Adrian Bejan | Flying, from Design in Nature

Jul 24, 2025

In this video, Adrian Bejan explores animal locomotion through the lens of configuration, freedom to change, and effort. He begins by connecting design in space with design in time, describing rhythm as an essential expression of flow. Using the flight of the condor as a case study, Bejan breaks down the movement of animals into cycles of vertical and horizontal motion, each with distinct effort. He emphasizes how freedom allows animals to adjust their movement, especially speed, to minimize total effort, revealing a natural tendency toward economical flow. From muddy roads to penguins in Antarctica, Bejan shows that all creatures, like fluids, flow through resistance, and their shapes and speeds are optimized by nature for persistence.

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Bejan presents flight as a cyclical process, involving two efforts: one vertical (lifting against gravity) and one horizontal (overcoming drag).
He introduces design in time as rhythm, using analogies from cars on snowy roads, string instruments, and cruising birds, to illustrate how flow systems pulse and repeat.
Using scale analysis, he shows how birds choose their cruising speed by balancing these efforts, highlighting speed as a key degree of freedom that birds can design.
Bejan connects the density of the body and the density of the environment to a resistivity factor, explaining why birds catch fish and not the reverse; resistance to flow determines directionality in the food chain.
He emphasizes that economic speed depends on mass, gravity, and the density difference between the animal and its surroundings, deriving a prediction that bigger animals are faster movers.
Bejan reframes flying as a flow architecture, where movement, like all flow in nature, is governed by the freedom to morph, interaction with the environment, and the minimization of effort over distance.

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