CHAPTER IIIFIRST FRUITS OF STUDY

Fig. 10.—The Phillips Wing-Curve.

The shape and curve of a plane, is of vital importance. A machine may be built, and an engine and propellers fitted, but the question is: Will the planes support through the air the load they have been given to carry? Phillips made many experiments, and in the end he produced a wing-shape which he patented. He pointed out that an advantage might be gained in lifting effect if the main curve or camber was situated near the front edge of the plane, and not in the centre (Fig. 10). The theory Phillips worked upon was this—and it is interesting if it can be expressed clearly. Taking a plane curved as he recommended, with this “hump” towards the front, and forcing it through the air as would be the case were an aeroplane in flight, the rush of wind which meets the edge of the plane is split into two currents—onesweeping above and one below. The air current below the plane, following its curve, is thrust downward, and in being so thrust down imparts a lift to the plane; while the current thrown above the plane—rushing up and over the “hump” which, as has been shown, is situated close to the front edge—will sweep rearwards in such a way that there is a partial vacuum or air space between the fast-moving wind current and the curved-down section of the plane behind the “hump.” The value of such a vacuum is this: it has a raising effect upon the surface of the plane, which is thus not only pushed up from below, but drawn from above.

Fig. 11.—Suction above a Cambered Surface.

How a vacuum is caused, by air passing over such an arched surface as Phillips recommended, may be shown in a simple experiment. Take a sheet of paper and curve it in the way shown inFig. 11, allowing the rear portion to hinge in such a way that it will move freely up and down. Then, if the sheet of paper is held between the finger and thumb and one blows across the top edge, the hinged flap at the rear will be found to raise itself—drawn up by the influence of the vacuum, such as Phillips describes.

Apart from his theory as to the dipping front edge of a plane, Phillips agreed with a suggestion made by Wenham; and this was that a plane, in order to be most effective in its “lift,” should be narrow from front to back. This theory meant that, as a plane movedforward, it was the curving front section which gripped and acted up the air; and that, if the plane was carried too far towards the rear, its lifting influence fell away, while the surface that was superfluous acted as so much resistance to the machine’s progress through the air.

Fig. 12.—Phillips’s Experimental Craft.

In furtherance of his views, Phillips built the strange-looking machine which is seen inFig. 12. It resembled, more than anything else, a huge Venetian blind; and he adopted this form so as to introduce as many narrow planes as possible. There were, as a matter of fact, fifty in the machine, each 22 feet long and only 1½ inch wide. The craft, as can be seen, was mounted on a light carriage which, having wheels fitted to it, ran round and round upon a railed track. A steam engine was used as motive power, driving a two-bladed propeller at the rate of 400 revolutions a minute. The machine was so arranged on its metals that, although the rear wheels could raise themselves and show whether the planes exercised a lift, the front one was fixed to its track—thuspreventing the apparatus from leaping into the air, overturning, and perhaps wrecking itself. Tests with the machine were successful. The lifting influence of the planes, when the engine drove them forward, was sufficient to raise the rear wheels from the track; and they did so even when a weight of 72 lbs., in addition to that of the apparatus, had been placed upon the carriage. In his main object, then, Phillips succeeded; and that was to show the lifting power of his planes. But his apparatus had not the makings of a practical aeroplane. He gained for himself, nevertheless, a name that has lived and will live. Even to-day, in discussing the wing-shape of some machine, draughtsmen will speak of the “Phillips entry.” Other workers did not pin themselves exactly to his shapes or theories, but these paved the way for a series of further tests.

Science was forging link by link indeed the chain that would lead to an ultimate conquest. Sir George Cayley suggested an arched plane; Wenham devised a machine in which narrow planes should be fitted one above another; and Phillips laid down the rule for a curve or camber of special shape, which should exercise most “lift” when thrust through the air. But still men lacked many things; all the links in the chain were far from being in their place; and one of the greatest flaws was that no man, even supposing he was able to build a machine that would fly, had learned as yet to balance that machine when it was in the air.

The building of large machines—Sir Hiram Maxim’s costly work—A steam-driven French craft which flew—Professor Langley’s research in America.

Of the way research next tended, it may be said that it was the first putting into practice of the theories science had laid down; for now, having an idea as to the shape of planes, and knowing that these planes could be made to carry a load through the air, there were engineers who began to build man-carrying, power-driven machines. In so doing, however, they may be said to have tried to run before they could walk. What they did was to provide the world with powerful flying craft before there were men who could handle them.

One of the most interesting and ambitious designs was that of Sir Hiram Maxim; and it was one to which he devoted years of labour and large sums of money. He is said, indeed, to have expended £20,000 upon aerial research. After a number of experiments with plane shapes, following the theories of Phillips, he began to build a very large machine, which he set upon a miniature railway as Phillips had done, using the same precaution of a check-rail to prevent it from rising more than a certain distance in the air. His apparatus, whenbuilt at Baldwin’s Park, Kent, weighed 8000 lbs.: it was, in fact, the largest machine ever built. The span of its planes was 105 feet, and they offered a total supporting surface of 6000 square feet.

Fig. 13.A. Elevating Plane; B.B. Outriggers; C.C. Operating Wires;D.D. Position for ascending; E.E. Position for descending.

Fig. 13.

A. Elevating Plane; B.B. Outriggers; C.C. Operating Wires;D.D. Position for ascending; E.E. Position for descending.

The inventor employed the suggestion made by Wenham, and fitted his lifting planes one above the other; while he used a horizontal plane in front of the machine to act as an elevator. This plane could be tilted up and down; and the idea was that, when it was tilted upward as the machine ran forward upon its rails, it would exercise such a lifting influence that the front of the craft itself would be raised, and so cause the main-planes to assume a steeper angle to the air; and the result of the planes being inclined thus more steeply would be to give them a greater lift, and so induce the whole machine to raise itself from the ground. This system is explained inFig. 13, and is important becausesuch lifting planes, for rising or descending, have now come into general use.

Sir Hiram Maxim employed another controlling surface which has become a feature of present-day aircraft, and this was an upright plane, which could be swung from side to side, and by which his craft was to be steered. Such a rudder-plane is illustrated inFig. 14. By this means, as will be shown later, practically all aeroplanes are steered to-day. The action of the aerial rudder, when it is moved from side to side, is like that of swinging the rudder of a ship; but for the same reason that propellers have to be made large—owing to the small density of the air—so an aeroplane rudder needs to be a comparatively large plane, in proportion to the size of the craft, before it will exercise an adequate turning influence.

Fig. 14.A. Vertical steering rudder; B.B. Outriggers; C.C. Operating wires;D.D., E.E. Positions assumed in turning.

Fig. 14.

A. Vertical steering rudder; B.B. Outriggers; C.C. Operating wires;D.D., E.E. Positions assumed in turning.

To drive his machine Sir Hiram used two specially-built and lightened steam engines, which developed a total of 360 h.p., and yet weighed only 640 lbs.; that is to say, they gave one h.p. of energy for each 1¾ lb. of weight. But they were only suitable for purposes of experiment. Sir Hiram himself wrote:

“The quantity of water consumed was so large that the machine could only remain in the air for a few minutes, even if I had had room to manœuvre and learn the knack of balancing it in the air. It was only too evident to me that it was no use to go on with the steam engine.”

The engines drove two canvas-covered wooden screws, each 18 feet in length, and the general appearance of the machine is indicated byFig. 15. In these trials, although it was always captive, the aeroplane demonstrated much that its inventor had set himself to prove. In Sir Hiram Maxim’s own words, it showed that it had “a lifting effect of more than a ton, in addition to the weight of three men and 600 lbs. of water.” He adds: “My machine demonstrated one very important fact, and that was that very large aeroplanes had a fair degree of lifting power for their area.”

Fig. 15.—The Maxim Machine.

So unmistakably did this craft show its lifting power, that—in one fierce effort to rise—it broke a check rail which kept it upon its metals, with the consequence that it became unmanageable, swerved sideways, and was wrecked. At this stage Sir Hiram, having no faith in the future of such steam engines as he was using, and having spent a large sum of money, was compelled to relinquish his tests. His trouble was that he was, as the saying goes, “before his time.” The machine was too ambitious and too large. That it would have lifted itself into the air was proved; but there was no man living who could have controlled it. To put in charge of such a craft a man who knew nothing of the navigation of the air, would have been like placing a novice at the levers of a 60-mile-an-hour express. Picture such a huge aircraft in the hands of a man who had never flown. It would rise, it is true; but how could one who was not an expert so adjust the angle of its lifting plane that it would glide smoothly from the ground and not rear itself upward and fall with a crash? A machine is struck by wind-gusts, too, when it is aloft; and there is the delicate art of making a descent, without damaging one’s craft by a rough contact with the ground. Besides, it would have been unlikely that this machine, being purely experimental, would have been perfectly balanced as it flew: it might have shown a tendency to slip sideways when in the air, or dive steeply. All of which goes to show this: that the inventor might have wrecked one costly machine after another before he obtained a practical model, even were he lucky enough to escape with his life. Sir Hiram Maxim’s machine, while it settledproblems as to weight-lifting and power, lacked the man who could fly it; and so did others of these man-lifting craft which were built before their time. A child must learn to walk before it can run, and must learn to crawl before it can walk. And what had not been realised, at this stage of the conquest, was that there must be some stage between building a model and a full-sized, motor-driven machine: some step, in a word, by which a man might learn, without too great a risk of death, to balance himself when in the air.

While Sir Hiram Maxim in England was devoting time and money to the quest, there was another skilled engineer, a Frenchman, who was working at the problem, and also by means of large machines. This was Clement Ader, one of the European pioneers of the telephone, and he experimented for many years. One of his first machines had wings like those of a bird, and these the would-be flier was to operate by his own muscular power. But this failed, seeing that men are not provided with sufficient power, by their unaided efforts, to wing their way through the air in a flapping flight. As Giovanni Borelli, a seventeenth century writer, quaintly puts it: “It is impossible that men should be able to fly craftily by their own strength.”

PLATE I.—THE LANGLEY MACHINE.This craft, a double monoplane, was launched from a platform over the river Potomac, loaded with a weight equivalent to that of a man. The trials were unsuccessful; but recently—after a lapse of many years—the Langley machine has been tested again, and has proved its ability to fly.

PLATE I.—THE LANGLEY MACHINE.

This craft, a double monoplane, was launched from a platform over the river Potomac, loaded with a weight equivalent to that of a man. The trials were unsuccessful; but recently—after a lapse of many years—the Langley machine has been tested again, and has proved its ability to fly.

Ader next turned to steam-driven craft, and built a series of queer, bat-like machines, which he called “Avions,” one of which is illustrated inFig. 16. Its wings were built up lightly and with great strength by means of hollow wooden spars, and had a span of 54 feet, being deeply arched. The whole machine weighed 1100 lbs., and was thus far smaller and lighter than Maxim’s mighty craft. To propel it, Ader used a couple of horizontal, compound steam engines, which gave 20 h.p. each and drew the machine through the air by means of two 4-bladed screws. The craft was controlled by altering the inclination of its wings, and also by a rudder, the pilot sitting in a carriage below the planes. In 1890, after its inventor had spent a large sum of money, the machine—which, unlike those of Phillips and Maxim, ran upon wheels and was free to rise—did actually make a flight, or rather a leap into the air, covering a distance of about fifty yards. But then, on coming into contact with the ground again, it was wrecked. Ader’s experiments were regarded by the French Government as being so important that he received a grant equalling £20,000 to assist him in continuing his tests; and this goes to show how, even from the first, the French nation was—by reason of its enthusiasm and imagination—able to appreciate what its inventors were striving to attain, and eager to encourage them in their quest. For just an opposite reason—because, that is to say, it had not this imagination nor intuition—England neglected her experimenters, or merely regarded their efforts with an amused tolerance, as though they were children playing with toys.

Fig. 16.—Ader’s “Avion.”

Ader’s greatest success came in 1897. With an improved machine, he obtained a flight through the air of nearly 300 yards; and this goes down to history as being the first ascent by a power-driven aeroplane having a man on board. Ader’s name will never be forgotten, and one of his machines is exhibited, as a relic beyond price, at the Institute of Arts and Science in Paris. But the flight ended in damage to the machine, as the other had done. A wind gust threatened to overturn the craft, its engines were shut off, and it descended so heavily that it was wrecked. Through constant difficulties in regard to motive power, and the heavy cost of his experimental work, Ader was unable to make a definite success, or produce a machine which could be called a practical craft. In his case again, as in that of Maxim, there was a great and apparently insurmountable defect. The aeroplane would rise; its engines and propellers would drive it through the air; but the steersman had not his machine under control: he had not, in a word, learned to fly. The prospect, therefore, was unpromising, because one machine after another might share the same fate—rising into the air, flying a hundred yards or so, and then over-balancing and crashing to earth: thus, in fact, might thousands of pounds be squandered.

But this stage of putting into practice what science had taught, although disheartening for those who passed through it, was still of value; it made a stepping-stone to the next. One of the men who thus laboured, without himself seeing his work brought to the goal of success, was Professor S. P. Langley, an American scientist connected with the SmithsonianInstitution, and a man of original ideas and great resource. He made a methodical investigation of the action of lifting planes and the shape of propellers, using a large revolving table so that he could test the latter while they were moving through the air. Then he began building models which took a double monoplane form, as indicated byFig. 17, with wings set at dihedral or upturned angle. This uptilting of the wings was to give the models stability while in flight: and the fixing of planes at the dihedral angle was tested, by later experimenters, in regard to full-sized machines. But while it gave an undoubted stability when a craft was flying under fair conditions, it was declared by some experts to be a disadvantage in gusty winds. There seemed also a risk that a machine so built might slip sideways when upon a turn. But in some machines to-day a modified dihedral angle is used, and with satisfactory results.

Fig. 17.—Langley’s Steam-driven Model.

Professor Langley’s models, tested over the river Potomac, flew many times for distances of half a mile. One, weighing 25 lbs., flew for appreciably more than half a mile, and at a speed of 20 miles an hour;and with another, which was slightly larger and weighed 30 lbs., a three-quarters of a mile flight was obtained. This model measured a little more than 12 feet across its wing tips, and was about 16 feet long. The miniature steam engine which drove it, developing 1½ h.p., weighed about 7 lbs., and operated a couple of two-bladed propellers which were fitted behind the main wings, and turned in opposite directions at the rate of 1200 revolutions a minute.

So successful were Professor Langley’s models that the United States War Department became interested; and the result was that an official grant was made for the building, according to the Professor’s plans, of a machine of man-carrying size. But with this craft, which weighed 830 lbs., and was driven by a 52 h.p. engine—and is shown inPlate I—there was a record of failure: launched from the roof of a house-boat over the Potomac, it fell several times into the water; and ultimately, and largely owing to the heavy cost of tests with such large machines, the trials had to be abandoned.

But that the Langley machine would have flown, had it been launched more carefully, has been demonstrated recently, and in a remarkable way. On June 28th, 1914, obtaining permission to make tests with the actual Langley machine, which had been preserved as a relic. Mr. Glenn Curtiss fitted the craft with floats, and drove it across the surface of the water at Hammondsport, New York, using the same engine that had been in the machine during its early and unsuccessful trials. After skimming the water for a short distance the monoplane rose, flying steadily and well, and vindicating its constructor’s theories, although he himself was dead.

How two German schoolboys built wings which they tested on moonlit nights—The beginnings of a great and patient quest—Otto Lilienthal’s theories and study of the birds.

Much of the ground has now been cleared, and—apart from such a story as may be told merely from facts and figures, and is apt to prove unsatisfying—we have striven to show the inner meaning of this great quest: how each of these pioneers, although he may have seemed to spend money in vain, and build models only to meet with failure, was really playing a useful part; was in fact—although he himself did not realise it—forging one of the links in the chain.

It has been shown how men passed from an ill-judged, haphazard stage; how science threw upon the problem the clear, cold light of wisdom; and then, further encouraged by the data that was to hand, how there were engineers who were ready to build large machines and demonstrate that, even in a crude and early form, an apparatus with curved planes would lift itself from the ground.

But still there remained this problem: how were men to learn to balance themselves when in the air?And, in considering the equilibrium of the aeroplane, it must be remembered that the air in which a machine must fly is a disturbed and turbulent sea. So, even were a man to build himself a craft which would, without the need of a hand upon its levers, balance itself accurately when in still air, there would still be the problem of the wind gusts; there would, that is to say, still be the risk of a machine being struck by an air-wave, particularly when flying near the ground, and being thrown out of its balance and dashed to earth.

Fig. 18.—Flow of the wind over hills.

As waves roll across the surface of the sea, so in the aerial ocean are there breakers and eddies and many dangers unknown; and men cannot see, but only feel them. The air does not flow in regular streams over the earth’s surface; could we follow its movements with our eyes, we should see that it is full of whirls and eddies, with currents of warm air flowing upward, streams of cool air moving downward; and with all the obstructions on the face of the earth, such as hills and woods, causing an interruption and a disturbance in the air flowing over them (Fig. 18). The face of a cliff, for instance, will deflect a current upward, leaving a partial void at its summit; and into this void the air will rush in the form of a whirling eddy.

The man who would learn to fly has to launch himself into a treacherous, quickly-moving element; and one which, to add to his perils, he cannot see. The rower in a boat, who sets out upon a stormy sea, can watch the flow of the waves and turn the prow of his vessel to a breaker that threatens him. But the aerial navigator moves in a medium that is invisible; gusts that rush upon him are unseen; he is unaware of their onslaught until his craft heels before the shock. This risk, from the sudden sweeping up of an air-wave, was put clearly by Wilbur Wright when he wrote:

“A gust, coming on very suddenly, will strike the front of a machine and will throw it up before the back part is acted on at all. Or the right wing may encounter a wind of very different velocity and trend to the left wing.”

In the aerial sea a machine will pitch and roll as does a ship upon the water; and the man who would fly must learn to check his craft, should it threaten to overturn; must be ready instantly with some system of controlling gear so as to correct the influence of each driving gust. And his task is made the harder because his machine, when struck suddenly by a gust, may fall towards the earth at any angle. On the road, when one learns to ride a bicycle, the machine will topple to one side or the other; but a craft in the air may fall forward or backward as well as from side to side, or partly forward and partly backward—or may slip and dive at any possible angle, either forward or backward or upon either side. A pioneer wrote, after his first experience in learning to fly:

“It is rather like trying to steer a motor-car along an exceptionally greasy road; you seem to slip all ways at once; and to slip so quickly also that, unless you make the right balancing movements without an instant’s delay, you find your machine has gone beyond control.”

If he were to succeed, if he were to fly like a bird, then a man had to learn this art of balancing himself in the air. Futile it was, as has been shown, to build some powerfully-engined machine that no one could control; futile also, and perilous as well, to make a pair of wings and jump from a tower. Another way must be found, or the quest abandoned and admitted hopeless. Here was the need; and here too, as we shall tell, came the man; a man who was not famous, who worked without reward and struggled to find time for his experiments; who died before he could see the final triumph; yet who won a fame that cannot die, and whom men call “the father of the aeroplane.”

To Germany one turns in telling the story of this man’s work. He was an engineer, Otto Lilienthal by name, and from the days of his boyhood he and his brother Gustav, living in Anklam, a small German town, were builders of model aeroplanes and students of the flight of birds. When the boys were thirteen and fourteen years of age respectively, they designed a flying machine; and in describing it afterwards, Gustav Lilienthal wrote:

“Our wings consisted of beech veneer with straps on the under sides through which we pushed our arms. It was our intention to run down a hill and to rise against the wind like a stork. In order to escape the gibes of our schoolmates, we experimented at night-time on the drill-ground outside the town; but there being no wind on these clear, star-lit summer nights, we met with no success.”

But they were not discouraged, and continued to build simple, easily-constructed machines—from each of which, although it would not fly, they learned a useful lesson. One, for instance, they made with wings of goose feathers, sewn upon tape and fixed to wooden spars. These wings, when finished, they fastened upon hoops which were strapped to the operator’s chest and hips; and he could, by means of a lever and a stirrup arrangement, beat the wings up and down by movements of his legs. This machine they hung from a beam in an attic in their house; but although the wingsdidflap, and actually showed some tendency to lift, the apparatus was soon consigned to a lumber-room, and they were busy with plans for another.

What impressed Otto Lilienthal was the fact that, even when provided by Nature with a perfect flying apparatus, the birds of the air had to learn to use it. They could not just leap upward and “ride the wind” as men had tried to do; they needed to take their first fluttering flights—beating their wings anxiously and often falling back to earth, because they did not know as yet how to use these wings. Particularly did Lilienthal study the flight of storks. He obtained young birds from neighbouring villages, and fed them in his garden with meat and fish while he watched their efforts to learn to fly, and studied that marvellous piece of mechanism—the wing Nature had given them. Writing of his observations in a book he afterwards prepared, calledBirdflight as the Basis of Aviation, Lilienthal describes the antics of young storks upon the lawn behind his house:

“When the actual flying practice begins, the first attention is devoted to the determination of the wind direction; all the exercises are practised against the wind, but since the latter is not so constant on the lawn as on the roofs, progress is some-what slower. Frequently a sudden squall produces eddies in the air, and it is most amusing to watch the birds dancing about with lifted wings in order to catch the wind which changes from one side to another, all round. Any successful short flight is announced by joyful manifestations. When the wind blows uniformly from an open direction over the clearing, the young stork meets it, hopping and running; then turning round, he gravely walks back to the starting-point and again tries to rise against the wind.

“Such exercises are continued daily: at first only one single wing-beat succeeds, and before the wings can be raised for the second beat, the long, cautiously placed legs are again touching ground. But as soon as this stage is passed,i.e.when a second wing-beat is possible without the legs touching the ground, progress becomes very rapid, because the increased forward velocity facilitates flight, and three, four, or more double beats follow each other in one attempt, maybe awkward and unskilled, but never attended by accident, because of the caution exercised by the bird.”

Lilienthal was fascinated by the mechanism of the bird’s wing. He and his brother built one machine after another to determine the exact amount of lifting effort that a man could obtain by imitating the wing-beat of a bird. One such apparatus is illustrated inFig. 19. This had a double set of wings; a wide pair in the centre and narrower ones in front and at the rear. These wings beat alternately, by movements of the operator’s legs; and the machine was suspended by a rope and pulleys from a beam, being counterbalanced by a weight. The tests showed this: that, after some practice in working the wings, a man could raise with them just half the weight of himself and of the machine; but the muscular effort proved so great that he couldonly maintain this rate of wing-beating for a few seconds. Here, incidentally, a fact may be mentioned: the energy a man can produce, at all events for a prolonged effort, has been estimated at about a quarter of a horse-power; and this—in tests so far made—has been insufficient for the purpose of wing-flapping flight. Lilienthal himself thought that, with some perfect form of apparatus, a man might fly with an expenditure of 1·5 h.p. of energy; but other experimenters have put the minimum power necessary, even if mechanism could be devised, at 2 h.p. And another fact must be remembered: even had Lilienthal been able, with such a machine, actually to raise himself in the air, he would still have had the problem of balancing himself, in addition to the working of his wings.

Fig. 19.

Fig. 20.—Lilienthal Kite.

After many tests such as these, carried out over a number of years, during which the brothers grew from boys to men, Lilienthal decided that no good results could be obtained unless a machine was made to moveforward through the air, instead of seeking to rise straight upward. By such forward motion if rapidly made, and with a suitably shaped wing or surface, he calculated that a definite support might be obtained from the air, and without any great output of energy. The value of forward motion is seen when a large bird seeks to rise. The first few flaps are heavy and laboured; but after this, as soon as it begins to travel forward, the wings exercise a lifting influence apart from their beat; and, as the bird flies faster, so its wing-beats become less violent. An instance of the need for a bird to move forward when it begins to fly, is provided in the case, say, of a sparrow imprisoned in a chimney: even if the chimney is wide, and there is plenty of room for the bird to fly straight upward and escape, it has not the power to lift itself vertically for any appreciable distance, because it cannot obtain the lifting assistance of a leap forward through the air; it is in fact a prisoner within the chimney.

Lilienthal studied the gliding or soaring flight of many birds; that form of flight in which, with its wings outstretched and held almost motionless, a bird such as the falcon will hover in the air, using no apparent effort and yet supporting itself with ease; diving, rising again, and wheeling in a perfect mastery of the medium in which it moves. Lilienthal built and flew kites, to which he gave curved wings in imitation of those of birds (Fig. 20). With one of these he obtained, althoughonly for a few moments, an actual gliding flight. The incident is described by his brother Gustav:

“It (the kite) was held by three persons, one of whom took hold of the two lines which were fastened to the front cane and to the tail respectively, whilst the other two persons each held the line which was fastened to each wing. In this way it was possible to regulate the floating kite, as regards its two axes. Once, in the autumn of 1874, during a very strong wind, we were able to so direct the kite that it moved against the wind. As soon as its long axis was approximately horizontal the kite did not come down, but moved forward at the same level. I held the cords controlling the longitudinal axis, and my brother and my sister each one of the cords for the adjustment of the cross-axis. As the kite maintained its lateral equilibrium, they let go the cords; the kite then stood almost vertically above me and I also had to free it. After another thirty steps forward my cords got entangled in some bushes, the kite lost its balance, and in coming down was destroyed. Yet, having gained another experience, we easily got over the loss.”

From kites, in quest of a curved surface which should give a maximum lift with a minimum resistance to its own passage through the air, Lilienthal embarked upon a series of tests with wing shapes; setting these up in the wind upon suitable recording machines, and noting patiently the data that could be procured. There are many problems to be considered when planning a wing for flight. If it is given a deep curve or camber from front to back this may, while exercising a powerful lift, offer too high a resistance as it passes through the air, and thus waste the energy needed to propel the craft; or, if its front edge is dipped too sharply, this may cause the air to act upon its upper surface, and send a machine diving headlong to the ground. The planning of a successful wing becomes a compromise,having for its object a surface which shall give the greatest lifting influence with the least resistance. Lilienthal, after much experiment and the examination of the wings of many birds, decided that the curve, camber, or upward arch of a plane should measure, at its maximum depth, about one-twelfth of whatever width the plane might have from front to back.

How a man may use gravity as a motor—Theory of the “glider”—The craft Lilienthal built—A problem of balance—The centres of gravity and pressure.

Up to this point in his research Lilienthal had moved more or less upon the lines of other experimenters. Had he continued to follow in their footsteps, he would have planned some large and impracticable machine—and perhaps gone no further. But although he desired to test the lifting power of the planes he had built, Lilienthal had always in mind this vital fact—that a man must learn to balance himself in the air before he can hope to fly. His own words, in summing up this problem, were: “stability first; propulsion afterwards”; and by this he meant a man must acquire the art of handling a craft in the air, before he dares to fit a motor and attempt power-driven flight.

But if a man used neither wing-beats nor a motor to drive him through the air, how was such practice to be obtained? Lilienthal solved the problem—and made his name immortal—by devising a system in which he used the force of gravity as his motor.His plan was this: first he would build a pair of large, light wings—so light in fact that, even with the woodwork that was in them and with the additional weight of a balancing tail, he could raise them to his shoulders and run forward. With these wings he would go to the summit of a sloping hill and face what wind might be blowing—as he had seen the young storks do. Then he would run forward with his wings, so as to obtain the lifting influence necessary before they could act upon the air. And then, when the wind was sweeping under his curved wings, he would raise his legs from the ground and seek to soar or glide; his own weight, and that of his machine, providing a gravity motor or downward pulling influence, while the sustaining power of his planes, resisting this drag, would send him gliding through the air, only a few feet from the ground, at an angle which tended gradually earthward.

Fig. 21.Paper glider, in which the cardboard weight (A.) should be 3/10 inch wide, and 1/16 inch thick, slightly arch planes upward (B.B.). Turn up a little flap at each end (C.C.). An eighth inch is sufficient. Hold between finger and thumb (the cardboard weight uppermost); then allow to dive, as indicated by the dotted line.

Fig. 21.

Paper glider, in which the cardboard weight (A.) should be 3/10 inch wide, and 1/16 inch thick, slightly arch planes upward (B.B.). Turn up a little flap at each end (C.C.). An eighth inch is sufficient. Hold between finger and thumb (the cardboard weight uppermost); then allow to dive, as indicated by the dotted line.

This power of a weighted plane to glide, even when no motive power is attached to it, may be demonstrated quite simply by the little paper model seen inFig. 21. If, when you have made this, you allow it to flutter from your hand without any weight attached, the model plunges, dips, and dives; it has no forward motion, therefore it has no stability or poise. But when you gum the small cardboard weight to its fore-plane, the action of the model is changed. By the use of this tiny strip of cardboard you have, so to say, given it an engine; you have provided it with means whereby it can obtain forward motion, and so glide through the air. When youhold it as shown in the sketch, with the weighted fore-plane tilted downward, and release it without a jerk, the tendency for the model is to fall to the ground as it did before. But now there is the weight to reckon with: this pulls the model forward and downward, tending to fall more quickly, of course, than the paper by itself would do. But there is also the plane behind the falling weight to be taken into consideration: jerked forward and downward through the air, this begins to exercise a sustaining influence, and so resists the falling movement of the weight. Still the weight, actuated by the force of gravity, pulls downward. But the plane refuses to fall sheer to the ground; and yet the weight must have its way. So, as in most situations of this kind, thereis a compromise. The falling weight pulls; the plane resists; and in a flash the model starts upon a graceful glide. Its plane is fulfilling its task of bearing it through the air; and the weight is carrying out its mission also, in causing the glide to tend earthward. So, pulled down by its weight and yet partly sustained by its plane, the model will pass across a room; and if its plane and its weight are in a nice adjustment, one may see a pretty manœuvre before it reaches the floor. As it is swept faster through the air, owing to the increasing drag of the weight, the plane of the model acquires a greater lifting influence; and the moment comes when this “lift,” reaching a maximum, checks altogether the descending movement, and causes the model actually to ascend. Up indeed it goes, for a second, in a sudden swerve. But this ascending impulse is soon checked; gravity cannot be denied. The model loses speed; and, as it loses speed, so does its plane lose lift. Hence the weight is again the predominating partner; it pulls down the fore-plane, converts the rise into a fall, and brings the model with another dive to the floor.

But here, at all events, is a demonstration of this theory of gliding flight—one that can be carried out without a motor. Of course such flying has its restrictions: a man must start from the summit of a hill, and the glide is in the form of a descent towards the ground below; but still he is passing through the air; and above all—and this proved the advantage of the scheme for such a pioneer as Lilienthal—there is no need, during any such glide,to pass high above the ground. The operator may, in fact, if the side of his hill slopes gently, skim within only a few feet of its surface; and this means that, should he lose his balance at first, as he may expect to do, he will not share the fate of those who leapt from towers, but will be able to alight without mishap.

Fig. 22.—Lilienthal Glider.

This, then, was Lilienthal’s plan; he would build a machine with wings and a tail, stand facing the wind as the storks had done, then seek to glide through the air in the manner of a soaring bird. The idea underlying his scheme was that he hoped, by a series of such gliding flights, to learn the adjusting movements he knew would be necessary to preserve his balance in the air. The gliding craft Lilienthal built, as illustrated inFig. 22, has become an historical machine. The framework of its wings and the supports of its tail were of willow, and the wings and tail, to give them their grip upon the air, were covered with a smoothly stretched fabric. Then the whole structure was braced and tightened; and though it weighed less than 50 lbs., it was strong enough to bear its operator though the air. Lilienthal could raise the apparatus upon his shoulders—passing his head through the aperture between the planes, which will be noted in the sketch—and walk or run forward; and to hold the machine, as he carried it thus, he gripped two wooden rods. The tail was flexible, being allowed an automatic movement, thus giving the craft a certainnatural stability. The main wings had a span of 24 feet, and the machine measured 18 feet from front to tail. The wings were cambered, according to the curve Lilienthal had decided most efficient, and contained about 180 square feet of lifting surface. In giving them this area, Lilienthal was relying upon experiments he had made; these showed that, as his machine glided through the air, each square foot of its surface should bear a weight equal to about 1 lb.

Although enthusiastic, Lilienthal was not impatient: he had the priceless gift of judgment, allied to common-sense. So, when he had his glider built, he made no wild nor dangerous tests. He contented himself, in fact, with a leap from a springboard no more than 3 feet high; and this height he increased gradually to 8 feet. By such humble beginnings, and without risking his life, he proved that his glider would sustain his weight in the air; or, to be more precise, that its wings would exercise a lift sufficient to permit him to glide rather than fall to the ground. So now he began more elaborate tests, seeking hills which had gently-sloping sides, so that he might glide down them. But with many the difficulty was this: the winds near the surface, being broken and disturbed, blew fitfully and in gusts, while what Lilienthal needed was a steady, uniform wind. At length he found favourable conditions at some gravel-pits at Südende; and here, on the brink of a pit, he built a shed and housed his gliders.

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