Delfly Flex: A Flapping Wing Micro Air Vehicle with a Bio-Inspired Unibody Composed of Compliant Joints

Flying insects' thorax houses the flight muscles that provide efficient, multi-axis wing actuation. Such bio-inspiration is essential for developing future flapping wing micro air vehicles (FWMAVs) that combine advanced maneuverability with design simplicity, low weight, and high power efficiency. In this work, we propose a novel unibody with distributed compliant joints that mimic the multiple degrees of actuation freedom of an insect thorax, yielding a compact multifunctional structural component capable of active pitch and yaw for the 24.6-gram FWMAV: Delfly Flex. All of these functions are achieved within a single 3.73-gram 3D-printed integrated airframe. To design this unibody, we provide an analytical framework that guides compliant joint geometry using differential flexure beam analysis, along with an optimal joint orientation analysis for seamless integration into the unibody. To ensure sufficient structural endurance, we investigate various resin materials and printing configurations, resulting in a robust resin-printed unibody that incorporates two compliant joints and wing-root stabilizers. This single structure replaces the conventional multi-component FWMAV body composed of rigid-hinge-based dihedral pitch & yaw mechanisms attached to a rod-like fuselage. We characterize the flight capabilities of Delfly Flex through tethered experiments measuring force and moment generation. The results show thrust generation and yaw moment arms equivalent to its predecessor, while the pitch moment arm is approximately 50% smaller due to the concentrated mass distribution inherent to the unibody design. Free-flight experiments further validate the concept, demonstrating controlled pitch and yaw maneuvers enabled by compliant beams as thin as 0.4mm. Combined with simplified assembly and more than 10% mass reduction, this unibody concept opens pathways toward future designs with increased deformability and expanded control authority. Overall, this study highlights the synergy between aero-mechanical design and additive manufacturing, achieving enhanced body intelligence through insect-thorax-inspired FWMAV structures.