Aerofils are so designed that the speed of air

The aerofils are so designed that $P_{\text{upper side }} < P_{\text{lower side }}$ so that the aerofils get a lifting force in upward direction. According to Bernoulli's theorem, where the pressure is large, the velocity will be minimum or vice-versa. Thus, $\quad v_{\text{upper side }} > v_{\text{lower side }}$