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.

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 in Fig. 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 you hold 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, there is 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 manoeuvre 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.

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 in Fig. 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 certain natural 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.

Now, patient and assiduous, he began to teach himself the art of aerial balance. Raising his wings to his shoulders he would face the wind—which in his first tests he did not care to be blowing at more than ten or fifteen miles an hour. Then, running against the wind to increase the pressure beneath his wings, he would raise his legs and begin to glide, moving forward and at the same time downward. How he appeared when in flight is indicated by Fig. 23.

His first tests were brief, for the reason that his craft would either dip too sharply, or incline its planes steeply and so check its forward speed. In either event the result was the same: the glide came to an end. But Lilienthal’s caution saved him from being injured in an involuntary descent. It must be remembered he was always moving close to the earth; therefore he had only a short distance to fall. To safeguard himself still further he fitted below his machine a shock absorber, which came into contact with the ground first and lessened the force of any impact.

But the difficulties of preserving his balance were great, as he had foreseen; for not only did his glider dip down, or rear itself up, but also—under the influence of wind-gusts—threatened to slip sideways. It was Wilbur Wright, lecturing afterwards upon problems of aerial equilibrium, who said crisply:

“The balancing of a gliding or flying machine is very simple in theory; it merely consists in causing the centre of pressure to coincide with the centre of gravity; but in actual practice there seems to be an almost boundless incompatibility of temperament between the two, which prevents their remaining peaceably together for a single instant.”

Fig. 24.—Centres of gravity and pressure.