# Design, Manufacture and Wind Tunnel Test of a Modular FishBAC Wing with Novel 3D Printed Skins
## 1 Introduction
The stringers and spine were printed together as a single internal'skeleton'. Furthermore, a silicone elastomer skin was pre-stretched and bonded to this skeleton, whereas the pulley mechanisms were manufactured separately and later installed. Kevlar tendons were used to transmit the actuation loads from the servos to the trailing edge of the morphing device. These were anchored to the pulleys on one end and hand-stitched to the trailing edge on the other end [20,21]. Although the 3D printing process is ideal in terms of reducing part count, as the stringers do not need to be later attached to the spine, 3D-printed ABS plastic is not an aerospace-grade material, and its use is far from ideal in terms of its stiffness and strength. However, these prototypes were still key for investigating the aerodynamic benefits of the FishBAC via wind tunnel testing [22]. The second generation of FishBAC devices was designed and manufactured around incorporating carbon fibre-reinforced composite to the spine (Figure 2), which presents significantly improved material properties when compared to 3D-printed plastics, and also allows for stiffness tailoring by exploiting variations in the fibre angle [23]. This generation of FishBAC devices was useful to further study the aerodynamic behaviour of the FishBAC by conducting comprehensive wind tunnel experiments that showed the clear aerodynamic  
Figure 1: First generation of FishBAC devices based on 3D-printed plastic [19]. The copyright of this photograph belongs to [PERSON] (co-author of this article), and has been used with his permission. benefits that this morphing device has when compared to a hinged trailing-edge flap of similar dimensions [24]. However, one clear disadvantage of using a carbon fibre laminate for the spine is that the stringers have to be bonded to the spine, which increases the time and complexity of the manufacturing process, as well as the part count. Additionally, the first and second generation of FishBAC devices required the tendons to be hand-stitched and sealed with adhesive, whereas the elastomeric skin had to be manually bonded. Thus, neither the tendons nor the stringers could be replaced without removing the skins, and the tendons' tension themselves could not be adjusted as they were bonded to the device. The aim of this work is to address these limitations in the first and second-generation FishBACs by designing, manufacturing and wind tunnel testing a FishBAC wind tunnel wing model where: _(a)_ each structural member can be removed and replaced, _(b)_ the tendon tension can be adjusted and _(c)_ the skins and stringers can be attached without using any adhesive. This is achieved by combining traditional manufacturing techniques, such as machining for the actuation mechanism and composite hand layup for the spine, with a novel morphing skin design that is manufactured by 3D printing the skins and the stringers together using two different materials that are fused during the printing process. This paper presents the successful design, manufacture and wind tunnel testing of the first third-generation FishBAC device.