Just in time for Halloween, scientists are learning the secrets of how bats fly. As this ScienCentral News video explains, the research may lead to some pretty agile tiny flying machines.
A bat on the wing is a marvel of aerodynamic engineering. "They can do amazing things in the air. They're agile, maneuverable, they can turn on a dime," says Brown University engineering professor Kenny Breuer. "They can fly in large swarms, they can fly in forests or caves… When they leave the roost, they fly long distances, they can consume a huge amount of food, put on a huge amount of weight and fly back. So they can fly with excess body weight– the females fly pregnant, which is pretty impressive.
"So one of the interests from an engineering perspective is how can we use nature's examples, in particular bats' examples, as the opportunity to fly in a maneuverable way," Breuer says.
But Breuer wasn't always such a big bat fan. When biologist Sharon Swartz proposed they team up to study how bats pull off their fantastic feats of flight, she recalls, "Like a lot of people, I definitely had the sense when I met Kenny that he would just as soon never come within, you know, a mile of a live bat."
"Flight is such an important part of what makes bats extraordinary," says Swartz, who's been studying bats for 20 years. "We think there are probably about 5,000 species of mammals on the planet today. Of that 5,000, over 1,200 of them are bats– so bats represent a quarter of all the diversity of mammals. But in addition to all the diversity, they occupy this incredible range of habitats. There are bats almost as far north as any other animals go, there are bats everywhere in the tropics. They occupy almost any habitat that humans can live in, and they do it without buildings or clothing or anything like that."
Breuer was persuaded, and with funding from the Air Force and the National Science Foundation, the team uses a wind tunnel, sensors, smoke and lasers to make the invisible forces generated by bats' wings visible.
"They fly forward but they stay still because the wind is keeping them stationary," explains Breuer. "We take high-speed video of that flight from different angles, and we also put a mist in the air which we illuminate with a laser and we take photographs of the mist as it moves, and from that we can figure out the kind of motion the bat is creating. And from the motion the bat is creating, we can work out the forces the bat is generating in order to fly."
image courtesy Breuer Lab, Brown University
They've found that bats' multiple joints and wings made of skin make them the ultimate shape-shifters. "The flexibility of their skeletons, the flexibility of the wing membrane that they use, the level of maneuverability that they exhibit– I just don't think we really appreciated until we actually started measuring it."
Their early experiments have focused on demonstrating that these techniques can be successfully used to make such comprehensive measurements, with many mysteries still to be explored. For example, Breuer says, it's believe that the very hair cells in bats' wings affect their flight, "but no one really knows what they do and how they control flight, or if they have any role– which we assume they do."
Swartz adds that bats' aerodynamic designs work on a scale so small that the principles of aircraft engineering don't apply. "Because bats are so much smaller and so much slower than human-engineered aircraft, the physics of flight are really different for a bat," she says.
In fact, Breuer says, bats are so complicated, "I don't think that we will build, in any foreseeable future… something that looks exactly like a bat."
Instead, they are using models to break down that complexity into features that engineers can mimic to push micro air vehicles to new limits.
"We can engineer things that have characteristics of biological flying machines, and use the biological inspiration to help with our designs without having to create the complete complexity of a real organism" that needs to eat, reproduce and raise offspring, says Swartz. "A micro air vehicle (MAV) only has one job. All it has to do is fly."
Breuer says because existing MAVs have tended to work like small planes or helicopters, "They're limited in how fast and how slow they can go," he says. "They’re also limited in maneuverability– their ability to turn and to negotiate small spaces and hover around and loiter and look around."
But by mimicking certain aspects of bats, or – as the University of Florida's Rick Lind does– birds, researchers can create a new generation of morphing MAVs.
Breuer can think of lots of uses for those abilities. "It might be in a mine looking for people who have been injured, or in a forest where you can't easily get access to the space, to do reconnaissance or search for things inside buildings."
image courtesy Breuer Lab, Brown University
The work has given him a new bat-attitude. "Over the years and months, I've become much more familiar and comfortable and enthusiastic– very enthusiastic– about the project and really have enjoyed the experiments and the collaboration," he says. "So I've changed."
Still, he prefers that other lab members do the actual bat handling.
Speaking of which, Swartz points out that not harming the bats was a huge part of planning their experiments. She says it turns out that some bats seem to like the wind tunnel while others don't. Those that don't, don’t participate.
She hopes their research also increases awareness of the importance and beauty of bats, which, despite their evolutionary success, are now threatened by habitat loss in many parts of the globe.