At first glance, FlexiQuad looks like a small drone. Just one more buzzing thingy doing its flying thing. Then you see it crashing against a tree. A regular drone would crash and fall to the ground, but this one bounces back from the tree trunk and happily keeps flying along. The key to this trick is a hollow ring design crafted from flexible materials. [Image: Courtesy Luca Girardi] Today, almost every quadcopter uses four stiff arms bolted to a central body that holds its battery, brains, sensors, and sometimes other payloads. ETH Zürich researchers trashed that core and the arms, placing the rotors on a ring, with the batteries and the rest of the electronics spread evenly around the rim. The FlexiQuad’s middle is empty, like a vertically flat donut. [Image: Courtesy Luca Girardi] The ring is flexible. That’s its superpower. Hit a wall and it buckles, absorbing the force of the impact and bouncing back. As the structure stabilizes, it keeps itself in the air.  [Image: Courtesy Luca Girardi] Developed by engineer Luca Girardi and Stefano Mintchev, a former professor at ETH Zürich’s Environmental Robotics Lab, the drone takes its design inspiration from how the wings of birds and insects bend in some directions but not others. “We were interested in how stiff their wings are and how they bend them—or don’t bend them—in different directions,” Girardi said in a statement.  Wings yield in some directions but can also hold rigid in others. Engineers call this “anisotropic stiffness,” the resistance of a material or a structure to deformation, which changes depending on the pushing force’s direction.  Girardi has worked on the idea for about five years. “At first, I was skeptical whether the drone would fly without the central reinforcement,” he said in the statement. “But I really liked the approach. Then we tried it—and it worked.” “The key was to rethink where the weight sits, as well as where the structure bends,” Girardi tells me. “A heavy central bo

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