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Computer drawing showing the forces acting on the control line.

An excellent way for students to gain a feel for aerodynamic forces is to fly a 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. A kite is heavier than air and relies on the motion of the wind past the kite to generate the aerodynamic lift necessary to overcome the weight of the kite. The movement of the air past the kite also generates aerodynamic drag which is overcome by constraining the kite with a control line. The control line is attached to the kite bridle by a knot at the bridle point.

In flight, the kite rotates about the bridle point due to the torques created by the forces. The action of the forces is transmitted through the control line by tension in the line. The flyer feels the pull of the tension at the other end of the line. You will notice that the control line produces a gentle curve from the hand of the flyer to the kite; the line sags under its own weight. Mathematical equations have been developed which describe the shape of the control line. The line has a certain length s and the weight of the control line is evenly distributed along the length at p ounces per foot of line. The total weight of the control line is designated g and

g = s * p

Because the shape of the line is not a straight line we can not use trigonometry to determine the height at which the kite flies for a given length of line.

The mathematical equations describing the curve of the control line are pretty messy! So we have written a computer program to solve them for you. You can use the KiteModeler computer program to solve these equations for a kite which you design. You can then build your own kite from your design and compare the flight performance to a computer prediction of the performance. KiteModeler predicts the height at which your design flies based on mathematical models of the lift, drag, weight, and control line conditions. You can determine the altitude at which your kite actually flies by using some simple math techniques and some graph paper. With a little more mathematical knowledge you can even calculate the altitude at which the kite is actually flying.

Enjoy flying ... but always fly safely.


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Editor: Nancy Hall
NASA Official: Nancy Hall
Last Updated: May 13 2021

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