Popular Science Monthly/Volume 28/November 1885/Flying-Machines
THE subject of my paper—flying-machines—in a general way, is of interest to everybody. But, to those who have given it more particular attention, it is not only interesting but fascinating, and a little dangerous. The pathway has been strewed with wrecks; and I fear there is a feeling prevalent that, after all, it leads nowhere in particular, unless it be to the almshouse or lunatic asylum.
Still, there are times when we heartily envy the birds their wonderful power. I remember in reading, I think, Mr. Wallace's book on the Amazons, that he was once standing on the shore of the mighty river, confronted by an impenetrable wall of green, concealing within itself doubtless no end of new plants and beetles; and when a gayly painted macaw came sailing lazily along and disappeared behind the tree-tops without any sort of trouble, he gave vent emphatically to the general wish to fly, and to a feeling of surprise that apparently so simple a problem should have remained so long unsolved.
I propose here to give an account of some of the attempts to fly that have been made in the past, and are now being made; and to try to explain the principles involved, and why success has not been achieved.
The old Greeks and Romans very sensibly appear to have been content to give the gods and birds and butterflies a monopoly of the air; for, excepting the story of Dædalus and Icarus, little mention has been made by classical writers of attempts to fly, or of flying-machines.
Dædalus, it seems, had killed a man in Athens, and with his unfortunate son fled to Crete, where King Minos very properly detained him; but, determined to escape, he made wings of feathers cemented with wax, and, instructing Icarus to fly neither too high nor too low, but to closely follow him, launched himself into the air, and took a bee-line for Greece. The young man, however, was ambitious, and, flying too near the sun, the wax melted, and he perished in the sea—a warning to future generations.
After Dædalus, we next hear of Archytas of Tarentura in Sicily, a famous geometrician who lived about 400 years b. c. He is credited with a dove made of wood, so contrived, we are told, "as by certain mechanical art and power to fly; so nicely was it balanced by weights and put in motion by hidden and inclosed air." One is surprised at the amount of talk and speculation that these few words have caused. If the dove were put in motion by inclosed air, then probably it was constructed on the principle of a balloon. If so, then of course the air must have been heated; or, better, since wood will crack and warp from heat, not unlikely a light gas was used; and since hydrogen is light, possibly hydrogen; and if so, how did Archytas prepare it? Others seriously try to throw ridicule on the whole affair, saying that a wooden dove could not possibly get support in such a way—that necessarily it would be too large and heavy, and that the material would not stand the strain, and so on.
For my own part, however, I think that old Lauretus Laurus had the true theory and explanation. He says that "the shells of hen's eggs, if properly filled, and well secured against the penetration of the air, and exposed to the solar rays, will ascend to the sky, and sometimes suffer a natural change; and if the eggs of the larger description of swans, or leather balls, stitched with fine thongs, be filled with niter, the purest sulphur, quicksilver, or kindred materials, which rarefy by their caloric energy; and if they externally resemble doves they will easily be mistaken for flying animals.
"If we should desire to give aërial motion to a wooden and ponderous machine, we must apply fire. Should there be any apprehension of the dove being burned, it can be covered over with some incombustible coating, and tubes of tin introduced, so that the fire may be kept alight in its bosom without injury to it. . . . To prevent the crackling of flames, and the emission of sparks, the powder may be deprived of force by the mixture of ochre and butter. . . . An artificial throat may be formed to change the crackling of the flames into an imitation of the cooing of a dove. Tubes could have been easily" (and probably were) "constructed to ascend one after the other at convenient intervals, so that the bird would apparently be endued with life."
After Archytas, we hear little or nothing of flying-machines until the middle ages. Then the astrologers and alchemists and witches, in league with the evil-one on the one hand, and the friars and monks helped by good spirits on the other, did many wonderful things. The competition was strong. To simply fly was a mere bagatelle, a ready means to the sinful or good end in view. The broomstick took a pre-eminent position as a flying-machine. What a pity it is that our ancestors should have so persistently fought against and finally succeeded in surpressing the broomstick! What could be more simple and effective? Perhaps by proper treatment the witches might have been persuaded to instruct the rest of the world in its use. In those days, dragons and magicians and good and evil spirits made out-of-doors at night rather dangerous, and good people remained at home, with holy water on hand for an emergency. Here is an example from Remigius. Says he: "There is no doubt the following will be considered incredible by all and ridiculous by many; yet I can aver that two hundred persons testified to its truth. On regular and stated days these people assembled in a crowd on the banks of some lake or river, secluded from the observation of passers-by; and there they were in the habit of lashing the water with wands received from demons, until such time as vapors and mists were produced in large quantities, and with these they were wont to soar on high. The exhalations thus provoked condensed themselves into thick and darkling clouds, agitated and swept the heavens, assisted in their atmospheric war by the evil spirits whom they wrapped in their folds, and at length in a hail-storm smote the earth in their fury. … Salome and Dominica Zabella, however, add that, before they thus agitated the water, they were in the practice of throwing into it an earthen pot, in which a little previous a demon had been inclosed, together with some stones of such size as they wished the hail to be. … Decker Maygeth states that he and his confederates in crime used to receive candles from a demon of an azure color, and sail with them some distance from the margin of the lake, hold the light downward and let it drop freely into the water; that after that they scattered and spread some medicinal powder over the surface; that they then, with black rods, bestowed on them by demons, most vehemently lashed the waters, accompanying the action with a repetition of incantations to produce the desired results. Then the sky became overcast with clouds, and discharged torrents of rain and hail on those localities which they had pointed out." This incantation, Romigius says, "is not an invention of modern ages. It is not the invention of old hags whose mental powers were depraved by demons, or perverted by visions or dreams. It was practiced by men of keen intellects and acute investigation, who minutely observed, critically examined, and deliberately adopted their convictions."
Here is a description, according to Kircher, of a flying-machine invented by one of the fathers of the Church: Some of the fathers in India had been "cast into prison, and while they continued ignorant of any means of effecting their liberation, some one, more cunning than the rest, invented an extraordinary machine, and then threatened the barbarians, unless they liberated his companions, that they would behold in a short time some wonderful portents and experience the visible anger of the gods. The barbarians laughed at the threat. He then constructed a dragon of the most volatile paper, and in this inclosed a mixture of sulphur, pitch, and wax, and so artistically arranged all his materials that when ignited it would illumine the machine and exhibit this legend—'The wrath of God.' The body being formed and the ingredients prepared, he affixed a long tail, and committed the machine to the heavens. Favored by the wind, it soared aloft toward the clouds. The spectacle was terrific. The barbarians beholding it were smitten with the greatest astonishment and fear. . . . Thereupon without delay," says Kircher, "they threw open the gates and suffered the prisoners to go forth in peace."
In the middle ages, anybody at all distinguished by knowledge of science was credited with the art of flying, and indeed in many cases did not scruple to claim it. Albertus Magnus was one of these, but refused to give particulars to the world at large. He tells us, however, how to make thunder. Says he: "Take one pound of sulphur, two pounds of willow carbon, and six pounds of rock-salt, ground very fine in a marble mortar; place where you please in a covering made of flying-papyrus to produce thunder. The covering, in order to ascend and float away, should be long, graceful, and well filled with this powder; but to produce thunder the covering should be short and thick, and half full."
Roger Bacon, an eminent philosopher of the thirteenth century, also claimed to have knowledge of the art of flying, but believed also in the wisdom of silence concerning the details. But in his writings we find flashes of real light. He speaks of the possibility of constructing engines of great power to traverse land and sea; and seems to have been the first to have tolerably clear ideas of the principles involved in the construction of balloons. He describes a large hollow globe of copper or other suitable metal wrought extremely thin. It must then, he says, "be filled with ethereal air or liquid fire, and then be launched from some elevated point into the atmosphere, where it will float like a vessel on the water."
In his day the air was supposed to have a well-defined upper limit, like the water.
Friar Bacon too has been credited with the invention of gunpowder. He was of course accused of holding communion with the devil. Good Pope Nicholas placed his writings under a ban, and his wings were effectually clipped.
Shortly after his time, the project of training up children from infancy to fly received a good deal of attention, and, if we can trust the accounts, considerable progress was made, for it is said that, by combined running and flying, individuals could skim over the ground with great rapidity.
Regiomontanus, a famous mathematician, is said like Archytas to have formed an artificial dove, which flew out to meet the Emperor Charles V at his public entry into Nuremberg. But, if this is true, the dove must have survived its inventor for at least twenty years. Then we are told of a monk who attempted a flight with wings from the top of a tower in Spain. He broke his legs, and was afterward burned as a sorcerer. Another similar trial was made from St. Mark's steeple in Venice; another in Nuremberg; and so on—legs or arms were usually
broken, occasionally a neck. In the sixteenth century we read of a certain Italian who went to the court of James IV of Scotland, and attempted to fly from the walls of Stirling Castle to France. His thigh was broken; but, as a reason for the failure, he asserted that some of the feathers used in constructing his wings were from barn-yard fowls, with a natural affinity for the dung-hill; whereas, if composed solely of eagle-feathers, they would have been attracted to the air. However, he docs not appear to have carried the experiment further.
Many other trials have there been of the same character. The results were generally discouraging, but men can always be found ready to risk life and limb in striving to attain something much less important than the art of flying; without a knowledge of the principles involved, ignorant of the nature of the atmosphere, without machinery or power, fettered by a superstition that looked upon all learning outside of the Church as coming from the prince of darkness, it was a struggle in the dark—brave but hopeless.
Still, those old fellows were quite as reasonable in their attempts as many of our inventors are now. In looking through Patent-Office reports, we shall find devices only slightly different in detail from those tried five hundred years ago.
One of our illustrations shows the plan proposed by Rétif de la Bretonne away back in the dark ages; and another an apparatus patented in this country in 1872. It is only one of numbers of the same sort. Rétif had an advantage, in that he carried a lunch-basket and umbrella, and did not need so many ropes and spars; but otherwise the later arrangement seems equally good.
In 1783 the Montgolfiers invented the balloon. Friar Bacon, as we have seen, had speculated upon the possibility of such a construction. In 1670 Francis Lana, a Jesuit, had described an apparatus which, although impracticable in so far that it could not be built, nevertheless was correct in principle. The same idea had occurred to others; and there are even shadowy accounts of actual ascents. But to the Montgolfiers certainly belongs the honor of first actually building and bringing the balloon before the public as an accomplished fact. They used hot air only, but the substitution of hydrogen gas by Professor Charles speedily followed, and in a few years the balloon was made as perfect, excepting in a few details, as it is now.It would be difficult to describe the excitement which followed this invention. The most extravagant hopes and anticipations were entertained. The problem had been solved. The birds and insects would no longer have a monopoly. Every gentleman would have a balloon hitched to his gate-post, or, wafted along by summer breezes, would look down in luxurious pity upon the poor plodders. Sails and rudders were to be used as on ships to direct the course. Regular lines of aerial passenger and mail coaches were to be established. There seemed no limit to the possible speed. Rome, or St. Petersburg, or even America, might be reached in a few hours, and for the comfort of travelers the arrangements proposed went far ahead of our palace cars. Floating hospitals were to be built; methods of warfare would need to be entirely reorganized; and England's boasted supremacy on the sea would be of no avail, unless she also maintained supremacy in the air.
The Modern Flying Man. (Taken from United States Patent-Office Reports.)
Of course an invention of such importance could not escape condemnation. Balloons were manifestly contrary to the will of Divine Providence, for, if it had been intended that man should fly, wings would have been given to him. Moreover, the barriers of virtue and morality would be broken down by permitting aëronauts to descend into gardens and balconies; and, above all, the boundaries of empires would be practically annulled, and nations in consequence engage in continual war.
Well is it, then, for humanity that balloons have not proved a very great success. Many extensive voyages and many interesting observations have been made; but as a flying-machine the balloon has no place. It is the servant of the air, not the master. It must obey a will, pitiless, fickle, sometimes kind, but never trustworthy. The expectation that headway could be made against the wind by means of sails and rudders had no basis in sound theory or sense. A sailing-ship is immersed in two fluids of widely differing densities, and its sail is only effective because the water, while supporting, at the same time allows the vessel to move more readily in one direction than another.
Fig. 3.—Sullivan's Flying-Machine. (Taken from United States Patent-Office Reports.)
A balloon, on the other hand, is totally immersed in an ocean of air, and being of the same weight bulk for bulk, and subject to no external forces, must necessarily follow the slightest current. One might as well attempt to steer a boat, swept along by a great stream, without wind or oar. It forms an integral part of the current itself. It is a thistle-down blown by an autumn gale.
Of course we may provide our balloon with wings or propeller, and fly as the birds fly. This has been and continues to be a favorite combination with our inventors. One patented in this country in 1880 has been chosen as an illustration. The balloon, oblong in shape and divided for safety into compartments, supports a car containing the propelling machinery, and also a gas-generator to make up such loss of hydrogen as may occur. Two immense rudders steer the machine. It is propelled by four paddle-wheels, which would act, one would think, very much as the wheels of our river-steamers would act, if totally immersed in the water, and would be about as likely to drive the balloon backward as forward.
Generally, however, in machines of this class the propeller is one gigantic screw, or a number of screws, and the balloons have a variety in shape and grouping which is quite remarkable.
It is strange that people have not realized that a thing necessarily so big and light as a balloon can not be made strong and durable enough to stand the pressure of the wind at comparatively low velocities. Floating with the current, the velocity would have no destructive effect; but brought into opposition to this current, or forced at any great speed through the air, the resistance would be much greater than a silk bag could safely stand.
It may be well here to refer to a table giving the relation of pressure to velocity of air, experimentally determined and verified time and again—results very important in the study of flying and flying-machines:
|VELOCITY OF THE WIND.||Pressure on one square
|Character of the wind.|
|Miles per hour.||Feet per second.||Pounds.|
|10||14·67||0·492||Pleasant brisk wind.|
|150||. . . . . .||. . . . . .||Sometimes reached.|
Now let us suppose that a balloon only forty feet in diameter should resist the pressure of wind blowing at the rate of twenty miles an hour, or, what is the same thing, that the balloon should be traveling through still air at this speed. The surface presented to the wind would be about twelve hundred square feet, and the pressure on each square foot, from our table, would be 1·9 pound, and the total pressure over a ton. A calculation is hardly necessary to show that such a pressure, acting constantly upon our silk, would be likely to rupture it; and when we consider that sudden gusts might readily increase the pressure five-fold, it will be admitted that terra firma would be decidedly safer, if less exciting.
More than all this, balloons as hitherto constructed are at best but temporary affairs, quickly losing their gas and buoyancy, expensive and unwieldy, and, however valuable for certain kinds of work, must be considered as simply floating, not flying machines. If we expect to gain the respect of the birds or butterflies, we must go to work in a much less clumsy way.
In the excitement following Montgolfier's invention, simple flying machines dropped out of sight almost entirely, and it was only after a long series of disappointing trials that the old ideas came to the surface again. The balloon craze, however, brought about a more careful study of aëronautics generally; but at the same time there has been and is a strong current of misguided thought and invention, particularly to be noticed in our Patent-Office reports.
Inventors of flying-machines, as a rule, belong rather in a lower class. Just as we still find old-new arrangements for producing perpetual motion, so in the attempts to fly the old story is repeated. The perpetual-motion man is likely also to know just how to make a successful flying-machine. lie only lacks the means. Still, particularly in England and on the Continent, many able men have been working intelligently, perseveringly, quietly. Before building a flying-machine they have thought best to study the examples Nature has provided, thinking that, while we need not necessarily imitate the mechanism, we may in this way get a better idea of the principles and action involved.
The broad principle governing either natural or artificial flight is quite simple, but the difficulty of applying it very great. Our flying machine, one that is much heavier than the air, and depending entirely upon its own power, in the first place, must be able by acting on the air to lift itself, and, while maintaining a position at any desired height, to propel itself forward. It must be prepared to encounter and take advantage of, and overcome currents of air sometimes hardly perceptible, sometimes perhaps a roaring gale—currents, too, not unlikely to suddenly change both in direction and velocity. It should be able to fly continuously for a long while, and should be tolerably safe.
On the water, if the machinery gives out, we can float or swim; but in the air any little difficulty of the sort would be likely to end unpleasantly. And even if, like a parachute, the machine could be made to drop slowly, in a brisk wind the final landing-place would for a while be a matter of uneasy conjecture.
It may easily be understood, then, that the problem is not a simple one, and yet, to a person watching, for example, the flight of a flock of gulls following in the wake of a steamer, the exquisite ease and grace and apparent simplicity of the movement are very striking. Sweeping around in circles, occasionally elevating themselves by a few flaps of the wings, they glide down and up the aerial inclines without apparently any effort whatever. But a close observation will show that at every turn the angle of inclination of the wings is changed to meet the new conditions. There is continual movement with power—by the bird it is done instinctively, by our machine only through mechanism obeying a mind not nearly so well instructed.
The study of the flight of birds and insects has of late years received a great deal of attention, and, in a general way, the motions of the wings are fairly well understood. We could probably very closely imitate these motions, but the question at once arises, in doing so, would we be applying our power in the most effective way? While somewhat similar, the movement and construction of the wings of flying creatures vary considerably. What is best for a heavy body with short wings is by no means best for a light body with long wings; nor does a sea-bird, constantly on the wing, but perhaps not a rapid flier, fly in the same way as a pigeon or humming-bird; and, in any particular case, it does not necessarily follow that Nature has provided the most efficient apparatus; or, in other words, that the power the bird possesses could not be utilized more effectively. Nature can not always be trusted. We can study and understand her laws, but she does not pretend to apply them on economical principles. Fish and marine animals swim in a great variety of ways, they have all sorts of propelling arrangements, but there can be no doubt that a screw-propeller is vastly more efficient than any of them; and why should we try to copy the motions of a bird's wing any more than those of a fish's tail? The motions are very complicated in any case, and our machine, imitating them, would be complex and liable to get out of order. And one can not help thinking that we are about as likely to make a steam road-wagon by imitating the action of a horse, as we are to make a practicable flying-machine by copying the motions of a bird. The desired results can probably be obtained in a much more simple and effective way.
Still, the study of flying creatures has brought out many interesting and suggestive facts, and has given us, too, some encouragement.
In the first place, we notice that all birds are heavy, and that the expanse of wing generally diminishes in proportion to the increase of weight. The following is a table prepared by M, Lucy, showing this very clearly:
Table giving the Expanse of Wing-Surface for each Pound of Weight.
We see that the gnat, one of the lightest of insects, has an expanse of wing of no less than 48·9 square feet for each pound of weight, while the heavy cockchafer has only 5·1 square feet for each pound. With birds, the sparrow has 2·7 square feet of wing-surface for each pound of weight, while the great Australian crane has only 0·41 of a square foot, and yet this bird undertakes remote journeys, and, the eagle excepted, Hies higher, and keeps on the wing longest, of all the travelers.
It would appear, then, that our flying-machine, while heavy, need not necessarily have a very broad expanse of flying surface. Indeed, Paradoxical as it may seem, weight is really an essential feature. Set in motion by muscular effort, the weight of a bird acts somewhat like the fly-wheel of an engine: the power is stored up during the downward stroke of the wing, to be given out again on its upward stroke, and probably it is weight also that enables the bird to successfully combat and take advantage of the force of the wind. It is noteworthy that all sailing-birds, like the hawk or vulture, have comparatively heavy bodies. The magnificent albatross, in rising from the water, is said to beat the air with great energy, but, when fairly launched, in a brisk gale, will sweep around in broad circles for hours together, hardly ever deigning to flap a wing. Darwin, in his "Voyage of the Beagle," speaks of watching the condor sailing in a similar way at a great height, without, so far as he could notice, any flapping action whatever.
At the same time, it is hard to understand how such a condition of affairs could exist. The condor's wings, inclined to the wind, have been compared to a kite, and if there were a string stretching from the bird to some fixed point, the whole thing would be clear; but every boy knows to his cost that, if the string slips or breaks, the kite quickly seeks some other point of support—probably a telegraph-wire. But Professor Pettigrew has suggested that the string is the invisible one representing the attraction of gravitation, and that "the string and the hand are to the kite what the weight of the flying creature is to the inclined planes formed by its wings." This, however, does not make the matter much clearer, for the force of gravity acts in vertical lines, and a vertical kite-string, with the kite flying directly overhead, is a thing, it is safe to say, no boy ever saw. Why should not our bird drift with the wind unless he uses some muscular effort to overcome its force or to keep himself from falling?
Once elevated, he can utilize his weight in a number of ways. A body will naturally fall along a line of least resistance, and if ^the front edge of the wings be tipped slightly downward the bird will glide forward while falling, gaining velocity and momentum; and then, by reversing the inclination of the wings, he can again glide up an aerial incline until this stored-up energy has been expended. But the resistance of the air must be overcome, and there must be continual loss from the imperfect sustaining power of the wings.
We shall see presently that the force of the wind can be utilized to a certain extent to make up these losses, but still some muscular effort should be required. If our vulture or albatross would only occasionally deign to flap a wing, all would be well. His obstinacy is very perplexing.
Leaving the birds to their own peculiar devices, let us now consider what principles should guide us in constructing a flying machine.
In the first place, by acting on the air, the machine should be able to lift itself from the ground; and, leaving out of account small models, this is a preliminary no one appears so far to have succeeded in. Many pictures may be seen of flying-machines booming along through the air with all sails set, passengers evidently happy, some serenely smoking, others promenading the deck in the usual way, with perhaps a couple behind the wheel-house; but a representation of a machine just on the point of starting out is not to be met with.
In order to produce an upward pressure or reaction, the wings or propeller acting on the air evidently should drive it downward. Suppose now that our machine weighs 600 pounds, and that it has the same propelling surface in proportion to its weight as the Australian crane, we should then need about 246 square feet, and a pressure of 2·4 pounds acting upward on each square foot would lift it from the ground.
Referring again to the table giving the relation between wind velocity and pressure, we notice that a pressure of 2·4 pounds would be occasioned by a velocity of about twenty-two miles an hour.
If, then, we should cause our propeller—be it a screw or wings, or any other form—to drive downward a current of air at this rate, the cross-section or area of the current being 246 square feet, the total upward reaction would be great enough to raise the machine.
Of course, for any other proportion of wing-surface to weight, our table would give other results; or if the air is already in motion, it will tell us what increase of velocity should be given to produce the desired pressure.
The results given in the table can also be readily found in a purely theoretical way, and they seem so important that it is a wonder investigators have given them little or no attention.
A machine possessing weight can fly only by doing something to the air. It must put the air in motion, and it can be shown that the amount of this motion will be a measure of the work done and reaction obtained.
If air is already in motion, we can not utilize its force, not wishing to drift along, except by changing in some way its velocity.
Granting all this, our table or formula will tell us, not only what volume of air must be used to gain the desired reaction or motion, but also the least power necessary. Knowing the weight of and velocity impressed upon the air, downward or in any other direction, it becomes an easy matter to determine the power.
For example, in the practical case just considered, to lift the machine from the ground would require an expenditure of at least eighteen horse-power. This is the least power that would do the work—the actual power would depend entirely upon the efficiency of the propeller.
Having at last succeeded in getting away from the ground, we wish to fly in any direction—to set the birds an example of how the thing ought really to be done.
Here, again, we must apply the principles just announced. To go forward, the air must be driven aft. Knowing the speed proposed, our table will give us at once the resistance for each square foot; and knowing the size or bulk of our machine, we can readily estimate the power required.
The management of the wind unquestionably will be a very important factor in the construction of a flying-machine; indeed, it may be considered the most troublesome part of all. Properly handled, the wind might be made a useful servant, otherwise a dangerous master.
The only plan that suggests itself is through the use of an inclined plane. Here, at any rate, we must imitate the birds.
My attention was not long ago called to an article on Aëronautics, in the Proceedings of the New Zealand Institute for 1878, and in it was a table from experiments by Mr. Skye, giving the lifting power of the wind, blowing at the rate of twenty-three miles an hour upon a plane surface, one square foot in area, inclined at various angles. These figures lead to some very surprising and interesting results:
|Angle plane makes
|Lifting force, In pounds.||Drifting force, In pounds.||Ratio between the two.|
It will be seen from the second column that while the greatest lifting effect occurs at about an angle of 40°, even at so small an angle as 5° it is still considerable. The third column gives values for the corresponding horizontal pressures; that is, the force which tends to move the plane in the direction of the wind. The fourth column gives the ratio between the two.
It will be seen that the drifting force diminishes at a much faster rate than the lifting force, as the angle of inclination of the plane becomes less.
Consider again the flying-machine weighing 600 pounds, and suppose that, in addition to the propeller, we furnish it with an inclined plane having the same area, or, perhaps after the manner of birds, make the propeller act also as an inclined plane; and let it be inclined five degrees, with the wind blowing at the rate of twenty-three miles an hour. Then the table shows us that the total lifting force due to the wind would be 278 pounds, leaving 322 pounds to be supported in some other way. The horizontal or drifting force would be 023 pounds on each square foot, or only 56 pounds altogether. To counteract this, let us make our propeller act as a kite-string by sending backward the air at an increased velocity. Our other table tells us how great this velocity should be, and makes the necessary power amount to only about half a horse-power. To support the balance of the weight, we should need also to send downward a current of air, involving an additional expenditure of about seven horse-power.
Combining the two, we get this extraordinary result, that while nearly nineteen horse-power was necessary to lift our machine from the ground, it could hold its own in a breeze of twenty-three miles an hour with an expenditure of only seven and a half horse-power.
No account has been taken of the wind blowing against dead surfaces, such as the body of the bird or machine. This, of course, would depend upon the shape. A bird's body is long and narrow, cleaving the air without great resistance, and a flying-machine should be fashioned similarly.
Other losses have not been considered, but still the broad result holds that it is possible in this way to utilize part of the energy stored up in the wind. The accuracy of the results will depend upon that of Mr. Skye's table; but if future experiment should verify it, we can understand why it is that the albatross, and wild-duck, and heavy birds generally, while rising with great difficulty, when once up keep on the wing with so much apparent ease.
However, there is still the necessity for a kite-string of some sort. There is a force tending to carry the bird along with the wind which must be overcome somehow, and I still fail to understand how the albatross can sail in the air indefinitely without some muscular effort.
From Mr. Skye's table, in connection with the other, we get this important practical result—that in a flying-machine, properly constructed, the greatest power required will be that necessary to lift it from the ground; and that once off, up to a certain limit, the stiffer the breeze the better.The efficiency of a propeller of any sort will depend not only upon its area, but also upon its ability to send the air away in parallel streams. If we wish to go forward, the air must be driven aft, and a forced current in any other direction will at best give us back but a fraction of its energy. Ordinary screw-propellers have not proved very effective, for the reason, probably, that revolving at great speed, they send off a large amount of air tangentially.
What, now, should be the mechanical construction of a successful flying-machine? How should it be built? In what way should the power be applied? I have tried to make clear what seem to me the principles involved, but the best method in which to apply them can only be found by patient and intelligent study and experiment. Many men have been and are now working at the problem, and that it will be eventually solved seems certain. A bird's muscles, while strong, are not as strong as steel, and while his power in proportion to his weight is great, we can exceed it; and let us not admit that we can not equal his intelligence in applying it.
One of our illustrations shows the flying-machine invented by Mr. Henson in England in 1842, and deserves mention as being the first
of importance designed to fly without the aid of muscular power. The chief feature was the very great expanse of its sustaining planes, which were larger in proportion to the weight than in many birds. The machine advanced with its front edge a little raised, and the air acting upon the lower surface, when the proper speed had been attained, was expected to lift and sustain it. This speed at the start-off was to be got by running down an inclined plane or hill, and the object of the screw-propeller was simply to keep up the motion. It is unnecessary to say that this machine did not work, and yet Henson evidently had a glimmering of what is required. He introduces the inclined plane and propeller, but does not apply them in a practical way. Such a machine, of course, would be completely at the mercy of the winds; and while he might find a convenient hill to roll down in order to get the required velocity, in coming to earth again there might be trouble,
Landell's flying-machine, invented in 1863, was also provided with an extensive aëro-plane, but differs in having screws acting vertically to sustain the machine in addition to those for driving it forward. Capping all are two parachutes, intended to open and prevent a sudden fall in case of accident. There are four sets of blades on each vertical screw-shaft, on the principle, one would think, that if one set would be a good thing, four sets would be four times as good. They would be likely to act somewhat like four screw-propellers, one behind the other, on an ocean-steamer. The mechanism was to be driven by a steam-engine. The dark object suspended below may be ballast to counteract any superfluous energy of the steam.
In 1868 Mr. Stringfellow built and exhibited a model of a flying-machine at the Crystal Palace, in London, where it took a prize. There are three aëro-planes, one above the other, with a broad tail behind. As in Henson's machine, no provision was made for lifting it
from the ground, the power being applied simply to produce or keep up horizontal velocity, the reaction of the air against the inclined planes serving to sustain the weight.
At the exhibition the model ran down an inclined wire, but refused to rise into the air. It weighed only twelve pounds, including an engine exerting one third of a horse-power, boiler, water, and everything. Of course, even if the model had been a success, no large machine constructed in such a way could be of practical value.
The machine designed by Mr. Moy in 1874 was somewhat similar to Henson's and Stringfellow's. There are two inclined planes, one behind the other, and two horizontal screws. The necessary speed to lift the machine was to be obtained by a preliminary run along the
ground on the wheels underneath. In coming to earth again we should only need to look out for some favorable locality, strike tangentially, and the resistance of the wheels over stones, fences, and the like would speedily bring us to rest.
These are the more important inventions of this class—that is, self-raising and self-propelling machines—and it must be confessed the results are far from encouraging. M. Pénaud and others have constructed flying models, but on too small a scale to be of much practical importance.
But still there are the birds; they completely refute the arguments of those who say, "It is impossible to build a successful flying-machine."