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The Answer, My Friend: is Blowing in the Wind

PepperJack

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blog-0291881001368747550.jpgSummer is here boys and girls, and what better way to be cool in the sun, than flying a kite! It’s fun, safe, and somewhat boring when it stays in the sky for twenty minutes! So you may have to get creative with other things you can put on the string to fly up… But the point of this blog is the physics of flying a kite.

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A kite is light weight so it’s aero-dynamic, and also whimsical. The light small string is strong enough to not break when winds are high, yet light enough so it freely flows in the sky. There is a tail added to a kite for stability. Those little attachments that are like the bones of the kite—they are usually made of plastic or wood and are thin little frames that hold the body of the kite together. (Usually three or four points if the kite is shaped like a diamond.) They are attachment points for stability as well. A kite is kind of like a mini airplane without people on it. The kite flies because it is heavier than the air itself—thus demonstrating aero-dynamics.

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When I was flying this kite with my friends, we were wondering what would happen if we let go of this 30 foot string kite? Also, how far would it go if we followed it in our car? Where would it land? Would it land?—it was a very windy day. And more importantly, how many kites would we need to lift us off the ground while holding on to them? We needed a lawn chair or something. Maybe on a more windy day we will have to test this out again. All of these things depend on the lift and drag of the kite and its aero-dynamics. (You cannot have one without the other.)

To get a kite off the ground you have to run with it. Get a little crazy, and just fly with the kite. This creates “apparent wind” which creates a lift and pushes up the kite. Once the kite reaches a high enough altitude where the wind becomes strong enough, you can stop running and the kite will remain aloft. This is extremely boring. Like more boring than you would image. The first 10 minutes are fun and all, but after a while you start to think about what else you should do. Sit down? Sleep? Let go of the kite? Fly your shorts up the kite string? So many possibilities. On this specific day, there was a lot of velocity in the air flowing through the kite, so it stayed up for a very long time. It was also pulling towards a specific direction and it made you feel like it was leading you somewhere. Stand your ground, and don’t let any kite pull you around. Tell him whose boss.

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The change in velocity from the surface of the earth to some altitude is caused by the boundary layer of the atmosphere. Inside the boundary layer the velocity is low and may be unsteady—this is why your kite may fall down very fast and do a few flip tricks against the pavement before it flies away. But with enough altitude, the velocity (and the lift force) become fairly constant. Once the kite has been launched, it will cruise at an altitude and in an attitude where all the forces and the torques are balanced. By pulling on the control line, you can slightly increase the velocity of the kite—this is called lift. This is a very fun feeling. It feels like your sailing in the air and you feel very in control of this little kites journey.

There is actually a computer program called KiteModeler which solves equations to get an approximation of the flight characteristics of your design in the kite. By using some simple math techniques, a little graph paper and a little more mathematical knowledge you can even calculate the altitude at which the kite is flying.

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Happy flying everyone!



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Kites are fun, but I have never thought about how they work from a physics perspective!

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