# Design, Manufacture and Wind Tunnel Test of a Modular FishBAC Wing with Novel 3D Printed Skins
## 3 FishBAC and Wing Design and Manufacture
### Actuation Mechanism & Tendon Anchoring  
The actuation mechanism of the latest FishBAC generation has been envisaged to improve access for assembly, replacement of the components and adjustment of the tendons, as well as to provide higher structural performances required to operate under high actuation loads. To achieve this, the main individual parts have been designed with all the features required to mount and operate the servo-tendons system allowing for  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Thickness (mm)** & **Ply Count** & **Stacking Sequence** & \(\mathbf{w_{\text{min}}}\)/\(\mathbf{w_{\text{max}}}\) & \(\mathbf{w_{\text{max}}}\) (mm)** \\ \hline
1 & 8 & [90/90/90/0]\({}_{\text{S}}\) & 0.883 & \(-15.5\) \\
0.78 & 6 & [90/90/0]  
quick disassembling from the main structure and relatively easy components replacement or adjustment.  
The actuation is provided by a high torque brushless digital servo motor (Figure 5), with a nominal stall torque of 45.5 kg cm, that meets both the actuation torque requirements and geometric constraints imposed by the Bo-105 blade cross-section. This servo has an operating travel range of \(\pm\)45\({}^{\circ}\), and an operating speed at no load of 0.1 s/60\({}^{\circ}\) (at a maximum supply voltage of 8.4 V). The torque is transmitted to the FishBAC's trailing edge through a tendon on either side of the spine, both wound around and connected to a 6.35 mm steel shaft that is then attached to the servo spline through an adaptor with the set screw. This setup is defined as the \"spooling pulley\". A 2 mm-diameter high-performance polyester-coated Vectran rope was used for the tendon. This material choice gives the rope excellent stiffness and strength, along with very low cold creep properties (under tension) and reduced tendon-pulley friction.  
The tendons are anchored to the D-shaft using aluminium collars with clamp screws. The collars clamp the wound-up tendons to the shaft on either side of the loading path, leaving enough clearance between them to allow the winding and unwinding of both tendons. Finally, the tendons connect to the trailing edge through a tendon anchor, a two-piece aluminium component screwed together to clamp to the spine, as seen in Figure 6.  
A monolithic actuation frame, machined from 6082-T6 aluminium alloy billet, is mounted in between the front and rear spars. This removable bracket hosts the actuation mechanism (mounts the servo, supports the shaft and tension the tensioning arm) and also connects the two spars, ensuring full load transfer between them. Tendon tensioning is achieved via a tensioning arm, which pivots off of the actuation frame to support a pulley around which the tendon wraps. The arm can be drawn towards the leading edge via a nut on a threaded rod, which increases the path length the tendon must travel, thereby creating variable tendon tension. The initial resting position of the FishBAC can also be adjusted by introducing differential tensioning of the upper and lower tendons via offsetting the tendons at their clamping points on the spooling pulley. Figure 6 shows the components of this actuation mechanism.  
Figure 5: FishBAC spine mounted on the 3D-printed adapter and tendons anchored at the trailing edge before skin assembly.  
Figure 6: Actuation mechanism of wind tunnel model from different points of view: (a) actuation mechanism perspective view (**left**); (b) actuation mechanism top view (**right**).