Page "Lift (force)" Paragraph 31

From Bernoulli's principle, the pressure on the upper surface where the flow is moving faster is lower than the pressure on the lower surface where it is moving slower.

The pressure difference thus creates a net aerodynamic force, pointing upward and downstream to the flow direction.

The component of the force perpendicular to the free stream is lift ; the component parallel to the freestream is drag.

In conjunction with this force by the air on the airfoil, the airfoil imparts an equal-and-opposite force on the surrounding air that creates the downwash, in accordance with Newton's third law.

Measuring the momentum transferred to the downwash is another way to determine the amount of lift on the airfoil.