The flight of fruit bats is being used to inspire the design of miniature drones known as ‘micro air vehicles’ with flapping wings.
Virginia Tech researchers used experimental measurements of the movements of the bats' wings in real flight, and then used analysis software to see the direct relationship between wing motion and airflow around the bat wing.
"Bats have different wing shapes and sizes, depending on their evolutionary function. Typically, bats are very agile and can change their flight path very quickly showing high manoeuvrability for mid-flight prey capture, so it's of interest to know how they do this," explained Danesh Tafti, from the university’s Department of Mechanical Engineering.
Among the biggest surprises in store for the researchers was how bat wings manipulated the wing motion with correct timing to maximize the forces generated by the wing. "It distorts its wing shape and size continuously during flapping," Tafti said.
In a paper published in the journal Physics of Fluids, the researchers note that the bat increases the area of the wing by about 30 per cent to maximize favourable forces during the downward movement of the wing, and it decreases the area by a similar amount on the way up to minimize unfavourable forces.
The force coefficients generated by the wing are "about two to three times greater than a static airfoil wing used for large airplanes," said Kamal Viswanath, a co-author who was a graduate research assistant working with Tafti when the work was performed and is now a research engineer at the US Naval Research Lab's Laboratories for Computational Physics and Fluid Dynamics.
“Next, we'd like to explore deconstructing the seemingly complex motion of the bat wing into simpler motions, which is necessary to make a bat-inspired flying robot."
The researchers want to keep the wing motion as simple as possible, but with the same force production as that of a real bat.
"We'd also like to explore other bat wing motions, such as a bat in level flight or a bat trying to manoeuvre quickly to answer questions, including: What are the differences in wing motion and how do they translate to air movement and forces that the bat generates? And finally, how can we use this knowledge to control the flight of an autonomous flying vehicle?" Tafti added.