How Kites Fly

When a kite is flown the operator may have wondered how this is made possible. A kite is heavier than air, and yet the air supports it, just as water supports a boat. Not only that, the air lifts the kite, as it lifts the wings of an aeroplane, and so the kite climbs upwards. But it may happen that the wind tends to drive it backwards and downwards, and this is where the kite line plays its part. When it is taut the backward and downward travel is checked, and the kite is enabled to climb. This upward move­ment is improved, on occasions, by the operator as he runs along, holding the line and lengthening it as required. After the flight the kite is brought down to the ground again.

In the above statement four forces are mentioned or implied which need further explanation. They are the forces of resistance; upward thrust or lift; downward pull or gravity, and propulsion. It may be said that a kite flies because ways and means have been found to use these forces to the best advantage. This may be seen as each is considered in turn.

The first is called resistance. By this it is meant that air exerts a force against, that is, opposes, an object moving through it, as the following simple examples will show. Take a piece of card­board, hold it in a horizontal position, and move it from side to side. The board cuts easily and quickly through the air, the reason being that there is very little resistance offered to the board. Now use the latter as a fan, that is in an upright position. The difference is immediately felt. There is definite opposition to the movement of the board, as if the air were acting like a brake to slow it down.

Again, this resistance is felt whenever anyone is out walking. Even on a calm day he is aware of the air brushing against him; and on a windy day the air becomes a strong opposing force, to overcome which the walker has to increase his efforts.

In these examples the moving objects are displacing the air as a boat displaces water, and the air opposes the action. It rubs against the objects, and this action is like that of a brake. Air clings to the surfaces of things, tending to slow them down and to stop them, in other words, to overcome the power which is moving them along.