Originally Posted by
studi
Corpjet, the previous two answers cover low speed stalls, not high speed stalls.
A low speed stall is when the airflow over your airfoil separates due to a too high angle of attack.
It is a function of angle of attack and not of speed. Many people mix this up with speed, because for a certain weight, speed (IAS) and angle of attack are in relation to each other, e.g. the slower you fly the more angle of attack you need to produce a certain amount of lift. But separation is a function of angle of attack and not of speed.
With higher altitude (= reduced air-density), you need a higher TAS to have the same IAS, thus the higher you fly, the higher will be your TAS to carry a certain weight (good for traveling).
A high speed stall is when the airflow over your wing reaches the speed of sound (M = 1 at this point of the wing) due to acceleration around the wings shape. A shockwave emerges and the airflow separates. [Note: you might have heard of the term Mcrit or critical machnumber. This is the aircrafts theoretical speed when its airflow over the wing reaches M = 1]
The higher you fly the lower is your TAS needed to reach M = 1 (due to lower temperature). The higher you fly, the easier you end up in a high speed stall.
So now we combine low and high speed stall: at a certain altitude, the minimum TAS you need to avoid a low speed stall will be the maximum TAS you can fly before entering a high speed stall. This point is called "coffins corner" and theoretically the highest altitude an aircraft can fly.
Just what is wrong about having airflow separate? Especially at high Mach?
Look at it this way: the lower surface of the wing intercepts airflow, which is forced to accelerate around the wing. The wing creates lift at all angles of attack except approximately 0 degrees, approximately 90 degrees, approximately 180 degrees and approximately 270 degrees.
At approximately 0 degrees, there is no lift because there is exactly as much air accelerated above wing as below wing. At 90 degrees, the same applies.
Now, from 0 to certain stalling angle of attack, the larger the angle of attack, the larger the lift. This should provide the necessary vertical and therefore lateral stability.
So what if the speed of airflow exceeds the speed of sound over the wing, or under the wing, or in free airflow? Yes, shockwaves are generated, and the airflow separates easily over the wing - but the airflow under the wing has nowhere to separate and has to support the wing for simple Newton second law reasons, whether its speed is 0,8 M, 1,8 M or 18 M...