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Future air transport
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Little John



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PostPosted: Thu Sep 28, 2017 3:04 pm    Post subject: Reply with quote

Am I right in thinking that as long as you get enough altitude, it should be possible to glide any arbitrary distance. Thus, a glider passenger aircraft service across, say, the English channel should be possible so long it is initially towed to an appropriate height.

Assuming that height is too high to be done in a single straight line, presumably it could be done in a large circle.

Is all of that just a daft conjecture or would it work? I don't actually know the altitude involved.
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Little John



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PostPosted: Thu Sep 28, 2017 3:09 pm    Post subject: Reply with quote

Continuing with this conjecture - one could even have it set up so that the towing line would be initially held aloft with a helium balloon. Then, a glider aircraft could attach itself to the bottom of the towing line at the ground and slowly elevate up the line till it reached a specified height and then release itself from the line and glide away. Meanwhile, other gliders could be climbing the line behind in a queue - each one releasing as it reached the specified height.

The balloon, mentioned above, would be needed to get the line to a specified altitude initially and so would be responsible for holding all of the line's weight. However, once the gliders were making their way up the line, they would progressively take the majority of that weight as a function of their lift.
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Little John



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PostPosted: Thu Sep 28, 2017 3:22 pm    Post subject: Reply with quote

Actually, an amendment to that is that you wouldn't need a balloon on the top of the line. You would just need a bloody big set of wings acting as a permanently stationed glider at the top of the line to hold it taught.
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PS_RalphW



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PostPosted: Thu Sep 28, 2017 3:52 pm    Post subject: Reply with quote

I used to work in aviation research. Gliding range is limited by the practical height you can get to - the air get thinner and you need more speed to avoid stalling. At 35,000 feet (jet cruising altitude) you can glide 2-300 miles in a jet aircraft. Gliders travel further by finding thermals - patches of rising air - which lift them higher. They are also designed for minimum drag, but low speeds. When I used to fly London to Paris the jet would climb to 28000 feet and then immediately start it's glide descent into Paris on engine idle. Also, at high altitude winds are faster - it is up to 2 hours quicker New York to London than vice versa because of the jet stream.

A glider won't stay airbourne forever unless it can find rising air. I think you need to revise you your basic energy calculations.

Unless you can build a space elevator, of course. Razz

[edit]

correction
A jet aircraft will descend over a range of 2-300 miles with engines running on low power. They still input energy into the system. A glider will have an ideal glide angle of 20-1.


Last edited by PS_RalphW on Thu Sep 28, 2017 4:06 pm; edited 1 time in total
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adam2
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PostPosted: Thu Sep 28, 2017 3:54 pm    Post subject: Reply with quote

Little John wrote:
Am I right in thinking that as long as you get enough altitude, it should be possible to glide any arbitrary distance. Thus, a glider passenger aircraft service across, say, the English channel should be possible so long it is initially towed to an appropriate height.

Assuming that height is too high to be done in a single straight line, presumably it could be done in a large circle.

Is all of that just a daft conjecture or would it work? I don't actually know the altitude involved.


It would only work for relatively short distances. All fixed wing aircraft can glide to an extent that is determined by the "glide ratio" of the type of aircraft used. A fairly typical glide ratio for large passenger jets is about 10% meaning that for every mile of altitude gliding for about 10 miles is possible.
Crossing the channel is just possible given a great enough initial altitude.
Nothing would be gained by taking a circular path, unless this path took the aircraft into a strong up draught.
I very much doubt that gliding would be allowed for routine passenger transport.
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adam2
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PostPosted: Thu Sep 28, 2017 3:59 pm    Post subject: Reply with quote

Little John wrote:
Actually, an amendment to that is that you wouldn't need a balloon on the top of the line. You would just need a bloody big set of wings acting as a permanently stationed glider at the top of the line to hold it taught.


A tethered glider wont work, they glide by passing through the air, anything tethered is more like a kite than a glider. And either option would fall to earth if the wind dropped.

A balloon is a theoretical possibility but I suspect that the size and cost of the balloon would render the idea non viable.
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Little John



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PostPosted: Thu Sep 28, 2017 9:19 pm    Post subject: Reply with quote

adam2 wrote:
Little John wrote:
Actually, an amendment to that is that you wouldn't need a balloon on the top of the line. You would just need a bloody big set of wings acting as a permanently stationed glider at the top of the line to hold it taught.


A tethered glider wont work, they glide by passing through the air, anything tethered is more like a kite than a glider. And either option would fall to earth if the wind dropped.

A balloon is a theoretical possibility but I suspect that the size and cost of the balloon would render the idea non viable.
It wouldn't drop if it was being constantly towed in a large circle by the line
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woodburner



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PostPosted: Fri Sep 29, 2017 12:35 am    Post subject: Reply with quote

Very efficient gliders have a minimum sink rate of around 3 feet per second. Your glider holding the line taut would still be decending. It looks like you are imagining a perpetual motion machine. Also consider the weight of the line. Go to a gliding site and sit in the winch. The cable is a frightening piece of equipment when it starts dropping even with a parachute on. When you see how much is on a drum, you might imagine it would be a tad heavy. That is for a small aircraft at low speed, and it might be about 3000 to 5000 feet of cable. Imagine the cable needed to pull a passanger aircraft at 200 knots to 30,000 feet.
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Little John



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PostPosted: Fri Sep 29, 2017 1:52 am    Post subject: Reply with quote

If the line is towing it, it would not be descending because the forward motion of the glider would be powered by that tow. Otherwise, towed launches would not work, would they. There is no perpetual motion here. The energy for the lift on the glider is coming from the energy it takes to provide the tow on the line.

As for the weight on the tow line, that is of course a potentially technologically limiting factor.

There are two questions here. The first one is do the laws of physics preclude such a system? The second is what are the current technological limitations on such a system?

The laws of physics question is the primary one that needs to be answered first because, if the answer is negative, the technological one is moot.
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woodburner



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PostPosted: Sun Oct 01, 2017 10:11 am    Post subject: Reply with quote

Quote:
.........acting as a permanently stationed glider at the top of the line to hold it taught.


You made this statement, then talk about a powered tow. Which is it?
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adam2
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PostPosted: Sun Oct 01, 2017 11:45 am    Post subject: Reply with quote

woodburner wrote:
Quote:
.........acting as a permanently stationed glider at the top of the line to hold it taught.


You made this statement, then talk about a powered tow. Which is it?


I was imagining a tethered glider at the top of a long tether, and that the lower end of the tether would be attached to a powered vehicle on the ground.
That should work in theory, but I see insurmountable practical problems.
The ground vehicle would have to be of very considerable weight and power, and have to run in a very large circle, many miles across.
An HGV would be ruled out by fuel consumption, and the need for regular replenishment.
That leaves a purpose built electric railway. Apart from the cost and the nimbyfests involved, just where could you build a new railway perhaps 30 miles in length, and WITHOUT A SINGLE BRIDGE OR CABLE over any part of the route.
And what happens when the railway breaks ? Does the glider break free and land automatically at an airfield ? or crash to earth to the detriment of persons and property below ? And the 2 miles of heavy cable where is that to land ?
That just leaves the problem of attaching your passenger aircraft to the moving train, hoisting it up the cable and releasing it to glide to France.

I have got a better idea. Build the railway instead in a tunnel, under the sea from England to France. Passengers could then travel in a train instead of the hazards and complications of being hoisted up on a fast moving cable and then gliding across the sea.
The glider would have a limited range and require a change to some other mode of transport near the coast.
The train could by contrast go anywhere on the existing rail network. Scotland to Spain ? why not !
Such a tunnel might be considered an undue fire risk, and would not exactly be scenic. A rail bridge over the Channel could be considered as an alternative. No risk of smoke build up, fresh air and a sea view. In the event of accident or emergency, rescue by sea or air could be arranged.
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Little John



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PostPosted: Sun Oct 01, 2017 12:35 pm    Post subject: Reply with quote

woodburner wrote:
Quote:
.........acting as a permanently stationed glider at the top of the line to hold it taught.


You made this statement, then talk about a powered tow. Which is it?
Both. It is permanently stationed at the top of a line that is towing it. It's not complicated.
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Little John



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PostPosted: Sun Oct 01, 2017 12:45 pm    Post subject: Reply with quote

adam2 wrote:

I was imagining a tethered glider at the top of a long tether, and that the lower end of the tether would be attached to a powered vehicle on the ground.
That should work in theory, but I see insurmountable practical problems.
The ground vehicle would have to be of very considerable weight and power, and have to run in a very large circle, many miles across......
Yes. I can imagine the technological problems may well make it impractical.

However, one point I would make about the size of the circle at ground level: I am wondering if it may not need to be as big as you suggest. That is to say, the arc of the circle at ground level may be relatively small, whilst the arc of the glider at the top, may be much larger depending on the length of the tow line.

Or, even, we could get even more exotic (but still within the realm of existing technologies) and have the aircraft at the top of the line powered electrically. So, it would no longer be "towed". But would, instead, be pulling the line taught itself. Given this craft would be flying in a very specifically defined circle, A device at ground level could be set up to provide continuous power via lazar vertically up to the craft. Thus enabling it to fly for an arbitrary length of time. Or, less exotically, the power could be provided directly up the line itself to the craft.

As for how to get the other craft up the tow line and then raised to appropriate height., This would need to be on some kind of roller mechanism that attached to the line at the bottom and then winched itself up the line. One it reached the appropriate height, it would release the winch and glide away.

As for the line it self. If the tethered glider (or powered craft) at the top became accidentally detached, then some kind of parachute device at the top of the line (or series of parachutes down the length of the line) would need to be deployed to reduce the speed of fall of the the line

Again, I do understand the massive technological issues involved. But, I am just trying to establish if there is any in principle bar to this kind of system.
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Little John



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PostPosted: Sun Oct 01, 2017 1:54 pm    Post subject: Reply with quote

Or, even simpler, an electrically power quad-copter type aircraft at the top of a vertical line with it's power coming directly up the line - so able to fly for an arbitrary period of time.
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kenneal - lagger
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PostPosted: Sun Oct 01, 2017 2:04 pm    Post subject: Reply with quote

A glider tow line is of limited length and will only tow the glider to a certain height. When that point is reached the glider has to detach itself or the forward and upward motion of the tow line ceases and the glider is dragged to the ground. On release the glider then starts on a slow glide, at about 1:20, to the ground or until it finds a thermal air current to lift it up again.

My initial suggestion of using a winch was aimed at removing the requirement for an electric aircraft to carry sufficient battery power to get the aircraft into the air, such power being an order of magnitude greater than the power required to move the craft through the air. The winch would only need to supply part of the power requirement of the aircraft as the on board batteries and motors would supply the power for the actual flight.

I was not advocating gliding for the whole flight. I think this point has been lost somewhere in the conversation.
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