Humans can’t fly, but we’re determined to work out how others do. Last month a Canadian team suggested that the way birds move their wings, rather than the shape of those wings, determines how they fly.
Now researchers from Stanford University, US, have watched five parrotlets in flight and discovered that – counterintuitive as it might sound – they used drag to help with their take-off and lift to assist with landing.
Conventional wisdom tells us that drag is a force that slows an object down and lift is a force that counters gravity. However, in this case drag supported up to half of the birds’ (admittedly low) body weight at a crucial time, while lift helped with braking.
David Lentink and Diana Chin made the finding after encouraging Gaga, Gary, Oreo, Aurora and Boy to make repeated flights from perch to perch through an instrumented flight chamber developed specifically for the purpose.
It was only 80 centimetres long, but then the birds only weigh 30 grams – and a grain of millet was sufficient inducement for each trip.
To measure the horizontal and vertical forces instantaneously, Chin built a setup with sensor panels on the floor, ceiling, front and back of the birds’ flight paths.
Each panel contained three sensors, as did the two perches for bird liftoff and landing. That’s 18 sensors in all, to measure the minuscule forces the birds generated.
Windows built into the panels allowed Chin to film wing movements with five high-speed cameras shooting at 1000 frames per second. By combining the measured motion from the images with the force measurements from the sensors, she and Lentink could determine the magnitude of lift and drag.
They found that the repurposing of drag during take-off maximised the birds’ generated forces, while reorienting lift helped them slow down without the power costs of braking before making the controlled collision they call a landing.
“Many other flapping animals probably make similar use of lift and drag during take-off and landing,” says Chin.
And we may have to reassess the historical record, she and Lentink suggest.
Protobirds (extinct animals seen as an evolutionary link between fossil reptiles and fossil birds) also had wings that primarily generated drag, they say, so there may be some species previously classified as flightless that could have used drag to become airborne.
After 150 million years of bird evolution, “if modern birds still make use of it, that tells you a lot,” says Lentink. “That doesn’t mean its efficient, but it’s effective.”
And, he suggests, we may have to revisit the way the evolution of bird flight and aerodynamics are taught.
“None of the aerospace literature came up with using drag to support weight,” he says. “That standard drawing has to be revised.”
The findings are reported in the journal Nature Communications.