The planes, as already described are, of course, a necessary part of the aeroplane.

The propeller supplies motive power to the aeroplane. This moves in a circle much like the blades of the electric fan or the propeller of a motor boat or modern stern ship. By driving the air backward it propels the aeroplane forward. While the blades of the propeller are of considerable length they are usually inconspicuous in photographs, and as one who has never seen an aeroplane looks at a photograph he naturally asks, “What moves it through the air?” The propeller is driven by the engine.

The engine is usually of the gasoline type which develops high power with light weight, frequently one horse power for every three pounds of weight and in rare instances as high as one horse power for every pound of weight. These powerful little engines are marvels of mechanism and they have had much to do in the rapid modern progress of aeronautics.

The rudder, as its name indicates, guides the aeroplane in its flight. It consists in the main of small horizontal and vertical planes under the control of the pilot. These may be in the front of the machine, but they are usually placed in the rear. By skillful manipulation of these the aeroplane can be guided upward, downward, to right or left at will. It is also guided and controlled as we shall see, by the “warping” or “curving” of the wings or planes.