An excellent way for students to gain a feel for aerodynamic forces is to fly a kite. Kites come in a wide variety of shapes and sizes, and each kite flies a little differently from another kite. On this page we discuss the basics of kite flying with a particular interest in the physics involved in launching and flying the kite. The motion of the kite through the air is the result of forces being applied to the kite. The kite responds to these forces according to Newton's laws of motion.
To launch the kite (1), we have to create a lift force which is greater than the weight of the kite. The size of the lift force depends on a lot of factors, with the most important factor being the velocity of the air going by the kite. It does not matter whether the air blows over the kite, or the kite is pulled through the air; the important factor for generating lift is the relative velocity between the air and the kite. Now if we face the kite and have the wind at our back, we already have some relative velocity provided by the wind. On windy days, this velocity plus a small tug on the line is usually enough velocity to lift the kite into the air. On less windy days, we may have to move backwards or run into the wind to get the kite flying. As the kite rises during launch (2), we can usually stand still and the kite will fly just fine. This occurs because the velocity of the wind normally increases as we increase altitude. 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. But with enough altitude, the velocity (and the lift force) become fairly constant.
Once the kite has been launched, it will cruise (3) at an altitude and in an attitude where all the forces and the torques are balanced. If the forces are changed, the kite moves until the forces are in balance once again. In particular, if we pull on the control line, we can slightly increase the velocity of the kite. The increased velocity increases the lift, which causes the kite to climb (4). If the wind velocity is even slightly higher at the new altitude, the kite remains at that altitude. Letting out line initially causes the kite to drop slightly due to the increased weight of the line, and decreased tension in the line (slight decrease in velocity). But pulling on the line can move the kite back up to a higher altitude.
There are mathematical equations which describe the forces and torques on the kite and the sag in the control line. You can use the KiteModeler computer program to solve these equations to get an approximation of the flight characteristics of your design. You can then build your own kite from your design and compare the actual flight performance to the computer prediction. You can determine the altitude at which your kite actually flies by using some simple math techniques and a little graph paper. With a little more mathematical knowledge you can even calculate the altitude at which the kite is flying.
Enjoy flying ... but always fly safely.
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