Tsevi Beatus, John Guckenheimer and Itai Cohen,
The Journal of the Royal Society Interface 12, 20150075 (2015) PDF
The flight of flapping insects is a complex process that is beautiful to watch. One of the reasons flapping flight is so difficult is that it is inherently unstable: similar to balancing a stick on one's fingertip, flapping flight is subject to rapid instabilities that must be constantly controlled to allow stable flight. For flies, the most unstable motion is rotation about their long body axis, called roll. If a fly did not control its roll angle, it would roll over and crash within just a few wing-beats. Yet flies manage to control this rapid instability and even perform extreme maneuvers, better than any man-made flying device.
We use common fruit flies, like the ones we often find in our kitchen, to study the mechanism insects use to control their unstable roll angle. To study how flies do it, we came up with a way to trip them in mid-air and film how they recover from these stumbles. Specifically, we glue a tiny magnet to the back of each fly and use a magnetic pulse to roll it over in mid-air. We film the fly’s correction maneuvers using three high-speed cameras and measure how the fly is using its wings to recover from the perturbation.
We found that flies manage to correct for large perturbations that roll them up to 100 degrees within 30 milliseconds. This means that by the time you blink, the fly could have performed this entire correction maneuver 10 times. The flies start to respond to the perturbation within 5ms. This puts the roll correction reflex among the fastest in the animal kingdom.
Flies correct for these roll perturbations by flapping with one wing harder than the other for 2-5 wing-beats. The resulting left-right force imbalance leads to corrective torque. We managed to describe the asymmetric wing motion using a controller model that is mathematically similar to controllers in air-conditioning and cruise-control systems. The model is termed “Proportional-Integral (PI) Controller.”
Finally, we tried to challenge the flies with perturbations that they cannot correct for. Rather than exerting a single perturbation pulse that rolls them once, we challenged them with a series of pulses that rolled them over eight full turns. Surprisingly, we find that the fruit flies managed to recover from this extreme perturbation very quickly, within a few wing-beats. We have not yet managed to find a perturbation from which the flies cannot recover. Although these tiny insects are common and often a nuisance, we now have greater appreciation for what amazing fliers they are.
The work was supported by the Cross Disciplinary Postdoctoral Fellowship of the Human Frontier Science Program, and by the Army Research Office.