If you will scale a sheet of tin, or a thin, flat stone, or even a
slate from a roof, into the air, you will have the simplest form of
an aeroplane. The stone, or tin, is heavier than the amount of air
it displaces, but it stays up for a comparatively long time because
it is in motion. The moment the impulse you have given it by
throwing fails, then it begins to fall.
The engine, or motor, aboard an aeroplane keeps it constantly in
motion, and it glides along through the air, resting on the
atmosphere, by means of the planes or wings.
If you will take a clam shell, and, holding it with the concave side
toward the ground, scale it into the air, you will see it gradually
mount upward. If you hold the convex side toward the ground and
throw it, you will see the clam shell curve downward.
That is the principle on which airships mount upward and descend
while in motion. In a biplane there is either a forward or rear
deflecting rudder, as well as one for steering from side to side.
The latter works an the same principle as does the rudder of a boat
in the water. If this rudder is bent to the right, the craft goes
to the right, because of the pressure of air or water on the rudder
twisted in that direction. And if the rudder is deflected to the
left, the head of the craft takes that direction.
Just as the curve of a clam shell helps it to mount upward, so the
curve of the elevating or depressing rudder on an airship helps it
to go up or down.
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