The migratory white stork has been a welcome visitor to Europe for centuries. It seems to display a peculiar affinity for human beings, building its huge stick nests on their rooftops and chimneys, foraging in their fields, and playing starring roles in their myths and legends. It eats mice and rats, is thought to bring good luck and provides prudes with a polite explanation of the provenance of babies.
Like all large birds, the stork is an arresting sight in flight, its slow, powerful wing beats punctuated by periods of still, straight gliding. Among those arrested was a pair of young German brothers, Gustav and Otto Lilienthal (whose surname, by the way, in keeping with the bucolic tone of this narrative, means “valley of lilies”). Around 1860, they would trek to meadows outside Anklam, their hometown in northern Germany, to watch the browsing birds. They found they could approach quite close to the storks, which have a poor sense of smell, even from upwind. When they approached from upwind, they noticed a startled stork would make several hops toward them before becoming airborne. From this observation, they deduced it must be significantly easier for the birds to take off into the wind because, as Gustav later wrote, “without some compelling cause, the shy bird would not advance toward us.”
Thirty years later, Otto Lilienthal would publish his classic work, of which the title, Der Vogelflug als Grundlage der Fliegekunst, translates into English as Bird Flight as the Basis of Aviation. My philological proclivities oblige me to note that “aviation” feels anachronistic in this context. Fliegekunst is really the art — meaning technical skill — of flying, not the broad field of human flight.
A paperback facsimile of the 1909 English translation of Otto Lilienthal’s classic is available from Markowski International Publishers in Pennsylvania ([email protected]). It is a remarkable document full of surprises, even if you already know that Otto Lilienthal invented the weight-shift hang glider and made 2,000 flights before the 19th century had come to an end. Otto even pioneered a classic type of aeronautical mishap: He died in 1896 after stalling at a height of 50 feet in a gust of wind.
Admirers of the Wright brothers praise their methodical, carefully documented analyses and experiments. The Lilienthals’ research was of the same kind. Well trained in the sciences, they dismantled the puzzle of flight into its parts and studied each in isolation, testing their hypotheses against the known principles of physics.
Otto and Gustav — Otto traditionally gets the credit, but Gustav, a trained engineer himself, was at his side — spent hours watching storks fly, sketching the braided paths of bodies and wingtips and the wing twisting that accompanies each stroke. They observed the big birds soaring and attempted, not always successfully, to understand the role of wind — actual wind, not the relative motion of air over the bird — in supporting them. Soaring flight, they perceived, demonstrated that there existed “a method of flight requiring almost no effort.” To discover it should be “the supreme aim of the science of aviation (Flugtechnik).”
Indeed, the Lilienthals had discerned a key fact: There is something about the forward motion of the wing, not its beating, that sustains the bird with the most economical balance of speed and effort. Presumably, it has to do with the shape of the wing — but what shape? Birds’ wings come in so many different shapes.
Studying the wings of storks, they derived airfoil shapes thick and rounded in front, where the flesh and muscles were, tapering to a point at the rear. The cross sections of the birds’ wings were cambered. They correctly inferred that camber assisted with flow attachment at the leading edge. They recognized the phenomenon of the stall and drew remarkably accurate pictures of streamlines over flat and cambered plates in stalled and unstalled conditions.
Beginning with metal rectangles mounted on whirling arms — the only way available at the time to achieve a known wind speed — they mapped the effects of angle of attack on lift and drag, producing the first “polar diagrams” that made obvious the importance, for achieving flight, of the first 15 or so degrees of angle of attack — the prestall range — where the lift increases linearly and is much larger than the drag. They always related their findings to their observations of birds. Nature, after all, had already solved the problem.
Ninety percent of the insights of the Lilienthal brothers were accurate, but not all. For one thing, they seem to have believed that wind, in and of itself, contributed to lift, and birds flew more readily on windy days than on calm ones. One of their experiments seemed to demonstrate that there was an upward component to all wind, even close to the surface. Most likely, the effect they observed was indirectly due to a velocity gradient, but they did not interpret it that way. They believed, too, that when an aircraft that could rise from the ground under its own power was built, it would propel itself by flapping its wings; in this, at least, their loyalty to Mother Nature led them astray.
As rich in insights as Otto Lilienthal’s book was, it was no more a guide for building a flying machine than Euclid’s Elements is for building a skyscraper. Nevertheless, around 1890 they began to apply what they had learned toward constructing a glider. To their credit, the brothers did not try to build an oversize stork. Clearly, they understood a rigid glider posed a somewhat different problem than an intelligent variable-geometry device like a bird. They built, instead, a light, cambered wing with a fixed vertical stabilizer — something no bird had, but a rigid glider absolutely required. They placed the pilot at what they had discovered to be the center of lift, not in the middle but closer to the leading edge, and relied on his ability to shift his weight to control roll and pitch, something the bird did with subtle movements of its feathers.
In 1891, a dozen years before the first successful powered flight by the Wrights, Otto Lilienthal began making glides from a hill in the village of Derwitz, 25 miles west of Berlin. Later, he had a tall conical mound built from which to launch himself. Crowds came to watch. Over a period of five years, he made 2,000 flights, amounting to an estimated five hours in the air. Truly, Otto Lilienthal was the first pilot, and his glider the first airplane.