_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.