Der Vogelflug als Grundlage der Fliegekunst (1889) [Bird flight as the Basis of Aviation]

Theory

From experiment to construction

From their childhood onwards, the Lilienthal brothers are convinced that self-propelling flight is not limited to birds only. Otto Lilienthal, in particular, does his utmost to prove this in theory and practice. In order to "eavesdrop on the bird's art of flying" he proceeds systematically and with great perseverance: He researches the special body structure of animals, studies their flight performance, their wing stroke and the learning process of young birds trying to fly. He calls the white stork his "master", whom he tries to simulate with his own experimental equipment.

What is the secret of the wing? With their rotating apparatus the Lilienthal brothers try to get to the bottom of this question and to physically describe the properties of the wing. They attach artificial wings to this device and move them through the air. This allows them to experiment with different shapes and materials. By dividing the forces on the wing into a restraining and a lifting component, they develop the concept of describing wings by lift and drag, which still holds true today.

Technical drawing of the rotating apparatus, in: Otto Lilienthal, Der Vogelflug als Grundlage der Fliegekunst (1889) [Bird flight as the Basis of Aviation]

Model wing for aerodynamic force measurements, Otto Lilienthal, Wax, 1888, reproduction according to originals in the Deutsches Museum

Otto Lilienthal, Der Vogelflug als Grundlage der Fliegekunst (1889)
With the construction of his experimental machines and his meticulous working methods, Otto Lilienthal develops the so-called polar diagram of the joint representation of lift and drag, also known as Lilienthal polar. Knowledge of the curved profile of the bird wing, which is superior to a flat surface, is considered the quintessence of Lilienthal's long-term studies, which became the prerequisite for his flight experiments.Polar diagram of the aerodynamic forces: lift and drag. In: Otto Lilienthal, Der Vogelflug als Grundlage der Fliegekunst (1889) [Bird flight as the Basis of Aviation]

Zwei Jahrzehnte widmet Otto Lilienthal der Erforschung des Vogelfluges und seinen systematisch durchgeführten Experimenten. Erst nach sorgfältiger Prüfung und Wiederholung von früheren Messreihen veröffentlicht er sein Buch „Der Vogelflug als Grundlage der Fliegekunst“. Es ist allgemeinverständlich geschrieben und gilt zugleich als die wichtigste flugtheoretische Publikation des 19. Jahrhunderts. Neben zahlreichen technischen Zeichnungen enthält es auch ein Gedicht und ein Aquarell einer „kreisenden Storchenfamilie“ von Otto Lilienthal.

Der Vogelflug als Grundlage der Fliegerei. Ein Beitrag

zur Systematik der Flugtechnik. Auf Grund zahlreicher von O. und G. Lilienthal ausgeführter Versuche, bearbeitet von Otto Lilienthal, Berlin: R. Gaertners Verlagsbuchhandlung, 1889

Otto Lilienthal, Der Vogelflug als Grundlage der Fliegekunst (1889)
Storks are Lilienthal's role models. Since the stork lives closer to man than any other large bird, it is almost predestined to teach man how to fly, according to Lilienthal. In order to study them as closely as possible, he tries to settle storks on the roof of his house in Berlin in 1889. When that fails, he acquires young storks to watch their maiden flight attempts. Storks serve Lilienthal as biological models for the design of his flying machines’ wings.Technical drawing, in: Otto Lilienthal, Der Vogelflug als Grundlage der Fliegekunst (1889) [Bird flight as the Basis of Aviation]
Technische Zeichnung des „Modells Möwe“, in: Otto Lilienthal, Der Vogelflug als Grundlage der Fliegekunst (1889)
With the "Seagull Model" Otto Lilienthal makes his first standing tests in 1889 with a pair of wings measuring ten square meters. He tries to master the artificial wings in the wind from the garden of his Berlin residence. In the spring of 1891, he also makes his first attempts from a springboard at a height of one metre. In doing so, he feels how his body weight is being carried by the wings in the air for the first time.Technical drawing of the "Model Seagull", in: Otto Lilienthal, Der Vogelflug als Grundlage der Fliegekunst (1889) [Bird flight as the Basis of Aviation]