Swallowtails are unique among butterflies in that their wing area is very large relative to their body mass, and their overlapping fore wings means that their flapping frequency is comparatively low and their general wing motion severely restricted.

As part of the study, the “ornithopter” was filmed in flight using a high-speed video camera, to mimic how these characteristics limit the ability to actively control the aerodynamic force of their wings, making their body motion a passive reaction to the simple flapping motion, and not, as is typical in other butterflies, an active reaction to aerodynamics. It was found that the simple flapping of their flight requires little feedback control of the feathering angle.

Hiroto Tanaka and Isao Shimoyama, from the School of Engineering and Applied Sciences at Harvard University and the Graduate School of Information Science and Technology at the University of Tokyo, found that without feedback control of the wing motion, the body movement is directly affected by change of aerodynamic force due to the wing deformation.

The study, published in Bioinspiration & Biomimetics [Tanaka, H., Shimoyama, I. Bioinspir. Biomim. (2010) DOI: 10.1088/1748-3182/5/2/026003], revealed that stable forward flight could be realized without active feathering or feedback control of the wing motion, and that while flying the artificial butterfly’s body moved up and down passively in synchronization with the flapping, as it followed an undulating flight trajectory like an actual swallowtail butterfly. The experiment confirmed that the undulating body motion during the flight of swallowtails is caused passively by these morphological features, rather than intentionally.

Although there have been many studies of small flies with high-frequency flapping, there has not been much research into large butterflies that have a low-flapping frequency. The team showed that flight mechanism of butterflies is different in terms of significant interaction between aerodynamic force and passive body motions.
The research proved that the swallowtail achieves forward flight through simple flapping motions, with motion analysis software monitoring the ornithopter’s aerodynamic performance. This showed that flight can be realised with simple flapping motions without feedback control, a model which could be applied to future aerodynamic systems.

Having explored the aerodynamic coefficients of butterflies, the researchers hope to now examine their power efficiency, using flow visualization to examine how wing deformation and passive body motion affect the aerodynamics, as well as comparing other flight modes.