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Cayley's glider is now regarded as the earliest version of the modern airplane. The glider—also known as a sailplane—differs from a modern airplane only in that it has no power source of its own. Instead, it uses updrafts and winds for propulsion and maneuvering. Cayley was well aware of the need for a powerful engine for moving a heavier-than-air machine, but only steam engines were then available, and they were much too heavy for use in an aircraft. So Cayley designed an airship (his glider) that could make use of natural air movements.

The B-2 Stealth Bomber. Photograph by P. Shambroom. Photo Researchers, Inc. Reproduced by permission.

The modern sailplane retains the characteristics that Cayley found to be crucial in the design of his own glider. In particular, both aircrafts have very large wings, are made of the lightest possible material, and have extremely smooth surfaces. The protrusion of a single rivet can produce enough friction to interfere with the successful maneuvering of a sailplane or glider. Properly designed sailplanes can remain in the air for hours and can travel at speeds of up to 150 MPH (240 km/h) using only natural updrafts.

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Science EncyclopediaScience & Philosophy: Adrenoceptor (adrenoreceptor; adrenergic receptor) to AmbientAircraft - Early Theories Of Air Travel, Lighter-than-air Aircraft, Heavier-than-air Aircraft