Most sensible air travelers dread turbulence. A little atmospheric hiccup can shake airplanes, rattle nerves and spill beverages. A Cornell-led collaboration has found that birds don't mind at all.
By combining wind speed data with the measured accelerations of a golden eagle outfitted with GPS tracking instruments, the researchers suggest that, rather than hindering flight, turbulence is a source of energy that birds may use to their advantage.
This counterintuitive discovery could revise what we know about avian flight, and help the aerospace industry develop faster, more efficient ways to fly in turbulent environments.
The group's paper, "Turbulence Explains the Accelerations of an Eagle in Natural Flight," published June 8 in PNAS. The lead author was doctoral student Kasey Laurent.
While the flight of birds may appear easy and graceful to earthbound spectators, winged animals are actually navigating air flow that is structured, textured and constantly in flux, according to Gregory Bewley '00, assistant professor in the Sibley School of Mechanical and Aerospace Engineering, who led the team.
"The air is always turbulent. This is really hard to convey, because unless there's a cloud or smoke, it is transparent to us," said Bewley. "I suspect these animals know a lot about turbulence in a practical sense. They're not just responding to this as if it were an annoyance. They feel it and can probably anticipate it and respond in clever ways that I think would be useful for us to implement in our own flight vehicles."
Bewley's research background is in fluid mechanics, with a focus on the intrinsic properties of turbulence and its role in environmental settings. Ever since childhood, he's been fascinated with the mechanical feat of bird flight; about five years ago, he connected with David Winkler, professor in the Department of Ecology and Evolutionary Biology, and began exploring the topic from an aerospace engineer's perspective.
In order to take his experiments out of the lab and into the sky, Bewley's team partnered with two groups - Conservation Science Global and Cellular Tracking Technologies. Scientists from these companies captured a female golden eagle in Alabama, rigged it with a solar GPS telemetry unit with an accelerometer weighing less than 3 ounces, then released the bird.
Over the course of 17 days, as the eagle migrated north along the Appalachian Mountains toward Canada, the GPS "backpack" transmitted more than 200 hours of data - including location coordinates, altitude, ground speed and tri-axial acceleration - via cellular networks.
Bewley's lab then obtained wind speed data from the National Centers for Environmental Prediction's weather history databases and mapped it onto the eagle's flight measurements, identifying the bird's various flying and nonflying behaviors.
They found a "highly irregular, fluctuating pattern" in the eagle's accelerations, which resembles the typical trajectories of particles in turbulent airflows. At timescales ranging from 0.5 to 10 seconds - which translates to approximately 1 to 25 wingbeats - the eagle's accelerations and atmospheric turbulence were completely in synch.
"We asked how much of that acceleration can be explained by the turbulence in the atmosphere," Bewley said. "And the answer is, within an interval of frequencies between about 0.1 and 1 hertz, the whole signal can be explained by atmospheric turbulence. It's almost as if, instead of the eagle, you put a feather out in the wind. You would see the same thing. This turbulence has such a strong effect that everything about this eagle that makes it alive, such as hunting, is subordinate."
And just how intense are these accelerations? As a point of comparison, people riding in a car or aboard a commercial flight experience less than 0.1 g, or one factor of earth's gravitational acceleration. Meanwhile, the accelerations of birds exceed 1 g - which would throw those human passengers out of their seats.
Over longer timescales, the eagle exerts more and more control over its flight. Bewley surmises this connection with turbulence is even more pronounced in smaller creatures.
"The eagle is a rather large bird," Bewley said. "So if this holds for this large, relatively heavy, relatively fast animal, I think it must only be more true for smaller, slower, lighter-weight animals. I imagine they are more susceptible to the turbulence or more involved with it."
Of course, aeronautical engineers strive to reduce turbulence as much as possible, and no airline passenger or pilot wants a bumpy ride. But Bewley believes there are opportunities to harness the energy of turbulence, particularly for person-less transport and small reconnaissance aircraft.
"If you could find a path in which every vortex is pushing you the right way, then obviously you get there a little faster with a little less energy," Bewley said. "We're still working hard to understand turbulence by itself. I think it's fascinating that there might be some practical empirical knowledge embodied in wildlife that we don't appreciate yet."
Co-authors include Tobias Ginsburg '21 and researchers from Cellular Tracking Technologies, Conservation Science Global, West Virginia University and SABER Consulting.
The research was partially funded by the Friends of Talladega National Forest.
