Researchers have studied birds in flight for years, with an eye toward applying the tricks they use to navigate changing conditions in the real world to design better aerial robots.

Most of the insights gained so far have resulted from painstaking study, involving calculations of wing force dynamics inspired by footage captured in the wild.

Now, they hope the construction of a new bird wind tunnel will 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,” says David Lentink, assistant professor of mechanical engineering at Stanford University.

With the recent boom in drone use, it's easy to forget that the robots frequently fail in windy conditions.

“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.”

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, allowing researchers to study how birds fly in smooth-flowing air such as that found at higher altitudes.

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.

Bird tricks

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.

To protect them, Lentink says this would be the maximum speed he would consider letting larger birds fly to keep them comfortable, even though the tunnel can blow much faster, with speeds up to 50 m/s.

Nearly two meters long, the six-sided windowed observation section of the tunnel provides a variety of ways to study bird flight. The researchers currently zero in on specific aspects of birds’ wing beats, using high speed cameras and motion capture techniques more commonly utilized in Hollywood films, to record 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.

“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 use,” Lentink says.

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