MECHANICAL BIRD ACTION
In order to understand the theory of the modern flying
machine one must also understand bird action and wind
action. In this connection the following simple experiment
will be of interest:
Take a circular-shaped bit of cardboard, like the lid of
a hat box, and remove the bent-over portion so as to
have a perfectly flat surface with a clean, sharp edge.
Holding the cardboard at arm's length, w
hand, leaving the cardboard without support. What is
the result? The cardboard, being heavier than air, and
having nothing to sustain it, will fall to the ground.
Pick it up and throw it, with considerable force, against
the wind edgewise. What happens? Instead of falling
to the ground, the cardboard sails along on the wind,
remaining afloat so long as it is in motion. It seeks
the ground, by gravity, only as the motion ceases, and
then by easy stages, instead of dropping abruptly as in
the first instance.
Here we have a homely, but accurate illustration of
the action of the flying machine. The motor does for
the latter what the force of your arm does for the cardboard--
imparts a motion which keeps it afloat. The
only real difference is that the motion given by the
motor is continuous and much more powerful than that
given by your arm. The action of the latter is limited
and the end of its propulsive force is reached within a
second or two after it is exerted, while the action of the
motor is prolonged.
Another Simple Illustration.
Another simple means of illustrating the principle of
flying machine operation, so far as sustentation and the
elevation and depression of the planes is concerned, is
explained in the accompanying diagram.
A is a piece of cardboard about 2 by 3 inches in size.
B is a piece of paper of the same size pasted to one edge
of A. If you bend the paper to a curve, with convex
side up and blow across it as shown in Figure C, the
paper will rise instead of being depressed. The dotted
lines show that the air is passing over the top of the
curved paper and yet, no matter how hard you may
blow, the effect will be to elevate the paper, despite the
fact that the air is passing over, instead of under the
In Figure D we have an opposite effect. Here the
paper is in a curve exactly the reverse of that shown in
Figure C, bringing the concave side up. Now if you
will again blow across the surface of the card the action
of the paper will be downward--it will be impossible to
make it rise. The harder you blow the greater will be
the downward movement.
Principle In General Use.
This principle is taken advantage of in the construction
of all successful flying machines. Makers of monoplanes
and biplanes alike adhere to curved bodies, with
the concave surface facing downward. Straight planes
were tried for a time, but found greatly lacking in the
power of sustentation. By curving the planes, and placing
the concave surface downward, a sort of inverted bowl
is formed in which the air gathers and exerts a buoyant
effect. Just what the ratio of the curve should be is a
matter of contention. In some instances one inch to the
foot is found to be satisfactory; in others this is doubled,
and there are a few cases in which a curve of as much as
3 inches to the foot has been used.
Right here it might be well to explain that the word
"plane" applied to flying machines of modern construction
is in reality a misnomer. Plane indicates a flat,
level surface. As most successful flying machines have
curved supporting surfaces it is clearly wrong to speak
of "planes," or "aeroplanes." Usage, however, has made
the terms convenient and, as they are generally accepted
and understood by the public, they are used in like manner
in this volume.
Getting Under Headway.
A bird, on first rising from the ground, or beginning
its flight from a tree, will flap its wings to get under
headway. Here again we have another illustration of
the manner in which a flying machine gets under headway--
the motor imparts the force necessary to put the
machine into the air, but right here the similarity ceases.
If the machine is to be kept afloat the motor must be
kept moving. A flying machine will not sustain itself;
it will not remain suspended in the air unless it is
under headway. This is because it is heavier than air,
and gravity draws it to the ground.
Puzzle in Bird Soaring.
But a bird, which is also heavier than air, will remain
suspended, in a calm, will even soar and move in a
circle, without apparent movement of its wings. This
is explained on the theory that there are generally vertical
columns of air in circulation strong enough to sustain
a bird, but much too weak to exert any lifting power
on a flying machine, It is easy to understand how a
bird can remain suspended when the wind is in action,
but its suspension in a seeming dead calm was a puzzle
to scientists until Mr. Chanute advanced the proposition
of vertical columns of air.
Modeled Closely After Birds.
So far as possible, builders of flying machines have
taken what may be called "the architecture" of birds as
a model. This is readily noticeable in the form of
construction. When a bird is in motion its wings (except
when flapping) are extended in a straight line at right
angles to its body. This brings a sharp, thin edge
against the air, offering the least possible surface for
resistance, while at the same time a broad surface for
support is afforded by the flat, under side of the wings.
Identically the same thing is done in the construction of
the flying machine.
Note, for instance, the marked similarity in form as
shown in the illustration in chapter II. Here A is the
bird, and B the general outline of the machine. The
thin edge of the plane in the latter is almost a duplicate
of that formed by the outstretched wings of the bird,
while the rudder plane in the rear serves the same purpose
as the bird's tail.