Flying like a bat

Bats in the laboratory

Model BatWhile 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 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.

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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