THE ORTHOPTER.--The term orthopter, or ornithopter, meaning bird wing, is applied to such flying machines as depend on wing motion to support them in the air.

Unquestionably, a support can be obtained by beating on the air but to do so it is necessary to adopt the principle employed by nature to secure an upward propulsion. As pointed out elsewhere, it cannot be the concaved type of wing, or its shape, or relative size to the weight it must carry.

As nature has furnished such a variety of data on these points, all varying to such a remarkable degree, we must look elsewhere to find the secret.

Only one other direction offers any opportunity, and that is in the individual wing movement.

NATURE'S TYPE NOT UNIFORM.--When this is examined, the same obscurity surrounds the issue.

Even the speeds vary to such an extent that when it is tried to differentiate them, in comparison with form, shape, and construction, the experimenter finds himself wrapt in doubt and perplexity.

But birds do fly, notwithstanding this wonderful array of contradictory exhibitions. Observation has not enabled us to learn why these things are so. High authorities, and men who are expert aviators, tell us that the bird flies because it is able to pick out ascending air currents.

THEORIES ABOUT FLIGHT OF BIRDS.--Then we are offered the theory that the bird has an instinct which tells it just how to balance in the air when its wings are once set in motion.

Frequently, what is taken for instinct, is something entirely different.

It has been a.s.sumed, for instance, that a cyclist making a turn at a rapid speed, and a bird flying around a circle will throw the upper part of the body inwardly to counteract the centrifugal force which tends to throw it outwardly.

Experiments with the monorail car, which is equipped with a gyroscope to hold it in a vertical position, show that when the car approaches a curve the car will lean inwardly, exactly the same as a bird, or a cyclist, and when a straight stretch is reached, it will again straighten up.

INSTINCT.--Now, either the car, so equipped possesses instinct, or there must be a principle in the laws of nature which produces the similarity of action.

In like manner there must be some principle that is entirely independent of the form of matter, or its arrangement, which enables the bird to perform its evolutions. We are led to believe from all the foregoing considerations that it is the manner or the form of the motion.

MODE OF MOTION.--In this respect it seems to be comparable in every respect to the great and universal law of the motions in the universe.

Thus, light, heat and electricity are the same, the manifestations being unlike only because they have different modes of motion.

Everything in nature manifests itself by motion.

It is the only way in which nature acts.

Every transformation from one thing to another, is by way of a movement which is characteristic in itself.

Why, then, should this great mystery of nature, act unlike the other portions of which it is a part?

THE WING STRUCTURE.--The wing structure of every flying creature that man has examined, has one universal point of similarity, and that is the manner of its connection with the body. It is a sort of universal joint, which permits the wing to swing up and down, perform a gyratory movement while doing so, and folds to the rear when at rest.

Some have these movements in a greater or less degree, or capable of a greater range; but the joint is the same, with scarcely an exception.

When the stroke of the wing is downwardly the rear margin is higher than the front edge, so that the downward beat not only raises the body upwardly, but also propels it forwardly.

THE WING MOVEMENT.--The moment the wing starts to swing upwardly the rear end is depressed, and now, as the bird is moving forwardly, the wing surface has a positive angle of incidence, and as the wing rises while the forward motion is taking place, there is no resistance which is effective enough to counteract the momentum which has been set up.

The great problem is to put this motion into a mechanical form. The trouble is not ascribable to the inability of the mechanic to describe this movement. It is an exceedingly simple one.

The first difficulty is in the material that must be used. Lightness and strength for the wing itself are the first requirements. Then rigidity in the joint and in the main rib of the wing, are the next considerations.

In these respects the ability of man is limited.

The wing ligatures of flying creatures is exceedingly strong, and flexible; the hollow bone formation and the feathers are extremely light, compared with their sustaining powers.

THE HELICOPTER MOTION.--The helicopter, or helix-wing, is a form of flying machine which depends on revolving screws to maintain it in the air. Many propellers are now made, six feet in length, which have a pull of from 400 to 500 pounds. If these are placed on vertically-disposed shafts they would exert a like power to raise a machine from the earth.

Obviously, it is difficult to equip such a machine with planes for sustaining it in flight, after it is once in the air, and unless such means are provided the propellers themselves must be the mechanism to propel it horizontally.

This means a change of direction of the shafts which support the propellers, and the construction is necessarily more complicated than if they were held within non-changeable bearings.

This principle, however, affords a safer means of navigating than the orthopter type, because the blades of such an instrument can be forced through the air with infinitely greater speed than beating wings, and it devolves on the inventor to devise some form of apparatus which will permit the change of pull from a vertical to a horizontal direction while in flight.

CHAPTER VI

THE LIFTING SURFACES OF AEROPLANES

THIS subject includes the form, shape and angle of planes, used in flight. It is the direction in which most of the energy has been expended in developing machines, and the true form is still involved in doubt and uncertainty.

RELATIVE SPEED AND ANGLE.--The relative speed and angle, and the camber, or the curved formation of the plane, have been considered in all their aspects, so that the art in this respect has advanced with rapid strides.

NARROW PLATES MOST EFFECTIVE.--It was learned, in the early stages of the development by practical experiments, that a narrow plane, fore and aft, produces a greater lift than a wide one, so that, a.s.suming the plane has 100 square feet of sustaining surface, it is far better to make the shape five feet by twenty than ten by ten.

However, it must be observed, that to use the narrow blade effectively, it must be projected through the air with the long margin forwardly.

Its sustaining power per square foot of surface is much less if forced through the air lengthwise.

Experiments have shown why a narrow blade has proportionally a greater lift, and this may be more clearly understood by examining the ill.u.s.trations which show the movement of planes through the air at appropriate angles.

_Fig. 22. Stream lines along a plane._

STREAM LINES ALONG A PLANE.--In Fig. 22, A is a flat plane, which we will a.s.sume is 10 feet from the front to the rear margin. For convenience seven stream lines of air are shown, which contact with this inclined surface. The first line 1, after the contact at the forward end, is driven downwardly along the surface, so that it forms what we might term a moving film.

The second air stream 2, strikes the first stream, followed successively by the other streams, 3, 4, and so on, each succeeding stream being compelled to ride over, or along on the preceding ma.s.s of cushioned air, the last lines, near the lower end, being, therefore, at such angles, and contacting with such a rapidly-moving column, that it produces but little lift in comparison with the 1st, 2d and 3d stream lines. These stream lines are taken by imagining that the air approaches and contacts with the plane only along the lines indicated in the sketch, although they also in practice are active against every part of the plane.

THE CENTER OF PRESSURE.--In such a plane the center of pressure is near its upper end, probably near the line 3, so that the greater portion of the lift is exerted by that part of the plane above line 3.

AIR LINES ON THE UPPER SIDE OF THE PLANE.-- Now, another factor must be considered, namely, the effect produced on the upper side of the plane, over which a rarefied area is formed at certain points, and, in practice, this also produces, or should be utilized to effect a lift.

RAREFIED AREA.--What is called a rarefied area, has reference to a state or condition of the atmosphere which has less than the normal pressure or quant.i.ty of air. Thus, the pressure at sea level, is about 14 3/4 per square inch

As we ascend the pressure grows less, and the air is thus rarer, or, there is less of it. This is a condition which is normally found in the atmosphere.

Several things tend to make a rarefied condition. One is alt.i.tude, to which we have just referred.

Then heat will expand air, making it less dense, or lighter, so that it will move upwardly, to be replaced by a colder body of air. In aeronautics neither of these conditions is of any importance in considering the lifting power of aeroplane surfaces.

RAREFACTION PRODUCED BY MOTION.--The third rarefied condition is produced by motion, and generally the area is very limited when brought about by this means. If, for instance, a plane is held horizontally and allowed to fall toward the earth, it will be r.e.t.a.r.ded by two forces, namely, compression and rarefaction, the former acting on the under side of the plane, and the latter on the upper side.

Of the two rarefaction is the most effectual, and produces a greater effect than compression.

This may be proven by compressing air in a long pipe, and noting the difference in gauge pressure between the ends, and then using a suction pump on the same pipe.

When a plane is forced through the air at any angle, a rarefied area is formed on the side which is opposite the one having the positive angle of incidence.

If the plane can be so formed as to make a large and effective area it will add greatly to the value of the sustaining surface.

Unfortunately, the long fiat plane does not lend any aid in this particular, as the stream line flows down along the top, as shown in Fig. 23, without being of any service.

_Fig. 23. Air lines on the upper side of a Plane._