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Flying Trains

Planes without fuel? or trains without tracks?
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Summary: Planes carry a large amount of fuel for long journies (comparable to the total weight of people and cargo). Some electric trains on the other hand get their power from overhead power lines, but don't have nearly the same speed as planes due to limits on the tracks. Combining the two, transmitting power to via wire to a low altitude aircraft could overcome both problems.

Detail: Currently the fastest trains are based on maglev technology. These are capable of speeds in excess of 500km/h (about half the speed of a 747). Unfortunately maglev technology is extremly expensive. Conventional trains are limited to much lower speeds by the quality of the track.

The idea would be to have an long skinny aircraft (the shape of a train but with small wings attached down its whole length) fly following the overhead power line. This produces significant weight savings since the craft no longer needs to lift the weight of the fuel.

Obviously for a high speed system it would be necessary to have a computer do all the piloting (it would be very difficult for a human pilot to maintain contact with the power line) but with a wire to follow this should not be difficult.

Flying close to the gound has an added bonus of reducing lift induced drag through the use of ground effect (http://www.avweb.com/news/airman/185905-1.html) On the other hand it also increases the drag on the fuselage due to increased air pressure compared to that which comercial air lines experience. But then we can reduce the fuselage drag my makeing the craft much longer and skinnier than aircraft are normally made

So how fast can the craft go? Well I guess that all depends on how much you want to spend on electricity to power it. Maglev trains achieve 500km/h but I don't see why this limit exists. Afterall planes have not trouble going faster than sound. At supersonic speeds you can use a ram jet, only insead of attempting to use combustion to heat the air, you use an electric arc and the flame wouldn't keep going out when you tried to go to fast.

In general planes are not allowed to fly at supersonic speeds over land because of noise constraints, and choose not to fly supersonic over sea because of the extra fuel costs. A train traveling close the the ground at supersonic speeds on the other hand will not create a sonic boom that spreads out over a large area of land, instead the area affected by the boom will be relativly small, and the boom will be reflected off the ground into the air. Infact it may be possible that the train will generate lift and save even more power by surfing on its own sonic boom. Even if its not possible to save power in this way, the use of electricity rather than jet fuel may produce enough savings to make supersonic travel practical over land at least.

So could the scheme be applied to intercontinental travel? Well it would be difficult to run overhead power lines over the ocean, but not necessarily impossible. It may be possible to suspend the power lines from blimps that themselves use power from these lines to hold their position. It would end up depending a lot on how much tension you can put in the power lines, which would determine how close you needed to space your support structures.

What about catastrophic failure? With a high speed craft flying close to the ground the possibility of a nose dive in the event of say an explosion on top of the train is a scarry possibility. Computer or sensor failure is just as scarry. Engine failure would presumably result in the craft skidding to a halt on the ground just bellow. On the other hand the engines will be much less complicated than a turbo jet and far less likely to fail.

smeghead, Apr 01 2004

Ground Effect http://www.avweb.co...irman/185905-1.html
URL from [smeghead]'s article. [jutta, Oct 04 2004, last modified Oct 21 2004]

Wikipedia: Ekranoplan http://en.wikipedia.org/wiki/Ekranoplan
Ground effect air craft. [jutta, Mar 01 2008]


       how does it stop??? Then what....it falls over?
hatch, Apr 01 2004

       Yes, an ekranoplan following its own power lines to minimise fuel costs would be an effective way to describe it.   

       hatch: you still need an undercarriage at the begining and end of a "flight" just like a plane :-)
smeghead, Apr 01 2004

       Sounds like an upside down flying tram to me. Failure of engines etc leaves it sitting on the power lines? Definitely "scarry".
unclepete, Apr 01 2004

       Couldn't you fly underneath the wires instead? You might have to build them a bit higher, but it would avoid crashing into live cables.   

       Or maybe you could have a very fast (possibly unmanned) train running along under the power lines and beaming power up to the airplane via microwaves.
kropotkin, Apr 01 2004

       What propulsion system are you using again? An electrically powered jet engine? I wasn't aware that such a device was able to produce any significant quantities of thrust. You will be stuck with heavy electrical motors and propellors, so no chance of that sonic boom. Also, have you considered the influence of changing weather conditions? It might be possible to fly a large aircraft on a very specific heading at low level in good weather, but in a storm (or even a light changeable breeze) keeping contact with the electrical supply cable will be next to impossible.
suctionpad, Apr 01 2004

       The proposed propulsion was electric motors driving turbines at low speed (below mach 1) and ram jets with electric arcs rather than combustive heating at high speeds. As for not producing thrust, its not so much that electric engines of any sort can't produce meaningfull thrust, but rather that the weight of batteries they usually have to lug along is excessive. Hence power transmission via wires. I believe (and this is infact what started me thinking about electric arc ram jets, which lead to this idea) that supersonic wind tunnels usually use an arc heating system (primarily because lacking a flame, they can't be blown out by air moving at high speed).   

       As for weather conditions, I did consider the problem. I decided to ignore it without performing any actual calculations just on instinct. The reason for this is a) because the aircraft is so close to the ground, and infact the ground in integral to the aerodynamics of the craft, the conditions are likely to change far more slowly then for a craft in the air. b) if a human can hold a small light fighter craft on a small target for midair refueling, a computer should have no problem holding a much larger (more inertia) craft in position to draw power from a cable
smeghead, Apr 04 2004

       it would be bitchinglyhard to do, and would need an all new rail network. By the time we have one of those we may also have fast enougn computers to do the course holding in real time.
modge, May 07 2004

       Well, I’d vote for old-fashioned electric propellers, like my granddad used to fly. The Electric Jenny, he called it. Yep. Had a power line attached. All he could do was circles, but people came all the way from Cincinnati to see it. One day, he said, one day there’d be some genius come along and make a power line long enough to go from Detroit to Chicago. We was young then, and we believed him.
ldischler, May 07 2004

       Perhaps some sort of tether system... The aerotrain would have a power cable permanently affixed to its fuselage, with the other end being affixed to the main powerline by a dovetail type of joint. This connection would slide the length of the main powerline as the aerotrain traveled. A configuration such as this would allow for sufficient slack to adjust to atmospheric conditions and very slight course deviations.

Of course, at the point where you're going through the time and expense of slinging cables from city to city, why not just make the train gravity powered? Just have 2 giant zip lines angling sharply downwards towards their respective destination cities. Then the only power you'd be using would be in getting the passengers up to the departure terminals and braking the damn thing at the destination.
justaguy, May 07 2004

       Fact is, if you had a frictionless maglev car in an evacuated tube, cutting through the earth between two cities, the trip would always take about 45 minutes--and ideally no power would be required. Actually, there'd be some--for the mag units and for keeping the tunnel free of air.
ldischler, May 07 2004

       I love Idischler's Idea! Get rid of the rail, But it would be good to have a tunnel. then you wouldn't have to use extremely hightech guiding systems. You won't be able to get a completeley evacuated tube, but you could reduce friction. Not having to carry fule would be great. You could produce the energy using various sources of alternative energy along the path of the tunnel also, and just feed it into the line. Having a tube takes the weather variable and expensive guiding equipment out of the haystack.
fity, Dec 21 2004


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