Bat wings inspire new
breed of drone
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[June 27, 2016]
By Matthew Stock
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.
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"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.
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