For a long time, people thought that birds must be immensely strong – otherwise, how could they stay up in the air? But actually, the crucial factor is the shape of their wings.... The Lilienthals realised that, so they turned their attention to the shape of the aircraft wings. After extensive experiments with flat wing-surfaces, they examined curved bodies and discovered a simple effect based on physics. This was what they wrote:
"Because the surface is curved, the air flowing across it [...] is deflected from its path in an arc. [...] This curvilinear movement of the air particles corresponds [...] to a very specific centrifugal force. The portion of air that passes beneath the surface exerts pressure on the surface from below, while the portion that glides across the top is impelled away from the surface and causes a suction effect, which is also directed upwards.
These days, this "suction effect" – or buoyancy – is part and parcel of physics lessons. When an air flow meets a curved surface, different flow velocities lead to a build-up of different levels of pressure on the top and bottom of the surface. The air has to travel further across the top of the curved surface, so it flows faster. That creates lower pressure – what Lilienthal called the “suction effect”.
This upward pull is called lift. It was only when he realised this that Lilienthal was able to carry out successful practice flights. And that insight into the physics remains fundamental to the design of every aircraft even now.
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Zitat Vogelflug: Der Vogelflug als Grundlage der Fliegekunst, Berlin: 1889, p. 80.
All depictions: © Lilienthal-Centrum Stölln
