Ben Parslew had a problem: His robots weren't very good at jumping. Parslew, an aerospace engineering researcher and lecturer at the University of Manchester, studies the mechanics of flight. Along with his research team, he had turned to robots to better understand how flying machines might be able to launch themselves into the sky like birds. But while avians seem to effortlessly become airborne, Parslew's robots, which were relatively simple constructions, didn’t find it quite so easy. Some would flip over in the air and land upside-down. Others remained stable in the air but jumped in the wrong direction. And still others fell over before they got off the ground in the first place.
Something was clearly not working here. So Parslew and his team decided they needed to back up and first study how exactly a bird launches itself into the air. “That was kind of motivation for doing this study, to understand why our robots are failing and why birds succeed with such apparent ease,” Parslew says.
The study Parslew's team conducted was published last month in the journal Open Science. Using computer analysis, the researchers found that when birds take off, they simultaneously control two motions: the direction they’re jumping in and the amount they rotate (pitch) their body as they accelerate, Parslew says. Such coordination allows them to remain balanced during launch.
To conduct the study, Parslew’s team created computer models using data from two studies of birds with different takeoff styles: one was led by Pauline Provini, who researches evolutionary biology at the French National Museum of Natural History in Paris, using perching Diamond Doves, and another was led by Havalee Henry, now an orthopaedic surgery resident at Yale New Haven Hospital, on ground-jumping Guinea Fowl.
"That’s the kind of approach we took," Parslew says, "looking at birds in nature as a sort of dataset, but then also building our own theoretical models and computational models.”
In addition to discovering the importance of pitch and direction, the researchers found that birds are specially built for this kind of takeoff because of a certain sponginess, or cushioning, in their leg joints that lets them extend their legs for a smooth and fluid movement during the jump. This extra flexibility in combination with their jump direction and proper rotation allows for a stable takeoff, he says.
Diamond Dove taking off. Video: Dr. Pauline Provini/Functional Morphology Lab/Muséum National d'Histoire Naturelle
The team also confirmed that the wings don’t contribute at all to the physics of the initial jump—it’s all about the legs. This can be seen in Provini’s video of a Diamond Dove taking off. The bird lowers its trunk by flexing its legs, and then extends its hip, knee, and ankle to push the body in the direction of takeoff.
“What is interesting is the fact that the wings are still up when the bird leaves the perch," Provini says, "meaning that they are not involved in the first propulsion.”
Parslew’s modeling also showed that a bird's ability to grip a perch is beneficial for taking off: It allows them to launch at more angles than when they are standing with their feet on flat ground.
"The big impact of having a perch to apply torque to is that it means you can jump in many more directions—from very shallow to very steep," Parslew says. "Without a perch you have a much narrower window of jump directions, and if you try anything outside of this window you tip over."
The Diamond Doves, for instance, jumped at angles around 20 degrees shallower than birds with feet that aren't made for gripping, Parslew says. Contrast that with the Guinea Fowl: With their fee flat on the ground, they have a much more limited range of jumping angles.
While this new research helps scientists understand bird flight and takeoff better, Parslew hopes that it will also translate to robotics research. These findings could make drones more efficient, he says, and perhaps one day they will even be able to perch and take off themselves—without doing any unexpected flips.
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