Nature

18
Feb

Flappy Birds and Flippy Fish Obey Same Laws of Fluid Dynamics

Why don’t airplanes flap their wings? Flexible-wing flight could offer many advantages over rigid wings, including greater maneuverability and lighter weight. Yet airplanes still have rigid wings, in part because we don’t know enough about the dynamics of flappy-winged flight to understand how to replicate it, except for in a few very recent experiments.

Some scientists, though, have suspected that wing, tail, and fin flexibility is key to natural animal motion, and that understanding it could help us improve aeronautical engineering. Biologist John Costello and his team of researchers at Providence College in Rhode Island decided to test this theory, and they started with YouTube. Here’s Philip Ball, writing for Nature News:

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”.

orange-bird
Believe it or not, a bird's delicate wings flex and deform to the same degree as a killer whale's powerful fins.

The researchers combed video websites including YouTube and Vimeo for footage of species ranging from fruit flies to bats and from molluscs to humpback whales. For all the vast diversity of propulsor shapes and structures — gossamer-thin membranes, feathered wings, thick and heavy whale tails — the researchers found little variation in certain variables, which they measured essentially by hand. Specifically, across 59 species the distance from the point where bending starts to the wing base tended to be around two-thirds of the total wing length; and the maximum angle of bending was confined within the range of about 15° to 38°.

It seems, then, that all of these animals have evolved around the same universal principles of motion—a phenomenon known as convergent evolution. No matter how minute the insect or how mammoth the whale, fins and wings flex in accordance with the same physical laws that maximize thrust. If scientists can pinpoint the specific mechanisms involved in this bending, a new generation of bio-inspired technology could be waiting in the wings.

Watch "Making Stuff Wilder" to learn more about how scientists are taking cues from nature in developing new tools.