# Design, Manufacture and Wind Tunnel Test of a Modular FishBAC Wing with Novel 3D Printed Skins
## 7 Wind Tunnel Test
### Wind Tunnel 2D Insert  
A bespoke 2D wind tunnel insert with a 1 m \(\times\) 1 m cross-section by 1.2 m-long test section was designed and built. The 2D insert consists of two splitter plates made of 10 mm thick acrylic sheets mounted on square frames and four vertical uprights connecting them and mounting the wing section. This frame is made from extruded aluminium slotted rail sections for ease of assembly and modification. The uprights support the top splitter plate and maintain the 1 m section height. Moreover, the connections at all corners are made of four distinct brackets, each to ensure high rigidity and strength (Figure 20). The bottom splitter plate is mounted to a base frame, where the automatic pitch drive system also sits, through four shorter vertical extrusions that lift the cube-like structure to the centre of the tunnel section (Figure 21). The splitter plates work as walls for the reduced test section where 3D effects are reduced, and a desired quasi-2D flow is produced on a 1 m span. The leading and trailing edges of the splitter plates were cut at a 45\({}^{\circ}\) angle to cleanly cleave off a fresh boundary layer. The two sides, which correspond to the walls on the suction and pressure side of the flow, are open, allowing the free expansion of the freestream along the airfoil thickness. Figure 21: Two-dimesional wind tunnel inserted inside the 7 ft \(\times\) 5 ft test section. Figure 20: Two-dimesional test section: acrylic walls and splitter plates. A precision right-angled worm gearbox driven by a synchronous servo motor was installed with the axis in a vertical orientation and used to control the wing's pitch angle. An industrial digital drive with 6 A rated output was installed to drive the servomotor and set up with closed-loop control of position and analogue output to be acquired during testing. The servo produces a stall torque of 2.9 N m with a resolver measurement accuracy of \(\pm 45\)'' and is equipped with a braking system. The two-stage worm gearbox has a reduction ratio of 400:1 and can support a maximum axial load of 25 kg. The system was designed to produce a maximum pitching torque of 100 N m and operate between 0.1 and 12 rpm, with an overall accuracy of 0.0667\({}^{\circ}\) (driven mostly by gearbox backlash). The dedicated software has been set up to perform sequential tasks corresponding to the angle ranges and step increases defined for each test. Two ME-systems K6D80 6-axis force sensors with a forces and moments range of 2 kN and 100 N m, respectively, were used to measure the aerodynamic forces and moments acting on the wing. Two load cells are required to fix the demonstrator at both ends and obtain 2D aerodynamic coefficients. The top force sensor was mounted to the 2D insert using a series of aluminium brackets and is integral with it, whereas the bottom force sensor was attached to a shaft connected to the pitch drive that rotates inside a flanged bearing to change the wing's pitch angle. Mounting and attaching the wing directly to both force sensors allows bypassing the aerodynamic forces acting on the 2D insert (e.g., additional drag). However, this setup does not eliminate the effect that the presence of the uprights may have in terms of flow quality and smoothness. Furthermore, the entire 2D insert structure is mounted to an AMTI Biomechanical Force Platform (1000 lbs max measurable force), which measures the forces and moments that the entire setup experiences during the wind tunnel test. The force plate is fixed to a foundation frame that connects to the wind tunnel structure. These forces and moments are measured as a 'backup' to the wing's force sensors. Therefore, this data set would only be used if the force sensors present any issues. Additionally, two pitted tubes were mounted to the two front uprights (i.e., facing the inlet of the test section) to measure the local flow velocity where the quasi-2D flow is produced. Each pitted tube is connected to one NXP MPXV7002 differential pressure sensor, which measures the dynamic pressure.