Reflexions, le site de vulgarisation de l'Université de Liège
Flying like a bat
5/10/11
For thousands of years, the animals that have fascinated us the most are undoubtedly birds and their amazing
ability to conquer the earth's gravity. The invention of the first flying machine was therefore a technological
turning point in the history of humanity! It was on 17th December 1903. That day, two Americans, the Wright
brothers, managed to take off from the ground over a distance of 250 metres, on a beach in North Carolina.
Unlike the wings of a bird, those of the Wright Flyer didn't flap. The engineers understood that in order to fly,
the solution lay rather in the combination of thermal or electric propulsion with fixed wings in order to lift the
machine. The entire history of aviation over the past 100 years is based on this principle. But today, man
is developing smaller and smaller planes: drones. And yet, below a certain size, efficient flight can only be
achieved through the flapping of wings. Hence the renewed interest of aeronautical engineers in the work of
animal locomotion specialists. Researchers from the University of Liège are particularly interested in bat flight,
which still has many hidden secrets.
On a warm summer night, how is it possible to distinguish a bat from a bird? A bat flies in a far jerkier manner
than a bird. Bats make sudden, rapid and repeated changes of direction; the curves of a bird are gentler and
more fluid. "Bat flight is a mixture of bird and insect", summarises Professor Grigorios Dimitriadis from the
Aerospace and Mechanical Engineering Department at ULg. "Some large bats are capable of gliding, like
birds, while others flap their wings as fast as an insect." To study bat flight, Greg Dimitriadis joined forces
with biologists from the University of Manchester (James Gardiner, Jonathan Codd and William Seller),
specialising in animal locomotion. There are several ways to study animals in flight. For instance, by filming
them in their natural habitat. But while it is possible to obtain beautiful images of birds in full flight in the sky
(from a plane, for instance), they are not always suitable for scientific research.
This is why researchers have invented more standardised laboratory techniques, such as flying in a wind
tunnel. This is no easy matter. It is necessary to teach the birds to fly against an artificial source of wind, so that
the animal flaps its wings without moving forward. In this relative motion the birds can be observed through
a camera lens for many minutes. And by modifying the power of the wind tunnel, researchers can vary an
important parameter: the speed of flight. In some laboratories, it is also possible to vary the altitude by altering
the atmospheric pressure inside the wind tunnel. Researchers can then analyse the images from every angle
and even digitise them to develop computer models. Based on these models, it is possible to vary parameters
other than speed or altitude and answer much more sophisticated questions such as: what kind of muscular
effort must such a bird make to fly such a distance, at such a speed and at such altitude?
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Bats in the laboratory
While the experiment has already
been successfully carried out with large birds such as geese, it is very difficult to teach a bat to fly in a wind
tunnel. Rectilinear flight and the discipline of a squadron, are not part of this flying mammal's locomotive
register. That's not a problem, thought the researchers at ULg. Let's put a model of a bat in a wind tunnel
instead, an artefact that resembles the animal as closely as possible. It was the researchers in Manchester
who carried out this part of the work, a sophisticated model mainly composed of latex to reproduce the wing
membranes and a metallic structure for the skeleton. The model was a Plecotus auritus (brown long-eared
bat), characterised by its large ears. "It's a static model", stresses Greg Dimitriadis. "The wings are deployed
but they don't flap. It's as though we are studying the bat gliding." Placed in ULg's wind tunnel, the model
was subjected to forces which were recorded by aerodynamic load sensors. The wind tunnel's airspeed can
reach 60 metres per second (216 kph), but it is limited to 10 m/s (36 kph), which corresponds to this bat's
maximum flight speed. An initial series of measurements related specifically to the role of the animal's long
ears. "It's obvious they play an aerodynamic role", explains G. Dimitriadis. "But what is it? Our study showed
that in a horizontal position, the ears give the animal lift. They complement the role of the wings. In a vertical
position, they serve as a brake. And in a differential position, i.e. with one ear raised and the other horizontal,
they allow the bat to turn even more quickly." These results were published in 2008 in the specialised review
Acta Chiropterologica.
A second series of measurements related to the animal's tail and legs, which have the particularity of being
very short and attached to the wings. The study, published in the review PLoS ONE (1), showed that the legs
and tail contribute towards the dynamic stabilisation of the bat's flight (stability is an object's propensity to
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spontaneously return to its initial position if its balance is disturbed) but also its manoeuvrability. A bat in flight is
like a pencil standing on its tip - unstable whatever its position. But unlike a pencil, which will certainly fall over,
the bat rebalances itself by the movement of its body. "Therefore, it is more correct to refer to dynamic stability
rather than instability", emphasises Greg Dimitriadis. In aeronautical terms, the bat is closer to a fighter plane
than an Airbus A380! A small amount of turbulence has practically no effect on the position of a large transport
aircraft, which can return to its initial position with no stabilising force. A fighter plane, on the other hand, is
very quickly destabilised and can only maintain its trajectory through multiple rapid corrections calculated by
the onboard computer.
The myth of Icarus revisited
Could the flight of the bat be of interest to the aeronautical industry? In Greek mythology, Icarus attempted
to escape the palace of Knossos on the island of Crete by air, sticking bird feathers to his arms with wax.
Flying too close to the sun, the wax melted, the feathers fell off one after another and he plunged into the
sea. Just like this classical myth, the first "flying" machines flapped their wings, but all of them broke owing
to one fact: man is too big to take off through the power of his arms alone, even if they are transformed into
wings. In nature, moreover, there aren't any birds of the size and weight of a human being (fossil birds such
as the Argentavis weighed probably 80kg but it is not known if they could flap their wings or just glided) .
By inventing the reciprocating engine, engineers found a sufficiently powerful method of propulsion for man
to defy the earth's gravity without flapping a pair of wings. All the studies in progress on the flight of birds,
bats or insects are therefore of no use for the development of tomorrow's aircraft, whether military or civil, a
fighter plane or a transport plane. But the race for miniaturisation over the past few years has changed things.
The standard aeronautical model, with static wings and propellers, is becoming less efficient as the size of
the aircraft becomes smaller. And below a certain size, no object can fly efficiently without flapping its wings.
Furthermore, in nature, the smaller the winged creature, the faster its wings have to flap to stay airborne: those
of a large bird flap less than 10 times a second, while those of a hummingbird flap up to 80 times and those
of a fly hundreds of times!
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In the coming years, the development of drones could benefit from new research on wing flapping. Drones are
small, pilotless aircraft that can perform military or civil tasks (territorial surveillance, spying, surveillance of
polluted sites, etc.). The existing models are one to two metres long. But the drones of tomorrow may not be
any bigger than a hand. And to develop such small drones, researchers have to reintroduce flapping wings into
the design. Researchers working for NASA have thus developed an "entomopter", a sort of large mechanical
insect suitable for a trip to Mars, where the atmosphere has so little density that a traditional aircraft couldn't fly.
It would require wings ten times bigger than on earth and it would be impossible for a spacecraft to transport
such an aircraft.
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Frankenbird at the university
Within this context of renewed scientific interest in flapping wings, a researcher from Greg Dimitriadis' team,
Norizham Abdul Razak, has developed an exceptional tool: a robot bird, which the researchers in Liège are
hesitating to call "Frankenbird" for fear of giving it a bad reputation. The aim of the machine isn't to terrorise
the population, but to simulate a bird in flight. Frankenbird - the result of two years' work - resembles a
large winged suppository, with a one-metre wingspan and 60 centimetres in length. Inside the robot, and
electric motor and high precision mechanics provide the wings with two different movements: an up and
down movement and a pitching movement. "In nature, the combination of these two movements increases
flight efficiency", explains Greg Dimitriadis. With this exceptional machine, the researchers in Liège will be
able to study flight from almost every angle. "For instance, we can modify the rhythm of the beating, the
combination of movements and of course the shape and size of the wings." Frankenbird is designed to fly
with wings measuring from several dozen centimetres, up to approximately one metre. "We had to make a
choice", explains Greg Dimitriadis. "In nature, the largest birds have a wingspan of three or four metres and
the smallest ones a few centimetres. We chose the intermediate size."
Frankenbird is an amazing research tool that will perhaps help aeronautical engineers to develop the drones
of tomorrow. On the other hand, it can also shed light on the distant past, especially on dinosaurs, the
ancestors of birds. One of the major questions in animal palaeontology is indeed to find out how certain
prehistoric animals flew, such as pterosaurs. Did they flap their wings? Did they take off by running or did
they launch themselves off a cliff? Researchers at the University of Manchester are currently reconstructing
a computer model of a pterodactyl based on a genuine skeleton. The software used allows them to
extrapolate biological data such as muscle structure, body fat, skin and even the animal's kinematics. The
shape and size of the wings will be calculated. "Based on this computer model", Greg Dimitriadis explains,
"our colleagues in Manchester will create the wings of a pterosaur that we can test with our Frankenbird."
The flight of an animal that lived several hundreds of millions of years ago, reproduced in a laboratory by a
robot, is a fascinating research area. The work has just begun.
Gardiner James D, Dimitriadis Grigorios, Codd Jonathan R & al, A potential role for bat tail membranes in flight control, in PLoS ONE (2011), 6(3), 18214
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