The quest to develop flying machines based on the aerodynamics of flapping wings is hampered by a lack of information about how birds achieve stability and control. So even though the earliest dreams of human flight, from the story of Icarus to the designs of Leonardo da Vinci, attempted to emulate birds, practical designs from the Wright brothers onwards have focused on the stationary aerofoil wing. Working flapping-wing devices have been built only very recently.

Scientific opinion has varied on whether the fact that animal wings are flexible, rather than rigid like the wings of an airplane, helps or hinders the production of thrust. John Costello, a biologist at Providence College in Rhode Island, and his colleagues thought that the flexibility might be key to natural fight, so they decided to look at just how real animal wings deform.

Feathers and fins
They suspected that similar bending effects would be evident in wings and in fins and flukes used for propulsion in water. In fact, they were initially motivated by their work on a project for the US Office of Naval Research to develop a biologically inspired “jellyfish vehicle”. That work, says Costello, showed that “the addition of a simple passive flap to an otherwise fairly rigid bending surface resulted in orders of magnitude increases in propulsive performance”.