The UAV in question was spotted at ~10,000ft above sea level, but it was only about 800-900ft above the local ground level at the time.
Multi-rotors using BLDC motors cannot really be compared to small general aviation aircraft with infernal combustion engines.
"Reasonable duration" is a nebulous phrase. Some of the airframe configurations we use here couldn't manage more than about 10 minutes at 100ft, whereas others could quite happily carry a gopro at 10,000ft for 10 minutes (assuming it didn't have to climb from sea level!)
>Multi-rotors using BLDC motors cannot really be
It's got nothing to do with the type of motor or fuel.
>"Reasonably duration" is a nebulous phrase. So
So is "quite happily carry". I'll admit that it appears that it's not impossible for such a flight, but I don't agree that any drone is "happily carrying" anything at that altitude.
I don't have time to run the model for this specific case, but from a similar case with a lower air density of 0.71kg/m^2 (40c at 4km altitude) a simple quad uses ~20% more power in a stationary hover than at sea level.
Interestingly at high flight speeds (15-20m/s) you could actually start to see power savings at this altitude, as the parasitic drag of the vehicle body is reduced.
https://i.imgur.com/Z5toIVm.jpg
Assuming air pressure at 10000 ft is ~69kpa, and sea level is ~100kpa, there really doesnt appear to be much of a difference, except for medium-sized props, which had very interesting thrust outputs- gaining efficiency at low rpm and loosing a lot at high rpm.
Anyway, point being is a 250 class quad could pretty easily fly at 10000 feet. Not particularly well, but considering 1:4 (or even 1:8) thrust ratios arent uncommon with 250 class quads they could still pull it off.
Now, idk about bigger quads like the phantom/iris, their props have quite different geometries which probably act quite differently.
Do you mind if I ask where you are getting that pressure value from?
The ICAO Standard Atmosphere gives a value of 70kPa for 10,000ft.
Not really... you need to move the same MASS of air, just more air VOLUME. Bigger props solve that problem, at the expense of moving a large prop (greater moment of inertia, slower acceleration).
Most small drones these days are electric, so that doesn't happen. It's a bit harder to generate lift from the props in thinner air like that, but you also experience less drag, which balances that out to a large extent.