Unlike the aerodynamics of birds – aka fixed-wing flight – the flight of the insects is not well understood. Researchers at New York University have revealed some surprising insights to this long-standing question. Namely, they have found that being top-heavy provides an unintuitive aid to maintaining stability while undergoing rapid flapping-wing flight. They published their results in the February 2012 issue of Physical Review Letters.
Fixed-wing flight refers to a stationary wing traveling through a medium like air. We’ve all seen birds engaged in this sort of flight. The mechanism for lifting the wing is the difference of air pressures above and below the wing, caused by the wing’s forward motion. Fixed-wing flight is well understood. In fact, our airline industry utilizes it. This working understanding makes for over 87,000 flights per day gracing the skies above the United States, not counting the rest of the world.
The vast majority of nature’s pilots, however, are winged insects. Understanding the flight of insects could make a similar impact on the way we move from place to place.
In their February 2012 paper, Bin Lui and colleagues outline the experiment that led to the counterintuitive result that being top-heavy is an aid to flapping-wing flight.
First, they had to construct a model ‘bug’ that can fly by flapping a set of wings – a major task. Lui and collaborators bypassed this difficulty by causing the air to move instead of a set of wings. This type of air flow, generated by an acoustic sub-woofer, mimics the turbulent behavior of air flow around flapping wings without necessitating the technically difficult task of creating mechanically precise flapping wings.
With this difficulty out of the way, the task of creating a ‘bug’ became much simpler. The team devised a light triangular pyramid as their flying bug.
Inside the pyramid, on a small vertical beam, they positioned a weight that could be adjusted up or down to control the height of the center of mass. The adjustment from top-heavy – to centrally balanced – to a low center of gravity was the factor they focused on during their experiment.
In order to determine which arrangement is the most stable, the bugs were placed in a vertical wind tunnel with the sub-woofer at the bottom providing the periodic, wing-like, lift. By taking a series of photos, the experimentalists were able to reconstruct the effective force acting to keep the bug upright.
They used a common tool to physicists, the concept of potential energy. If potential energy is graphed as a function of position, or in this case, angle of inclination, the downward slope of the graph represents the force acting on the bug. In this case, the slope of the graph is approximately zero for angles between -20 and +20 degrees, resulting in no restoring force. However, once the angle of inclination goes beyond 20 degrees, the potential energy rises sharply, representing a strong restoring force acting to restore the bug to its upright position.
By constructing these potential energy graphs for various weight positions, they found that the top-heavy arrangement provided the most stable flight pattern for their bug. Lui and his team were able to prove the mechanism for this greater stability was a phenomena known as ‘vortex shedding.’ As the bug tilts to one side, a vortex of air is released from that side, returning the bug to an upright orientation. The results of this experiment show that this process is apparently aided by a high center of mass.
Whether the results of this simple, intriguing experiment can be applied to more realistic models of insects – such as actual bugs with actual wings – remains to be seen. However, Bin Lui and colleagues have highlighted an important nuance of flapping-wing flight, namely that high center of gravities can actually stabilize the orientation of the flier.
Just as an increased understanding of fixed-wing flight led us to revolutionize the way we travel, one can only imagine the further effects of utilizing the motion of insects through the sky.