Downwash Behind an Airfoil

This photo is posted on the excellent NASA educational web site dealing with aerodynamics. The photo is credited to Paul Bowens and was supplied by Jan-Olov Newborg, of Stockholm, Sweden. The Cessna Citation has just flown above a cloud deck, and the downwash from the wing has pushed a trough into the cloud deck. Note the swirling flow from the tip vortices.

The air across the top of a conventional airfoil experiences constricted flow lines and increased air speed relative to the wing. This causes a decrease in pressure on the top according to the Bernoulli equation and provides a lift force. An alternate view is to model the lift from Newton's laws in terms of the angle of attack. Part of the modeling of lift force involves attachment of the boundary layer of air on the top of the wing with a resulting downwash of air behind the wing. If the wing gives the air a downward force, then by Newton's third law, the wing experiences a force in the opposite direction - a lift.

Aerodynamicist's such as Eastlake consider the Bernoulli approach and the Newton's Law's approach to be ultimately equivalent ways of describing airfoil lift. Eastlake states that the Bernoulli approach is the preference of aerodynamicists. Numerous discussions of Bernoulli vs Newton continue, but this photo makes one thing clear. There is a pronounced downwash behind the aircraft wing.