How Do Kites Fly?

Q: How do kites fly?
Suzanne, Albuquerque, New Mexico

A: A kite flies because it deflects the wind downward like prop wash from a helicopter. The kite string keeps the kite at an angle of attack to the wind. Figure illustrating how a kite flies.

The wind hits the kite and flows down--the path of least resistance. The air exerts an equal and opposite--upward--force on the kite and the kite rises due to this lift.

In other words,
the kite blocks the wind, which deflects the wind downward.
The kite pushes down on the air.
The air pushes back up on the kite and lifts it into the sky.
That's how kites fly.

We've all experienced the same phenomenon, when riding in a car with the window open. You rest your arm along the bottom of the window and maybe, just for fun, cup your hand and raise your arm to increase the angle of attack. Almost instantly your hand shoots up like a rising kite. The event is so simple and straight forward it's a shame most kite books and physics books mess up the explanation. If you want to read more, Further Surfing has good references.

Whatever you do, do not pick up a high school physics text. It will be wrong, more than likely, giving an explanation of aerodynamic lift in terms of Bernoulli's principle. As Author Klaus Weltner says this explanation--based on the principle that a fluid's pressure decreases with a increase in velocity-- "is incomplete and often wrong".

Further Reading:

Klaus Weltner, "Aerodynamic Lifting Force" and "Bernoulli's Law and Aerodynamic Lifting Force" published in the February 1990 edition of The Physics Teacher, pages 78 through 86.

Gomber Kite Productions: Kite science

(Answered Sep. 13, 2002; updated Sep. 14, 2007)