CHAPTER XIPRESENT DEFICIENCIES AND FUTURE POSSIBILITIES OF THE MILITARY AËROPLANE
Inthe portion of this handbook which especially dealt with airships, certain advantages possessed by them over aëroplanes were noted; several of their disadvantages were also a matter of comment. It was hinted that in the future it might be possible to impart to aëroplanes also those very advantages of which the airship can still certainly make boast. Should this be done by engineering skill—and it is well within the limits of reasonable possibility—then it would seem that the lighter-than-air machine must entirely yield its claim as an adjunct of war to the heavier-than-air principle. The free balloon “mounting heavenwards,” as Carlyle said, “sobeautifully, so unguidably,” is now merely a past reminiscence, and even so, too, will be the mammoth motor-impelled gas envelopes. When the din of war ceases, the still greater perfection of the aëroplane should be the object of the attention of British engineering skill. The endowment of the aëroplane with certain qualities in which it is still deficient appears to be merely a matter of engineering detail based on principles that have been already elucidated.
Since the brothers Wright made their epoch motor flights which gave to man the attribute of the bird, so long his envy, progress in flight records has been largely made in the attempt to win a money prize. In one sense the pilot has progressed at a faster rate than has the evolution of the machine. He has accomplished heights, durations, and distances on machines in which the margin of safety is indeed small. It might be well if the next series of prizes should be devoted to the further development of the machine itself—prizes which would, in their turn, stimulate the genius of the aëronautical engineer.
Four essential points in the future development of flying machinesare:—
1. Variable speed.2. Immediate rising into the air.3. Hovering in the air.4. Stability.
1. Variable speed.
2. Immediate rising into the air.
3. Hovering in the air.
4. Stability.
1.Variable speed.
The aërial machine that cannot vary its speed, so as to be able to go fast, at moderate pace, or quite slow, must from one point of view be in a crude state of development. Yet aëroplanes are as yet in this stage of growth.
More than one plan has been suggested for endowing the aëroplane with the power of variable speed, which would make its use in war still greater. One of these plans is the extension and reducing at will of the sustaining surfaces, so that for high speeds the practical minimum of surface may be utilised, for low speeds the practical maximum. A machine to produce this result has been already planned by Mr. C. F. Webb. Itwas described at a meeting of the Aëronautical Society of Great Britain in 1906. At the time of the reading of the paper the world was hardly ready to realise the importance of considering this problem; at the present moment all military aëronautical experts agree as to the advisability of the production of a variable speed flying machine, though they shirk the complexity of structure the variable speed machine would seem to necessitate. In Mr. Webb’s design is a form of aëro-surface which, by special adaptation, can vary its area in accordance with the requirements of, and in proportion to, the constants, speed, and weight, and thus automatically adapt itself to the requirements of the varying speed of the wind. In this machine the two wings are situated on each side of the car in such a way that the centre of support of each is some distance above the centre of the mass of the machine. Each wing is fan-curved from front to rear, with the outermost segment longer than the innermost. The fan wings are opened or contracted by a hand-leverarrangement, and besides the hand levers there is an automatic pendulum mechanism which regulates their area to the requirements of the wind. Whether or no the inventor’s exact arrangements may prove on trial to be successful is a matter on which decisive opinion cannot be given; but the principle of expanding and diminishing surface is thoroughly sound, and is worthy of lavish expenditure and experiment. Other ways of attaining variable speed machines have been suggested, though the method of a variable surface would seem likely to carry the regulation of speed to a greater nicety than do the other plans. One of these projects is to alter the angle of the incidence of the planes while the machine is in flight; the angle would have to be steep for slow speed, and gradually flatten for increase of speed.
2.Immediate rising into the air.
It is undoubtedly a disadvantage of the aëroplane that it has to run on the ground on wheels to get the initial velocity necessary for flight.In some of the earlier military experiments with aëroplanes the machines were made to run over ploughed fields, for it was recognised that machines which could only rise when running on smooth ground would be useless for military work. But one can imagine that it may often be expedient in military operations for machines to rise from land so unequal that with the present method flight would be impossible.
The perfect military aëroplane should be able to rise in the air at any time and from any place. The application of horizontal lifting screws beneath the flying machine would make this a possibility, though it would be necessary to have two of such screws revolving in opposite directions. It is indeed curious that so little has been done in the way of such experiments. It will be said that each added screw means engine multiplication and complication; but these difficulties are details of engineering that are not unsolvable.
In the case of such large aëroplanes as the Russian type that has been described, it wouldseem specially feasible to attach the lifting screws.
3.Hovering in the air.
One great advantage of the lifting screws would be that by their use the machines could hover in the air. Now, when the vertical screw is stopped, the aëroplane must fall to earth unless the aviator makes the “vol-plané.” This necessity brings into strong relief the present imperfection of the flying machine. When horizontal screws are attached to a flying machine we really have the essential feature of sustentation, and the existence of the ordinary supporting surface becomes superfluous. The flying machine has, in fact, become of the “Hélicoptère” type, though doubtless for some time the supporting surface will be retained as a means of additional security; in time it may vanish altogether, and support as well as progression depend upon revolving screws.
4.Stability.
It has been stated that the properly constructedairship is stable when in the air; it has not got to fear the more treacherous side gust which over and over again has brought the aëroplane to earth, and coupled its name with tragedy. The vexed problem of the stability and equilibrium of aëroplanes is the most important that has yet to be solved; until this is done it is not likely the airship will completely disappear as an instrument of war. In speaking of the remarkable exploits of Pégoud, it was said that they were an object-lesson on the materiality of the air, and we have yet to learn how to use this materiality to the best advantage, so as to afford us continual stability. Until the problem is solved, man cannot be said to have brought himself to the level of the soaring bird; the latter, indeed, makes good use of the very attributes of the wind which at present tend to upset the aëroplanist—the vertical component of the wind, its internal work,i.e., its gustiness; its non-uniformity,i.e., its different velocities at different levels. Every light, therefore, that can be thrown experimentallyor mathematically on the difficult subject of equilibrium and stability should be eagerly sought.
Professor G. H. Bryan’s mathematical researches are indeed epoch-making, and their study by the aëronautical engineer should be prolific of practical result. He does much to elucidate points of the problem of stability that before had been imperfectly grasped. For instance, take the case of his remarks as to distinction between equilibrium and stability.
We say that the motion of a flying machine is steady when the resultant velocity is constant in direction and magnitude, and when the angle of the machine to the horizontal is constant. If this motion is slightly disturbed the machine may either return after a time to the original motion, or it may take up a new and altogether different mode of motion. In the first case, the steady motion is said to be stable, and in the second unstable.It is evidently necessary for steady motion of any kind that there should be equilibrium—i.e., that there should be no forces acting on the machine (apart from accidental disturbances) which tend to vary the motion, and hence it follows that the numberof modes of steady motion of which a machine is capable is, in general, limited, and that when an unstable, steady motion is disturbed, the new mode of motion taken up is entirely different from the old.It is necessary to distinguish carefully between equilibrium and stability, as the two are very often confused together. Equilibrium is necessary to secure the existence of a mode of steady motion, but is not sufficient to ensure the stability of the motion. The question of the stability of a rigid body moving under the action of any forces has been solved by Routh. In order to apply his results to the stability of flying machines, it is necessary to know the moment of inertia of the machine about its centre of gravity, the resistance of the air on the supporting surfaces as a function of the velocity and angle of incidence, and also the point of application of this force—i.e., the centre of pressure for different angles of incidence. If these are known for the surfaces constituting any machine, then the problem of its stability for small oscillations can be completely solved. Unfortunately, our knowledge of these points is very unsatisfactory. Several valuable series of experiments have been made to determine the resistance on planes, but there is still some doubt as to the position of the centre of pressure at small angles of incidence, especially for oblongplanes, and very little indeed is known as to the movement of the centre of pressure on concave surfaces. Until experiments are made on this point it will be impossible to solve the problem of stability for machines supported on concave surfaces.
We say that the motion of a flying machine is steady when the resultant velocity is constant in direction and magnitude, and when the angle of the machine to the horizontal is constant. If this motion is slightly disturbed the machine may either return after a time to the original motion, or it may take up a new and altogether different mode of motion. In the first case, the steady motion is said to be stable, and in the second unstable.
It is evidently necessary for steady motion of any kind that there should be equilibrium—i.e., that there should be no forces acting on the machine (apart from accidental disturbances) which tend to vary the motion, and hence it follows that the numberof modes of steady motion of which a machine is capable is, in general, limited, and that when an unstable, steady motion is disturbed, the new mode of motion taken up is entirely different from the old.
It is necessary to distinguish carefully between equilibrium and stability, as the two are very often confused together. Equilibrium is necessary to secure the existence of a mode of steady motion, but is not sufficient to ensure the stability of the motion. The question of the stability of a rigid body moving under the action of any forces has been solved by Routh. In order to apply his results to the stability of flying machines, it is necessary to know the moment of inertia of the machine about its centre of gravity, the resistance of the air on the supporting surfaces as a function of the velocity and angle of incidence, and also the point of application of this force—i.e., the centre of pressure for different angles of incidence. If these are known for the surfaces constituting any machine, then the problem of its stability for small oscillations can be completely solved. Unfortunately, our knowledge of these points is very unsatisfactory. Several valuable series of experiments have been made to determine the resistance on planes, but there is still some doubt as to the position of the centre of pressure at small angles of incidence, especially for oblongplanes, and very little indeed is known as to the movement of the centre of pressure on concave surfaces. Until experiments are made on this point it will be impossible to solve the problem of stability for machines supported on concave surfaces.
The subject of the stability of aëroplanes falls under twoheads:—
1. Automatic stability.2. Inherent stability.
1. Automatic stability.
2. Inherent stability.
Attempts have been made to produce the first by the aid of moving gyroscopes and pendulums without much success, and Professor Bryan has pointed out, apart from the fact that movable parts are likely to get out of order, they also increase the degree of the friction of the machine, thus further adding to the number of conditions that have to be satisfied for stability.
It would seem, therefore, that the desideratum is inherent stability. Professor Bryan considers that there is hope of attaining longitudinal and lateral stability by the use of exhaustive mathematical researches; these will result in the fixing of independent auxiliary surfaces in aëroplanesin such happy positions as will secure stability in all conditions of atmosphere. Or it may well be that through some unlooked-for observation or simple experiment the answer will come. In the shape of the aëroplane surfaces alone may be the solution of the problem. But if the aëroplane be still an imperfect instrument, it is sufficiently developed to be already one of the greatest factors of modern warfare.
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