Spry robot built to zip like the butterfly

JOHNS HOPKINS (US) — High-speed video of butterflies’ agility in flight may help researchers build tiny robots that mimic the insects’ maneuvers.

The Air Force, which funded the research, is supporting the development of bug-size flyers to carry out reconnaissance, search-and-rescue, and environmental monitoring missions without risking human lives. The devices are commonly called micro aerial vehicles (MAVs).

“For military missions in particular, these MAVs must be able to fly successfully through complex urban environments, where there can be tight spaces and turbulent gusts of wind,” says Tiras Lin, a Johns Hopkins University undergraduate conducting the high-speed video research.


Johns Hopkins undergraduate Tiras Lin used high-speed video cameras to analyze the flight dynamics of painted lady butterflies. “Butterflies flap their wings about 25 times per second,” Lin says. “That’s why we had to take so many pictures.” (Credit: Will Kirk, JHU)

“These flying robots will need to be able to turn quickly. But one area in which MAVs are lacking is maneuverability.”

To address that shortcoming, Lin has been studying butterflies. “Flying insects are capable of performing a dazzling variety of flight maneuvers,” he says. “In designing MAVs, we can learn a lot from flying insects.”

“This research is important because it attempts to not only address issues related to bio-inspired design of MAVs, but it also explores fundamental questions in biology related to the limits and capabilities of flying insects,” says Rajat Mittal, a professor of mechanical engineering in the university’s Whiting School of Engineering, who supervises Lin’s work.

Twists and turns

Lin has been investigating how changes in mass distribution associated with the wing flapping and body deformation of a flying insect help it engage in rapid aerial twists and turns.

The junior mechanical engineering major recently presented some of his findings at the annual meeting of the American Physical Society’s Division of Fluid Dynamics. He also won second prize for a presentation at a regional meeting of the American Institute of Aeronautics and Astronautics.

“Ice skaters who want to spin faster bring their arms in close to their bodies and extend their arms out when they want to slow down,” Lin says.

“These positions change the spatial distribution of a skater’s mass and modify their moment of inertia; this in turn affects the rotation of the skater’s body. An insect may be able to do the same thing with its body and wings.”

‘So many pictures’

Butterflies move too quickly for someone to see these wing tactics clearly with the naked eye, so Lin, working with graduate student Lingxiao Zheng, used high-speed, high-resolution videogrammetry to mathematically document the trajectory and body conformation of painted lady butterflies.

They accomplished this with three video cameras capable of recording 3,000 1-megapixel images a second. (By comparison, a standard video camera shoots 24, 30, or 60 frames a second.)

The researchers anchored their cameras in fixed positions and focused them on a small region within a dry, transparent aquarium tank. When a butterfly veered into the focal area, Lin switched on the cameras for about two seconds, collecting approximately 6,000 views of the insect’s flight maneuvers. From these frames, the student typically homed in on roughly one-fifth of a second of flight, captured in 600 frames.

“Butterflies flap their wings about 25 times per second,” Lin says. “That’s why we had to take so many pictures.”

The arrangement of the three cameras allowed the researchers to capture three-dimensional data and analyze the movement of the insects’ wings and bodies in minute detail. That led to a key discovery.

An insect’s delicate wings possess very little mass compared to the bug’s body. As a result, earlier researchers concluded that changes in spatial distribution of mass associated with wing flapping did not need to be considered in analyzing an insect’s flight maneuverability and stability.

“We found out that this commonly accepted assumption was not valid, at least for insects such as butterflies,” Lin says. “We learned that changes in moment of inertia, which is a property associated with mass distribution, plays an important role in insect flight, just as arm and leg motion does for ice skaters and divers.”

He says this discovery should be considered by MAV designers and may also be useful to biologists who study insect flight dynamics.

Lin’s newest project involves even smaller bugs. He has begun aiming his video cameras at fruit flies, hoping to solve the mystery of how these insects manage to land upside down on perches.

The U.S. Air Force Office of Scientific Research and the National Science Foundation funded the insect flight dynamics research.

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