BForce due to the pressure difference between the upper and lower surfaces of the wings, created by different air speeds on the surface
b
(b) Upper surface of wing is more curved than its lower surface, therefore, the speed of air above the wings is larger than the speed of the air below the wings. According to Bernoulli's theorem, the pressure above the wings becomes less than the pressure below the wings. Due to this difference of pressure on the two sides of the wings, a vertical lift acts on the aeroplane. When this lift is sufficient to overcome the gravity pull on the aeroplane, the aeroplane is lifted up.