Raise the angle of attack and the lift grows, for a while. Then something changes. At the critical angle of attack, about 15° in the toy model, the smooth airflow over the top of the wing separates from the surface and breaks up into swirls. The lift drops and the drag rises. This is called a stall.

Two things to fix in your head straight away:

• A stall has nothing to do with the engine stopping. A plane can stall with the engine at full power, and a glider has no engine at all.
• A stall is caused by angle of attack, not by speed. A wing stalls when α goes past the critical angle, and that can happen at any speed.