Artificial butterfly mixes high, low tech

View a video of an artificial butterfly in flight

A new artificial butterfly is revealing the importance of wing veins and of upsy-downsy waverings to swallowtails’ flight.

The tiny machine flaps a pair of wings made of thin polymer film and can stay airborne for four or five meters at a time, until the coiled rubber band powering it unwinds, Hiroto Tanaka of Harvard University and Isao Shimoyama of the University of Tokyo report in the June Bioinspiration & Biomimetics.

The team designed a model swallowtail butterfly and analyzed its flight patterns as a way to investigate basic questions about flight that they couldn’t study in live butterflies. “We can’t ask insects, like ‘Hey, please just flap your wings at 10 hertz,’” Tanaka says.

One question the researchers asked was whether simply flapping big wings up and down could reproduce a swallowtail’s undulating flight pattern without the butterfly tweaking wing angles.

Swallowtails, like other butterflies, have two wings on each side of the body, but the forewings overlap with the hind ones. That overlap might force the butterflies to flap the hind and fore pair like one big wing, the researchers speculated. Such overlaps are common among butterflies but not among other four-winged flying insects, Tanaka says.

To mimic the way the butterflies might use their front and rear wings together, the researchers created one big wing for each side of the body. They used a silicon-etching technique to make a mold for creating realistic wing veins and made a balsa wood “body” to keep the model’s weight at 0.4 grams, near that of a real swallowtail.

With high-speed cameras, the researchers got enough images of the model’s few seconds of flight for motion analysis. They conclude that by simply flapping its wings straight up and down, the machine recreated the bobbing flight of real butterflies. And by comparing fully veined with veinless wings, the researchers found that veins stiffened the wings and helped them achieve greater lift. The bobbing motion of the body also increased lift to help keep the flier aloft.

The team’s analysis fits into the basic framework of what’s known about flight, says Jane Wang of Cornell University, who also studies insect aerodynamics. “The basic picture that an up-and-down motion generates a forward thrust, thus forward flight, is known in classical aerodynamics going back to 1930s, if not earlier,” she says.