If you've ever watched a flying bird weaving its way through a forest, you may have wondered how it could do so without hitting its wings on the trees. Well, birds actually do hit trees with their wings. Unlike the rigid wings of an aircraft, however, birds' wings simply fold back under impact, then immediately fold open again to maintain flight. Now, scientists from Stanford University have developed wings for flapping-wing drones that do the same thing.

Created by researchers David Lentink and Amanda Stowers, the bird/bat-inspired wings are made from carbon fiber and Mylar film. Each one consists of two joined sub-wings: an "arm" wing and a "hand" wing. A 3D-printed "wrist" joint hinges the two together, allowing the hand wing to fold back over the arm wing. The arm wing is likewise joined to the body by a "shoulder" joint.

In regular flapping-wing flight, centrifugal force keeps the hand and arm wings extended, creating a full flight surface. When hit with a steel rod, however, the wing temporarily gives way and folds back without any damage. As the flapping motion continues, the two sub-wings are pulled back open as they were before, within a single wing beat.

Additionally, no electronics are necessary, as the wing-morphing process is completely passive. This makes it much lighter and more reliable than a system requiring motors or computer activation.

"This finding will greatly help make flapping winged drones much more robust," said Dr. Lentink. "This is essential if we ever want to safely fly through a forest or land in a tree like a bird. The Office of Naval Research wanted us to find solutions to enable drones to fly in such cluttered environments, and this is a promising step forward."

A paper on the research was recently published in the journal Bioinspiration and Biomimetics.

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