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Self-righting for fixed-wing drone, inspired by the elytra of ladybug


Micro Aerial Vehicles (MAVs) are being used in a wide range of applications such as surveillance, reconnaissance, inspection, and search and rescue. However, due to their size and mission profiles, they are prone to tipping over, jeopardizing their operation. Self-righting is an open challenge for fixed-wing drones since existing research focuses on terrestrial and multicopter flying robots with solutions that increase drag and structural weight. Until now, solutions for winged drones remained largely unexplored. Inspired by beetles, A research team at the Laboratory of Intelligent Systems, Ecole Polytechnique Fédérale de Lausanne in Switzerland propose a robust and elegant solution where they retrofit a fixed-wing drone with a set of additional wings akin to beetles shell structured wings called elytra.

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Self righting for fixed wing drone inspired by the elytras of ladybug 01 Ladybug (Coccinellidae) with spread wings. Ladybugs, as all flying beetles, have two sets of wings: the hind-wings and the elytra.  On the 2nd picture, the MAV Ely has a set of fixed-wings akin to the beetle's hind-wings and a set of artificial elytra (Picture source: IEE)


Over the past few years, drones have displayed great potential for operating in difficult environments across a great span of applications and mission profiles. These include being deployed in arduous weather conditions, confined spaces, or areas cluttered with debris. During these missions drones are prone to being tipped over. To takeoff again and continue their mission, robots must be able to self-right. To date, studies on self-righting robots have been limited to terrestrial robots and multi-rotor drones, but have not considered winged aircraft.

The research team accomplished this by taking inspiration from the coleoptera order, commonly known as beetles. Beetles have shown a remarkable ability to self-right after falling to the ground by using an outer set of hardened wings called elytra, (singular: elytron) . These elytra serve to provide the insect with self-righting capacity as well as producing auxiliary lift during flight. In this way, the added weight and complexity of the second set of wings is offset by the additional lift they produces. Similarly, they incorporated a set of artificial elytra made from a hybrid carbon fiber and Kevlar composite fabric onto a fixed-wing MAV. The elytra are attached through two sets of two servos allowing them to be swept back and pitched 180° forward. By first sweeping the wings backwards and then pitching them forward, the aircraft can flip itself over.



Test have been made with a winged drone, code-named Ely. Ely is a conventional fixed-wing MAV with a single electric motor in tractor configuration. The hind-wings were made from Expanded Poly Propylene (EPP); a resilient and highly flexible foam material. The elytra were 3D printed with Acrylonitrile Butadiene Styrene (ABS) plastic and were re-enforced with carbon-Kevlar composite fabric and epoxy adhesive for added resilience

The flight tests consisted of first dropping Ely onto the ground such that it landed in an inverted position. At impact, the elytra absorbed the landing loads and immobilized the drone. Then, the self-righting function was manually triggered. The pre-programmed self-righting function autonomously began after 5 seconds. As described in previous sections, the elytra are first swept back 90  and then pitched forward 180  to flip the plane into its upright position. After uprighting, the elytra move back to their flight position. Next the pilot engages full throttle and the plane takes off. Ely flew steadily at an approximate speed of 8 m/s. The flight test was carried out in calm wind conditions of less than 2 km/h. The plane successfully flew for 45 seconds before landing in short grass.


Self righting for fixed wing drone inspired by the elytras of ladybug 03 The winged drone, code-named Ely, with detail of the self-righting mechanism composed of elytra that rotate in pitch and sweep through a pair of servos controlled by an on-board micro-controller (Picture source: IEE)


The proposed solution is suitable for fixed-wing drones at the Micro Aerial Vehicle scale . In this study, the research team used widely available materials. Elytra materials with different mechanical properties could be used to enable self-righting in a variety of challenging environments with diverse surface composition in terms of temperature, humidity and friction coefficients. Elytra geometries with differing levels of camber or airfoil shapes could be employed depending on the aerodynamic performance required. Moreover, the simple yet robust mechanical design of the self-righting mechanism makes the system fit for use not only in aerial vehicles, but also in terrestrial and marine robots that require self-righting capabilities.

 

Nature is always a source of inspration for researchers. For other examples, see biomechanical small-UAS based on dragonfly or UAV flying like a bird, or also obstacle avoidance system based on mosquito night navigation.