The flapping motion of insect wings is qualitatively different from fixed airplane wings or even the rotation of helicopter blades. Over the past two decades, the importance of the unsteady flows created by the flapping motion of insect wings has become better understood. Wang's calculations join earlier experimental work on insect flight 2,3 in identifying the responsible mechanisms. This motion combines pitching and heaving, that is, rotational and translational movement, as illustrated in the figure on page 23. The plane of the stroke during hovering varies from insect to insect.
Insects are in an intermediate regime in which neither effect can be neglected.
Consequently, the analysis of dynamics in this regime can be quite messy, and researchers have turned to empirical studies, models, and computers for insight. In 1996, Charles Ellington and coworkers at the University of Cambridge used smoke to image the airflows around a tethered hawk moth, and built a large-scale flapping model with the same Reynolds number as the moth to better study the dynamics. They found that the vortex that forms on the leading edge of the wing spirals out away from the insect's body and toward the tip of the wing. These observations confirmed earlier work by Tony Maxworthy. 4 Last year, Michael Dickinson and colleagues at the University of California, Berkeley, reported studies on their own dynamically scaled model insect, a robotic fruit fly, complete with sensors for monitoring the time-dependent aerodynamic forces.