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 move forward 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, although only 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.
CHAPTER V
GLIDING FLIGHT
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.