The unique mechanical properties of bat wings could lead to a new breed of nature-inspired drone. A prototype built by researchers at the University of Southampton shows that membrane wings can have improved aerodynamic properties and fly over longer distances on less power.
"The unique aspect of a bat-wing is that it's made of muscles, and when it starts to flap the wing what it can do is the wing can actually deform and change shape. And the change in shape could make it more efficient and make it fly better," said Professor Bharath Ganapathisubramani of Southampton's Aerodynamics and Flight Mechanics Group, who oversaw the project.
Using a paper-thin rubber membrane, the team designed wings that mimic the physiology of the muscles in a bat's wing, changing shape in response to the forces it experiences.
Robert Bleischwitz, who led the project for his PhD, said that the wing's structure creates a series of vortices as air passes over it. These give the structure added lift.
"In cases where the flow is at higher risk of detaching from the surface; so your vehicle has a risk to sag down, to fall down. In that period a membrane wing can keep it afloat because the dynamics in the surface trigger vortices which roll down the wing, and these vortices produce lift. So you can use this vortex generation to produce lift. And you need the membrane to excite, generate these vortices," Bleischwitz told Reuters.
He added that membrane wings in the future will also incorporate electro-active polymers that make them stiffen or relax, depending on an applied voltage, further increasing their performance. This replicates the control that bats have over their wings during flight. Research conducted in 2014 by scientists at Brown University showed that bats have a tiny network of muscles - called plagiopatagiales - in the skin of their wings that enables them to control the stiffness and curvature of their wings when they fly.
Two electric rotors produce a cushion of air under the wings, helping it to lift like a hovercraft. Once airborne, these rotors are tilted back into a horizontal position, allowing it to fly much like an ordinary airplane.
"The fans in front help to produce an air cushion, because the wing is tilted like a wedge and so the air is trapped below the wing surface and like a hovercraft it can elevate up at the beginning, so it can really lift off at nearly zero speed. And later you can tilt these thrusters in the front to get into a more streamlined, so which is then in the performance, better. So you use tillable rotors in the front to lift off at lower speed and fly at higher speed," said Bleischwitz.
The membrane wing was tested using a combination of experimental work at Southampton and computational research at Imperial College London.
According to the researchers, the proof of concept wing will eventually enable flight over much longer distances with a much higher payload than currently possible.
"The thing that we really wanted to was to test how the flexibility of the wing actually improves the aerodynamic performance. So we ended up making these membrane wings and we put it in the wind tunnel and we put a flow over it, and we could see that the membrane actually acts very much like a sail of a ship - when the flow hits it, it sort of changes shape and it gives you some aerodynamic performance. And not only does it change, it also starts to vibrate and fluctuate. And that fluctuation improves the aerodynamic performance even more," added Ganapathisubramani.
They used the data to build their 0.5m-wide test vehicle, designed to skim over the surface of the sea. Even so, the vehicle can fly just above any smooth ground surface, and it is able to overcome higher obstacles for a short period of time.
The next step is to incorporate their bio-inspired research into typical unmanned aerial vehicle (UAV) designs, with deployment in real-world applications possible in the coming years. Drones are increasingly used in a wide variety of civil and military applications, such as surveying remote and dangerous areas. The team says that incorporating wings that respond to their environment could represent a paradigm shift in drone design; helping them achieve better in-air performance, the ability to transport higher payloads and the efficiency to fly much further.