A tethered flying device that supports itself and the cable that connects it to the ground by means of the aerodynamic forces created by the relative motion of the wind. This relative wind may arise merely from the natural motions of the air or may be caused by towing the kite through the agency of its connecting cable.
Kites take many forms (see illustration); the bow and box kites are common in the United States. In many countries, particularly in Asia, kites are frequently used in rituals and festivals; their bizarre forms and shapes are traditional, some having been developed centuries ago.
The lifting force of all kites is produced by deflecting the air downward, the resulting change in momentum producing an upward force. To be successful, a kite must have an extremely low wing loading (weight/area) so that it can fly even on days when the wind velocity is not high. It must be completely stable, since the only controls available to the operator are the length of cable and the rate at which it is taken in or let out. Efficient design requires that its lift-to-drag ratio be as high as possible. See also: Aerodynamic force; Aerodynamics
Experiments on the possible application of efficient aircraft-type lifting surfaces, in which most of the lift arises from the low pressures created by the air flowing over the upper surfaces, have shown them to be too sensitive to changes in wind force and direction. Under normal atmospheric conditions the use of this type of lifting surface results in a kite that behaves in a violent and unpredictable manner. For this reason, the higher drag associated with a surface from which the flow has separated is tolerated, and most of the lifting force is obtained from pressure on the lower surface, because stalled surfaces are much less sensitive to wind changes.
Both the lift-to-drag ratio and the stability of the kite are functions of the length of cable. The more cable released, the more drag created. The increased drag, combined with the increase in weight being supported, causes the kite to sag off downwind, reducing the flight angle, which is the angle formed between the horizontal and a line passing through the kite and the operator.
Most kites with a properly located cable pivot point, generally slightly ahead of the center of gravity, demonstrate longitudinal stability. Lateral and directional instabilities generally couple to produce violent motions. The longer the cable, the more these motions are damped. Lateral and directional stability are improved by the use of effective dihedral (the bow of the bow kite) and a flexible tail, which provides both directional stability and yaw damping.