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
thdraw your

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

curved surface.

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.