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To enter deeply into a discussion of the theory of the aeroplane would not only tire the reader but would waste valuable space in endeavoring to explain that which has been more adequately dealt with in more notable works.

In order to gain a clear understanding of the following chapters, however, it will be necessary to first grasp the elementary principles underlying the flight of an aeroplane. In setting these forth, I shall try, as far as possible, not to hamper or confuse with unnecessary terms or technicalities, except where such might be of worth in rendering a better conception of that to which they apply.

FIG. 1. Diagram showing a kite held in the air by the action of a wind. The dotted lines and arrow heads represent the direction and force of the wind.

An ordinary kite is one of the best examples of the action of an aeroplane. It is scarcely necessary to define the kite; it is a rigid frame of wooden sticks, on which is stretched a surface of cloth or paper. A string attached to the kite by means of a "bridle" serves to hold the apparatus to the ground.

In Fig. 1 is represented a kite against which the wind is blowing as indicated by the dotted lines. The string is so arranged that the kite is inclined at an angle to the wind and thus is sustained in the air by the force of the wind, viz., the molecules of air in striking against the slanting surface exert a pressure upon it which both calculation and experiment show to be perpendicular to the surface and tending to lift it. The kite also exerts a strong pull on the string which holds it in position.

But on days when there is no breeze or when the wind suddenly dies out; what is to be done then?

Wind is not an absolute thing. It is a relative movement of the surrounding air in comparison to a body. The effect is the same, and the relative movement takes place whether the air is still and the body in motion, or the air is in motion and the body motionless.

It is therefore an easy matter to "create a breeze" and a kite can be kept in the air providing the person flying the kite and holding the string commences to run.

Although no wind is blowing, the pressure of the air through which the kite is moving will cause it to remain in the air. In other words, the kite would be sustained in the air by virtue of its own relative motion to the wind. In order that the kite may fly, it makes no difference whether the wind moves against the kite or the kite moves against the wind.

An aeroplane, in fact, is nothing but a kite which creates its own breeze. If an aeroplane were attached to a strong wire serving as the string in the case of the kite, it would fly in the same manner as the kite, providing of course that the wind were sufficiently powerful. If the wind were not blowing at all, or not blowing hard enough, the other end of the wire could be attached to an automobile and by driving the automobile fast enough the required relative motion of the air would be produced and the aeroplane would fly.

There could be no direct benefit derived, however, from an aeroplane which must remain attached to a machine running over the earth and travel in its wake. Some other means of producing the required relative motion is necessary so that the aeroplane may be free to fly in any direction and either with or against the wind. This is accomplished by a propeller driven by a motor revolving at high speed in the aeroplane itself.

The action of an aerial propeller is similar to that of its marine prototype employed for driving ships through the water. Each depends for its action upon the imparting of a sternward motion to a column of fluid, in the one case air and in the other water. A propeller screws itself forward into the surrounding media in identically the same manner that an ordinary screw forces itself into a block of wood. An aeroplane therefore essentially consists of the wings or supporting surfaces, also sometimes called planes, driven through the air in an oblique manner by the propeller and motor.

FIG. 2. Diagram representing a typical monoplane. The only remaining requisition is that the aeroplane may be guided at will, caused to rise or fall or be steered to the right and left. The devices used to accomplish this are two rudders called respectively the "elevator" and the "steering rudder." The "elevator" takes the form of a small surface carried either in front or behind the main supporting surfaces and enables the machine to take an upward, a horizontal or downward course accordingly as it is adjusted. It acts as a rudder to steer the aeroplane up or down or to hold it to its course in exactly the same manner that a ship's rudder steers it to the right or left. When it is desired to direct the aeroplane upwards, the front edge of the elevator is raised so as to set it at a greater angle with the horizontal. If the aeroplane's course is required to be downward, the front edge of the elevator is lowered.

Aeroplanes are usually of two general types, monoplanes and biplanes. A monoplane, as its name implies, is a machine having a single pair of wings or supporting surfaces. The Bleriot, Antoinette and Santos Dumont machines are the most prominent representatives of this type of aeroplane.

The "elevator" on a monoplane is usually in the rear of the main supporting surfaces. When in this position it also acts as a tail to furnish longitudinal stability to the machine in the same way that a feather on an arrow steadies its flight.

FIG 3. Diagram showing the makeup of a biplane (Wright).

The most prominent machines of the biplane group are the Voisin, Wright, Curtiss and Farman aeroplanes. The old practice of placing the elevator in the front of a biplane is gradually being abandoned and it is safe to say that by the time this book has been printed all these machines will be of the "headless" variety with the elevator in the rear.

The vertical fins shown between the planes of the elevator in the old type of biplane, counterbalance the effect of gusts of wind striking the vertical rudder from the sides and also act as a pivot for turning to the right and left. Together with the steering rudder, they constitute a sort of keel which keeps the machine straight to its course.

In order for an aeroplane to fly in the accepted sense of the word, it must possess supporting surfaces, an elevator or tail and a propeller driven by a motor. These are essentially the sustaining, propelling and steering members of the machine.

FIG. 4. Two methods of controlling the lateral stability of an aeroplane.

The machine must, however, also possess "lateral stability," that is, the wings of the apparatus must not incline from the right to left or vice versa during the flight. The machine must be so constructed at it rights itself by its own effort or is under the immediate control of the aviator.

This is accomplished by "warping the wings," that is, the extreme tips of the planes can be moved up and down so as to present a greater or lesser angle and corresponding increase or decrease the lifting capacity of those portions.

The same result is also reached by means of small subsidiary moving planes attached to the rear of the main supporting surfaces called "aileron." When one aileron is lowered, the other is raised. The action of the air on the ailerons is to depress the one which is raised and to raise the one which is lowered as shown by the arrows in the illustration.