Flying in Thin Atmosphere

Question

How thin could at atmosphere be before it could not be flown in using a modern plane?
Does speed affect this to a reasonable degree?

Unless you [edit] to include some [space.se] aspect to your question, this would fit [aviation.se] better if you're asking about practical limits of high altitude flight in Earth's atmosphere, or [physics.se] if you're asking about theoretical limits. But, in a nutshell, you're essentially looking for Kármán line, or at least its more contemporary equivalents. Oh, and please don't cross-post, if your question isn't specifically about [space.se]. Comment here or raise a custom flag and mention where to, and we'll migrate it for you. Thanks! — TildalWave, Oct 01 '15 at 21:30
Also potentially relevant: What if XKCD: Interplanetary Cessna — TildalWave, Oct 01 '15 at 21:33
Related: Why are many jet aircraft designed to cruise around FL350-370? — mins, Oct 02 '15 at 07:37

score 5 · Answer 1 · answered Oct 02 '15 at 05:32

There are two major constraints on aircraft operating in thin atmospheres:

Lift produced by aircraft (wing)
Thrust produced by the engine

The lift produced is directly proportional to the density; as altitude increases, density and hence lift reduces. This can be compensated to a certain extent by increasing the speed (or by increasing the lift coefficient). However, as the speed increases, the drag also increases and the required thrust goes up.

However,the engines need air to generate thrust (it doesn't matter if they are piston + propeller or gas turbine) and as the density falls, so does the thrust. As a result, the aircraft won't be able to generate enough speed to produce the required thrust to fly in a thin atmosphere. Usually this limitation is reached before the wings stop producing lift and the airplane would never reach theoretical considerations like the Kármán Line.

For anyone interested that doesn't know, the Kármán Line is 100km, the point at which traveling fast enough to produce lift would exceed orbital velocity. — Ezra Bailey, Oct 02 '15 at 14:18

score 3 · Answer 2 · answered Oct 01 '15 at 21:33

Depends on the plane.

Airliners generally cruise at 10km altitude, where atmospheric pressure is about 1/4 of sea level.

Speed does indeed matter; lift is proportional to the square of airspeed, and higher-performance planes like the SR-71 can maintain some lift up to 25km.

As mentioned by TildalWave, the Kármán Line at 100km is the point at which you need to go at orbital speeds to develop any aerodynamic lift.

Flying in Thin Atmosphere

2 Answers2