Bats Conserve Energy During Flight by Drawing in Wings on Upstroke

Posted on April 11, 2012

Bat Wings Flight at Brown University

A new Brown University study found that bats take advantage of their flexibility by folding in their wings on the upstroke to save inertial energy. The researchers estimate that folding and unfolding comprises roughly 44% of the inertial cost, but that the total inertial cost is only approximately 65% of what it would be if the wing remained extended and rigid throughout the wingbeat cycle.

Unlike insects, bats have heavy, muscular wings with hand-like bendable joints. The study suggests bats use their flexibility to compensate for this larger mass. The research suggests engineers looking at flapping flight should account for wing mass and consider a folding design.

Attila Bergou, who along with Daniel Riskin is co-lead author of the study, says, "Wing mass is important and it's normally not considered in flight. Typically you analyze lift, drag, and you don't talk about the energy of moving the wings."

The researchers filmed 1,000 frames-per-second videos of 27 bats (from 6 different species) performing five trials in a flight corridor or wind tunnel. They tracked markers on the bats and measured how frequently the wings flapped, how far up and down they flapped, and the distribution of mass within them as they moved. They measured the mass by cutting the wing of a bat that had died into 32 pieces and weighing them. The team fed the data in to a computer model that allowed them to determine what the inertial energy costs of flapping were and what they would have been if the wings were kept outstretched.

Here is a video of one of the bat flights the team recorded with a computer animation of a flying bat next to it. Take a look:

The study can be found here in the Proceedings of the Royal Society B.

Photo: Breuer and Swartz/Brown University