WHY A KITE STAYS UP
What is it that makes a plane or a kite stay in the air? The answer is inertia. The balloon shows us that air possesses weight; the aeroplane shows us that air possesses inertia. This is a natural consequence. Every body possesses inertia and the heavier the body the greater its inertia. By inertia we mean resistance to change of motion or rest. The pressure of air against the face of a fan is due to its resistance to a change from state of rest to a state of motion, while the pressure of wind against a surface represents the resistance of air in motion to being brought to a state of rest. The more sudden the change the higher is the resistance or pressure developed. If an open newspaper be laid over one half of a ruler, while the other half extends beyond the edge of the table, the ruler may be broken by a sharp blow on the overhanging end, not because the other end is held down by the weight of the newspaper, but because the inertia of the air bearing on the broad area of the paper prevents the ruler under the paper from rising in response to the sudden blow at the overhanging end. It is the inertia of the air, i. e., its resistance to rapid displacement that keeps a parachute from falling like a solid shot to earth.
Figure 62 shows how a kite is maintained in the air. The line AB represents the plane of the kite, the line CO at right angles to this plane is the pressure against the center of the kite surface. The wind pressure DO is resisted by the pull of the kite string and exerts a lift EO, which resists the vertical pull of gravity. The sum of the forces DO and EO must be equal to the force CO. If EO is greater than the force of gravity the kite will rise, and if it is less the kite will fall. The magnitude of the force EO depends upon the velocity of the wind DO and the angle of the kite AB to the wind. If the plane of the kite were parallel to the direction of the wind the angle would be zero and the lift would also be zero.