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Students capture the flight of birds on very high-speed video

07 July 2013

Slow-motion video footage of the fastest wings in the bird world may provide design cues for improving the performance of flying robots.

Eirik Ravnan, a mechanical engineering graduate student, trains small birds called parrotlets to fly from perch to perch, or to fly through narrow passageways. In exchange for their flight displays, the birds receive their favourite seeds as a reward

Stanford mechanical engineering professor David Lentink and his students have captured slow-motion video of birds with the fastest wing motion in their quest to build better, more capable flying robots.

"The best way to prevent a small drone from spying on you in your office is to turn on the air-conditioning," said David Lentink, an assistant professor of mechanical engineering at Stanford. That little blast of air, he explained, creates enough turbulence to knock a hand-size UAV off balance, and possibly send it crashing to the floor.

A pigeon, on the other hand, can swoop down busy city streets, navigate around pedestrians, sign posts and other birds, keep its path in all sorts of windy conditions, and deftly land on the tiniest of hard-to-reach perches.

"Wouldn't it be remarkable if a robot could do that?" Lentink wondered. If robots are to become a bigger presence in urban environments, they will need to.

In order to build a robot that can fly as nimbly as a bird, Lentink began looking to nature. Using an ultra-high-speed Phantom camera that can shoot upwards of 3,300 frames per second at full resolution, and an amazing 650,000 at a tiny resolution, Lentink can visualize the biomechanical wonders of bird flight on an incredibly fine scale.

Hummingbirds beat their wings about 50 times per second, nothing but a green blur to human eyes.

"Our camera shoots 100 times faster than humans' vision refresh rate," Lentink said. "We can spread a single wing beat across 40 frames, and see incredible things."

Every time Lentink's students take the camera into the field, they have the potential to make a groundbreaking discovery. Thousands of birds have never been filmed with a high-speed camera, their secret flight mechanics never exposed.

Students Andreas Peña Doll and Rivers Ingersoll filmed hummingbirds performing a never-before-seen 'shaking' behaviour: As the bird dived off a branch, it wiggled and twisted its body along its spine, the same way a wet dog would try to dry off. At 55 times per second, hummingbirds have the fastest body shake among vertebrates on the planet – almost twice as fast as a mouse.

The shake lasted only a fraction of a second, and would never have been seen without the aid of the high-speed video.

Though Lentink's lab has amassed hours of short clips of bird flight, it's difficult to frame up a perfect shot in the wild, so his students supplement this footage with carefully orchestrated laboratory-based experiments.

"In the field, you can observe social interactions near other birds, how they fly through the wind or through clutter," Lentink said. "This is very valuable. But the conditions aren't always ideal for examining discrete motions."

Eirik Ravnan, a mechanical engineering graduate student, trains small birds called parrotlets to fly from perch to perch, or to fly through narrow passageways. In exchange for their flight displays, the birds receive their favourite seeds as a reward.

Repeating and videotaping these actions in controlled conditions, he said, makes it possible to look more carefully at, for example, exactly how a bird tilts its wings to slow itself when landing, or how birds corner. 

A better bird 'bot

Search-and-rescue is one of the more attractive applications for robotic aircraft, particularly scanning a wide urban area for survivors after a natural disaster. The unpredictable environment will demand robots that can better deal with changing conditions.

Mini-copters and planes often stall at steep angles, or when they get caught in a gust of wind. They have difficulty avoiding other airborne objects, and fly clumsily near buildings.

Lentink and his students have already begun applying the lessons they've learned from birds to various robotic designs.

"Hummingbirds are amazing at hovering, but it's not a very efficient form of flight," said Waylon Chen, a graduate student in Lentink's lab. "A swift flies a lot, so it has a very efficient wing platform, but its legs are too short to land. As we lay out the goal of our robotic design, we can pick and choose which natural mechanisms will be useful, and incorporate only those."

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