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Chapter 8

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.

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.

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.

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.

Flying A Kite

In dealing with this subject, for the sake of clarity, some things, which have already been dealt with, will be mentioned again, and, if necessary, will receive further comment.

First to be considered is the place from which the kite is flown. There are certain do's and don't's which operate here. For example, avoid a spot where the kite might become entangled with trees or overhead wires, or where it might sail over a busy road. A good site is one where there is room to move about freely without causing annoyance to others. This applies particularly when several kites are being flown. It is exasperating if the lines get mixed up. Kite flyers, like anglers, give one another sufficient room in which to enjoy their pastime.

Next, a word about weather conditions. It is the aim and intent of the enthusiast to fly his kite successfully under varying conditions. He regards them as a challenge to his skill. At the same time he is not so misguided as to see a challenge in a gale. He knows that because a kite is, after all, but a frail craft, there are limits to its capabilities. In this respect, common sense is a good guide.

The following are instances of different flying conditions. On a warm calm day the air seems to be still. But this is only comparatively so. Although there is little movement in a horizontal direction, there is upward activity. This is due to rising currents of air, called thermic currents. They will be found, for example, above ploughed fields, moorland and where buildings are grouped together. The familiar heat shimmer is an indication of this rising air. On the other hand, on such a day, there will be colder descending air where there is water, marsh or meadow.

Then there comes a day when the wind is blowing. There can be variation in its movement. For example, it may be a fitful wind. Again, and to the point here, it may meet an obstacle such as a hill, a cliff, or a building, in which case it is deflected upwards. The strength of the up current will depend upon the force of the wind, and the size of the obstacle it meets.

Rising currents are a means whereby birds can soar, that is, fly without flapping their wings. Again, the glider pilot derives benefit from them. It follows, therefore, that, on occasions, they may prove to be helpful to the kite flyer.

(Please note. Further information on weather conditions is given in Chapter 10.)

We turn now from the weather to the kite. Before flying it, its size must be taken into account. Sometimes the enthusiast is tempted to build an outsize one. Though it may be the object of interest and admiration, the owner might not have realized that in flying large kites both skill and strength are needed. Someone has said that a 6-ft. kite pulls like a cart-horse. It follows then that one half this size, in a fairly strong wind and when a considerable amount of line has been released, can exert a strong pull. For this reason, the size in this book is limited to 3 ft. 6 in. This is a convenient and manageable size range for the beginner. Later on, if desired, the reader may make larger sizes by increasing the measurements given.

The next thing to do is to inspect the kite. First, check the bridle and the line, to see that they are secure and that the line runs freely on the reel. Secondly, test the bracing and bowstrings to make sure that they are taut. The method of tying these, which is shown in Chapter 7, under the heading 'Knots and Hitches*, will ensure that they can be tightened, if necessary. Thirdly, inspect the cover to ascertain whether it is secure and in good condition. This is very important in the case of a paper cover, as it can easily get damaged.

The method of flying the kite is as follows. A length of line is released. A friend holds the kite, or if no help is at hand, it is laid on the ground. The operator then runs forward against the wind. The length of the run and the speed required depend upon the force of the wind. The kite will gradually rise at a shallow angle. The operator, still moving, releases more line, the kite meanwhile responding to upward thrust. From then on it is largely a matter of movement on the part of the operator when called for, and of manipulating the line. Even on a fairly calm day the action of running causes a wind that will lift the kite, and this is aided by a gradual release of the line, and by choosing a place where there may be rising currents of air.

The height which a kite may reach is dependent very largely upon the amount of line which is released. If conditions be favourable, it will climb steadily until the weight of the line begins to be felt. It will rise all the time that the upward thrust is strong enough to overcome the downward pull, due to the weight of the kite and the line. The speed of the ascent is increased by a series of steady pulls on the line. In bringing the kite down, allow it to describe a descending curve, meanwhile move towards it, and carefully wind in the line. The kite should not be pulled down by brute force. Care is particularly needed for the last few feet of descent. The kite will behave wildly if the line is wound in too fast.

Success in kite flying comes, as it comes in other activities, by acquiring skill through practice. Theory may be good, but, for example, it is only when the pilot or the driver takes over that he gets the necessary 'feel' of his machine; it is only by handling his machine that he discovers what he and it can do in a given situation. So it is with kite flying. And let it be said that it is in this very fact that much of the interest of kite flying is found. If a kite were as mechanical in its movements as a clockwork train on a circular track, then the interest would be lost. It is the spontaneous response of a kite to variable weather conditions which sustains interest. The operator is in control, and finds much pleasure in this fact. There is a certain analogy between kite flying and both gliding and sailing. One person, as it were, becomes part of a craft which is very responsive to the elements and to his controlling hand.

As far as kite styles are concerned, it may be said that today they mainly belong to two classes: the box and the flat type. The box kite flies after the manner of an aeroplane, that is, at a small angle to the wind. Its shape helps to keep it from side-slipping, which means a sideways and downwards movement. The shape improves stability. The box kite has a good lift, that is to say, it climbs well, and can be readily brought to a nearly overhead position. The flat-type kite flies at a larger angle to the wind, and in many cases balance is obtained by the use of a flexible tail. It has a lively manner and is more suited than the box to aerial acrobatics. By skilful use of the line it will dive and dart about in the air in a most interesting way.

It will therefore be seen that the choice of a kite depends upon what one wishes to do with it. If the aim is to fly high, then the box type is the choice. If on the other hand, the idea is to carry out aerial manoeuvres, then the flat or plane kite, as it is also called, is the one to use.

Note 1. A Useful Hint.

For steadying a kite in flight, when other methods fail, tie a piece of thin material about the size of a man's handkerchief, to the end of the tail string. The author has found this to be very effective.

Note 2.    Important.

Readers are advised to make inquiries at their local police station as to whether there are any bye-laws or local regulations, governing kite flying, particularly regarding the height to which they may be flown.

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