Hidden away in the Lentink lab at Stanford University, a dedicated team of engineers – and a parrotlet named Gary – have been uncovering the secrets of avian flight. With a brand new method to record how a bird’s wing changes shape in flight, the team hopes to better understand the forces that keep them in the air.
These forces are never more challenged than inside city limits. As space to grow outwards is limited, our cities grow ever taller, bringing with them a ‘skyscraper wind’ effect. When wind collides with the side of a tall building it is directed towards the ground, creating downdraughts and gusty conditions at street level. Clusters of skyscrapers add to the effect, squeezing wind through narrow corridors.
City-dwelling birds have adapted well to their blustery new environments, gliding through urban jungles with ease. When it comes to developing drones that can zip between high-rise buildings just as effortlessly, engineers have had to turn to the city’s avian residents to find some answers.
“We’re trying to figure out how birds are capable of flying so well in these complex, turbulent environments and a lot of that comes from how they deform the shape of their wings, left versus right, to adjust to gusts quickly,” said David Lentink, an assistant professor of mechanical engineering.
Lights, camera, action!
Animal movement has inspired innovation in the past, but collecting this level of detail has never before been possible. Current methods to record motion often rely on sticking markers to the animal or tracking their own distinctive patterns such as stripes or spots. These methods, however, are often too slow or produce poor quality, low resolution images.
To overcome these problems, the team have devised a system that can automatically reconstruct a bird’s body shape, or in this case wing shape, at high speed and high resolution. Because the system can render a 3D model quickly, it catches tiny changes in shape over time, allowing engineers to analyse each movement.
The setup consists of a video camera linked to a projector that produces two overlapping light patterns. The first is a grid-like pattern, while the second is a series of unequally spaced lines like a barcode, which is projected at a right-angle to the grid. The irregularity of the second pattern ensures no two areas of the light field look the same. As the bird flies across the patterns, its body acts like a screen and the straight lines deform over the shape of its body and wings.
Graduate student and the study’s senior author, Marc Deetjen, has developed an algorithm that matches the deformed pattern captured by the camera with the original projection. It can then build up a detailed, 3D reconstruction of the bird’s movement as it passes through the light field.
“The great thing about this system is that it’s the first fully automated, high-speed reconstruction of birds in the world,” said Deetjen.
Borrowing secrets from nature
To put their design to the test, the team needed a little help of the feathered kind. Gary, a four-year-old parrotlet, was trained to fly between two perches, though the projected light grid. His light-coloured body showed up the patterns perfectly, allowing the camera to capture the distorted image. While the team only needed one camera to capture the data for this experiment, multiple cameras could allow for a full-body reconstruction in the future.
With each wing-beat, birds morph their bodies through a range of different shapes, tweaking the angle, twist and asymmetries of their wings to keep them airborne. The first trial run was designed to put the algorithm- and Gary- to the test. What the engineers didn’t realise was how successful their attempt would be. By studying Gary’s flight pattern in detail, the team discovered that during take-off, birds can manipulate the drag and lift produced by the angle of their wings, using this to thrust themselves into the air.
“They’re actually able to generate more total force on lift-off,” said Deetjen. “That enables them to not only push up and overcome gravity, but to accelerate forward.”
By understanding how birds use nature to their advantage, engineers hope to borrow their secrets, applying them to small flying machines such as drones.
The next step for the team, and Gary, will see them testing the system in a specialised wind tunnel, analysing how wing shape changes to keep birds in the air in turbulent conditions. While this experiment has focussed on uncovering the secrets of flight, the team are confident it could be applied to any form of movement.
“This is a technique that goes all the way from animal locomotion to direct applications in engineering, where things deform fast,” said Lentink. “We only need to create one frame and then we can reconstruct the shape in 3D. This technique, in principle, does not have a speed limit.”
Image redit Kurt Hickman, Stanford News