- The Washington Times - Wednesday, June 29, 2005

Jim Hodges understands the basic physics that make each of his 25 kites soar through the skies around the District.

Not every kite enthusiast gives much thought to Newton’s Third Law of Motion or Bernoulli’s principle, which explains alike how airplanes and kites fly.

“It’s almost like driving high-performance cars. A lot of people want to know why the car performs like it does. The others just get a kick out of driving,” the Fairfax resident says.

However, scientific factors explain what keeps kites aloft and why they have been a boon for scientists seeking to study flight.

Mr. Hodges, owner of Sky Jewels, a kite store in Fairfax, and a member of the kite enthusiasts group Wings Over Washington (WOW), says modern kites are technically superior to those previous generations flew.

“Today, the kites are made of mostly fiberglass for the rods and higher levels of ripstop nylon, which is more resistant to tearing and lighter in weight,” Mr. Hodges says.

This year’s models are so efficient, he adds, they often can be flown indoors.

Tom Crouch, senior curator of aeronautics with the Smithsonian’s Air and Space Museum, says the Wright brothers owe a debt of gratitude to kites.

“It’s hard to imagine how the airplane would have been invented without kites,” Mr. Crouch says. The Wright brothers experimented with kites in 1899 to verify their blossoming flight theories.

“Kites were really the only heavier-than-air things we could build which were capable of staying in the air for a considerable amount of time,” Mr. Crouch says.

The kite’s impact on aerodynamics didn’t end there, he says, citing the subsequent development of hang gliders.

Kites are able to fly in the first place thanks to Bernoulli’s principle, which states that an increase in the speed of a fluid produces a decrease in pressure, and a decrease in the speed produces an increase in pressure.

For airplanes and kites, that means that as the speed of objects increases, air flows faster over the curved top of the wing than underneath. That makes the higher pressure exerted by the air under the wing stronger than the gravitational pressure pushing the wing downward, Mr. Anderson says.

Kites often fly in what Mr. Crouch calls a near stall, in which they are suspended midair but are not moving rapidly. It’s a moment when all the forces acting upon the kite are equalized, if only for a moment.

That’s when Newton’s Third Law of Motion — which says when a body exerts a force on a second body, the second body exerts an equal and opposite force back to the first body — comes into play.

John Anderson, curator for aerodynamics at the National Air and Space Museum and an honorary fellow at the American Institute for Aeronautics and Astronauts, explains that kites push on the air deflecting downward, causing the air to exert a force in the opposite direction, meaning up. That process is similar to what an airplane wing experiences.

The center of pressure for a kite is where the string or line should be affixed, Mr. Anderson says. That way, the kite stands the best chance of flying as straight and flutter-free as it can. Air pressure always acts in a perpendicular fashion to the kite surface, he adds.

“You are exactly balancing that aerodynamic force,” he says.

Jeanne Merry, owner of Cobra Kites in Toms River, N.J., says innovations in kite design have led to more reasonably priced models that are both lighter and more durable than their predecessors.

Very little wood can be found in the more expensive kites, and the most common element is hollow carbon tubing that serves as the kite’s structural element, says Mrs. Merry, whose company designs, manufactures and distributes kites.

“It’s remarkable to see what you can get today,” Mrs. Merry says, adding that fiberglass is often used instead of wood for a lighter, more flexible kite skeleton.

Harold Ames, past WOW president from King George, Va., says different kinds of kites use physics in different ways.

Delta kites, shaped like the delta symbol, tend to fly straight and often don’t require a tail to serve as a drag, Mr. Ames says.

Three-dimensional box kites, shaped as the name implies, also tend to fly in stable patterns and tend to be heavier than other kites, with a lower lift-to-weight ratio, meaning they need more wind to stay aloft.

Generally, the lift-to-weight ratio for a kite is the reverse of what it is for an airplane, Mr. Ames says.

Mr. Ames says a kite flier would benefit from hitting the physics books before the next kite outing.

“You need to understand the forces acting upon the kite so you understand when your kite is acting improperly,” he says.

Mr. Crouch agrees, especially for those keen on getting more involved with the activity.

“It’s more fun if you understand what’s actually going on, and it can help you build a better kite,” Mr. Crouch says.

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