Construction of an artificial hill—The building of larger craft—Peril of gusty winds—The accident which caused Lilienthal’s death.

So determined was Lilienthal to obtain the best conditions for his gliding that, finding no natural slope to meet his purpose, he ordered the construction of an artificial hill. This was built at Gross-Lichterfelde, near Berlin, and was 50 feet high and had gently sloping sides from which, at any direction of the wind, he could make a soaring flight. On the top of the hill, which is illustrated in Fig. 25, Lilienthal had a roomy chamber, and in it he stored his craft. In this illustration, also, the airman may be observed standing upon the hilltop, ready for a trial. By means of dotted lines, and representations of machines in flight, it is possible to show how he glided through the air.

In the upper of the two flights shown, profiting by a day when there were rising currents in the wind, Lilienthal had allowed himself to be lifted, for a moment or so, to a point in the air actually higher than that from which he started. Then, in order to obtain forward speed, he dived, only to incline his wings more steeply again, and allow the wind to bear him upward. In this way, by exercising skill in the balancing of his machine, he was able to prolong a glide, and under favourable conditions to traverse, before touching ground, a distance through the air of nearly 1000 feet.

In the second of the glides shown in Fig. 25, Lilienthal is making a swift, low flight—one of those during which he was never far from the ground, and with which he contented himself in early tests. On glancing again at this sketch it will be noted that the upper of the craft shown has two main sustaining wings, placed one over the other in the girder construction already described. It is in fact a biplane, whereas the lower machine is a monoplane—such a craft as was illustrated in Fig. 22. It was when Lilienthal became expert at balancing himself in the air that he built a machine on the biplane principle. His reason for doing so was that he required more surface in the sustaining wings, so that they might carry him farther through the air. What he wanted to do in each of his glides was to remain in the air as long as possible, and thus gain a maximum of experience. The difficulty, as explained, was to obtain enough practice. In five years, for instance, although assiduous in his experiments, Lilienthal was not more than five hours in the air.

Thus it was that he built a biplane, each of the wings being 18 feet in span, and containing 100 square feet of surface. The craft is shown in Fig. 26. Lilienthal did with it what he expected he would be able to do; he increased materially the length of his glides. But there were drawbacks in the use of this machine, and one introduced an element of danger. With the monoplane, so soon as he made balancing movements by instinct, Lilienthal found he could control his craft quite well; it was small, and responded quickly when he threw the weight of his body from side to side. But the biplane, being considerably larger, and having more surface upon which the wind could act, was sluggish in its response to his controlling movements. In the case of both monoplane and biplane, Lilienthal relied merely upon the weight of his body to counteract falling movements. In the biplane, therefore, although it required a greater leverage to restore its balance, he was unable to increase the correcting influence. This difficulty, in the use of a large machine, was faced subsequently by the Wrights, and how they solved it will be shown.

Lilienthal recognised the position, of course, and saw there might be peril in the use of a biplane; but he was content, none the less, to rely upon his skill. In each glide he made he became more expert; instead of allowing his machine to slip to the ground when struck by a gust, he restored its equilibrium by an instantaneous movement of his body: he was, in fact, like a man who had learned to ride a bicycle—balancing himself without pausing to think what he should do. But Lilienthal, in his navigation of the air, was facing a danger the cyclist has not to fear. He was braving dangerous wind-gusts; and he did not know, and had no means of knowing, with just what strength these gusts would strike his craft. Also—and this too was a danger no man on earth need fear—he had empty air below him should he fall. The peril grew greater as his skill increased, because he soared higher, and left greater distances between him and the ground below. In another way, also, he courted greater risk; and this was through gliding in stronger winds. At first, when he was unskilled, he had cared only to brave a wind of a velocity, say, of 10 or 15 miles an hour. But soon, feeling his balancing power grow greater, he ventured into the air when there was a wind of from 20 to 25 miles an hour.

In regard to this question of the strength of the wind, uncertainty often exists. What, for example, is a “stiff breeze”? What is a “strong wind”? And at what velocity must the wind blow before it is called a gale? Such questions are often asked, and the table below should prove instructive:

In gliding in a breeze, say of 25 miles an hour, Lilienthal had to face this danger, and it is one all airmen meet: whereas the average strength of a wind may be maintained at 25 miles an hour, there is no assurance that there will not be a sudden and heavy gust of a greater force than this. Sometimes, when the wind is uncertain, there will come a gust which has double the force of the normal pressure; and such a gust, sweeping unexpectedly against an aircraft, threatens to blow it over and send it headlong to the ground. Thus Lilienthal, having no more control over his machine than could be brought to bear by movements of his body, was running a considerable risk when he soared in gusty winds—particularly if using the biplane form of craft. Sometimes, when struck by a gust, his glider would heel and assume a dangerous position in the air such as is illustrated in Fig. 27. Here the craft threatens to fall backwards and partly sideways; and the operator can be seen throwing his body and legs forward, in a quick effort to check this overturning impulse.

One incident, indicating the risks Lilienthal ran, should be mentioned: he was gliding 50 feet high one day, in a fresh wind, when one of the wooden arm supports, which he gripped while in flight, broke suddenly and threw his craft out of balance. The machine, before he could right it, fell heavily to the ground; but, thanks to the shock-absorber below the wings, Lilienthal escaped with nothing worse than bruises.

He had set himself to master this art of balance, and master it he did, and was ready to risk his life in so doing. In the year 1896 he was bold enough to glide from hills 250 feet high; and from such a height he would come sweeping through the air, often traversing before alighting a distance of 750 feet. Sometimes, too, on a day when the wind was high, he would stand upon the hilltop and allow the wind pressure under his wings to raise him in the air; then, throwing his weight forward, he would start his craft on a downward glide. Frequently, when experimenting in strong winds, he would find himself higher than his starting-point, and would hang almost motionless for a moment or so, soaring in the air. But such hovering flight, though he practised assiduously, he found difficult to maintain. He could not keep his machine poised in an ascending current of wind; he had not that instinct of the birds which enables them to profit instantly by each rising gust, and hold themselves in it as they allow it to bear them upward. Soaring flight has a fascination for those who study the navigation of the air; but no man, as yet, has been able to indulge in it for more than the briefest space of time. It is only possible to hover thus, without effort or the use of a motor, when the forces that govern a machine are exactly in balance; that is to say, the power of gravity which is pulling downwards must be balanced perfectly by the strength of the wind, which is blowing under the planes of the machine and tending to force it upward. Should the wind fail in its thrust, then the craft will move forward and downward; should the wind blow more strongly, then it will drive the machine backward, and tend to throw it out of equilibrium. Some birds, profiting by the skill they have in the minute adjustment of their wings, are able to hover over a given spot at will, remaining motionless in the air, without flap or visible effort.

But Lilienthal, although he never attained such proficiency as might enable him to soar indefinitely above the hill from which he sprang, was always confident that some perfect glider would be invented, and men thus be able to imitate the birds. It was when writing upon this problem of soaring flight that he expressed the thought:

“It is not to be wondered at that birds are able to perceive the slightest variations in the movements of the air, because the whole of their body surface acts as an organ for this sensation; the long and widely extended wings constitute a sensitive feeling lever, and minute sensibility will be particularly concentrated in the follicles from which the feathers issue, just as is the case with our finger-tips.... Should it ever become possible for man to imitate the splendid sailing movements of birds, he will not require to use steam engines or electro-motors for the purpose; a light, properly shaped, and sufficiently moveable wing, and the necessary practice in its manipulation is all that will be required of him. He should, unconsciously, be able to draw the greatest advantages from whatever wind may be blowing, by properly presenting the wings.”

When three years of gliding lay behind him, Lilienthal thought he could go little farther in this research. He was able to balance himself in the air; he could glide in high winds; but always, seeing that he had no motive power with which to drive his craft, he must start from a hilltop and descend to the ground. Now he sought longer and bolder flight; and so he and his brother discussed the building of an engine which might propel a glider through the air. No petrol motor, unfortunately for Lilienthal, was then available; so he planned to construct a carbonic acid motor, and make this drive his craft by flapping the ends of its wings.

During the summer of 1896 he was busy with plans for this motor, while still continuing his flights; but in August he decided to cease gliding for a while, and await a test of the power-driven craft. So on Sunday, August 9th, he said he would travel out to Stollen, make one or two final flights, and pack up his machine. His brother Gustav was to have accompanied him as usual, but had a mishap with his bicycle, and so remained behind. What happened is described, in few but expressive words, by Gustav Lilienthal:

“Our families, whom we had intended to take with us, remained at home, and my brother drove out, accompanied by a servant. He intended to make some change on the rudder, but at the very first glide, the wind being uncertain, the apparatus, when at a considerable height, lost its balance. Unfortunately my brother had not fitted the shock-absorber, and the full shock of the fall took effect, so that the apprehension of our uncle was fulfilled. My brother fell, a victim to the great idea which—although at that time so little recognised—is now acknowledged in its full bearing by the whole civilised world.”

At the moment his machine lost balance, Lilienthal was more than 100 feet in the air. Striking the ground with great violence, he sustained injuries which were almost immediately fatal. In his work, to which he sacrificed his life, he had met with no encouragement or recognition. He suffered the fate of pioneers; his theories were so far ahead of his time that folk did not grasp their significance. Little interest seems to have been taken in his glides; there was no sensation; there were no crowds. Nobody, in fact, realised what he was doing, or appreciated the vast importance of these seemingly simple tests. But in the years that followed, when other men came to grips with the problem as Lilienthal had done, when they were able to use the data he had compiled and to profit by his experiences in actual flight, then this pioneer came into his own.

His work, summarised, may be said to lie in this: he provided a stepping-stone to power-driven flight. He showed men that they should learn to balance themselves in the air before, and not after, they had built themselves costly craft. How his example acted as a spur upon others, and how the work he had begun was carried to its triumph, will be the purpose of our next chapters to show.[1]