What is the direction of the downwash produced by wing-induced circulation in the horseshoe vortex model?

• A. Downwash is directed downward behind the wing.
• B. Downwash is directed upward behind the wing.
• C. Downwash is directed toward the wing.
• D. There is no downwash.

Answer: (A)

Across a lifting wing the circulation around the wing creates an induced velocity field in the wake. In the horseshoe-vortex picture, the wing has a bound vortex with trailing legs that extend downstream, and the velocity induced by this configuration has a vertical component pointing downward behind the wing. This downward deflection of the air, called downwash, is how the wing transfers momentum to the air to generate lift. An upward or inward (toward the wing) induced velocity would not match how the wake behaves for a lifting wing, and having no downwash would violate the observed wake pattern and momentum transfer. So the downwash behind the wing is directed downward.