Fruit bat-inspired flying vehicles

Bats and other flying creatures have inspired Virginia Tech researchers to design small flying vehicles known as ‘micro air vehicles’ with flapping wings.

The researchers used experimental measurements of the movements of the fruit bats’ wings in real flight and then used analysis software to see the direct relationship between wing motion and airflow around the bat wing. They report their findings in the journal Physics of Fluids.

“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 midflight prey capture, so it’s of interest to know how they do this,” said Danesh Tafti, the William S. Cross professor in the Department of Mechanical Engineering and director of the High Performance Computational Fluid Thermal Science and Engineering Lab at Virginia Tech.

The fruit bat weighs roughly 30 grams and a single fully extended wing is about 17 x 9 cm in length, according to Tafti.

Among the biggest surprises in store for the researchers was how bat wings manipulated the wing motion with correct timing to maximise the forces generated by the wing. For example, it increases the area of the wing by about 30% to maximise favourable forces during the downward movement of the wing, and it decreases the area by a similar amount on the way up to minimise 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.

This study was just an initial step in the researchers’ work. “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,” said Viswanath. The researchers also 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.