How Kites Fly. Part 3
It may be helpful at this point to provide a brief summary of the chapter so far. A kite in flight is subject to the influence of four forces, namely: resistance; upward thrust; downward pull; and propulsion. Resistance is the opposition exerted by the air to an object moving through it. Upward thrust is the part of this resistance which is converted into a lifting force. Downward pull is due to the weight of the kite, which, because of its weight, is attracted to the earth. Downward pull is said to be concentrated at that point in a body which is called the centre of gravity. Propulsion is the force exerted by the kite line, which acts as a motor to the kite. These forces act in opposition in the horizontal and the vertical direction: resistance and propulsion in the horizontal, and downward pull and upward thrust in the vertical. In order for these forces to act in such a way that makes flight possible, they must pass in continuous lines through the centre of gravity. If they do not, then the kite is unstable in flight, because the forces are being exerted at different points. The forces must be expended in opposite directions and along continuous lines.
In order to meet with this requirement the kite must present a correctly inclined surface to the wind. It must also be properly balanced. And because balance is so important it becomes the subject of further study. As has been said, balance is obtained by an even distribution of weight around the centre of gravity, or mass centre, as it is also termed. As far as the practical application of this principle is concerned, the writer has found it helpful to adopt the following procedure. Take a kite with a main central strut or backbone. There is a point on this backbone where the kite will balance on the end of a rod. When the point has been located it may be marked with a pencil. It is the centre of gravity or mass centre.
So far it has been established that at this centre the kite balances on the end of a rod, because there is an even distribution of weight around the centre. The force of gravity is pulling equally on all sides. The kite is showing what is called longitudinal and lateral balance. The next thing to do is to find ways and means of keeping this two-way balance when the kite is in flight. The first thing to do in this search for balance is to think of a kite in flight. The air, in meeting it and flowing around it, creates disturbances which tend to make the kite unsteady. For example, it may be liable to dip backwards and forwards. The kite does not show longitudinal balance. In order to improve this balance, the kite line must be tied in the right position. A kite with the line fixed at the right point, generally slightly ahead of the centre of gravity, shows longitudinal stability, in other words, it keeps on an even keel.
There is another fault which will upset a kite in flight. Owing to the disturbances it encounters, it may tend to sway from side to side. It lacks lateral balance. The latter is improved by the use of effective dihedral. In an aeroplane, dihedral is the angle between the horizontal surface of a wing and the fuselage. As seen from the front, the wings form a shallow V-shape. In a kite, dihedral is secured by the bowing of the crossbar or bars. In effect this means, that if the kite tips to one side, then the edge of that side which is forced down presses against the air underneath it. At the same time the edge of the other side which is forced up presses against the air above it. This increases the pressure of air in both places, with the result that one side is lifted up and the other side is pushed down and so balance is restored. Again, lateral balance is improved in those cases where the bridle is fixed crossways on the kite.
The longitudinal and the lateral balance of a kite is illustrated in the action of a pair of scales. If an equal weight is placed on the scale pans, and one side is tipped down, the arm will return to a position of equipoise. The scales demonstrate stable equilibrium, which means the tendency of a body to return to a position of rest when moved or disturbed. There is another item which should be mentioned, and it is directional steadiness. Without this a kite may tend to move erratically from its course. Such deviation is called yawing. Directional stability is improved by the use of a flexible tail, which acts like the rudder of a boat. The degree of perfection which is implied in the above requirements is not achieved by written instructions alone. They are but the signposts pointing the way to a desirable goal. When all has been said which may profitably be said, it still remains for the kite flyer to strive towards the goal by constant practice. By a process of trial and error, eliminating faults one by one, sure progress is made towards the goal of successful kite flying. The latter will now be further considered under a separate heading.