Scientists have long wondered how exactly birds generate lift during flight. A new device precisely—and humanely—measures the forces generated by a bird’s wings as it flies.
The work, published in the journal Interface, promises to answer many mysteries of bird flight. Those findings could be useful in the design of innovative and efficient unpiloted aerial vehicles, known as UAVs or, more recently, drones.
Measuring the lift forces of a bird in free flight has been a holy grail for biomechanical engineers, says lead author David Lentink, an assistant professor of mechanical engineering at Stanford University. But every technique developed so far has provided uncertain results.
Experiments that involve measuring airflow over the bird, and extrapolating force from that, suffer when the flow becomes turbulent. Measuring the flow also requires strong lasers, which can put the birds in harm’s way. Because of this, Lentink’s lab has developed special tiny goggles to shield birds’ eyes.
Alternative techniques rely on measuring the bird’s body motion to calculate the acceleration produced by its body parts, but that requires a post-flight dissection to determine the associated body masses in order to calculate how much force the bird exerted.
“We’ve developed a way for the bird to just freely fly in a nice environment. It’s a very animal-friendly method, and very precise, too,” Lentink says. “We reward the birds with seed for their flight. We have happy birds and happy researchers afterward.”
A very sensitive system
Lentink calls his device an aerodynamic force platform, and it works very similarly to the force platforms that have allowed bioengineers to study the forces that humans exert to walk or run.
It’s a box the size and shape of a large birdcage, with an acrylic observation window and two bird perches inside. Supersensitive force sensors are attached to the bottom of the box.
This force transfer is based on Newton’s third law of motion, which states that for every action there is an equal and opposite reaction. As the bird flies perch-to-perch, each beat of its wings pushes against the air, which in turn pushes against the bottom of the box and also sucks down the ceiling slightly.
These forces are recorded to produce a precise measurement for each stroke of the bird’s wings.
Each wing beat lasts 50 milliseconds, and the sensors take a new measurement every 1 millisecond. A very precise value can be determined every 10 milliseconds, producing highly detailed data of the bird’s lift. The system is so sensitive, Lentink says, that it registers vibrations in the air from the lab’s ventilation system.
“We have to turn off the air conditioning to conduct experiments, but we get very clean, precise data, so it’s worth it,” Lentink says.
Ray and Gaga in flight
For the proof of principle, Lentink’s team first calibrated their aerodynamic force platform with a quadcopter programmed to generate variable thrust, which they measured independently, and demonstrated the device is accurate within the sensors’ resolution of 0.2 gram.
Next they tested the device using two trained Pacific parrotlets—named Ray and Gaga—and already the work is producing interesting results.
They have found that the birds produce lift equal to two times their body weight during their downstroke, and generate virtually no lift on their upstroke, clarifying classic work done in the field.
Hummingbirds, bats, and drones
The engineers are putting the finishing touches on a refined version of their device, made of carbon fiber, and will soon begin testing more complex flight maneuvers made by birds.
They also hope to resolve an ongoing debate of how hummingbirds, whose wing strokes are more similar to insects’ than birds’, generate lift. Other animals of interest include bats, which fly with membranous wings controlled by tiny muscles under the skin.
Understanding how animals fly so effectively could lead to improved designs for drones, Lentink says. In particular, his group has built a flapping winged robot, with sail-like wings and carbon fiber stiffeners, but they had no way to measure exactly how its aerodynamics worked, or if it could be improved upon.
With the new device, he said, they’ll be able to measure the aerodynamic forces, and get instant feedback to improve upon their designs with greater certainty.
Source: Stanford University