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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.
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