What a “Stall” is in aerodynamics and why it’s an emergency for airplanes
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Normally when a plane flies it goes – roughly speaking – straight through the air and the wing deflects the air downwards, pushing the plane up. Both the bottom and the top deflect air downwards. It’s quite easy to imagine how the bottom deflects air by simply hitting it are an angle (probably not exactly accurate, but this is ELI5). The top is shaped so that it pulls a vacuum and the air rushes downwards to fill it. This only works if the wing curves gently so the air has time to deflect to follow it.
Stall is when the wing isn’t air stops flowing nicely over the top of the wing and being deflected downards. If you imagine a wing at a really steep angle to the air, like 45 degrees, the air isn’t going to follow over the top at all. From the perspective of an air molecule, it goes past the edge of the wing, and then the wing curves away from it way faster than the air molecule goes down. So the air molecule doesn’t follow the wing – it keeps moving in mostly a straight line. So the wing isn’t deflecting the air down, so the air isn’t deflecting the wing up, so the plane falls out of the sky.
Luckily pilots know how to avoid this and also how to get back to normal if it does happen.
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