Birds have been doing their flying thing with flexible and feathery wings for about a hundred million years, give or take. And about a hundred years ago, give or take, humans decided that, although birds may be the flying experts, we’re just going to go off in our own direction with mostly rigid wings and propellers and stuff, because it’s easier or whatever. The few attempts at making artificial feathers that we’ve seen in the past have been sufficient for a few specific purposes but haven’t really come close to emulating the capabilities that real feathers bestow on the wings of birds. So a century later, we’re still doing the rigid wings with discrete flappy bits, while birds (one has to assume) continue to judge us for our poor choices.
In a paper published today in Science Robotics, researchers at Stanford University have presented some new work on understanding exactly how birds maintain control by morphing the shape of their wings. They put together a flying robot called PigeonBot with a pair of “biohybrid morphing wings” to test out new control principles, and instead of trying to develop some kind of fancy new artificial feather system, they did something that makes a lot more sense: They cheated, by just using real feathers instead.
The reason why robots are an important part of this research (which otherwise seems like it would be avian biology) is because there’s no good way to use a real bird as a test platform. As far as I know, you can’t exactly ask a pigeon to try and turn just using some specific wing muscles, but you can definitely program a biohybrid robot to do that. However, most of the other bioinspired flying robots that we’ve seen have been some flavor of ornithopter (rigid flapping wings), or they’ve used stretchy membrane wings, like bats.
By examining real feathers, the Stanford researchers discovered that adjacent feathers stick to each other to resist sliding in one direction only using micron-scale features that researchers describe as “directional Velcro.”Image: Lentink Lab/Stanford University
Feathers aren’t just more complicated to manufacture, but you have to find some way of replicating and managing all of the complex feather-on-feather interactions that govern wing morphing in real birds. For example, by examining real feathers, the researchers discovered that adjacent feathers stick to each other to resist sliding in one direction only using micron-scale features that researchers describe as “directional Velcro,” something “new to science and technology.” Real feathers can slide to allow the wing to morph, but past a certain point, the directional Velcro engages to keep gaps from developing in the wing surface. There are additional practical advantages, too: “they are softer, lighter, more robust, and easier to get back into shape after a crash by simply preening ruffled feathers between one’s fingers.”
With the real feathers elastically connected to a pair of robotic bird wings with wrist and finger joints that can be actuated individually, PigeonBot relies on its biohybrid systems for maneuvering, while thrust and a bit of additional stabilizing control comes from a propeller and a conventional tail. The researchers found that PigeonBot’s roll could be controlled with just the movement of the finger joint on the wing, and that this technique is inherently much more stable than the aileron roll used by conventional aircraft, as corresponding author David Lentink, head of Stanford's Bio-Inspired Research & Design (BIRD) Lab, describes:
The other cool thing we found is that the morphing wing asymmetry results automatically in a steady roll angle. In contrast aircraft aileron left-right asymmetry results in a roll rate, which the pilot or autopilot then has to stop to achieve a steady roll angle. Controlling a banked turn via roll angle is much simpler than via roll rate. We think it may enable birds to fly more stably in turbulence, because wing asymmetry corresponds to an equilibrium angle that the wings automatically converge to. If you are flying in turbulence and have to control the robot or airplane attitude via roll rate in response to many stochastic perturbations, roll angle has to be actively adjusted continuously without any helpful passive dynamics of the wing. Although this finding requires more research and testing, it shows how aerospace engineers can find inspiration to think outside of the box by studying how birds fly.
The researchers suggest that the directional Velcro technology is one of the more important results of this study, and while they’re not pursuing any of the numerous potential applications, they’ve “decided to not patent this finding to help proliferate our discovery to the benefit of society at large” in the hopes that anyone who makes a huge pile of money off of it will (among other things) invest in bird conservation in gratitude.
With the real feathers elastically connected to a pair of robotic bird wings with wrist and finger joints that can be actuated individually, PigeonBot relies on its biohybrid systems for maneuvering.Image: Lentink Lab/Stanford University
As for PigeonBot itself, Lentink says he’d like to add a biohybrid morphing tail, as well as legs with grasping feet, and additional actuators for wing folding and twisting and flapping. And maybe make it fly autonomously, too. Sound good to me—that kind of robot would be great at data transfer.
Evan Ackerman is a senior editor at IEEE Spectrum. Since 2007, he has written over 6,000 articles on robotics and technology. He has a degree in Martian geology and is excellent at playing bagpipes.