# Design, Manufacture and Wind Tunnel Test of a Modular FishBAC Wing with Novel 3D Printed Skins
## 7 Wind Tunnel Test
### Results: Aerodynamic Forces and Moments  
The main objective of this wind tunnel test campaign was first, to ensure that the novel design and manufacturing techniques introduced in this latest generations of FishBAC wings lead to structurally sound FishBAC devices that can resist the different aerodynamic load cases, and second, to evaluate its aerodynamic performance and its ability to generate changes in lift coefficient with varying camber distribution (i.e., \(\Delta C_{L}\)). It is worth mentioning that, since (a) this wind tunnel setup consists of a 2D wind tunnel insert inside the \(7\arcmin\times 5\arcmin\) test section and (b) no local 2D drag measuring technique was implemented (e.g., wake rake), the drag measurements are expected to be higher than a purely 2D case. To correct for three-dimensional effects, standard wind tunnel corrections are applied to account for solid blockage and streamline curvature. Because the 2D insert is a closed section with no top/bottom covers (i.e., both suction and pressure sides are left open), the wake blockage is assumed to be zero. The correction factors implemented in this study are the classical ones given by [33]. All measurements were conducted at a freestreamvelocity (inside the insert) of \(30\,\mathrm{m}\,\mathrm{s}^{-1}\), which results in Mach and Reynolds numbers of \(M\approx 0.088\) and \(Re\approx 540,000\), respectively.  
Figure 22 shows a very clean set of data, where each increment in the actuation input angle generates a substantial change in lift coefficient. A maximum \(\Delta C_{L}\approx 0.55\) is observed in the linear region of the lift curve (i.e., for angles of attack of less than \(\alpha<10^{\circ}\)), whereas a \(\Delta C_{L}\approx 0.40\) is observed at around stall. Additionally, it is seen that the point of minimum drag does shift with increasing camber deflections, which is consistent with the results observed by [24]. However, it is worth noting that these drag coefficient results vary significantly with the angle of attack, which is not expected in a 2D wind tunnel test. Two-dimensional drag coefficients should remain relatively unchanged with increasing pitch angle (at a given camber deflection) when the flow is attached (i.e., in the linear region of the lift curve), and then \"spike\" as the flow starts to separate and stall (see [24] for example). These drag results suggest the presence of 3D effects that standard wind tunnel corrections ([33]) cannot correct. These 3D effects might have been introduced by the 2D insert inside the \(7^{\prime}\times 5^{\prime}\) tunnel section; therefore, they are most likely a limitation of the setup. Despite the lack of high-quality 2D drag measurements, it is interesting and encouraging that the minimum value of drag coefficient for any given morphing deflection is relatively constant, implying that the change in camber is essentially shifting the drag curves left and right by a consistent offset in the angle of attack. Lastly, Figure 23 shows that the FishBAC can be used for controlling pitching moment coefficients. A \(\Delta C_{M}\approx 0.10\) can be achieved by varying the camber distribution, which highlights the potential that the FishBAC has for use in moment driven control surfaces, such as servo flaps.  
Figure 23: Transverse displacement at the trailing edge in the presence of aerodynamic loads (freestream velocity of \(V=30\,\mathrm{m}\,\mathrm{s}^{-1}\)).  
Figure 22: Aerodynamic coefficients of the FishBAC wing tested at the University of Bristol’s \(7^{\prime}\times 5\) tunnel, using the 2D insert: lift (**left**), drag (**centre**) and moment (**right**) coefficients, respectively, vs. angle of attack.