## 2 Design of Experiment (DOE)
### Prototyping the UAS-S45 Wing Section and Its ETMs  
Prototyping processes have advanced significantly with the advent of 3D printers. This manufacturing technology has revolutionized the industry, making prototyping more accessible, easier and significantly less expensive than before. Most of the morphing wing prototypes in the literature have been fabricated using 3D printing [29; 38; 39]. Three-dimensional printing is an additive manufacturing process that has offered a fast and cost-efficient approach to manufacturing models for wind tunnel testing. It creates a three-dimensional object by depositing melted polymer layer by layer. The process begins with designing the object in CAD software (SolidWorks 2021). The design is then processed by a slicer, software that sets printing parameters and object orientations. The slicer divides the object into layers of defined thickness and generates G-codes, which are sent to the 3D printer (Ender 3 Max, Creality, China). The machine interprets these G-codes to control a  
Figure 3: Schematic of the UAS-S45 wing section equipped with an SMTE flap, elephant trunk mechanisms (ETMs), flexible skin and an actuation system. heated nozzle that extrudes the polymer filament, \"drawing\" the object line by line and layer by layer. The 3D printing parameters are critical for determining the mechanical properties and surface finish of the printed object, while the orientation of the object during printing is equally crucial. Similar to conventional composite materials, the mechanical properties of 3D-printed objects are anisotropic. In composite materials, fibers aligned in the direction of the applied load exhibit superior mechanical properties to those aligned in the perpendicular direction of the applied load. Likewise, 3D-printed objects, constructed layer by layer and line by line, display mechanical behavior resembling that of composites. In this case, the melted filament serves simultaneously as a fiber, as well as a matrix. Forces applied perpendicular to the layers can cause delamination, as the interface between layers, rather than the continuous fibers, must bear the load. This fact highlights the importance of optimizing printing parameters and arrangements to achieve desired mechanical performance. Figure 4 shows the vertical and horizontal arrangements. As shown in Figure 4, each layer in 3D printing has a specific height (\(\Delta h\)), representing a 2D slice of the studied object. Each layer is supported by the layer directly beneath it. However, if an object contains a cantilever or an overhanging part, the first layer of the cantilever will lack underlying support. To address this issue, the slicer automatically generates a block of support material beneath the overhang part. These supports are typically considered as waste material and are removed during post-processing. Optimizing the orientation and positioning of the object can significantly improve its mechanical properties while reducing printing costs and time by minimizing the need for support structures. In this study, the vertical arrangement is chosen for printing due to the wing's curved upper and lower surfaces. Printing these surfaces with greater accuracy is more easily achieved using the vertical orientation than with a horizontal arrangement. Figure 5 shows the vertically arranged wing section during its 3D printing. The material used for the wing section is PLA+, an enhanced version of regular PLA with additives that improve surface finish and simplify the printing process. The primary properties of PLA+ are provided in Table 1, as specified by the filament manufacturer [40]. Figure 4: Three-dimensional printing layer arrangements, (**a**) vertical, (**b**) horizontal. Based on the results obtained from the structural analysis in the previous work [37], the material selected for the ETMs was nylon. Nylon's exceptional mechanical properties made it an ideal material for structural components in the elephant trunk mechanism.