Bats are wonderfully adapted for their unique role as the only true flyers among mammals, and researchers have studied – and sometimes borrowed – those adaptations (echolocation, for instance) to benefit humans. Researchers at Brown University in Rhode Island are reporting on a robotic wing that’s probing the secrets of bat flight.
|Bat wings: the real thing and a robot. Photo courtesy of Breuer and Swartz labs/Brown University
The robot mimics the shape and motion of wings of the lesser dog-faced fruit bat of South and Southeast Asia. It is designed to flap in a wind tunnel and record aerodynamic forces, the university said in a news release. The wing’s seven movable joints are controlled by servomotors that let the researchers evaluate how much energy is required for various wing movements.
Testing showed the robot can match the basic flight parameters of bats, producing enough thrust to overcome drag and enough lift to carry the weight of the model species.
The robotic wing and preliminary research results were published in the journal Bioinspiration and Biomimetics. Brown University engineer Kenneth Breuer and biologist Sharon Swartz were lead authors, and Brown graduate student Joseph Bahlman led the project.
In addition to providing new insights into the mechanics of bat flight, the news release said, the research “could aid the design of small flapping aircraft.” The study was funded by the U.S. Air Force Office of Scientific Research and the National Science Foundation.
“We can answer questions like, ‘Does increasing wing-beat frequency improve lift and what’s the energetic cost of doing that?’” Bahlman said. The robot wing is necessary since bats would be unable to fly when loaded down with instrumentation, and they don’t take direction well, in any event.
Bats and some birds fold their wings back on the upstroke, and one experiment looked at the aerodynamic effects of wing folding, the university reports. Positive lift is generated by the downstroke, but some of that lift is lost in the subsequent upstroke, which generates negative lift. By running trials with and without wing folding, Brown said, the robot showed that folding the wing dramatically decreases that negative lift and increases net lift by 50 percent.
Bat wings are complex things. They span most of the length of a bat’s body, from shoulder to foot. They are supported and moved by two arm bones and five finger-like digits. Over those bones is a super-elastic skin that can stretch up to 400 percent without tearing. The eight-inch robot mimics that anatomy with plastic bones carefully fabricated on a 3-D printer to match proportions of a real bat. The skin is made of a silicone elastomer. The joints are actuated by tendon-like cables that pull on the joints.
“We learned a lot about how bats work from trying to duplicate them and having things go wrong,” Bahlman said.
During testing, for example, the joint of the robot’s elbow broke repeatedly. Bahlman eventually wrapped steel cable around the joint to keep it intact, similar to the way ligaments hold joints together in real animals.
The fact that the elbow was a weak point in the robot might help to explain the large muscle at elbows of bats, the university said. In humans, these muscles help us turn our palms up or down, but bats can’t make that motion. Bahlman’s experience suggests these muscles may be adapted to resist bending in a direction that would break the joint open.
The wing membrane provided more lessons. It often tore at the leading edge, prompting Bahlman to reinforce that spot with elastic threads. The fix ended up looking a lot like the tendon and muscle that reinforce leading edges in bats, underscoring how important those structures are.
Now that the robot is working, Bahlman said, “We’d like to try different wing materials, different amounts of flexibility on the bones, looking to see if there are beneficial tradeoffs in these material properties.”