This theory states that air molecules splitting at the front of a wing must meet at the back at the same time. Since the top of the wing is curved and represents a longer path, the air must travel faster. According to Bernoulli's principle, higher velocity means lower pressure, creating lift. The Reality This explanation is physically incorrect:
Real physics argues that lift is proportional to circulation (the Kutta–Joukowski theorem). But what is circulation? It is the net spinning motion of the fluid around the airfoil. When a wing moves, it sheds a starting vortex opposite in sign to the bound vortex around the wing. This vortex system creates downwash behind the wing. Induced drag is not a "mistake"—it is the price of generating lift in a three-dimensional, real fluid. understanding aerodynamics arguing from the real physics pdf
Viscosity does two essential things. First, it creates the boundary layer, a thin region near the surface where velocity changes from zero (sticking to the wing) to the free-stream speed. A healthy, attached boundary layer allows the flow to follow curved surfaces without separating. Second, viscosity is responsible for the starting vortex—a spinning blob of air shed from the trailing edge when the wing begins to move. This vortex induces the circulation around the wing (a measure of flow turning), directly linking lift to the real, unsteady process of pushing air. This theory states that air molecules splitting at