SIMPLE experiments using flags flapping in water have helped to show why a single number describes the locomotion of flying and swimming animals.
The movement of animals that flap wings, fins or their entire body is governed by something called the Strouhal number. This is equal to the frequency of flapping multiplied by its amplitude, then divided by the forward speed. In most cases, peak efficiency is reached when this number lies between 0.2 and 0.4 (Â鶹´«Ã½, 18 October 2003, p 14).
Michael Shelley’s group at New York University in New York city submerged a heavy but pliant sheet of copper-reinforced plastic in flowing…
