Bats save energy by drawing in wings on upstroke

Bat wings are like hands: meaty, bony and full of joints. A new Brown University study finds that bats take advantage of their flexibility by folding in their wings on the upstroke to save inertial energy. The research suggests that engineers looking at flapping flight should account for wing mass and consider a folding design.

PROVIDENCE, R.I. [Brown University] — Whether people are building a flying machine or nature is evolving one, there is pressure to optimize efficiency. A new analysis by biologists, physicists, and engineers at Brown University reveals the subtle but important degree to which that pressure has literally shaped the flapping wings of bats.

The team’s observations and calculations show that by flexing their wings inward to their bodies on the upstroke, bats use only 65 percent of the inertial energy they would expend if they kept their wings fully outstretched. Unlike insects, bats have heavy, muscular wings with hand-like bendable joints. The study suggests that they use their flexibility to compensate for that mass.

“Wing mass is important and it’s normally not considered in flight,” said Attila Bergou, who along with Daniel Riskin is co-lead author of the study that appears April 11 in theProceedings of the Royal Society B. “Typically you analyze lift, drag, and you don’t talk about the energy of moving the wings.”

The findings not only help explain why bats and some birds tuck in their wings on the upstroke, but could also help inform human designers of small flapping vehicles. The team’s research is funded by the U.S. Air Force Office of Sponsored Research.

“If you have a vehicle that has heavy wings, it would become energetically beneficial to fold the wings on the upstroke,” said Sharon Swartz, professor of ecology and evolutionary biology at Brown. She and Kenneth Breuer, professor of engineering, are senior authors on the paper.

The physics of flexed flapping
The team originally set out to study something different: how wing motions vary among bats along a wide continuum of sizes. They published those results in 2010 in the Journal of Experimental Biology, but as they analyzed the data further, they started to consider the intriguing pattern of the inward flex on the upstroke.

That curiosity gave them a new perspective on their 1,000 frames-per-second videos of 27 bats performing five trials each aloft in a flight corridor or wind tunnel. They tracked markers on the bats, who hailed from six species, 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 calculus-rich 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.

Bergou, a physicisist, said he was surprised that the energy savings was so great, especially because the calculations also showed that the bats have to spend a lot of energy — 44 percent of the total inertial cost of flapping — to fold their wings inward and then back outward ahead of the downstroke.

“Retracting your wings has an inertial cost,” Bergou said. “It is significant but it is outweighed by the savings on the up and down stroke.”

The conventional wisdom has always been that bats drew their wings in on the upstroke to reduce drag in the air, and although the team did not measure that, they acknowledge that aerodynamics plays the bigger role in the overall energy budget of flying. But the newly measured inertial savings of drawing in the wings on the upstroke seems too significant to be an accident.

“It really is an open question whether natural selection is so intense on the design and movement patterns of bats that it reaches details of how bats fold their wings,” Swartz said. “This certainly suggests that this is not a random movement pattern and that it is likely that there is an energetic benefit to animals doing this.”

- by David Orenstein

Professors Kenny Breuer and Eric Suuberg Named Associate Deans of Engineering

Brown University School of Engineering Dean Larry Larson has announced that Professor Eric Suuberg has agreed to accept an appointment as Associate Dean of Engineering for Research and Graduate Initiatives and Professor Kenny Breuer has agreed to accept an appointment as Associate Dean of Engineering for Academic Programs. Both will serve three-year terms.

“I want to thank them both for their willingness to serve the School of Engineering, and for the commitment of their time and energy in moving the School forward in the coming years. Professors Suuberg and Breuer bring decades of experience and wisdom to the School leadership.” said Larson.

The Associate Dean of Engineering for Research and Graduate Initiatives will be responsible for enhancing and expanding the research and graduate enterprise and profile of the School of Engineering, including development of an enhanced master’s program for the School of Engineering and development of a plan for improved engineering research laboratory and instructional space.

The Associate Dean of Academic Programs will be broadly responsible for the academic mission of the School of Engineering, and will work closely with the Director of Undergraduate Programs, Director of Graduate Programs and the Curriculum Committee on curriculum development, educational outreach, student career development and instructional technology.  

Professor Breuer received his Sc.B. from Brown and his M.Sc. and Ph.D. from M.I.T. He spent nine years on the faculty of M.I.T. in department of Aeronautics and Astronautics, before returning to Brown in 1999. His research interests are in fluid mechanics, covering a wide range of topics, including the physics of flows at micron and nanometer scales, animal flight (bat flight in particular), and the physics and control of turbulent flows. He is author of over one hundred refereed technical publications, has edited and co-authored several books, including “Microscale Diagnostic Techniques”, “A Gallery of Fluid Motion”, and “Multimedia Fluid Mechanics”. Breuer was elected a fellow of the American Physical Society in 2010.

Professor Suuberg has been at Brown since 1981, when he was one of the founding members of Brown's Chemical Engineering program. His research interests have been in the areas of energy and environmental engineering. He has served as Associate Dean of the Faculty (2002-2005), as Chair of the Psychology Department (2004-5) and as a member of the Executive Committee of the Division of Engineering. He is currently Co-Director of the Superfund Basic Research Program, and a co-founder of the Commerce, Organizations and Entrepreneurship concentration as well as a co-founder of the PRIME master’s program. He is a principal editor of the journal Fuel. He was elected fellow of the American Chemical Society (ACS) in 2011.

Professor Suuberg's research interests center on energy and environmental areas, involving study of fuel chemistry (coal, oil shale, biomass), activated carbons (production and properties), materials reuse (automobile tires, coal fly ash), fire safety and, most recently, the characterization and cleanup of lands and sediments contaminated with mixed pollutants with a focus on thermodynamics of mixtures of high molecular weight organic compounds and the related problem of vapor intrusion.

He received his bachelor’s degree in chemical engineering from M.I.T., a master’s degree in management science from M.I.T., and an Sc.D. in chemical engineering from M.I.T.