# Design, Manufacture and Wind Tunnel Test of a Modular FishBAC Wing with Novel 3D Printed Skins
## 3 FishBAC and Wing Design and Manufacture
### Skin and Stringer Design  
As briefly mentioned in the introduction, one of the main challenges in manufacturing FishBAC devices is how to attach the stringers to the spine, and then the skins to the stringers. To improve the manufacturing process, a novel technique is proposed: to manufacture the skins and stringer as a single component using a two-phase additive manufacturing process (combining two materials during 3D printing).  
Specifically, the two design objectives for the skin and stringers are: _(i)_ to manufacture them as a single component using multi-material Fused Filament Fabrication (FFF) and _(ii)_ to make the entire skin panel removable from the main wing-body without damaging other parts. These skin panels have three main structural components: the stringers, the skins and the rigid mounting tabs. The aerodynamic skins are comprised of a uniform TPU flexible membrane (Ninjatek Ninjaflex) that is stretched between the trailing edge and the main wing-body, whereas both stringers and mounting tabs are made of a stiffer formulation of TPU (Ninjatek Armadillo).  
The multi-material FFF printing approach allows printing the stringers and mounting tabs directly onto the skin sheet, which are themselves 3D printed in the same process. This will ultimately make the assembly much simpler compared with previous FishBAC iterations, as the skin sheet no longer needs to be separately stretched and bonded to the stringers in a second operation. Moreover, the total part count is reduced. The removable design allows the removal of the skin panel for inspection or replacement with a different design altogether. Mounting tabs that run along the spanwise direction were printed onto the membrane from the stiffer formulation TPU. These tabs are used to mount the skin to the trailing edge and the main wing-body using standard metal fasteners. Both the upper and lower skin were printed in one single step and then folded over at the trailing edge. The spine is clamped between the printed trailing-edge tabs using transverse screws. On the main wing-body side, the skin is held in place with a dovetail self-locking feature that hooks into the 3D-printed wing shell. The dovetail helps during the assembly process and uniformly distributes the load from the stretching of the skin along the entire span to the wing-body. Once again, the tabs are screwed in place with transverse fasteners. To follow the design objective of having a removable skin, the stringers were stitched to the spine using aramid thread, thus avoiding the use of adhesives. Channels were printed along the stringers to allow a needle and the thread to be pushed through and slots within the stringers. These features aligned with holes that were drilled into the composite spine so that both the upper and lower skin portions can be stitched together.  
The skin panel was further selectively reinforced with pultruded UD carbon fibre strips in key locations. It was found that the fasteners caused the tabs on both the wing-body side and at the trailing edge to warp, and due to the skin tension, the edges started to peel away from the wing-body and trailing edge. In order to stiffen the skin anchors and to better spread the load, a carbon fibre strip was embedded into the anchors with a spanwise orientation. A second carbon fibre pultrusion was also inserted at the end of the spine into a slot that spanned both the upper and lower portions of the skin's trailing edge, stiffening the trailing-edge section and providing an alignment feature for the spine and skin portions. Figure 7 shows the side view of the skin and stringers mounted on the wing.  
Figure 7: Skin panel (side view) mounted to the wing and stitched to the spine.