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A glider is able to fly unpowered indefinitely so long as it can exploit nearby thermals and the like to stay aloft.

The world record is 3,008km so perhaps there is a practical limit. I imagine this is limited by the pilot and weather conditions but perhaps there are other limits. I assume that a computer controlled glider with sufficient weather information could fly indefinitely. But feel free to attack this assumption.

Are there any limits in scale for gliders?

We also have many designs for airborne wind turbines. All of these that I've seen are tethered. I think this is both to exploit the difference in air vs ground speed to generate electricity and to keep the generator in one place.

If we removed the tether could we create wind powered devices able to stay aloft indefinitely?

I think this is just a matter of engineering but is there a theorhetical reason why this would not work?

For example this question suggests trying to explot air power in a plane is impractical.

For example could we create a cloudbase (i.e. a aerial platform) using the glider princple and scaling up? I presume it would not be able to stay on station permanently but would have to move with the wind or do something like circle around the jet stream.

I imagine there is a limit to the maximum area and maximum density* of such a platform.

Likewise could small drones re-charge in the air? I find it easy to imagine a drone keeping itself charged if it goes high enough but harder to imagine it being able to return to base rather than being blown far far away.

So to questions with a definite answer:

  1. Is this possible (I think yes in principle though an engineering challenge) An answer could refute this.

  2. If yes, what limits the maximum area and maximum density* of such a platform.

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The bigger you are, the heavier you are and the tendency you have to need to fly faster and have a larger turning radius. One of things limiting the size of a sailplane is that you need thermals large enough and strong, or ridge lift strong enough to support you.

"I assume that a computer controlled glider with sufficient weather information could fly indefinitely. But feel free to attack this assumption." We have real life examples: Birds. Thermal soarers and the albatross, in particular. As long as it doesn't starve, that is which is the equivalent of control power for aircraft.

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Thermals only exist during the day, over land, and in suitable weather conditions.

The record breaking glider flights do not rely on thermals, but on the upflow of wind blowing over a hill or mountain. That flow does continue at night, but depends on the weather conditions. The upflow extends to a much greater height than the hill itself. For example in the UK "high mountains" are typically only around 3,000 to 4,000 feet, but a glider that is "ridge soaring" can climb to altitudes where the pilot requires oxygen, and the flight plan needs to avoid commercial aircraft flying at 17,000 feet or higher.

AFAIK the UK no longer records long duration glider record attempts, because the only technical challenge is how long the pilot(s) can stay awake and still be capable of making a safe landing at the end of the flight. The historical world record for a solo glider flight was 56 hours, recorded in 1956. The record for a two-seater glider was more than 70 hours, in 1961.

Birds like the albatross do not use thermals but use "dynamic soaring" which uses the wind gradients in the boundary layer close to the ocean surface. That involves very low level flight, which is fine for a bird that can land anywhere on water, but probably not so practical for a drone.

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Wind will not help creating lift, because the drone's relative wind speed is zero. But it will glide down gently if the proper angle of attack on the blades is maintained. Same as a glider.

Unless it is tethered to the ground or to another drone flying in a defferent weather with different wind speed, of course with constant automatic adjustment to create optimal lift.

But a drone can climb like a glider soaring in a thermal. it is constantly descending in a gentle slope like gyro copter, even while soaring with a rising thermal.

It will climb with the effictive climb rate of vertical speed of thermal minus glide ratio of the drone.

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  • $\begingroup$ Using a dynamic soaring flight pattern, the glider can maintain an average speed close to 10 times the average wind speed. Radio controlled gliders have been flown in a repeating "closed loop" path with average ground speed exceeding 350 mph. $\endgroup$
    – alephzero
    Jul 8 '21 at 22:06
  • $\begingroup$ @alephzero, yes I had read about that in Wikipedia. I don.t think it is a proper answer to OP's question: a wind-powered platform to staying aloft indefinitely! $\endgroup$
    – kamran
    Jul 9 '21 at 2:53

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