# 3D-Printed Bio-Inspired Mechanisms for Bird-like Morphing Drones
## 4 Discussion and Conclusions  
There are more than 50 species of gulls distributed worldwide, ranging in size from the little gull, at 120 g and 30 cm long, with a 78 cm span, to the great black-backed gull, at 1.75 kg and 76 cm long, with a 1.7 m span. Gulls are generally uniform in shape, with heavy bodies, long wings, and moderately long necks [30]. Bird wings are multifunctional structures with an incredible balance between stiffness to withstand the varying aerodynamic load and flexibility to allow for morphing required for maneuvering. The wings have joints and muscles that enable them to change the orientation of their bones and expand their feathers. Bird bones are lightweight due to their porosity and low density, while feathers are super lightweight yet very strong and durable due to their associated fibers. Indeed, the benefits and superiority of morphing avian wings have been reported compared to traditional ones. However, despite advances in additive manufacturing, composite materials, and servomotors, constructing lightweight, functional morphing UAVs remains technically challenging. This paper expands on the developments of morphing technology with proposed sweep-morphing feathered wing and tail designs influenced by the anatomy of a common seagull. As highlighted by [PERSON] et al. [31] and [PERSON] et al. [20,21], the manipulation of two skeletal joints in the gull's wing (elbow and wrist) is responsible for most of the wing shape change. Hence, the proposed morphing wing design features planar sweep-morphing capability with feather expansion. This morphing action can operate symmetrically or asymmetrically. Asymmetric sweep-morphing generates roll moment. The musculoskeletal mechanism birds use to expand the feathers of their tails was described by [PERSON] and [PERSON] [32] and [PERSON] and [PERSON] [33]. Both highlighted the importance of the bulbi rectricium muscle, which holds the root ends of the feathers and acts independently of other tail muscles [33]. As the bulbi rectricium contracts, it changes shape, causing the rotational position of each feather to change. The tail feather expansion-compliant mechanism presented in this work mimicked this action to control the rotational position of the tail feather shafts. Achieving the optimal balance between flexibility, to allow morphing with the available actuation forces, and stiffness/strength, to carry the applied aerodynamic loads, is one of the most challenging tasks in designing morphing structures. Using rigid materials in morphing designs requires a relatively large number of components connected with joints, which could increase friction, complexity, and failure mechanisms. The 3D-printed materials proposed in this work and the geometric design of the wing and tail feather expansion mechanisms proved effective in producing the desired morphing deformations with the available servomotor actuation forces. In both designs, only one component was  
Figure 12: Proof-of-concept model of the proposed sweep-morphing feathered tail mechanism: (**a**) feather contraction, and (**b**) feather expansion. needed to hold all feathers and morph them once actuated, avoiding any mechanical joints in the design. The computational simulations and actuation tests performed on the manufactured proof-of-concept models justified the design decisions and demonstrated the effectiveness of the proposed designs. However, feathers are attached to these mechanisms, and the feather shafts act as cantilever beams that will bend under the transverse aerodynamic load the UAV would experience, depending on the shaft and the vane materials. The proposed shafts are composite rods with two options for the vane material. The laminated composite vanes would be stiffer with a small weight penalty. Further wind tunnel and flight tests on a full UAV would confirm these preliminary findings.