By folding their wings in toward their bodies on the upstroke, bats use 35% less energy and reduce aerodynamic drag, compensating for heavier, more muscular wings. Credit: Breuer and Swartz/Brown University
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.
team’s observations and calculations show that by flexing their wings
inward to their bodies on the upstroke, bats use only 65% 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.
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 the Proceedings of the Royal Society B. “Typically you analyze lift, drag, and you don’t talk about the energy of moving the wings.”
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.
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
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.
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.
team fed the data into 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.
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% of the total inertial cost of
flapping—to fold their wings inward and then back outward ahead of the
your wings has an inertial cost,” Bergou said. “It is significant but
it is outweighed by the savings on the up and down stroke.”
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.
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.”
Source: Brown University