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:
Is this possible (I think yes in principle though an engineering challenge) An answer could refute this.
If yes, what limits the maximum area and maximum density* of such a platform.