The air flow around the aerofoil has the effect of creating an upwards lift force that keeps the plane up in the air, and a backwards drag force that has the effect of slowing the plane down.
The upward lift force is created in two ways. Faster flowing air moving over the top of the aerofoil creates a negative pressure and a force pulling the wing upwards, whilst the slower air moving under the aerofoil creates a positive pressure and a pushing force on the wing.
Drag up and down on the end of the aerofoil in this animation to change the angle of incidence and watch what happens to the the lift and drag forces.
What do you think causes the lift force to suddenly drop as you keep on tilting the aerofoil up?
When an aerofoil is tilted up beyond a critical angle the air flow separates from the top surface of the wing and becomes turbulent. The wing is said to stall and the sudden loss of lift that results makes an aeroplane suddenly difficult to control. This usually led to a crash for the early pioneers as they were flying close to the ground and did not have time to recover. Otto Lilienthal died in this way, and in order to avoid a similar accident the Wright Brothers adopted a design for their planes that made recovery from a stall as easy as possible.

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