The fly has greater mobility than any other flying creature. By the combined action of its legs and wings it can spring eighteen inches in the tenth of a second; and when in flight can change its course instantaneously.

If a sparrow had the same dexterity, proportionally, it could make a flight of 800 feet in the same time. The posterior legs of the fly are the same length as its body, which enable it to spring from its perch with amazing facility.

_Fig. 55. Common Fly. Outstretched Wings._

The wing surface, proportioned to its body and weight, is no less a matter for wonder and consideration.

In Fig. 55 is shown the outlines of the fly with outstretched wings. Fig. 56 represents it with the wing folded, and Fig. 57 is a view of a wing with the relative size of the top of the body shown in dotted lines.

_Fig. 56. Common Fly. Folded Wings._

The first thing that must attract attention, after a careful study is the relative size of the body and wing surface. Each wing is slightly smaller than the upper surface of the body, and the thickness of the body is equal to each wing spread.

_Fig. 57. Relative size of wing and body._

The weight, compared with sustaining surface, if expressed in understandable terms, would be equal to sixty pounds for every square foot of surface.

STREAM LINES.--The next observation is, that what are called stream lines do not exist in the fly.

Its head is as large in cross section as its body, with the slightest suggestion only, of a pointed end. Its wings are perfectly flat, forming a true plane, not dished, or provided with a cambre, even, that upward curve, or bulge on the top of the aeroplane surface, which seems to possess such a fascination for many bird flight advocates.

It will also be observed that the wing connection with the body is forward of the line A, which represents the point at which the body will balance itself, and this line pa.s.ses through the wings so that there is an equal amount of supporting surface fore and aft of the line.

Again, the wing attachment is at the upper side of the body, and the vertical dimension of the body, or its thickness, is equal to four-fifths of the length of he wing.

The wing socket permits a motion similar to a universal joint, Fig. 55 showing how the inner end of the wing has a downward bend where it joins the back, as at B.

THE MONOPLANE FORM.--For the purpose of making comparisons the ill.u.s.trations of the monoplane show a machine of 300 square feet of surface, which necessitates a wing spread of forty feet from tip to tip, so that the general dimensions of each should be 18 1/2 feet by 8 1/2 feet at its widest point.

First draw a square forty feet each way, as in Fig. 58, and through this make a horizontal line 1, and four intermediate vertical lines are then drawn, as 2, 3, 4, 5, thus providing five divisions, each eight feet wide. In the first division the planes A, B, are placed, and the tail, or elevator C, is one-half the width of the last division.

_Fig. 58. Plan of Monoplane._

The frame is 3 1/2 feet wide at its forward end, and tapers down to a point at its rear end, where the vertical control plane D is hinged, and the cross struts E, E, are placed at the division lines 3, 4, 5.

The angles of the planes, with relation to the frame, are usually greater than in the biplane, for the reason that the long tail plane requires a greater angle to be given to the planes when arising; or, instead of this, the planes A, B, are mounted high enough to permit of sufficient angle for initiating flight without injuring the tail D.

Some monoplanes are built so they have a support on wheels placed fore and aft. In others the tail is supported by curved skids, as shown at A, Fig. 59, in which case the forward supporting wheels are located directly beneath the planes.

As the planes are at about eighteen degrees angle, relative to the frame, and the tail plane B is at a slight negative angle of incidence, as shown at the time when the engine is started, the air rushing back from the propeller, elevates the tail, and as the machine moves forwardly over the ground, the tail raises still higher, so as to give a less angle of incidence to the planes while skimming along the surface of the ground.

_Fig. 59. Side Elevation, Monoplane._

In order to mount, the tail is suddenly turned to a.s.sume a sharp negative angle, thus swinging the tail downwardly, and this increases the angle of planes to such an extent that the machine leaves the ground, after which the tail is brought to the proper angle to a.s.sure horizontal flight.

The drawing shows a skid at the forward end, attached to the frame which carries the wheels.

The wheels are mounted beneath springs so that when the machine alights the springs yield sufficiently to permit the skids to strike the ground, and they, therefore, act as brakes, to prevent the machine from traveling too far.

CHAPTER X

POWER AND ITS APPLICATION

THIS is a phase of the flying machine which has the greatest interest to the boy. He instinctively sees the direction in which the machine has its life,--its moving principle. Planes have their fascination, and propellers their mysterious elements, but power is the great and absorbing question with him.

We shall try to make its application plain in the following pages. We have nothing to do here with the construction and operation of the motor itself, as, to do that justice, would require pages.

FEATURES IN POWER APPLICATION.--It will be more directly to the point to consider the following features of the power and its application:

1. The amount of power necessary.

2. How to calculate the power applied.

3. Its mounting.

WHAT AMOUNT OF POWER IS NECESSARY.--In the consideration of any power plant certain calculations must be made to determine what is required.

A horse power means the lifting of a certain weight, a definite distance, within a specified time.

If the weight of the vehicle, with its load, are known, and its resistance, or the character of the roadway is understood, it is a comparatively easy matter to calculate just how much power must be exerted to overcome that resistance, and move the vehicle a certain speed.

In a flying machine the same thing is true, but while these problems may be known in a general way, the aviator has several unknown elements ever present, which make estimates difficult to solve.

THE PULL OF THE PROPELLER.--Two such factors are ever present. The first is the propeller pull. The energy of a motor, when put into a propeller, gives a pull of less than eight pounds for every horse power exerted.

FOOT POUNDS.--The work produced by a motor is calculated in Foot Pounds. If 550 pounds should be lifted, or pulled, one foot in one second of time, it would be equal to one horse power.

But here we have a case where one horse power pulls only eight pounds, a distance of one foot within one second of time, and we have utilized less than one sixty-fifth of the actual energy produced.

SMALL AMOUNT OF POWER AVAILABLE.--This is due to two things: First, the exceeding lightness of the air, and its great elasticity; and, second, the difficulty of making a surface which, when it strikes the air, will get a sufficient grip to effect a proper pull.

Now it must be obvious, that where only such a small amount of energy can be made available, in a medium as elusive as air, the least change, or form, of the propeller, must have an important bearing in the general results.

HIGH PROPELLER SPEED IMPORTANT.--Furthermore, all things considered, high speed is important in the rotation of the propeller, up to a certain point, beyond which the pull decreases in proportion to the speed. High speed makes a vacuum behind the blade and thus decreases the effective pull of the succeeding blade.

WIDTH AND PITCH OF BLADES.--If the blade is too wide the speed of the engine is cut down to a point where it cannot exert the proper energy; if the pitch is very small then it must turn further to get the same thrust, so that the relation of diameter, pitch and speed, are three problems far from being solved.

It may be a question whether the propeller form, as we now know it, is anything like the true or ultimate shape, which will some day be discovered.

EFFECT OF INCREASING PROPELLER PULL.--If the present pull could be doubled what a wonderful revolution would take place in aerial navigation, and if it were possible to get only a quarter of the effective pull of an engine, the results would be so stupendous that the present method of flying would seem like child's play in comparison.

It is in this very matter,--the application of the power, that the bird, and other flying creatures so far excel what man has done. Calculations made with birds as samples, show that many of them are able to fly with such a small amount of power that, if the same energy should be applied to a flying machine, it would scarcely drive it along the ground.

DISPOSITION OF THE PLANES.--The second factor is the disposition or arrangement of the planes with relation to the weight. Let us ill.u.s.trate this with a concrete example:

We have an aeroplane with a sustaining surface of 300 square feet which weighs 900 pounds, or 30 pounds per square foot of surface.

DIFFERENT SPEEDS WITH SAME POWER.--Now, we may be able to do two things with an airship under those conditions. It may be propelled through the air thirty miles an hour, or sixty miles, with the expenditure of the same power.