Shape-shifting wings make stealthy Batbot more agile

STEP aside Batman, there's a new show in town. A drone that mimics the way a bat changes its wing shape in flight could make small uncrewed vehicles far more agile.

"Bat wings are highly articulated, with wing skeletons very similar to those of human arms and hands," says Julian Colorado at the Polytechnic University of Madrid, Spain. "The way bats change the shape of their wings has great potential for improving the manoeuvrability of micro air vehicles."

The US military - which part-funded this research - is keen on drones that flap like birds because they can be far stealthier than faster, fixed wing aircraft. Colorado and colleagues in Madrid and at Brown University in Providence, Rhode Island, have built a drone with a half-metre wingspan inspired by a type of bat called the grey-headed flying fox.

The "Batbot" replicates the way a bat changes the profile of its wing between the downstroke and upstroke. On the downstroke, a large wing surface area is required to generate lift and propulsion. On the upstroke, however, that large area would create drag, so the bat folds the wing inwards. On the robot, the up/down flapping motion of the soft silicone wing is performed by an electric motor in the bat's body - as it is with other bird-like, flapping drones. The difference is that the extension and contraction of the wing is controlled by shape-memory alloy wires that switch between two shapes when different currents are applied in much the same way as the human triceps and biceps do.

See graphic: "Flexible flapping"

Situated between the "shoulder" and "elbow" of the robot, these wires rotate the elbow, pulling in the "fingers" to slim the wing profile on the upstroke (Bioinspiration and Biomimetics, DOI: 10.1088/1748-3182/7/3/036006). "This contracts and extends the wings in a similar way to the biological counterpart," says Colorado.

The Batbot is not flying stably yet, but Dario Floreano, director of the Swiss National Center of Robotics in Lausanne, already considers it "a major step towards more agile and autonomous flying robots".

"It is an example of how robotics is gradually moving from rigid mechanical components towards higher-agility bionic systems made from soft materials, elastic components and artificial muscles," says Floreano.

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