# Design, Manufacture and Wind Tunnel Test of a Modular FishBAC Wing with Novel 3D Printed Skins
## 3 FishBAC and Wing Design and Manufacture
### Skin Panel Manufacture  
The skin panels (Figure 8) were printed on an E3D Toolchanger FFF 3D printer using modified direct-drive extruders for flexible materials. Each material had its own extruder head so that the PID controller could be tuned for the materials printing temperature to avoid any temperature fluctuation throughout the print. Due to print size limitations, the individual panels for the wind tunnel demonstrator had to be assembled in four stages. These steps are described as follows.  
1. Assembly of the tabs that screw into the main wing-body. Then, a 1 m-long, 3 mm \(\times\) 1 mm carbon fibre strip was epoxied into a slot in the printed sections. The skin was held in place by screwing it to the main wing until the adhesive was fully cured. Release film was placed between the wing and the skin panel to avoid them getting permanently cured. The above steps were repeated on the opposite side.
2. Assembly of the trailing-edge section. Epoxy adhesive was applied to the spanwise ends of the stiffer TPU anchor pieces to create a single continuous panel. On one side and in an alternating fashion, the carbon fibre tabs were inserted using epoxy adhesive. Whilst still uncured, this entire assembly was screwed onto an especially designed assembly jig to ensure that all holes in the trailing edge will align once the adhesives are cured.
3. Thermoplastically welding the ends of the soft TPU skin membranes together to make a single air-tight skin surface. Kapton tape was temporarily placed over the joints to promote a smooth weld. A soldering iron with a specially shaped tip was drawn over the Kapton tape to melt and consolidate the adjacent skins together, as shown in Figure 9.
4. The final stage was to weld the stringers together. Pieces were printed to bridge between the two neighbouring panels stringers. They were then welded in place using a spot-welding technique, as shown in Figure 10.  
Figure 8: 3D-printed skin panels—(**a**) showing the inside and **(b)** showing the outer (aerodynamic) surface.  
Figure 9: (**a**) Fully finished weld. (**b**) Weld covered in Kapton tape ready to be welded.  
Once the skin panels were fully connected together, the final step in the manufacturing process was to attach them to the wing structure and stitch them to the spine. The skin panel was first folded around the trailing-edge fold and the screwed hand tight to the spine. The wing was then actuated to the maximum deflection to facilitate the pre-tensioning of the skin panels. The tab was screwed down hand tight. This was repeated on the opposite side, and once the alignment was checked, all fasteners were tightened. With the skin firmly attached to the wing, the skins were stitched to the spine. Pliers were used to force a needle with Kevlar thread through the narrow passage between the skin and the stringer. The holes of the opposing skins and the stringers aligned well, as the skin was under tension, which made stitching through the spine relatively straight-forward, as shown in Figure 11. It was made sure that the thread was always under tension as it was not possible to tension once stitched along the entire span due to frictional locking. The thread ends were tied up around a hole in a stringer and then secured using a quick-drying fastener lacquer.