How Kites Fly. Part 1
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. There is, however, another force which is present, acting in an upward direction. This may be proved in a very simple way. Place a piece of writing paper on a table and blow along the top of the table. The paper tends to rise and move forward. In passing, this was one of the ways in which Sir George Cayley demonstrated the lifting property of air. This force, called upward thrust, is evident when a leaf or a piece of paper are being blown about in the wind, and when an aeroplane or a kite is flying. Anyone who has ever flown a kite has used this force to prevent its falling to the ground. The operator sets out to fly his kite. He lays it on the ground, and holding the kite line, he runs forward against the wind.
In this action the front or leading edge of the kite is pulled against the air, causing it to rise off the ground. It gradually climbs at a shallow angle because the air is being exerted in an upward direction. As long as the correct angle or inclination of the kite is maintained, this upward thrust will operate effectively. In addition, as a result of the wind's flowing around the sides of the kite, a partial vacuum is formed above its upper side. This also helps the kite to rise in a steady manner, and the fact that this vacuum exists explains why the kite is said to sail on the air. It will be seen in what has just been said that in order to fly a kite it must present an inclined surface to the wind. In other words, the kite must meet the wind at an angle as it moves forward.
This angle is called the angle of incidence. The more the kite is inclined towards the horizontal, up to a certain fixed point, the better it will climb. As the kite moves nearer to the vertical position, it offers a larger target for air resistance, which will drive it backwards because the upward thrust cannot operate effectively. On the other hand, as the kite moves nearer to the horizontal position (up to a certain fixed point) part of the air resistance is converted into a force acting in an upward direction. Of course, if the kite approaches too much towards the horizontal position, then again the upward thrust is progressively weakened, and the kite will not climb.These points may be proved quite easily. Fly a kite from a fixed position in a gentle breeze. Left to its own devices it will tend towards the vertical position. Because of this position or angle the kite will be sluggish in rising, and at the same time will be driven backwards. It may eventually assume the vertical position, and consequently make one of those annoying dives to earth. In this event, the upward thrust has been made of no effect and the kite responds to a downward pull.