STUDY OF BIRD FLIGHT USELESS.--The study of the flight of birds has never been of any special value to the art. Volumes have been written on the subject. The Seventh Duke of Argyle, and later, Pettigrew, an Englishman, contributed a vast amount of written matter on the subject of bird flight, in which it was sought to show that soaring birds did not exert any power in flying.

Writers and experimenters do not agree on the question of the propulsive power, or on the form or shape of the wing which is most effective, or in the matter of the relation of surface to weight, nor do they agree in any particular as to the effect and action of matter in the soaring principle.

Only a small percentage of flying creatures use motionless wings as in soaring. By far, the greater majority use beating wings, a method of translation in air which has not met with success in any attempts on the part of the inventor.

Nevertheless, experimenting has proceeded on lines which seek to recognize nature's form only, while avoiding the best known and most persistent type.

SHAPE OF SUPPORTING SURFACES.--When we examine the prevailing type of supporting surfaces we cannot fail to be impressed with one feature, namely, the determination to insist on a broad spread of plane surface, in imitation of the bird with outstretched wings.

THE TROUBLE ARISING FROM OUTSTRETCHED WINGS.--This form of construction is what brings all the troubles in its train. The literature on aviation is full of arguments on this subject, all declaring that a wide spread is essential, because, --birds fly that way.

These a.s.sertions are made notwithstanding the fact that only a few years ago, in the great exhibit of aeroplanes in Paris, many unique forms of machines were shown, all of them capable of flying, as proven by numerous experiments, and among them were a half dozen types whose length fore and aft were much greater than transversely, and it was particularly noted that they had most wonderful stability.

DENSITY OF THE ATMOSPHERE.--Experts declare that the density of the atmosphere varies throughout, --that it has spots here and there which are, apparently, like holes, so that one side or the other of the machine will, unaccountably, tilt, and sometimes the entire machine will suddenly drop for many feet, while in flight.

ELASTICITY OF THE AIR.--Air is the most elastic substance known. The particles const.i.tuting it are constantly in motion. When heat or cold penetrate the ma.s.s it does so, in a general way, so as to permeate the entire body, but the conductivity of the atmospheric gases is such that the heat does not reach all parts at the same time.

AIR HOLES.--The result is that varying strata of heat and cold seem to be superposed, and also distributed along the route taken by a machine, causing air currents which vary in direction and intensity. When, therefore, a rapidly-moving machine pa.s.ses through an atmosphere so disturbed, the surfaces of the planes strike a ma.s.s of air moving, we may say, first toward the plane, and the next instant the current is reversed, and the machine drops, because its support is temporarily gone, and the aviator experiences the sensation of going into a "hole."

RESPONSIBILITY FOR ACCIDENTS.--These so-called "holes" are responsible for many accidents. The outstretched wings, many of them over forty feet from tip to tip, offer opportunities for a tilt at one end or the other, which has sent so many machines to destruction.

The high center of gravity in all machines makes the weight useless to counterbalance the rising end or to hold up the depressed wing.

All aviators agree that these unequal areas of density extend over small s.p.a.ces, and it is, therefore, obvious that a machine which is of such a structure that it moves through the air broadside on, will be more liable to meet these inequalities than one which is narrow and does not take in such a wide path.

Why, therefore, persist in making a form which, by its very nature, invites danger? Because birds fly that way!

THE TURNING MOVEMENT.--This structural arrangement accentuates the difficulty when the machine turns. The air pressure against the wing surface is dependent on the speed. The broad outstretched surfaces compel the wing at the outer side of the circle to travel faster than the inner one. As a result, the outer end of the aeroplane is elevated.

CENTRIFUGAL ACTION.--At the same time the running gear, and the frame which carries it and supports the machine while at rest, being below the planes, a centrifugal force is exerted, when turning a circle, which tends to swing the wheels and frame outwardly, and thereby still further elevating the outer end of the plane.

THE WARPING PLANES.--The only remedy to meet this condition is expressed in the mechanism which wraps or twists the outer ends of the planes, as constructed in the Wright machine, or the ailerons, or small wings at the rear margins of the planes, as ill.u.s.trated by the Farman machine.

The object of this arrangement is to decrease the angle of incidence at the rising end, and increase the angle at the depressed end, and thus, by manually- operated means keep the machine on an even keel.

CHAPTER IV

FORE AND AFT CONTROL

THERE is no phase of the art of flying more important than the fore and aft control of an airship.

Lateral stability is secondary to this feature, for reasons which will appear as we develop the subject.

THE BIRD TYPE OF FORE AND AFT CONTROL.-- Every aeroplane follows the type set by nature in the particular that the body is caused to oscillate on a vertical fore and aft plane while in flight. The bird has one important advantage, however, in structure. Its wing has a flexure at the joint, so that its body can so oscillate independently of the angle of the wings.

The aeroplane has the wing firmly fixed to the body, hence the only way in which it is possible to effect a change in the angle of the wing is by changing the angle of the body. To be consistent the aeroplane should be so constructed that the angle of the supporting surfaces should be movable, and not controllable by the body.

The bird, in initiating flight from a perch, darts downwardly, and changes the angle of the body to correspond with the direction of the flying start.

When it alights the body is thrown so that its breast banks against the air, but in ordinary flight its wings only are used to change the angle of flight.

ANGLE AND DIRECTION OF FLIGHT.--In order to become familiar with terms which will be frequently used throughout the book, care should be taken to distinguish between the terms angle and direction of flight. The former has reference to the up and down movement of an aeroplane, whereas the latter is used to designate a turning movement to the right or to the left.

WHY SHOULD THE ANGLE OF THE BODY CHANGE?

--The first question that presents itself is, why should the angle of the aeroplane body change?

Why should it be made to dart up and down and produce a sinuous motion? Why should its nose tilt toward the earth, when it is descending, and raise the forward part of the structure while ascending?

The ready answer on the part of the bird-form advocate is, that nature has so designed a flying structure. The argument is not consistent, because in this respect, as in every other, it is not made to conform to the structure which they seek to copy.

CHANGING ANGLE OF BODY NOT SAFE.--Furthermore, there is not a single argument which can be advanced in behalf of that method of building, which proves it to be correct. Contrariwise, an a.n.a.lysis of the flying movement will show that it is the one feature which has militated against safety, and that machines will never be safe so long as the angle of the body must be depended upon to control the angle of flying.

_Fig. 11a Monoplane in Flight._

In Fig. 11a three positions of a monoplane are shown, each in horizontal flight. Let us say that the first figure A is going at 40 miles per hour, the second, B, at 50, and the third, C, at 60 miles.

The body in A is nearly horizontal, the angle of the plane D being such that, with the tail E also horizontal, an even flight is maintained.

When the speed increases to 50 miles an hour, the angle of incidence in the plane D must be decreased, so that the rear end of the frame must be raised, which is done by giving the tail an angle of incidence, otherwise, as the upper side of the tail should meet the air it would drive the rear end of the frame down, and thus defeat the attempt to elevate that part.

_Fig. 12. Angles of Flight._

As the speed increases ten miles more, the tail is swung down still further and the rear end of the frame is now actually above the plane of flight.

In order, now, to change the angle of flight, without altering the speed of the machine, the tail is used to effect the control.

Examine the first diagram in Fig. 12. This shows the tail E still further depressed, and the air striking its lower side, causes an upward movement of the frame at that end, which so much decreases the angle of incidence that the aeroplane darts downwardly.

In order to ascend, the tail, as shown in the second diagram, is elevated so as to depress the rear end, and now the sustaining surface shoots upwardly.

Suppose that in either of the positions 1 or 2, thus described, the aviator should lose control of the mechanism, or it should become deranged or "stick," conditions which have existed in the history of the art, what is there to prevent an accident?

In the first case, if there is room, the machine will loop the loop, and in the second case the machine will move upwardly until it is vertical, and then, in all probability, as its propelling power is not sufficient to hold it in that position, like a helicopter, and having absolutely no wing supporting surface when in that position, it will dart down tail foremost.

A NON-CHANGING BODY.--We may contrast the foregoing instances of flight with a machine having the sustaining planes hinged to the body in such a manner as to make the disposition of its angles synchronous with the tail. In other words, see how a machine acts that has the angle of flight controllable by both planes,--that is, the sustaining planes, as well as the tail.

_Fig. 13. Planes on Non-changing Body._

In Fig. 13 let the body of the aeroplane be horizontal, and the sustaining planes B disposed at the same angle, which we will a.s.sume to be 15 degrees, this being the imaginary angle for ill.u.s.trative purposes, with the power of the machine to drive it along horizontally, as shown in position 1.

In position 2 the angles of both planes are now at 10 degrees, and the speed 60 miles an hour, which still drives the machine forward horizontally.

In position 3 the angle is still less, being now only 5 degrees but the speed is increased to 80 miles per hour, but in each instance the body of the machine is horizontal.

Now it is obvious that in order to ascend, in either case, the changing of the planes to a greater angle would raise the machine, but at the same time keep the body on an even keel.

_Fig. 14. Descent with Non-changing Body._

DESCENDING POSITIONS BY POWER CONTROL.--In Fig. 14 the planes are the same angles in the three positions respectively, as in Fig. 13, but now the power has been reduced, and the speeds are 30, 25, and 20 miles per hour, in positions A, B and C.

Suppose that in either position the power should cease, and the control broken, so that it would be impossible to move the planes. When the machine begins to lose its momentum it will descend on a curve shown, for instance, in Fig. 15, where position 1 of Fig. 14 is taken as the speed and angles of the plane when the power ceased.