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4. 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 analysis 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.

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.

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.

[Description: Black and white illustration: a technical diagram.]

In Fig. 13 let the body of the aeroplane be horizontal, and the sustaining planes B disposed at the same angle, which we will assume to be 15 degrees, this being the imaginary angle for illustrative 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.

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.

Fig. 15. Utilizing Momentum.

[Description: Black and white illustration: a technical diagram.]

CUTTING OFF THE POWER.—This curve, A, may reach that point where momentum has ceased as a forwardly-propelling factor, and the machine now begins to travel rearwardly. (Fig. 16.) It has still the entire supporting surfaces of the planes. It cannot loop-the-loop, as in the instance where the planes are fixed immovably to the body.

Carefully study the foregoing arrangement, and it will be seen that it is more nearly in accord with the true flying principle as given by nature than the vaunted theories and practices now indulged in and so persistently adhered to.

The body of a flying machine should not be oscillated like a lever. The support of the aeroplane should never be taken from it. While it may be impossible to prevent a machine from coming down, it can be prevented from overturning, and this can be done without in the least detracting from it structurally.

The plan suggested has one great fault, however. It will be impossible with such a structure to cause it to fly upside down. It does not present any means whereby dare-devil stunts can be performed to edify the grandstand. In this respect it is not in the same class with the present types.

THE STARTING MOVEMENT.—Examine this plan from the position of starting, and see the advantages it possesses. In these illustrations we have used, for convenience only, the monoplane

type, and it is obvious that the same remarks apply to the bi-plane.

Fig. 17 shows the starting position of the stock monoplane, in position 1, while it is being initially run over the ground, preparatory to launching. Position 2 represents the negative angle at which

the tail is thrown, which movement depresses the rear end of the frame and thus gives the supporting planes the proper angle to raise the machine, through a positive angle of incidence, of the plane.

THE SUGGESTED TYPE.—In Fig. 18 the suggested type is shown with the body normally in a horizontal position, and the planes in a neutral position, as represented in position 1. When sufficient speed had been attained both planes are turned to the same angle, as in position 2, and

flight is initiated without the abnormal oscillating motion of the body.

But now let us see what takes place the moment the present type is launched. If, by any error on the part of the aviator, he should fail to readjust the tail to a neutral or to a proper angle of incidence, after leaving the ground, the machine would try to perform an over-head loop.

The suggested plan does not require this caution. The machine may rise too rapidly, or its planes may be at too great an angle for the power or the speed, or the planes may be at too small an angle, but in either case, neglect would not turn the machine to a dangerous position.

These suggestions are offered to the novice, because they go to the very foundation of a correct understanding of the principles involved in the building and in the manipulation of flying machines,

and while they are counter to the beliefs of aviators, as is shown by the persistency in adhering to the old methods, are believed to be mechanically correct, and worthy of consideration.

THE LOW CENTER OF GRAVITY.—But we have still to examine another feature which shows the wrong principle in the fixed planes. The question is often asked, why do the builders of aeroplanes place most of the weight up close to the planes? It must be obvious to the novice that the lower the weight the less liability of overturning.

FORE AND AFT OSCILLATIONS.—The answer is, that when the weight is placed below the planes it acts like a pendulum. When the machine is traveling forward, and the propeller ceases its motion, as it usually does instantaneously, the weight, being below, and having a certain momentum, continues to move on, and the plane surface meeting the resistance just the same, and having no means to push it forward, a greater angle of resistance is formed.

In Fig. 19 this action of the two forces is illustrated.

The plane at the speed of 30 miles is at an angle of 15 degrees, the body B of the machine being horizontal, and the weight C suspended directly below the supporting surfaces.

The moment the power ceases the weight continues moving forwardly, and it swings the forward end of the frame upwardly, Fig. 20, and we now

have, as in the second figure, a new angle of incidence, which is 30 degrees, instead of 12. It will be understood that in order to effect a change in the position of the machine, the forward end ascends, as shown by the dotted line A.

The weight a having now ascended as far as possible forward in its swing, and its motion checked by the banking action of the plan it will again swing back, and again carry with it the frame, thus setting up an oscillation, which is extremely dangerous.

The tail E, with its unchanged angle, does not, in any degree, aid in maintaining the frame on an even keel. Being nearly horizontal while in

flight, if not at a negative angle, it actually assists the forward end of the frame to ascend.

APPLICATION OF THE NEW PRINCIPLE.—Extending the application of the suggested form, let us see wherein it will prevent this pendulous motion at the moment the power ceases to exert a forwardly-propelling force.

In Fig. 21 the body A is shown to be equipped with the supporting plane B and the tail a, so

they are adjustable simultaneously at the same angle, and the weight D is placed below, similar to the other structure.

At every moment during the forward movement of this type of structure, the rear end of the machine has a tendency to move upwardly, the same as the forward end, hence, when the weight seeks, in this case to go on, it acts on the rear plane, or tail, and causes that end to raise, and thus by mutual action, prevents any pendulous swing.

LOW WEIGHT NOT NECESSARY WITH SYNCHRONOUSLY-MOVING

WINGS.—A little reflection will convince any one that if the two wings move in harmony, the weight does not have to be placed low, and thus still further aid in making a compact machine. By increasing the area of the tail, and making that a true supporting surface, instead of a mere idler, the weight can be moved further back, the distance transversely across the planes may be shortened, and in that way still further increase the lateral stability.