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Experimental Fuel-Cell Airplane's 2nd NASA Test
js7a writes "The Helios prototype, holding the sustained flight altitude record, having unsuccessfully completed its first test with a fuel cell, is almost ready for its first night flight this Thursday, Friday, or Saturday. Helios uses solar panels for flight with payloads over 600 lbs. planned for up to six months using regenerative electrolysis. What good is a plane that will fly for six months without refueling? Besides providing a UAV alternative to AWACS, they can improve internet connectivity."
That, and thermals do go up high, but not up to 100,000 feet. And a plane that stays at 1000 feet isn't nearly as useful as one that stays at 100,000 feet.
Slope soaring would make more sense, as if you're in a coastal area is usually always windy and the wind direction is pretty well known, but this would require an appropriate slope and it's hard to gain signifigant altitude unless you have a full mountain for your slope.
You could probably keep a plane up indefinately over a constant source of massive heat like a nuclear power plant or volcano, but that wouldn't be very useful.
I believe that the world record duration for a R/C glider is 30 hours or so -- I don't know the specifics, but I'll bet this was on a slope. After 30 hours, he probably either got tired or his receiver battery died :)
There's more to it than that. Once you're above the troposphere, you get to use the ionosphere as a reflector with much less penalty. That's how over-the-horizon radars work.