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Finally, A Drone That Can Hit a Tree Branch and Keep Flying

Engineers have developed bird-like, flapping wings for drones that can withstand collisions and keep flying.

​Drones have evolved to do a lot of crazy shit over the last few years. They can ​fly 100 mph, dive underwa​ter, and identify faces from 1,000 f​eet away. But they still can't handle a few measly tree branches without snapping, spinning out, or slamming to the ground.

That's where new Navy-backed research published Wedne​sday in the journal Bioinspiration and Biomimetics comes in. Engineers from Stanford University took inspiration from bats and birds to develop a flapping drone wing that's able to easily squeeze through tight spaces and recover from midair collisions, all without any need for actuation.


Video: ​Lentink Lab

When the wing encounters an obstacle, the flapping motion propels its "origami-like" folds to close in and unfurl smoothly, all within a single motion.

"It's able to unfold within about one wingbeat, which is really fast," said David Lentink, an assistant professor of mechanical engineering at Stanford and co-author of the study. "It flaps its wings at 14 hertz, which is 14 times per second. If you can get your wing back into position within one wingbeat, that's super fast."

The design of the wing allows it to fold and unfold naturally, without the need for any controls from the drone operator. The wingspan is about 15 inches across and is made of carbon fibre and Mylar film, as well as special 3D-printed joints that allow for the folding mechanism. And it's able to withstand incredible force without breaking. Amanda Stowers, a PhD student and lead author of the study, physically tested the wing and found it easily recovered from the collision.

"Stowers found that the wing would automatically fold even if she pushed against it or hit it. I asked her if she could hit it as hard as possible, so she got a steel rod," Lentink told me. "She hit it as hard as she could, which is in the high speed video. It worked amazingly well and it surprised us."

The pair created a computer model that showed the wing design would fold when encountering an obstacle no matter how tiny or large the wings were. Numerous computer and real world simulations showed the wing successfully recovering over and over.


The wings haven't been tested for flight capability yet, but Lentink said he wasn't worried about that step as flapping-wing drones have​ already been proven to fly. He said their main focus is on finding a way to replicate the incredible morphology of bat and bird wings, with emphasis on the latter. As they continue their work, they hope to get as close as possible to the feathered, versatile wings of a bird.

"The Holy Grail is to end up with morphing wings that can change its size and shape dramatically," Lentink said. "Just imagine a falcon tucking its wings against its body and diving. Or just a pigeon. Even pigeons flying at high speeds can do similar things."

Stowers and Lentink are among many researchers who received grants from the Office of Naval Research to develop drones for the Navy that would be able to navigate through dense areas like forests and even land on trees. Current rotary-wing drones have a lot of advantages over wing-flapping crafts, Lentink said, like being more power efficient. But for navigating crowded spaces or heavy winds, a different design was needed.

Other researchers have been looking at solving the problem in different ways, like creating algorithms that can more quickly detect and react to possible obstacles. But Lentink said the bird-like wing design can complement those advances to make a nearly unshakable generation of drones.

"It's really difficult to completely avoid mistakes, even if the algorithm is perfect," Lentink said. "It's better if the vehicle can withstand a collision. That simplifies everything and we were really surprised that this relatively simple solution can withstand such hard impacts."