Humans have sought to build bird-like flying machines for centuries. With modern materials, engineers have come a long way since Leonardo Da Vinci drew a glider with flapping wings. And yet, even today’s most advanced drones struggle to navigate through turbulent air and gusty winds. Most birds, on the other hand, soar through rough skies with seemingly little effort, putting our high-tech imitations to shame.
“We actually know very little about how birds fly,” says Stanford University mechanical engineer David Lentink. To that end, Lentink and his colleagues recently discovered previously unknown intricacies in the mechanics of avian flight that allowed them to build the most bird-like flying robot to date, all with inspiration—and some real feathers—from that familiar but fantastically agile species, the Rock Pigeon. “There is a lot to learn from these accomplished flyers,” he says.
The team's new remote-controlled machine—aptly named the PigeonBot—can maneuver deftly in windy environments by rapidly adjusting its wing shapes, the scientists report in a study published last week in the journal Science Robotics.
In flight, birds can dramatically change their wing shapes: They tuck in their wings when flying fast or spread them out when gliding, while also making sharp twists and turns. “This enables them to fly more efficiently, fly longer, and maneuver better,” Lentink says. But scientists hadn’t quite understood how wing, skeletal, and muscular features aided this fluid wing morphing.
Lentink and his team advanced that understanding by filming the bending and stretching of real wings from dead pigeons in a university collection.
Bird wings, like human arms, have a humerus, radius, ulna, and wrist bones. They also bear a finger-like structure at the end of each wingtip. On film, Lentink and his team found that the wrist and finger joints move independently to align flight feathers. Rather than using individual muscles to adjust their feathers, birds can define their wing shape, direction, and speed with a slight bend at the wrist or twist in the finger joint. Also, an elastic ligament at the base of these feathers facilitates the graceful arrangement of primary and secondary feathers during flexing and extension.
Using this information, the team in Lentink’s Bio-Inspired Research and Design (BIRD) Lab built a simple, propeller-powered robot out of foamboard, rubber bands, and real pigeon feathers. When its fingers and wrists were rotated via remote control, the feathers automatically moved in proportion and recreated wing shapes as seen in pigeons.
The researchers also observed that even amidst turbulence, extended wing feathers worked in tandem to avoid creating gaps that could hinder a bird’s balance when flying. Scanning electron and X-ray microscope images of adjacent, overlapping feathers revealed minute hooks and barbs that helped these feathers stick together, Lentink’s team report in a parallel study published in Science last week. Like directional Velcro, “they lock with gigantic force” when spread, he says, “but when the wings fold, this fastening is released automatically.”
Pinning down the function of different bird wing parts has proven notoriously difficult, says Tobin Hieronymus, an anatomical scientist at Northeast Ohio Medical University who wasn’t involved in the research. The wing features Lentink and team describe in the studies were identified long ago, but showing how they work together has been a challenge, Hieronymus says. “I think the use of robots is a really elegant way of demonstrating function.”
PigeonBot accurately captures a gliding flight by its namesake bird, but it’s not capable of flapping its wings, let alone executing a hummingbird’s lightning-fast wing beats. Still, Lentink and team have taken a big step forward understanding and recreating the magic of bird flight—one that could yield advances in wing design for drones and other aircraft. Consider it yet another reason to appreciate the under-appreciated pigeon.