aerodynamics

Top-heavy ‘bugs’ show how to hover

NYU (US) — Top-heavy structures are more likely to maintain their balance while hovering in the air than those that bear a lower center of gravity, new research shows.

The findings, published in the Physical Review of Letters, run counter to common perceptions that flight stability can be achieved only through a relatively even distribution of weight—and may offer new design principles for hovering aircraft.

As the Wright brothers demonstrated 100 years ago, the key challenge of flight is maintaining balance. Yet, while insects took to the air 400 million years earlier, their flight stability remains a mystery because of the complex aerodynamics of their flapping wings.

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Researchers from New York University approached this question by creating experimental conditions needed to achieve stable hovering in mechanical flyers.

To do so, they created a range of pyramid-shaped “bugs” constructed from paper that hover when placed in an oscillating column of air, mimicking the effect of flapping wings. They captured the experiment with high-speed videos in order to analyze the nature of the airflow around the bugs.

To gauge which types of structures best maintained their balance, they created paper bugs with various centers of mass. Top-heavy bugs were made by fixing a weight above the pyramid, and low center-of-mass bugs bore this weight below.

Surprisingly, the top-heavy bugs hovered stably while those with a lower center of mass could not maintain their balance. The team showed that when the top-heavy bug tilts, the swirls of air ejected from the far side of the body automatically adjust to keep it upright.

“It works somewhat like balancing a broomstick in your hand,” explains Jun Zhang,  professor at the Courant Institute and one of the study’s co-authors. “If it begins to fall to one side, you need to apply a force in this same direction to keep it upright.”

For bugs, it is aerodynamical forces that provide this stability.The lessons learned from these studies could be put to use in designing stable and maneuverable flapping-wing robots, the researchers say.

The study was funded by grants from the National Science Foundation and the U.S. Department of Energy.

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