Flying like a bat

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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