How does an aircraft actually fly?

An aircraft can only take off if its engines provide enough forward thrust to cause an adequate volume of air to flow over its wings at a sufficient rate. The air flow separates when it comes up against the leading edges of the wings. As the wing has a cambered surface and the separated air molecules tend to join up again once they reach the end of the aerofoil, the air on the upper side of the wing flows faster than that beneath. This creates vacuum which provides the aircraft with lift. How fast the air has to flow over the wings depends, among other things, on the weight of the aircraft and the size of the aerofoil.