Flexible Fliers

Indoor maneuverability is an issue not only for crawling bots but for flying ones, too. Aerial robots that can get around quickly and nimbly-say, inside a stricken nuclear reactor-would be in high demand. And once again, insects are a natural source of inspiration. “Flying insects are much better at this sort of thing than any current robot,” Cowan notes.

What makes flying insects so maneuverable in tight spaces? A major factor is their flexibility-literally the flexibility of their bodies, which allows them to redirect the forces of their wing beats in a split-second. Cowan was on a sabbatical last year at the University of Washington at Seattle, and began working with a group of biologists, led by Tom Daniel, who study the flight dynamics of moths. They were able to demonstrate in experiments that when a moth flies, it automatically swivels its abdomen-representing about half its mass-in response to a changing visual scene, in order to quickly reorient its wing beats. “The visual-abdominal reflex we measured actually seems to be tuned for this purpose,” says Cowan.

To turn this biological insight into a new technology, Cowan and his students have acquired a micro-drone, a helicopter-like vehicle with four evenly spaced propellers. Normally, an unmodified quad-rotor craft like this has somewhat clumsy flight dynamics. It is slow to maneuver and can lose stability relatively easily. The reason is that it has to achieve six degrees of motion (up-down, left-right, forward-backward, roll, pitch, and yaw) with only four flight control elements. “If it needs to go up and to the right, for example, it first has to turn the propeller speeds up on the left side and down on the right side, to pitch the craft to the right, and then it can throttle up all its propellers and move off in that new direction,” Cowan says.

To improve stability and maneuverability, Cowan’s graduate student Alican Demir has slung the micro-drone’s battery pack-the analog of a moth’s food-stuffed abdomen-below the rest of the airframe, with a swivel and a small servomotor. “With one quick swing of this artificial abdomen, the craft can be reoriented, and even as it’s doing this, it can be cranking up all its propellers,” Cowan says. “In this way, a one- or two-second manuever becomes a split-second maneuver.” The team is just starting its flight tests of the mothlike craft, and Cowan sees plenty of engineering work ahead. “But the proof of concept is there already,” he says.

Landing on Ceilings

Insect Flight: Undergrad Tiras Lin ‘13 uses high-speed, high-resolution cameras to gain a new perspective on the mechanics of a painted lady butterfly’s flight pattern.

Mechanical engineering Professor Rajat Mittal is studying a different flexible flier: the butterfly. These insects can perform complex, intricate flying maneuvers, and, he says, “we’re starting to understand how their wing and body flexibility enable these aerial acrobatics.” He and his PhD student Lingxiao Zheng and Tiras Lin ’13 suspect that butterflies also assist their maneuvers by morphing their wings to alter aerodynamic forces and their moment-of-inertia. “That would be an amazing technology if we could figure out how to translate it from butterflies to engineering,” says Mittal.

https://youtu.be/azQeJLUWljc
What does Tiras Lin’s work tell us about how butterflies execute aerial maneuvers? Watch this video to find out.

Even more amazing would be the ability to land upside down on a ceiling or overhang, as some insects do routinely. Last autumn, Lin received a Provost’s Undergraduate Research Award for his proposal to study how fruit flies accomplish this feat. Since then, he and Mittal have set up experiments to film the tiny insects in the act, using the lab’s high-speed cameras.

It’s been a real challenge. For one thing, the fruit flies rarely land upside down; they generally want to land on the floor of their test chamber. “We’ve been trying to coax them, by putting a little bit of honey or vinegar or mashed fruit on the ceiling of the chamber,” Mittal says.

But the hardest problem by far is that fruit flies are too small for easy high-speed imaging. As the shutter speed of a camera increases, less of the ambient flux of light is available to illuminate any one frame. “For high-speed photography, you need high-intensity illumination, which typically means a lot of heat,” Mittal says. The fruit flies they’re trying to film are about half the size of common house flies, and their tiny body mass gives them almost no ability to resist the sudden heat pulse from the high-wattage camera lamps. “The flies tend to burn up faster than we can image them,” he says.

To get what they want, the engineers need to start the imaging just moments before the flies land, which is essentially a matter of luck. So Mittal and Lin have decided on a brute-force solution to the problem: They are setting up a fruit fly breeding colony, to ensure a near-limitless supply of the insects, and Lin is putting together a more automated fly-entry and imaging system in the observation chamber. “We’re trying to set up a system in which we can take hundreds, even thousands, of film sequences in a reasonable span of time, in the hope that a few of the sequences will show the perfect upside-down landings that we’re after,” Mittal says.

So far they have acquired a few somewhat blurry sequences which hint that under-landing fruit flies first grab the ceiling with their forelimbs and then-like circus trapeze performers-swing their lower halves up until their rear limbs can get hold. But Mittal and Lin haven’t yet ruled out their alternative hypothesis, which is that the fruit flies first climb toward the ceiling in an aerial loop-the-loop, and use their momentum to land squarely upside down. “We just haven’t done enough research yet to draw firm conclusions,” Mittal says.