# 3D-Printed Bio-Inspired Mechanisms for Bird-like Morphing Drones
## 1 Introduction
The more components added to make the drone more biologically accurate, the heavier and more complex the drone becomes. Meanwhile, birds are remarkably lightweight with hollow bones and feathers, allowing optimal weight efficiency. Realizing this in a drone has proven difficult due to the near-impossible task of finding such lightweight materials. In addition, lighter materials are generally weaker, making them less likely to hold the drone in the air [17]. Birds have novel aerodynamic traits compared to traditional aircraft beyond morphological control, including complementary wing and tail lift. In traditional aircraft design, the wings and tail can generate lift in opposing directions to produce competing pitch moments around the center of mass. While more dynamically stable than a bio-inspired tail, this induces additional drag, as does the use of a vertical control surface. By comparison, birds cradle their center of mass between the upward lift of their wings and tail [18]. They use their tails to achieve yaw control by pivoting the flat surface, splitting lift into pitch and yaw components. Pitch can be adjusted through planar sweep morphing and varying the angle of attack. Although this design is more efficient, it requires more active control and benefits greatly from morphing. Bio-inspired tail designs have been pursued in small ornithopters, however; hobbyists often use this design out of the need to maximize lift. A four-degree-of-freedom bio-inspired tail on a fixed-wing drone was evaluated in a wind tunnel by [PERSON] et al. [19] to demonstrate its pitch, roll, and yaw characteristics under various conditions. Seagulls and albatrosses, two very similar families of bird species, are some of the most adept dynamic soaring and gliding birds in the world. Their non-flapping flight performance highly compensates for their poor flapping. Due to the high aspect ratio of their wings, rapid and efficient level flight is enabled, further aided by their lean, streamlined bodies. Their wing structures have evolved to be light and rigid, two critical factors influencing flight performance and physical limits. The cross-section profile of a seagull's wing closely resembles an S1223 airfoil [20], optimized for high lift and low drag. Indeed, seagulls have been at the center of tremendous aerodynamic research in recent years, hoping to fully grasp the potential of their biologically-engineered and evolutionary design [21; 22; 23; 24]. Another interesting feature in avian flight is using a self-actuated covert flap to mitigate stall conditions over the wing [25]. The covert feathers on birds' wings can lift in response to flow separation during stall conditions, allowing birds to maintain higher angles of attack without losing control. This also results in higher lift efficiency. With the morphing benefits and challenges considered, this paper presents sweeping-morphing feathered wing and tail designs for a bio-inspired drone that benefits from the advances in 3D-printing technology and materials. Lightweight and flexible materials have been combined to construct the various components of the wing and tail designs, including plasticized copolyamide thermoplastic elastomer (PCTPE), thermoplastic polyurethane (TPU), Acrylonitrile Butadiene Styrene (ABS), balsa wood, carbon fiber and glass fiber composites. The wing design features a corrugated flexible PCTPE structure similar to the FishBAC design previously used in the flexible ribs of camber-morphing wings [26]. This design can also be supplemented with a dihedral angle morphing capability for fine-tuning flight characteristics. The proposed tail feather-spread mechanism features a 3D-printed TPU flexible structure with circumferential corrugations. This tail design can also be supplemented with a pitch and tilt mechanism. This article presents the nonlinear computational analyses of the designs and the manufacturing and testing of the proof-of-concept models.