# Downwash

Downwash
The effect of downwash from a hovering helicopter is clearly visible on the surface of water below.

In aeronautics downwash is the change in direction of air deflected by the aerodynamic action of an airfoil, wing or helicopter rotor blade in motion, as part of the process of producing lift.[1]

Lift on airfoil is an example of application of the law of conservation of momentum - the force required to create the downwash is equal in magnitude and opposite in direction to the lift force on the airfoil. Lift on an airfoil is also an example of the Kutta-Joukowski theorem - the Kutta condition explains the existence of downwash at the trailing edge of the wing and the faster speed of the air as it passes above the airfoil; Bernoulli's principle explains why the pressure in the faster-moving air is lower than the pressure in the slower-moving air below the wing. The pressure of the air above the airfoil amounts to a net downward force, and the pressure of the air below the airfoil amounts to an upward force. In flight, the latter force is greater than the former, and the difference between the two is lift.[1][2][3] Modern writings agree that both Bernoulli's principle and Newton's laws are relevant and either can be used to correctly describe lift. [4][5][6]