What a “Stall” is in aerodynamics and why it’s an emergency for airplanes
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Some good answers here that approximate the phenomenon. But the following should fill in the gaps:
The angle at which an airfoil meets the air is called angle of attack. As angle attack increases, lift and drag also increase, but lift increases faster than drag.
A stall is when an airfoil’s angle of attack exceeds its *stall angle*. The stall angle is where the ratio of lift to drag peaks, after which drag increases precipitously compared to lift, which may increase for a little while longer before precipitously decreasing.
This happens because past the stall angle, formerly smooth airflow that followed the airfoil’s top surface separates because its inertia overcomes atmospheric pressure holding it against the airfoil. Air leaks around the trailing edge into the cavity left by the separated flow. This leakage and turbulence both destroy lift and increases drag, effectively causing the wing to stop working.
In addition to forcing the airplane to descend, chaotic flow around the stalled wing reduces control effectiveness and can cause unpredictable departures from controlled flight, wnich for some aircraft may be unrecoverable.
[Here ](https://youtu.be/L2CsO-Vu7oc) is an example which demonstrates the hazard. This occurred during a test flight. The pilots intentionally stalled the aircraft, which unexpectedly rolled inverted and gathered an alarming amount of speed while losing an even more alarming amount of altitude. Only the pilot’s perfect recovery technique prevented the aircraft from breaking up due to aerodynamic stress or becoming an expensive lawn dart.
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