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I was browsing through Wikipedia when I saw this:

An electric aircraft is an aircraft powered by electric motors. Electricity may be supplied by a variety of methods including batteries, ground power cables, solar cells, ultracapacitors, fuel cells and power beaming.

This led me to wonder whether an aircraft could stay airborne/take off without fuel or electricity. (Other than gliders)

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    $\begingroup$ Gliders are pretty good at this... Take off is hard though, but hang gliders do it. $\endgroup$ – Ron Beyer Apr 3 '20 at 2:44
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    $\begingroup$ I’ve personally flown a man powered airplane. $\endgroup$ – Eric S Apr 3 '20 at 2:45
  • $\begingroup$ Wouldn't your kinetic energy be converted into electricity? $\endgroup$ – Super Apr 3 '20 at 2:45
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    $\begingroup$ Sure, pedal-powered airplanes $\endgroup$ – quiet flyer Apr 3 '20 at 2:54
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    $\begingroup$ Do you count solar-powered aircraft, which do not carry electricity but generate it from sunlight as they go along? $\endgroup$ – Guy Inchbald Apr 3 '20 at 9:38
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Yes, there are so many sources of energy you can use.

In the simplest form we have Buoyancy, Human power, Fly wheel, Rubber band.

More complicated mechanisms include compressed air, steam, hydrogen peroxide, which you can either use directly as a rocket/jet, or drive a piston engine or turbine to further drive a fan or a prop.....

Or, if you prefer, you can never go wrong with nukes

(Although one could argue nuclear and peroxide(and other mono-propellants) are fuels)

Here I'll just present my favorite out of all the possibilities. enter image description here Picture credit: TIHOOD

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    $\begingroup$ All are fuel... $\endgroup$ – Michael Hall Apr 3 '20 at 3:26
  • $\begingroup$ @MichaelHall - fuel is stuff you burn to gain energy, so compressed air, flywheels or rubber band motors don't count. $\endgroup$ – Robin Bennett Apr 3 '20 at 9:12
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    $\begingroup$ I think the key point is that all heavier-than-air craft need an energy source to overcome drag. A helium balloon can stay aloft for as long as it can contain the gas, with no energy input. $\endgroup$ – Robin Bennett Apr 3 '20 at 9:16
  • $\begingroup$ Steam needs fuel or electricity to boil it. $\endgroup$ – Guy Inchbald Apr 3 '20 at 9:38
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Human powered airplanes exist. They are powered directly from pedaling and don't store energy in any way. My personal experience goes back to my sophomore year at MIT when I helped out on the Chrysalis project in 1979. Chrysalis was a human powered biplane with a wingspan about the size of a DC-9. It was limited by the size of the hanger at Hanscom field outside of Boston. We flew early in the morning when the wind was light enough. I personally flew Chrysalis several times about a mile down the main runway. I also hand towed the plane including pilot when drag measurements were being made.

The principles on that team eventually developed Daedalus which flew from Crete to Santorini. A distance of over 115 km. There are a few YouTube videos of the flight. A detailed write up is available here.

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  • $\begingroup$ Last I checked, humans (including myself) run on fuel we call "food". Carbohydrates. Fats. Proteins. Our bodies oxidize them for energy we then use to move things like, say, the pedal crank that drives the propeller of our soap bubble aircraft. $\endgroup$ – Zeiss Ikon Apr 3 '20 at 16:14
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    $\begingroup$ @ZeissIkon Of course we don't want to violate the laws of thermodynamics. Even hang gliders require energy from updrafts to stay aloft. I believe my answer is in the spirit of the question since the plane itself stores no energy. It is able to take flight without external sources of power and can maintain level flight for extended periods unaided. $\endgroup$ – Eric S Apr 3 '20 at 17:35
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Not even counting the relatively recent electric aircraft and compressed air powered model airplanes, sailplanes depend only on gravity and air currents for their energy (and can be launched by rolling down a slope into the wind, on the right terrain and in the right conditions).

The only other class of aircraft I'm aware of that don't require fuel is gas-type balloons -- not hot air, as those require some kind of fuel (usually propane, these days) to heat the air, but a balloon lifted by hydrogen, helium, or coal gas/methane can float until too much of the gas leaks from the envelope and uses effectively no energy while it does (you could argue that production of the lifting gas uses fuel, but that depends on the gas used).

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