Science: Maneuverability and Stability Are Birds of a Feather

Cynopterus brachyotis flying in a wind tunnel.

[Image courtesy of Jose Iriarte-Diaz]

A poignant source of envy in humans, the gift of flight affords many creatures the ability to travel through the air, but the details of their specialized maneuverability and stability has been poorly understood. Now, researchers writing in the journal Science say they have developed a framework for predicting airborne turning dynamics in flying creatures, and have used it to envisage the flight motions of seven different creatures of various sizes.

In the future, this knowledge might be used to develop more effective wing movement in flying robots. It will probably spark new research into the neuromuscular control of aerial maneuvers in animals as well.

Tyson Hedrick of the University of North Carolina at Chapel Hill, along with colleagues from the University of Delaware, studied low-speed "yaw" turns of 60 degrees or more in a variety of winged creatures, including insects, bats, and birds, and found that these maneuvers are accomplished with a mechanism they call "flapping counter-torque."

Basically, when a flying animal turns, its wing velocity is enhanced on the outside wing during the down-stroke and on the inside wing during up-stroke. This kind of force asymmetry produces a torque that can slow the animal's rotation, even when they are flapping their wings symmetrically.

The researchers captured this information and incorporated it into a model for four species of insects, two bird species, and a bat, which they then compared to extensive video footage of real animals.

A female ruby-throated hummingbird, Archilochus colubris.

[Image courtesy of Edwin Yoo]

They discovered that geometrically similar animals also have similar turning dynamics as far as their wing beats go, regardless of their size. So, for example, fruit flies and hummingbirds both require the same number of wing beats to finish a turn.

"Given the considerable range of species examined, the different animals exhibited widely divergent yaw turning performance, slowing down at very different rates," the researchers wrote. "However, in contrast to their differences in deceleration rate, the fruit fly, stalk-eyed fly, bluebottle fly, and hummingbird all exhibited similar peak yaw rates of approximately 1600 degrees per second."

Hedrick and his team also say that as flying animals beat their wings faster, maneuverability and stability are enhanced—two properties that were previously believed to be in opposition.

Manduca sexta feeding on a flower.

[Image © Science/AAAS]

In a Perspective about the research, Bret Tobalske, whose research at the University of Montana focuses on flight and biomechanics, muses that an "almost limitless array of combinations of yaw, pitch, roll, and flight velocity are available to flying animals. Now that technology has developed to the point where detailed measurements of flapping maneuvers have become feasible, a world of comparative research is opening in which the flapping counter-torque model can be used to test the functional significance of flapping motions in maneuvering dynamics."

The research on in-flight maneuverability, along with Tobalske's assessment, is published in the 10 April 2009 issue of Science.

Brandon Bryn

9 April 2009