Aerial trickery revealed in the shadow play
By James Dacey
The fruit fly, like many winged insects, has to work very hard to stay in the air given the tiny size of its wings. To generate the vertical force necessary to maintain flight, it must beat its wings hundreds of times every second. But at these speeds and torques, how on earth does the fragile fruit fly manage to control its flight to make those sharp turns and pull off those difficult aerial manoeuvres?
Well, the answer, according to a group of researchers at Cornell University in the US, lies in a gentle, passive movement of the fly’s wings.
The Cornell research team have honed in on the turning kinematics of fruit flies by filming these insects as they fly around within a confined space. They capture the motion using three synchronized cameras, focused along orthogonal axes – the x, y and z – to capture 8000 frames per second or about 35 frames for each wing beat.
Then comes the clever bit – converting these 2D snapshots into a full 3D reconstruction of the insect’s flight motion. This was achieved using a technique known as Hull Reconstruction Motion Tracking (HRMT), which merged three separate images, one from each camera, at distinct time steps. Attila Bergou, a member of the team, says that this was like painting three silhouettes from the three cameras onto the faces of a box. “The volume you get by using a cookie cutter to cut out the shadows and peeling all but the centre away is the visual hull,” he explains.
What they found is that the fly does something very smart. By allowing aspects of their wing motion to be passively dictated by the aerodynamic and inertial forces, they end up being able to control their flight through the air in a very simple and elegant manner. Bergou says that the fly does this without “thinking”, comparing the motion to the wiggle of a boat’s oar as it cuts through the water.
Bergou believes that the manoeuvrability and efficiency of flapping flight at small length scales will be of interest to aircraft engineers. “There is a large amount of interest in the development of micro-air vehicles that use such flapping strokes to fly,” he says.
This research is documented in a new paper in Physical Review Letters.