Imagine insect-like aircraft capable of military or civilian surveillance missions, impossible for current fixed-wing or rotary-wing vehicles – tiny flying machines able to access buildings reduced to rubble by earthquakes, or act as a fly-on-the-wall in the meeting rooms of enemy leaders. Such aircraft may be one step closer to realization, thanks to a breakthrough in our understanding of how flapping wings work.

The quest to mimic the action of flapping wings has puzzled humans ever since Icarus took the plunge. Researchers at the Georgia Institute of Technology (Georgia Tech) have come up a computer model that appears to have unraveled some of the mystery of flapping-winged flight, providing similar lift and maneuverability to that seen in insects and birds.

Emulating the efficiency, maneuverability, agility and hovering capability of flapping-wing flight has thrown up significant technical challenges for researchers in the past. This has been due to an incomplete understanding of the physics of flapping flight at small size scales. The key to the breakthrough, made by assistant professor Alexander Alexeev and his graduate student Hassan Masoud, has come through using flexible wings that are driven by a simple oscillating flapping motion. "We found that the simple up and down wavelike stroke of wings at the resonance frequency is easier to implement and generates lift comparable to winged insects that employ a significantly more complex stroke," Alexeev said. "When you want to create smaller and smaller vehicles, the aerodynamics change a lot and modeling becomes important. We tried to gain insight into the flapping aerodynamics by using computational models and identifying the aerodynamic forces necessary to drive these very small flying machines."

Alexeev and Masoud used three-dimensional computer simulations to explore the lift and hovering aerodynamics of flexible wings. The wings were oscillated vertically, and by using a flexible wing structure were able to be tilted horizontally. The simulations showed that lift comparable to that of small insects using a significantly more complex stroke was achievable. The simulations also identified flapping regimes that enabled maximum lift and maximum efficiency.

"This information could be useful for regulating the flight of flapping-wing micro air vehicles since high lift is typically needed only during take-off, while the enhanced aerodynamic efficiency is essential for a long-distance cruise flight," said Masoud.

The next step for the researchers is to design micro-scale air vehicles that apply this model and that would be controllable in real-world conditions. They are also investigating whether wings with non-uniform structures and wings driven by an asymmetric stroke may further improve the resonance performance of flapping wings.

Details of the research were presented at the 63rd Annual Meeting of the American Physical Society Division of Fluid Dynamics, and appeared in the May issue of the journal Physical Review E.

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