Engineers at Johns Hopkins University in Maryland are studying how butterflies flutter to help design bug-size airborne robots that can mimic those manoeuvres to carry out reconnaissance, search-and-rescue and environmental monitoring missions without risking human lives.

The research is funded by U.S. defence agencies, which hope to use these small devices, which are about the size of a penny are commonly called micro aerial vehicles or 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," said Tiras Lin, a Whiting School of Engineering undergraduate who has been leading the high-speed video research.

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

Lin and graduate student Lingxiao Zheng used high-speed, high-resolution videogrammetry to mathematically document the trajectory and body conformation of painted lady butterflies, which have movements that are too fast to see clearly with the naked eye.

In a dry aquarium tank, the researchers mounted three video cameras capable of recording 3,000 one-megapixel images per second, compared to a standard video camera, which shoots 24, 30 or 60 frames per second.

Several butterflies were released inside the tank and when a butterfly veered into the focal area, they switched on the cameras for about two seconds, collecting approximately 6,000 three-dimensional views of the insect’s flight manoeuvres. They concentrated on roughly one-fifth of a second of flight, captured in 600 frames.

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

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The butterfly research will aid the development of flying bug-size robots, like this insect-inspired flapping-wing micro air vehicle under development at Harvard. (Robert J. Wood/Pratheev Sreetharan/Harvard Microrobotics Lab/Harvard University)

Earlier, scientists believed that an insect’s delicate wings possess very little mass compared to the bug’s body so that changes in spatial distribution of mass associated with wing-flapping did not need to be considered in analyzing its flight manoeuvrability and stability.

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

Moment of inertia is the term used to measure or quantify the amount of mass located at an object's extremities

When ice skaters want to spin faster, they bring their arms in close to their bodies and extend their arms out when they want to slow down. The positions change the spatial distribution of a skater's mass and modify their moment of inertia, which in turn affects the rotation of the skater’s body.

Lin's research shows an insect may be able to do the same thing with its body and wings.

The discovery will be useful for MAV designers and biologists who study insect flight dynamics.

As for Lin, his next mission is to solve the mystery of how fruit flies manage to land upside down on perches.