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The Aerodynamic Forces of Kite Flying
Even though there are many shapes and sizes of kites, the aerodynamic forces that act on a kite are always the same. There are complete books written about the science of aerodynamics, and I'm not going to try and replicate it here. I am, however, going to try and explain the aerodynamic forces on a kite as simply as possible.
There are four basic forces that affect the flight of your kite. Gravity, Lift, Drag and Thrust.
The first force that is exerted on your kite is Gravity (also referred to as Weight in some mathmatical formulas). The same force that keeps us on the ground, is the same that pulls on a kite. There is no way to eliminate this force, so the only way to counter act it is by making kites as light as possible to minimize the gravitational pull.
Lift is the force that will help overcome gravity and send your kite upward. The angle of your kite in relation to the wind creates an airfoil, much like a wing on an airplane.
Kites are built in such a way as to have an "angle of attack" (or the angle between your kite and the wind flow) that is designed to keep the face of the kite angled in such a way to generate sufficient lift to overcome gravity.
An easy way to demonstrate this, is by sticking your hand out of a moving vehicle. If you hold your hand flat, your hand will stay vertical, but if you turn your hand up (increasing the angle of attack), you can feel that your hand will want to start moving up. This is because the amount of lift generated is higher than gravity. This is the same effect that is on your kite.
Drag can also be called air resistance in this case. It is the force that is pushing against the face of your kite in order to keep it from moving forward. Using the same demonstration as above, if you hold your hand out, the pressure you feel pushing your hand backward is drag. You will notice as you turn your hand, that the more your hand becomes straight up and down, the more it feels like your hand is being pushed backward. That is because you are creating more drag by increasing the surface area that the wind is hitting.
This air resistance actually helps with the stability of your kite. You can increase the amount of drag on a kite by adding a tail. The increased drag on the bottom portion of the kite, will keep the kite in the direction the wind is blowing.
Thrust is a force that propels an object in the direction of motion. When most people think of thrust, they think in terms of a rocket or airplane that uses their engines to propel them forward.
But its slightly different with kites. On a kite, the tension of the kite line pulling on the kite is thrust. If you were to let the kite line loose, the kite would lose forward motion and float backwards. If you were to run with your kite line in hand, the kite would move forward. That is because the thrust is higher than the drag.
How this all works together to fly a kite
In order for a kite to fly steady, the forces that interact with your kite must be in balance. Thrust and Drag must be equal to keep the kite from moving forward or backward. Lift and Gravity have to be equal in order to keep the kite up in the air.
By changing the bridle angle (to increase/decrease the angle of attack) or adding drag by using a tail, you can change the flight characteristics of your kite.
Further Reading and Mathmatical Formulas
For more information and mathmatical formulas related to aerodynamics and kites, visit the Glenn Research Center's "Forces on a Kite" webpage.