation and pilot of Harrier or F-35B can simply eject, jet-powered civilian VTOL would only have the remaining engines for it.

Now fortunately thrust and power are only indirectly related. To create thrust, force must be applied to the air. This force accelerates air, giving it some momentum and some kinetic energy. Thrust is (change of) momentum per time and momentum is mass time velocity while power is (change of) energy per time and energy is ½ mass times speed squared.

So we can produce a lot of thrust with little power by accelerating a lot of air by a little. Which means large propeller: a rotor. That's how helicopters get enough lift/thrust with reasonably small engine.

Unfortunately rotors are not very practical for flying fast. One would also be rather difficult to tilt. That's why V-22 Osprey and Augusta-Westland AW603 use a compromise that is somewhere between; large enough to still have good efficiency in hover and small enough to be possible to tilt.

What would definitely not be efficient for hover is a jet though. Jets are small and can't affect that much air; what they can is accelerate it a lot. That is necessary for flying fast, but it is terribly inefficient when flying slowly.

Military jets don't care. They want so powerful engine to fly very fast anyway and they don't care for efficiency much as decisive edge in combat is worth a lot of money. But for civilian transport the small advantage of being able to land closer to destination would not outweigh the disadvantage of being several times more expensive to operate. Flying the long distance with HTOL to a nearby airport and, if the destination is not reachable by car, hiring a helicopter just for the final part of the trip ends up being a lot cheaper.

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  • $\begingroup$ Thank-you very much Jan for the detailed answer. This actually was the perspective I was looking for. Do you recommend any readings or books about this topic? $\endgroup$ – M.Amin May 3 '16 at 12:54
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The Wikipedia page on VTOL has a whole section on the history of "jet lift" VTOL aircraft. It includes some pictures of the Harrier "Jump Jet", which is probably the most widely recognisable VTOL fixed-wing, and has a jet engine. I think the only reason "almost all" VTOL aircraft use propellors is that almost all VTOL aircraft are helicopters. VTOL fixed-wing aircraft turn out to be quite hard (read: expensive) to get right and not many designs (with props or jets) have made it into production.

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Jet propulsion is impractical for civilian VTOL, because the jet blast quickly erodes even a reinforced concrete landing pad, nevermind an asphalt driveway or a grassy field. Military installations can afford special pads, but that infrastructure cost would be daunting for an aircraft whose advantage is that it's supposed to land "anywhere."

Also, takeoff and landing would be objectionably loud: 125 dB at 100 feet away for the Harrier, for instance.

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Jet engines have already been used for VTOL aircraft, see the Harrier or the Joint Strike Fighter.

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  • $\begingroup$ Yeah, but is there any aircraft in the civilian space which uses jet engines for VTOL operations? Thanks. $\endgroup$ – M.Amin May 3 '16 at 10:32
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There is at least one civilian, jet-powered, thrust-hover aircraft: the Zapata Flyboard Air. This uses tiny (model aircraft size) turbojets to provide thrust for hover flight. It has good safety margins, can fly with one engine out and land safely with two down.

It is completely impractical; it's a toy.

The only other civilian aircraft remotely in this category to date is the various iterations (over the past fifty-five or sixty years) of the Moller Aircar (it's had various names). It's a ducted fan thrust hover craft that started out in the early 1960s looking like a tiny flying saucer and ended, without ever seeing production, around 2000 as a computer-controlled VTOL with transition capability and a purported top speed of 200 mph (though the prototype never got beyond tethered hover, as far as I'm aware).

Getting such a machine certificated with piston (or, for the last couple versions, rotary) engines would have been a nightmare; those engines, at the needed power and weight levels, can't be made highly reliable, which means the FAA won't let them carry people outside the Experimental category.

There was a brief effort to make the Williams "Flying Garbage Can" into a civilian craft; that never went beyond repainting a military prototype in a color other than olive drab -- same issue, reliability, as well as the fuel consumption of an engine several times the size of what's needed to fly with lifting surfaces.

A couple "jet pack" type devices have been built as one-off machines, but again, there's no practical application and they're too expensive and hazardous to make good toys.

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