Explainer #13
How does a wing create lift?
A wing is shaped and tilted so that it bends the oncoming air downward. Bending the flow creates lower pressure over the top and higher pressure underneath, and that pressure difference pushes the wing up. It's the same thing seen two ways: the wing pushes air down, and the air pushes the wing up.
The principle
Lift comes from turning the flow. Air only follows a curved path if the pressure changes across it, so the curved flow over a tilted, cambered wing ends up with lower pressure above the wing and higher pressure below. Where the pressure is lower, the air speeds up (Bernoulli), which is why the streaks over the top are faster.
Behind the wing the air leaves angled downward: the downwash. Pushing a mass of air downward every second requires a force, and by Newton’s third law the wing feels an equal force upward. Pressure and downwash are not competing theories, they are two descriptions of the same flow.
How much lift you get depends on air density, speed (squared), wing area and angle of attack, up to about 15° where the flow separates from the top and the wing stalls.
Common myth
Equal transit time (the "longer path" explanation)
The popular story says air over the curved top has a longer way to go, so it must speed up to meet the air from underneath at the trailing edge. There is no rule that the two have to meet, and they don’t: in the video, the green line of air released at the same moment splits at the wing, and the air over the top arrives first, well before the air underneath.
The path-length difference is also far too small to explain the lift a real wing makes, and it can’t explain why thin, flat or symmetric wings (or a plane flying upside down) still fly. And the top isn’t a vacuum: the pressure there is only a few percent below the surrounding air, but over a whole wing that adds up to a lot of force.
How this video was made: The airflow is computed, not hand-animated: 2D potential flow around a Joukowski airfoil close to a NACA 2412 (about 12% thick, 2% camber) at 6° angle of attack, with the Kutta condition setting the circulation. Colour shows pressure, streaks show speed. Real air also has viscosity and a thin boundary layer, which this simple model leaves out.
