FlexiQuad: the collision-resistant ring drone from ETH Zurich
Inventors file a patent application for an agile quadcopter that deforms on impact and maintains flight at speeds up to 80 km/h.
Robotics researchers at ETH Zurich have designed a quadcopter without a rigid central body that absorbs collisions without crashing. Invented by Luca Girardi and Stefano Mintchev, the ring-shaped FlexiQuad drone uses an elastic frame that flexes laterally on impact while retaining the vertical stiffness required for flight. The inventors have filed a patent application for the device, which has also earned a nomination for the university's Spark Award.
A ring frame inspired by insect wings
For over a century, multirotor drones have relied on a central rigid hub supporting outward-facing arms. As reported by TechXplore, Luca Girardi and Stefano Mintchev inverted that architecture by removing the central fuselage entirely. The components, including the rotors, control modules and power supply, sit distributed across a flexible, circular perimeter.
The mechanical behaviour of the frame draws directly from biological structures. The team analysed the anisotropic flexibility of insect and bird wings, which resist bending during flight strokes but flex when encountering obstacles. FlexiQuad operates on the same logic: the frame bends horizontally to absorb crash energy, yet remains stiff vertically so that propeller thrust transfers directly into vehicle acceleration. The prototype airframe costs less than 15 Swiss francs in raw materials, and the ring elements can be taken apart to fit flat into luggage.
“Soft doesn't automatically mean slow.”
— Luca Girardi, researcher at ETH Zurich
Surviving impacts at 25 km/h
Soft aerial vehicles often suffer from sluggish flight characteristics, but tests run by the inventors showed that FlexiQuad behaves like standard rigid quadcopters in free air. The prototype achieves linear and angular acceleration on par with conventional models, reaching maximum flight speeds of 80 km/h.
Crash survivability was assessed in physical obstacle tests described by TechXplore. Researchers flew the drone directly into solid barriers at speeds of up to 25 km/h. On contact, the elastic perimeter momentarily deformed, dissipated kinetic energy, and rebounded into shape without throwing the machine off balance. Girardi noted that test speeds were restricted primarily by the indoor flight corridor needed to accelerate the machine before impact, not by the physical limits of the frame.
Carrying payloads and reflex-based recovery
The open architecture expands the mechanical roles the vehicle can perform. Because the ring can actively compress around targets, FlexiQuad can grip and transport items using its central opening. In laboratory trials, the prototype successfully secured and transported a soft first-aid kit, carrying a payload heavier than its own structural mass. Applications outlined in the patent application include inspecting ballast tanks on cargo ships, navigating collapsed buildings during search and rescue operations, and deploying a net across the hollow frame to snare rogue drones.
Operating a deformable airframe introduces challenges for autonomous navigation. Vibrations after an impact can momentarily blur camera feeds and disturb state estimation. To resolve this, the researchers applied reinforcement learning algorithms that rely on onboard inertial motion sensors, mimicking automatic biological reflexes to stabilise flight posture immediately after an obstacle strike before handing control back to standard navigation routines.
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