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Union University Department of Physics

The Science Guys



Science Guys > April 2004

April 2004

Kites

While kites are considered a pastime toy, they have historically been used in many unique ways. They have been used to end wars by scaring the enemy, to lift people, to carry aloft scientific instruments, to fish, to aid people's escape, and for dozens of other activities. Most historians feel kites originated in China about 2000 years ago, and their use spread to Korea, Japan, and the South Sea Islands. Some islanders associate kites with gods, and even use them for fortune telling and funerals. The Romans had kites by 105 A.D. The actual word for kite first appeared in Japan in 981 A.D.

In the fifteenth century, Leonardo da Vinci developed a method to span a river with kites. Kites were actually used in making a suspension bridge near Niagara Falls in 1847. Everyone has heard of Benjamin Franklin and his flying the kite in a lightning storm in 1752. The Wright brothers flew kites often and even made some large enough to carry a person. Their kite flying was the forerunner to the invention of the airplane. Parachutes and hang gliders have been inspired by kites.

Kites come in numerous shapes, sizes, and types. Some of the major types are the diamond, delta, box, parafoil, and dragon. To fly a kite, a 10-20 mph wind is preferred, although some kite designs fly better in less wind than others do. If the wind gets too great, say over 25 mph, kite flying becomes difficult. A large open space and nice weather are ideal for kiting. Do think about safety - never fly your kite in a storm or near power lines. Ben Franklin was lucky he was not electrocuted.

Exactly what makes a kite fly? A kite experiences lift, created when the kite deflects the moving air (wind) downward. Newton’s laws tell us this change in the momentum of the wind results in an upward force on the kite. This is like a tennis ball exerting a force on a wall as it bounces off the wall and changes the direction it was going. Wind is redirected by the kite giving the kite lift. Also, the air travels above the kite was traveling faster than the air below, thus creating a pressure difference. Fast moving air creates less pressure above the kite, so the kite is forced or lifted upward. Since the kite is tied to a string, you can feel the force pulling on the string.

You can actually demonstrate this last principle to yourself. Hold a strip of notebook paper about 2" wide and 8" long at the top along the edge and roll your hands toward you a bit. You should have a rounded piece of paper with most of it hanging down on the side away from you. Bring it near your mouth and blow over the top of the strip. What happens? The strip rises because the air on top is moving faster than the air underneath, thus creating lift.

Although a kite can be flown only for pleasure, several things can be learned by the kite flyer. First, the higher the kite rises, the stronger the wind generally appears to be. The wind usually moves faster at higher elevations, since ground features slow the wind near the surface. Second, one can feel the wind pull on the string. Wind is a force of nature and can create very large forces when the speed becomes great. Recall the destruction of a hurricane or tornado. Third, wind has turbulence and is not always perfectly steady, eddies and swirls in the air make your kite dip, twist, and dance in the sky.

Kite flying is more popular than many people realize. There are contests and festivals held all around the world celebrating kite flying. There are even including fighting kite contests in which contestants try to cut the string on other’s kite with blades tied to their own kites. Kites may seem to be a child's toy, but there is much to be learned from the kite and much accomplished with this simple and enjoyable innovation of man.