Why does airflow separate from the wing during stall?
Air flowing underneath the wing at an angle is pushed downwards, regardless of the shape of the lower surface: high pressure has few practical limits.

Air flowing over the upper surface cannot suddenly change direction though because it is driven by an under-pressure gradient. It can follow a slowly curving surface to the limit of the pressure differential between ambient and suction, but too steep a curve and the airflow cannot follow and has to separate: a stall occurs.



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