# 3D-Printed Bio-Inspired Mechanisms for Bird-like Morphing Drones
## 1 Introduction  
Commercial aircraft are fixed-winged powered vehicles with discrete control surfaces, such as ailerons, rudders, elevators, and flaps, to control the flight path by altering the generated aerodynamic forces and moments. In contrast, birds have flexible wings that can flap to generate the needed lift and propulsion. Indeed, certain birds can fly for hours without a single flap. This is achieved by the highly efficient aerodynamic performance of birds' wings during gliding and soaring [1]. Birds can dynamically change their wing and tail postures to maximize adaptability and achieve diverse flight behaviors (e.g., maneuvering, gliding, and soaring flight) and flight environments (e.g., dense jungle, urban, and mountain environments) [2]. The deformation of birds' wing posture under aerodynamic loading is a complicated kinematic coupling process, primarily comprising two critical aspects: large-scale geometric wing deformations (e.g., changes in wing area, wingspan, chord length, sweep angle, or wing twist) and small-scale local deformations of each flexible feather (e.g., bending, twisting, and fluttering). Design concepts of bio-inspired wing configurations can be roughly classified into three major categories: (A) bio-inspired non-deformable wing configurations only considering biological structural elements of birds' wings, such as slotted wing-tips, feather flaps, and non-smooth wing surface features; (B) bio-inspired deformable wing configurations only considering basic morphing motion features, such as variable-camber, variable-sweep,twisting, folding, and inflatable deformations; (C) complex bio-inspired wing configurations considering biological structural elements and morphing motion modes [1]. Category B is the most common in literature. Multiple review papers have presented the various designs of morphing unmanned aerial vehicles (UAVs) from myriad aspects [3; 4; 5; 6]. The superiority of morphing wings over traditional wings has been demonstrated in numerous studies, including the work by [PERSON] et al. [7]. Recent examples of morphing UAVs include the Transformer Aircraft with span-morphing wings [8], MataMorph-2 with twist-morphing wings [9], and MataMorph-3 with camber-morphing wings and tail stabilizers [10]. Recently, researchers have developed new bio-inspired wing configurations that incorporate the biological structural elements and morphing motion of birds' wings (Category C) [11]. Although UAVs with flapping and non-flapping wing designs have been realized, non-flapping designs are more mature, scalable, arguably more practical, and capable of higher speeds [12]. Flapping designs (ornithopters) have poorer thrust-to-weight ratios and additional dynamic degrees of freedom to allow for flapping, which can lead to more mechanical issues. Examples of non-flapping bio-inspired UAVs include (1) \"RoboSwift\" ([[https://www.roboswift.nl/](https://www.roboswift.nl/)]([https://www.roboswift.nl/](https://www.roboswift.nl/)), accessed on 22 October 2023), which has a deformable wing structure inspired by discrete feather elements of swift birds that can be dynamically folded into different swept-wing shapes, (2) \"LisHawk\" [13], inspired by the Northern Goshawk, with sweep-morphing feathered wings and tail, (3) \"LisEagle\" [14] with wing folding and pitching, and (4) \"PigeonBot,\" inspired by the pigeon [15], with sweep-morphing wings and biological feathers. While fixed airplane wings have been considerably optimized in the last several decades, the design of each type of morphing wing has challenges. Moreover, no current standards exist for designing such new, innovative, and inspired concepts [16]. The primary challenges facing bio-inspired morphing wing design are system complexity, weight, stability, maintainability, scalability, and controllability. The motion of a bird's wing is highly complex, bending and rotating in six degrees of freedom to facilitate flight. However, achieving this mobility in a drone requires numerous components (servomotors, rods, hinges, etc.).