# How high can propeller airplanes fly?

Is there a maximum height a propeller airplane could fly?

Some lower bounds for maximum flying altitude:

Just for comparison: Low earth orbit is about 160,000m.

Upper bounds: I don't know. I guess at some point there is not enough air to get lifted. And nobody build an airplane with solar cells that flies to space.

I guess important factors for such an upper bound are:

• Weight of the airplane,
• Size, shape and speed of the propellers
• Speed of the propellers
• It seems like you already answered the question. What exactly are you expecting from an answer here? – Bernhard Jun 11 '14 at 20:22
• No, I just said that current prototypes (or rather: the best I found) can fly at least 29,523m high. I want to know if, for example, we had more efficient solar cells / lighter material they could even get to outer space. I guess not. I guess there might be a hight (below 160,000m) that cannot be reached. – moose Jun 11 '14 at 20:25
• And I would like to know if I got all factors which determine how high a plane can get. – moose Jun 11 '14 at 20:27
• If we extrapolate from the Helios, no matter how thin the air gets, if you can make a subsonic airfoil light enough, it should be able to be propeller driven. There is a limiting issue, which is that the speed of sound decreases with altitude, so a subsonic aircraft might have to travel very slowly (in indicated airspeed, which is not necessarily slow speed over the ground). – Mike Dunlavey Jun 14 '14 at 14:33
• Would Aviation be a better home for this question? – Qmechanic Jul 11 '14 at 23:05

It is a matter of power source (and engine aspiration in case of air breathing engines), wing loading and aerodynamic efficiency. With current technology, the limit is around 100.000 ft (30 km), as proven by Pathfinder and especially Helios. I doubt that much more is possible with really useable aircraft.

Aerodynamics first: The altitude factor of $c_l \cdot Ma^2$ tells you how much lift can be produced at a given flight Mach number, and the wing loading then gives you the minimum density for sustained flight. 0.4 is a good value for $c_l \cdot Ma^2$, and 30 kg/m$^2$ is a feasible wing loading for flight at 30 km. See this answer for more detail.

If the power source needs ambient air (piston engine), the plane needs triple-stage compressors or turbochargers, which have been tested up to 20 km altitude and should be good for maybe 24 km. They are finicky devices; Boeing Condor rarely flew at its maximum power because the stages of the turbochargers would oscillate in an alternating sequence of surges. One stage would race ahead, causing the other to surge, which made the first surge and free up the other to race ahead, and so on.

Above approximately 24 km, solar-electric propulsion looks like the best option currently. In all cases you can only fly subsonic, so the minimum practical wing loading will limit the maximum altitude. Aircraft like Helios are very delicate already, so they can only be launched in calm weather and are at risk of being blown away by high altitude winds. Payload is minimal, and depending what the aircraft is supposed to do besides flying high will give you a limit on the maximum altitude between 24 and 30 km.

Going to orbit in a propeller driven device is completely illusory. There is not enough matter to push against at higher altitudes, and the theoretical propeller diameter would be measured in Kilometers (or miles, if you prefer that unit). The structural mass would be prohibitive. Also, propeller thrust is inversely proportional to flight speed, and there is no way to accelerate with a propeller to escape velocity. The thrust would be just a rounding error away from zero at 7.9 km/s.

This speed is required to escape earth's gravity by flying fast enough around it so that the centripetal force equals the aircraft's weight and is called orbital velocity. The higher the orbit becomes, the more energy is required to reach it. In order to gain enough energy, a propeller aircraft would accelerate in the atmosphere to a speed quite a bit higher than the desired orbital velocity and then convert that kinetic energy to potential energy to lift its trajectory above at least 100 km, the internationally recognized altitude where spaceflight starts. Note that this phase of the flight requires inverted flight if the acceleration takes some time. The maximum flight Mach number would need to be maybe 12 or even 15 so this maneuver is possible at all.

In short: Going to orbit with a propeller? Forget it!

• @user2813274: You are right, I should be more explicit. – Peter Kämpf Oct 23 '15 at 7:26
• "The higher the orbit becomes, the faster the aircraft needs to go." – Other way around, isn't it? For circular orbits, the orbital velocity is slower the higher up you go. – Terran Swett Sep 18 '16 at 4:36
• @TannerSwett What I meant is that you need to start with a higher velocity in order to reach the higher orbit. I admit that was not expressed clearly. I hope my edit improved the answer. – Peter Kämpf Sep 18 '16 at 7:21
• @TannerSwett Sorry for replying to such an old post, but I wanted to say that Orbital Mechanics is bizarrely counter-intuitive. In order to reach a high, slow-speed orbit you need to accelerate to put energy into your orbit. Inversely, to move from a high-altitude low-speed orbit to a high-speed low-altitude one, you have to decelerate and remove energy from your orbit. Slowing down is speeding up and speeding up is slowing down. Crazy, right? – UIDAlexD Nov 15 '16 at 15:37