“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 wings did flap, 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, called Birdflight 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 in [Fig. 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 could only 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.