Stanford engineers build a one-of-a-kind wind tunnel for birds that paves the way for better drones

Most people look at a pigeon and see a chubby, common, boring bird. A “rat with wings,” in common New Yorker parlance. When David Lentink watches a pigeon dart around a building and land perfectly in its roost, however, he sees the future of robotic flight.

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Lentink, an assistant professor of mechanical engineering at Stanford, has been studying birds in flight for years, with an eye toward applying the tricks birds use to navigate changing conditions in the real world to design better aerial robots. Most of the insights he and his colleagues have gained so far have resulted from painstaking study, involving calculations of wing force dynamics inspired by footage captured in the wild.

Now, with the construction of one of the most advanced bird wind tunnels in the world, Lentink hopes to reveal even more of the magic of bird flight.

With the recent boom in drone use, it’s easy to forget that the robots frequently fail in windy conditions. Consider flying a drone down an “urban canyon” like Fifth Avenue in New York City. Turbulence varies wildly from the middle of the “canyon” to alongside the skyscrapers, and obstacles like traffic lights pop up frequently. Now, throw in a few dozen drones fighting for position like the taxis below. It’s a nightmare for drone operators.

“But you look up, and you’ll see a pigeon swoop by casually. It has no problem stabilizing itself, flying around corners, dodging cables and landing on a perch,” Lentink said. “It’s just something we haven’t accomplished in robotics yet. We need to study birds up close so we can figure out what their secret is to flying so stably under such difficult conditions, and apply that to aerial robotic design.”

Cruising speed for birds

The new wind tunnel works like a super tricked-out treadmill for birds. The windflow, generated by a fan roughly the size of a Volkswagen Beetle, is super smooth: Turbulence checks in around .015 percent, less than half of any other bird wind tunnel in the world. This allows the researchers to study how birds fly in smooth-flowing air such as that found at higher altitudes.

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Such conditions aren’t typical closer to the ground, particularly around trees and buildings, though, so the tunnel is fitted with a “turbulence generating system,” a series of computer-controlled wind vanes that can precisely simulate different turbulence patterns, creating up to 50 percent turbulence. In this state, the flow moves almost equally randomly in all directions, making it very unpredictable for the bird.

Wind speed is also highly tunable. The lovebirds, parrotlets, and hummingbirds that Lentink’s lab studies typically cruise around 7 meters/second, which the engineers can match perfectly to study sustained flight. They will occasionally crank the flow up to 15 m/s, which simulates a strong wind, maxing out at 20 m/s for large birds.

Lentink is fiercely protective of his birds, and said this would be the maximum speed he would consider letting larger birds fly to keep them comfortable. The tunnel can blow much faster, however, with speeds up to 50 m/s for the prototype drones he plans to test in the tunnel.

Nearly two meters long, the six-sided windowed observation section of the tunnel provides Lentink and his students a variety of ways to study bird flight. They currently zero in on specific aspects of birds’ wing beats, using high speed cameras as well as motion capture techniques more commonly utilized in Hollywood films, recording wing motion millisecond by millisecond. They then translate these measurements to precise calculations of the force dynamics experienced along the birds’ wings and in the surrounding air. Later this summer, Lentink expects to introduce two fluoroscopes to the mix, which will allow researchers to “see inside” the bird and visualize the exact muscular-skeletal movements it makes in different flight maneuvers.

Once his team has trained enough birds, Lentink plans to fly entire flocks in the tunnel to determine how turbulence created by one bird’s wing beats affects a nearby bird, and how they maneuver for position. Both of these measurements will provide critical foundational information for a future sky packed with drones.

Using the information gleaned from bird flights, Lentink envisions using the tunnel as a test-bed for new aerial robot designs. In addition to establishing better maneuverability controls for common quadcopter designs, he’s particularly interested in building bird-like, winged robots that quickly morph their wing shape in order to maintain stability in turbulent air flows.

Learning avian tricks

“Ever since Otto Lilienthal and the Wright Brothers studied birds to invent their airplanes, engineers have relied on talking with biologists to learn the tricks birds us,” said Lentink, who is a member of Stanford Bio-X. Although the wind tunnel will enable engineers to develop safer and more reliable drones that fly in urban environments as well as birds do, Lentink stressed that it is not only an engineering facility. It is a top-notch biology lab that meets and exceeds all animal research standards enabled by the very best technology Stanford offers.

Lentink, who is both a biologist and an engineer, teaches engineering students and biology postdocs how to collaborate.

“Our bird tunnel is really unique, and I’m incredibly thankful to my colleagues and the School of Engineering who thought it was an awesome idea to enable engineering students to study how birds fly to develop better flying robots and made this possible,” Lentink said. “The facility has been built with great care by people within the School of Engineering, and I’m really excited about the opportunity to study bird flight up close with engineering students who bring different interests ranging from biomechanics to fluid mechanics to aeronautics in our team of engineers and biologists.”

The wind tunnel was paid for by Stanford. The various measurement systems were acquired with support from the Air Force, Navy, Army, Human Frontiers Science Program, and Stanford Bio-X program.