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Setting aside the likely legal factors for a moment, what are the limiting factors when flying a model plane to high altitude?

As I see it these could be:

  1. Battery power consumption - Model planes seem to have a battery life of less than an hour. There is a large distance to be covered in a climb ~20KM.
  2. External forces - Strong winds/turbulence acting on a relatively small and light airframe could easily blow the plane off course. I'm not sure how severe these would be at higher altitude.
  3. Air pressure/oxygen content - I presume this could reduce flying efficiency.
  4. Low air temperatures affecting the operation of the electronics inside and perhaps the control surfaces.
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  • $\begingroup$ when going for high altitude/long duration you would use a liquid fuel (tends to have better total energy storage) $\endgroup$
    – ratchet freak
    Commented Nov 17, 2014 at 13:49
  • $\begingroup$ What kind of engine / drive? Isn't that crucial? $\endgroup$
    – curious_cat
    Commented Feb 8, 2016 at 8:12
  • $\begingroup$ Electronics tend to like it cold, hence the massive chillers in large computer rooms. Batteries, however, don't like producing electricity when they're cold, so that's an offset for an electric motored plane. $\endgroup$
    – FreeMan
    Commented Feb 8, 2016 at 20:44
  • $\begingroup$ @FreeMan: Remember that while on the ground server rooms have large cooling systems on spaceships and weather balloons electronics have heaters. Electronics tend to like it around 15 degrees celsius ambient air temperature. They start failing above 50 degrees and below 5 degrees. Military grade electronics can be made to work down to -10 degrees but the temperature at 20km is around -30 degrees. Also, stuff like your iPhone and Arduino (the most common hobbyist UAV platform) are not military grade. $\endgroup$
    – slebetman
    Commented Feb 10, 2016 at 11:31

2 Answers 2

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The same rules apply as for "real" airplanes, and the topic has been covered well in previous questions. With model airplanes, scaling laws make the task easier, rsp. the possible maximum altitude higher. I would suggest to hang the plane below a balloon to get it up without draining batteries. NASA has done it before with a remotely controlled glider.

For the limiting factors please see this question.

Here is a question for the maximum altitude of propeller aircraft.

Here is another on the feasibility of geostationary drones.

Depending on the acceptable fragility of the design, a model aircraft should easily go down to a wing loading of 10 $kg/m^2$ and then be flyable at 36 km of altitude. At a true airspeed of 155 m/s it will fly at half the speed of sound and the Reynolds number on the wing with an assumed chord of 0.25 m will be below 20,000. You need to fly it with a lift coefficient of 1.1, which will be a challenge at the low Reynolds number, and the L/D will probably be less than 10, so your propulsion needs to be quite powerful to hold the altitude up there. I haven't done the numbers, but the propeller will be huge. With back-of-the-envelope calculations, my answer for the maximum altitude is 36 km.

Turbulence should be of no concern - all flow patterns up there are of extremely large scale.

The low temperature should also be tolerable if all systems are electric. It is not so cold up there: The standard atmosphere gives -34°C.

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  • $\begingroup$ Uhh, 36 km propellers seem to push that into the unfeasible category, right? $\endgroup$
    – raptortech97
    Commented Nov 18, 2014 at 1:46
  • $\begingroup$ @PeterKämpf: The way you wrote it sounds like the propeller diameter is 36km. The "my answer is 36 km" part needs qualification if that is not what you meant. $\endgroup$
    – Jan Hudec
    Commented Nov 18, 2014 at 7:51
  • $\begingroup$ @JanHudec: Oops - thanks for pointing this out. The sentence stood in the context of maximum altitude before, and then I added the propeller sentence ahead of it. Fixed. $\endgroup$
    – Peter Kämpf
    Commented Nov 18, 2014 at 9:58
  • $\begingroup$ @raptortech97: My bad - I added the propeller sentence, which made the next sentence look like it refers to the propeller, not the altitude, and did not get your hint. Sorry! $\endgroup$
    – Peter Kämpf
    Commented Nov 18, 2014 at 10:01
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If you are relying on GPS to navigate that model UAV, note that some receivers have artificial limitations imposed on them due to CoCom paranoia. The answers at the following page may be of interest: https://electronics.stackexchange.com/questions/11868/gps-units-weather-balloon

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