Kite Terminology

Here is a picture of one of the simplest kites:

It consists of a sail to catch the wind. The sail is often made of fabric, and so it usually needs spars to maintain its shape.

These are classified as longerons running from top to bottom, and cross-spars or spreaders running from side to side. When a kite only has one longeron, it is often called a spine instead.

A tail may be added for stability, or just for a show!

A bridle attaches the flying line(s) to the spars or sail, and may consist of one or more legs. A compound bridle is often used when there are more than 3 bridle legs. In the case of a 4 legged bridle, the legs would be brought together in two pairs, and the two pairs would then be bridled together with a 2 legged secondary bridle.

The towing point is the point at which the legs join the flying line.

The relative lengths of the legs, and hence the position of the towing point determines the angle of attack, being the angle at which the sail is inclined to the wind.

Some kites, especially sports kites, have a leech line, which is a line running through a pocket or tunnel sewn in the edge of the sail. The line is held taught and stops the edge (usually a trailing edge) from flapping.

The ratio of width of the sail to its length is known as the aspect ratio. (A short, wide kite is said to have a high aspect ratio.) This is the main factor determining the flying angle, which is the angle at which the kite holds the flying line - a kite with a high aspect ratio tends to have a high angle of flight.

Since the above definition takes no account of the shape of the sail, a more precise definition is the square of the width divided by the area. Since the lift is equal to the rate at which downward momentum is imparted to the air passing across the sail, a given lift can be had by imparting a large momentum to a small amount of air (as in a low aspect ratio kite with a deep sail) or a small momentum to a large amount of air (a high aspect ratio kite with a wide sail). The latter is more energy-efficient since whilst momentum is proportional to velocity, energy is proportional to the square of the velocity. Notice how long and thin are the wings of a glider, in contrast to the space shuttle, which merely has to dispose of its launch energy.