According to Bernoulli's theorem for a flowing gas, where velocity is increased the pressure is decreased and vice-versa.
The wing of aeroplane is so designed that the curvature of its upper surface is greater than that of lowersurface and its frontend is shaped round while back end is flattened.
During flight flow of air above and below the wing is stream-lined. Air flowing above the wing has to travel longer distance compared to air below it hence, according to Bernoulli's theorem pressure of air at upper surface is less than the pressure at the lower surface. Because of this pressure difference the aeroplane receives the necessary lifting force.
