# Design, Manufacture and Wind Tunnel Test of a Modular FishBAC Wing with Novel 3D Printed Skins
## 3 FishBAC and Wing Design and Manufacture
### Spine Sizing and Manufacture  
The spine sizing was performed by using an already existing Finite Element Method (FEM) model of a composite FishBAC device [27], which included the presence of both skin and stringers. Since the spine is made of carbon fibre-reinforced polymer, the spine stiffness is determined by both its thickness and its stacking sequence (i.e., fibre angle orientation). It is worth remembering that the FishBAC is structurally designed to have a lower stiffness along the chordwise bending direction than along the span so that the energy required for shape change is reduced. However, the wing is not infinitely stiff along the span, and so there will always be some amount of elastic washout between actuation stations. For a full three-dimensional wing design, the FishBAC geometry and material properties can be tailored to provide the correct balance between stiffnesses. In the case of this wind tunnel test, the focus was on trying to have fairly consistent deflections along the span. Since the wing will be actuated at three discrete points along the span, it is crucial that the spanwise stiffness is high enough so that the morphed camber shapes can be maintained along the span. As mentioned in the introduction, the second generation FishBAC wind tunnel prototype showed significant elastic washout [23], which must be addressed in this new design.  
Since this wind tunnel wing model was designed for a quasi-2D wind tunnel test, the achieved deflections should be as uniform as possible (along the span). Therefore, only balanced and symmetric laminates were considered. The two main variables to consider are laminate thickness and stacking sequence. Because carbon fibre prepreg was used in this case, the laminate thickness is determined by the number of plies. Figure 4 includes the results for all the stacking sequences that were considered in the FEM analysis. Since the objective is to select a stacking sequence that both minimises the variations in the transverse deflection along the span and allows achieving large transverse deflections, the selection is performed in terms of the ratio between the minimum and maximum transverse deflections (along the span) and the maximum achievable deflection. It is observed in Table 1 that the best performing laminate according to these objectives is  
Figure 3: 2D profile of FishBAC wind tunnel wing model.  
\([90/90/0/90/90/90]\), as it presents the largest deflections under the selected actuation loads and has the lowest variation in transverse displacement along the span. However, it is important to note that this \"theoretical\" maximum displacement is likely an overestimation, as no losses in the actuation mechanism are being considered (e.g., due to mechanical friction in the pulley-tendon system or actuator efficiency factors).  
The composite spine was manufactured using Hexcel's 8552/IM7 carbon fibre-reinforced prepreg. The layup was performed by hand on a steel tooling plate and then cured under 8552/IM7's standard vacuum bag cured cycle--with a cure temperature of 180 \({}^{\circ}\)C, autoclave pressure of 7 bar and \(-1\) bar vacuum pressure inside the vacuum bag. The cure cycle was controlled using three thermocouples that were placed on the vacuum bag at three different locations. Along with the composite spine, three additional plates were manufactured to perform material characterisation and were cured under the same vacuum bag and cure cycle. Therefore, the results obtained via these material characterisation experiments should correspond to the material properties of this FishBAC's composite spine. Once the spine was cured, it was milled to size, and the holes and features were machined using carbide-coated crown point drill bits.