How Kites Fly. Part 2
Again, fly a kite in a strong wind. The kite will be eager to climb and to attain an overhead position. This is due to the fact that the greater wind pressure and the stronger pull on the line have caused the kite to assume an angle at which it responds readily to the upward thrust. But if for some reason the kite is prevented from maintaining the correct angle, and it moves too far in the direction of the horizontal position, then it becomes unsteady in flight and unless the position can be rectified, the kite will respond to the downward pull, and this is the next force to be considered. Downward pull is due to the weight of the kite, and weight is an important factor. A kite is heavier than air, and ways have to be found to overcome this disadvantage. Obviously, one thing to do is to make the kite as light as possible. A kite which is not made according to this rule is overloaded, and the effect of this will be the same as when an aeroplane is overloaded.
Nevertheless, even when a kite is as light as possible in relation to its size it is still subject to the influence of downward pull. This pull is called the force of gravity. Everything falls to the ground if it is not held up by some means. It is the earth which attracts things. If there were no such attraction, then everything would be shot off into space by the force of the earth's rotation. Gravity then is force exerted by the earth, to which all bodies are subject; a force manifested in a downward pull.
At this point reference may be made to what is called the centre of gravity. This may be defined as the point where the downward pull is concentrated; or again, as that point in a body about which the whole weight is evenly balanced. The centre of gravity may be found by the rules of geometry; or by experiment. As an example of the latter, a rod may be suspended by a string which is looped over a nail. If a weighted length of string is tied to the nail it will indicate the point where the centre of gravity is situated. This point comes where the string crosses the rod. It is here that the downward pull is concentrated; it is here that the whole weight is evenly balanced. But as far as kite flying is concerned, this is not the whole story. The centre of gravity is not only the point where downward pull is concentrated; it is also the point where the lines of the other forces should meet or intersect - these forces being upward thrust, resistance and propulsion. If this is to be achieved, then the kite must be properly balanced. Should there be any mistake in this respect, then the kite will be unsteady in flight.
To correct this instability and to bring the forces into line, the operator may have to make a good many adjustments, such as lengthening or shortening the tail or moving the kite line. Successful kite flying is very much the outcome of such practices. It has been explained, so far, that a kite in flight is subject to the influence of the forces of resistance, upward thrust and downward pull. To these a fourth must be added, which as yet has only been briefly mentioned, and this is propulsion. It might be wondered at first why this should be attributed to a kite, since it has no motor. A moment's reflection, however, will make it plain to the reader that propulsion is supplied by the operator and the kite line. The line, as it were, acts as a motor to the kite, or to put it in another way, transmits the power supplied by the operator to the kite.
Propulsion is effective according to the use which is made of the kite line. For example, suppose a kite is being flown in a light breeze. If the line is slack, the kite will tend to move backwards and downwards. To correct this the operator tightens the line and maybe runs forward. It is just as if a motor had been switched on. The kite tends to move forwards and upwards, in response to the power exerted by the line. This power causes the kite to assume an effective angle whereby resistance is converted into upward thrust. This general statement, however, must be qualified by the following facts. The more line there is released, the more resistance there is created. Again, the longer the line, the greater the weight the kite has to support. As a result, the kite may sag off downwind. This movement backwards tends to affect the angle of incidence, so that upward thrust decreases. Thus it will be seen that the propulsion supplied by the line is influenced by the amount which is released.