We have seen that the inventors of flying machines in the early days of aviation modelled their various craft somewhat in the form of a bird, and that many of them believed that if the conquest of the air was to be achieved man must copy nature and provide himself with wings.
Let us closely examine a modern monoplane and discover in what way it resembles the body of a bird in build.
First, there is the long and comparatively narrow body, or FUSELAGE, at the end of which is the rudder, corresponding to the bird's tail.The chassis, or under carriage, consisting of wheels, skids, &c., may well be compared with the legs of a bird, and the planes are very similar in construction to the bird's wings.But here the resemblance ends: the aeroplane does not fly, nor will it ever fly, as a bird flies.
If we carefully inspect the wing of a bird--say a large bird, such as the crow--we shall find it curved or arched from front to back.This curve, however, is somewhat irregular.At the front edge of the wing it is sharpest, and there is a gradual dip or slope backwards and downwards.There is a special reason for this peculiar structure, as we shall see in a later chapter.
Now it is quite evident that the inventors of aeroplanes have modelled the planes of their craft on the bird's wing.Strictly speaking, the word "plane" is a misnomer when applied to the supporting structure of an aeroplane.Euclid defines a plane, or a plane surface, as one in which, any two points being taken, the straight line between them lies wholly in that surface.But the plane of a flying machine is curved, or CAMBERED, and if one point were taken on the front of the so-called plane, and another on the back, a straight line joining these two points could not possibly lie wholly on the surface.
All planes are not cambered to the same extent: some have a very small curvature; in others the curve is greatly pronounced.Planes of the former type are generally fitted to racing aeroplanes, because they offer less resistance to the air than do deeply-cambered planes.Indeed, it is in the degree of camber that the various types of flying machine show their chief diversity, just as the work of certain shipmasters is known by theparticular lines of the bow and stern of the vessels which are built in their yards.
Birds fly by a flapping movement of their wings, or by soaring.We are quite familiar with both these actions: at one time the bird propels itself by means of powerful muscles attached to its wings by means of which the wings are flapped up and down; at another time the bird, with wings nicely adjusted so as to take advantage of all the peculiarities of the air currents, keeps them almost stationary, and soars or glides through the air.
The method of soaring alone has long since been proved to be impracticable as a means of carrying a machine through the air, unless, of course, one describes the natural glide of an aeroplane from a great height down to earth as soaring.But the flapping motion was not proved a failure until numerous experiments by early aviators had been tried.
Probably the most successful attempt at propulsion by this method was that of a French locksmith named Besnier.Over two hundred years ago he made for himself a pair of light wooden paddles, with blades at either end, somewhat similar in shape to the double paddle of a canoe.These he placed over his shoulders, his feet being attached by ropes to the hindmost paddles.Jumping off from some high place in the face of a stiff breeze, he violently worked his arms and legs, so that the paddles beat the air and gave him support.It is said that Besnier became so expert in the management of his simple apparatus that he was able to raise himself from the ground, and skim lightly over fields and rivers for a considerable distance.
Now it has been shown that the enormous extent of wing required to support a man of average weight would be much too large to be flapped by man's arm muscles.But in this, as with everything else, we have succeeded in harnessing the forces of nature into our service as tools and machinery.
And is not this, after all, one of the chief, distinctions between man and the lower orders of creation? The latter fulfil most of their bodily requirements by muscular effort.If a horse wants to get from one place to another it walks; man can go on wheels.None of the lower animals makes a single tool to assist it in the various means of sustaining life; but manputs on his "thinking-cap", and invents useful machines and tools to enable him to assist or dispense with muscular movement.
Thus we find that in aviation man has designed the propeller, which, by its rapid revolutions derived from the motive power of the aerial engine, cuts a spiral pathway through the air and drives the light craft rapidly forward.The chief use of the planes is for support to the machine, and the chief duty of the pilot is to balance and steer the craft by the manipulation of the rudder, elevation and warping controls.
