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Magic butterfly dust

Butterfly wing scales help flight.

Have you ever picked up a butterfly and found your fingers dusted with tiny scales from the wings? A monarch butterfly has more than a million, at least before you touched it. They amount to almost 1/10th of the mass of the butterfly. So, they must be important, right?

Aerospace engineer Amy Lang, Nathan Siegers, and colleagues set out to find out. They knew much about the sizes and shapes of the individual scales and how they sat on the wing surface. Next step: carefully clean off the scales. Voila: the butterfly’s efficiency in climbing in the air dropped by almost 1/3. The intricate pattern of airflow over the wing had changed.

It’s impossible to catch images of airflow at microscopic scale on a flying insect, so Lang and her colleagues built a model of the wing with its scales 300x larger than the real wing but filmed mineral oil flowing over it. The physical laws of aerodynamics made this gooey flow equivalent to the airflow over the butterfly wing. The researchers saw that the fluid flow got trapped into small eddies at the trailing edge of each simulated scale. Instead of the flow sticking at flat skin it formed tiny “roller bearings.” This effect decreased the estimated drag force on the wing by around 1/3 at the speed at which butterflies fly… which is up to 150 body lengths per second!

That’s one part of their amazing ability to fly 4000 km or 130 million body lengths as they migrate!

 This has been an outreach activity of the Las Cruces Academy, viewable at GreatSchools.org.


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