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Will commercial aircraft, of a similar size to a boeing 747, ever have jet powered VTOL systems?

Supplement from 02/10/2008 09:20am:
I remember something about a prototype circular aircraft with the engine in the middle : I think it was made in Canada. Anybody know anything about that one?

asked in commercial, aircraft, VTOL



mercman99 answers:

Unless a more economic and fuel efficient engine is developed the sheer weight of a 747 sized aircraft using a VTOL system is unsustainable.
The VTOL combat aircraft,the Harrier, used by the Uk forces and US Marine Corps uses vectored nozzles to alter the jet wash from the engine and create lift and thrust.It is a high fuel use system requiring a great deal of fuel to keep the aircraft in a hover or to take off verticaly.The reason why the Royal Navy have the ramp method of launching is because the fuel used to lift the aircraft in forward motion it considerably less than a vertical take off.This launch method enables the Harrier to carry more payload for a given fuel load and thus increase its operational range and combat available time.
A civilian aircraft would not be able to carry enough passengers or fuel to make the system viable in an economic sense.


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KentPDG answers:

Very unlikely.

In a VTOL aircraft, the engine thrust is pointed downward, either by rotating the engines or by some kind of adjustable channels. For the plane to rise vertically, the combined thrust must exceed the weight of the aircraft.

For the current model of 747, the 747-400, the maximum engine thrust is 58,000 pounds (with Rolls-Royce engines); for a total thrust of 232,000 pounds. The maximum takeoff weight of the aircraft is 875,000 pounds (could be somewhat less, depending on fuel, cargo, and passenger load). But there is obviously a considerable disparity.

If engines twice as powerful could be obtained, and six were installed instead of four, the plane could still not take off vertically. If engines 50% more powerful were fitted, and if ten of them were installed, in most cases the plane could just barely get off the ground.

Even granting that enough engines and enough power could be fitted to the aircraft, the airframe itself could not handle all that stress. Yes, the wings carry 875,000 pounds of downward weight, but that weight is concentrated at the roots of the wings -- where the structure is very, very strong. But in a VTOL configuration, the upward force would be spread out along the wings (or arrayed along the fuselage), so the forces and stress pattern would be quite different.

Yes, it would be possible to redesign the airframe to provide added strength where needed, but strength equates to weight. And the added engines would increase the weight quite significantly. So those would lead to the need for still more power, which would mean more strength needed, and so on.

If sufficiently powerful engines could be provided, there is probably a way to redesign the plane so that the weight and thrust would balance out, and the airframe would not fall apart. So it is theoretically conceivable to have a VTOL 747, but it would be enormously different from any 747 ever built previously.

Even granting that such a plane could be built, all that weight (doubtless, well over a million pounds) would require more fuel to keep it up in the air, and moving through the air. The vertical takeoff would also be a tremendous fuel hog. So it would be a much less fuel-efficient aircraft, but with no more carrying capacity than today's 747. And even though the wings would be much larger (to provide enough lift for all that added weight), it is virtually certain that the airplane's operating range would be reduced.

In short, yes, maybe, possibly, theoretically. But no one would want such an airplane. The effort to develop it would be ludicrously impractical.


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Aiming4777 answers:

There has been a lot of inventive effort put into this concept right since the conception of high speed, long haul aviation back in the post war years. Even more so now-a-days when world runway congestion is an acute and growing problem evoking costly delays for both air carriers and passengers. While aircraft noise and other environmental concerns form the chief restrictions to airport development, pervasive land use and high costs are also factors. This serious situation is forecast to go critical in the early years of the next century so serious consideration is once again being given to this type of aircraft design.

Clearly, there is an urgent need for a large, safe, vertical take-off and landing vehicle operating off single pads at major airports. These VTOL operations, mostly on short haul service, would help free up runways for conventional jets. A major customer for this aircraft is for short, popular flight routes such as New York to Washington.

The unique design of the VTOL aircraft addresses this congestion problem, and is useful as a vehicle for humanitarian and disaster relief. However, despite the research effort put into this design of aircraft, including from military sources where the advantages of being able to land troop or supply aircraft on makeshift or damaged runways are obvious, no real progress has been made.

This is because of the reasons given by Kent in his answer, the engine power needed to lift a fully laden airliner, even a small one, far exceeds that available from conventional engines. In fact, there is a reverse effect, the more engines you add, the heavier the aircraft becomes so the more power you need. There is also the noise problem from the thrust required to lift up a passenger plane without assistance from wing generated lift. Also, these days, it would not be seen as environmentally friendly to use fuel just to lift the plane off the ground to save runway space.

I think the type of aircraft you envisage will only become a reality after the anti-grav magnetic lift motor (or similar) have been invented.


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DragonsDen answers:

Designers, since 1946, have attempted to combine the best qualities of the airplane and the helicopter, while avoiding the limitations that each aircraft presents. The only true successes at this have been British (of course). The familiar Harrier military Jet which was a small aircraft, hardly in the class of a 747 but still had serious problems achieving a vertical take off system. The problem being that the fuel required to lift the aircraft vertically was so excessive that the plane was severely limited in its range especially if carrying any ordnance. Of course, adding more fuel meant more weight.

As a result, even these aircraft were re-designated as “short take off”, the Navy using shorter aircraft carriers with a ramp at the end and the US Marines abandoning their concept of support aircraft landing and being concealed easily near to the front lines. On most missions, the land-based Harriers were refuelled in flight after taking off. Clearly this design would not be practicable for a larger, heavier aircraft.

The other limited success was the "Rotodyne" (see picture) which was a compound autogyro invented by Westland in England, and was successfully demonstrated on test routes in Europe. It proved that it was indeed possible to design a safe VTOL airliner capable of lifting 70 passengers. This was a propeller driven aircraft but with a helicopter style rotor mounted on the top for take off and landing.

Following several fatal crashes of the aircraft during its introduction and trial services, the concept was abandoned, the British Government refusing to fund any further research. The flaw in the design was that, with two engine types, the conventional turbo props on the wings for forward flight and the main rotor engine for vertical flight, the weight needed to be kept to a minimum. This meant that many of the redundant back-up systems fitted to normal aircraft were not fitted, resulting in many of the crashes.
http://en.wikipedia.org/wiki/Fairey_Rotodyne

Again this was a small aircraft and the rotors needed to life a modern airliner off the ground would be multiple in number and immense and probably cause a problem in forward flight mode. Recently a similar aircraft has been developed as a joint venture between Bell Helicopter Textron and AgustaWestland, the Bell/Agusta BA609. This is a civil twin-engined tiltrotor VTOL aircraft but only carries a few passengers. It is really aimed at the executive market to give a helicopter that can travel at similar speeds to a private jet once aloft.


Supplement from 02/10/2008 07:55am:

Despite these minor successes, the air carrier industry still seems to doubt that a safe, fast, comfortable, 145-seat VTOL commercial airliner can be designed. Certainly nobody has even speculated about a plane any larger than this. Many designers believe that such aircraft cannot be designed with backups such as two and three hydraulic systems, redundant pumping, two electrical systems, etc., which are common on conventional jet transports.


Supplement from 02/10/2008 11:56am:

The aircraft you ask about in your supplement is the Avrocar, designed in the 1950s. Despite, yet again, having an English designer, it was an abject failure. It couldn’t lift its own weight safely more than a few feet off the ground and was not capable of high speed forward motion despite unbearable noise and heat. The heat was so oppressive that the test pilots flew it with the canopy off and all the instruments were baked brown and had to be replaced after just a few flights!

Despite the failure of the design, more of a hover car in practical terms than a VTOL aircraft, it only needed a rubber skirt to trap the downward thrust and they could have invented the first hovercraft, not achieved until 1959

Source: http://www.virtuallystrange.net/ufo/mufonontario/avro/avrocar.html


Supplement from 02/13/2008 12:10am:

I hope that's somebody walking along behind the craft and it hasn't got a "Flintstone's" under carriage!


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wumpus answers:

It seems unlikely.

There is considerable risk associated with an engine failure of a hovering aircraft; it can't glide and tends to plummet somewhat.

That level of risk can be acceptable in a single or twin seat combat aircraft fitted with ejector seats and parachutes. But it's not really feasible for maybe 200 passengers to eject from a falling aircraft.

As DragonsDen rightly points out, there have been many attempts at vertical flight for passengers, but they tend to rely on helicopter or autogyro principles because it's safer. Following an engine failure, a helicopter can still glide to a safe landing, using autorotation.

Most pilots will tell you that the Harrier is an extremely difficult aircraft to fly. When hovering, you're trying to balance the entire aircraft on a single "spike" of jet thrusts, with only some manouevering ducts to keep you balanced.
Also, the Harrier doesn't have much spare lift capacity, I've heard that it can't hover above 3000feet altitude due to the thinner air.

The F35 JSF has a far better design; it uses a bendy duct (Russian designed!) to divert the jet downwards at the back, while at the front it has a large cold-air lift fan, covered when in normal flight.
This configuration requires a monster of an engine, and should provide a better hovering capability than the Harrier. Also fly-by-wire should make it a far safer aircraft. But again, it's only a small fighter/strike aircraft.

As far as large VTOL aircraft go, there's only one in use by NATO at the moment - the V-22 Osprey tiltrotor. This can carry around 24 troops.


Supplement from 02/10/2008 09:19am:

A German company is currently trying to resurrect airship technology, using a helium-filled design with directable ducted fans for manoeuvring. This would have VTOL capability, along with a very large payload capability, and low running costs.

While it's not as quick as a Jumbo, it's still fairly nippy.
With suitable facilities made available it might be suitable for passenger variants - and could carry many passengers.

http://news.bbc.co.uk/1/hi/sci/tech/769642.stm


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