## 3 Preliminary Design
### Objectives and Test Section  
The objective of the IWT will be the testing of fixed-wing UAVs in icing conditions. The first step in the design of any wind tunnel, after defining its purpose and basic configuration, is to define the dimensions of the test section and the speed to be reached [40] (p. 61). The test section will determine the dimensions of the remaining sections, the total size of the installation and, together with the speed, will also determine the required fan power. The tunnel will be aimed at testing fixed-wing UAVs, and the 300 kg MTOW SIVA UAV has been used as a model for the design (see Figure 4). The SIVA is a medium-sized or Type II UAV according to the NATO classification, being in the 150 to 600 kg range, and is referred to as a tactical UAV [57]. It has a wingspan of 5.8 m, and its wing has an aerofoil, with a Reynolds number of 2 \(\times\) 10\({}^{6}\), based on a wing chord of 0.9 m and a cruising speed of 40 m/s. The wing profile is an Eppler E580 airfoil, with a maximum chord thickness of 16.1%. Defining a speed of 70 m/s in the test section, the Reynolds number per unit length is 5.6 \(\times\) 10\({}^{6}\) m\({}^{-1}\) and the Mach is 0.2, which classifies it as a low-speed wind tunnel, as the compressibility phenomena are negligible (Mach below 0.3). This speed allows us to use aerofoils with a minimum of a 14 cm chord, to maintain a Reynolds number (_Re_) of 8 \(\times\) 10\({}^{5}\) and thus ensure similar air behaviour with respect to the ratio between inertia and viscous  
Figure 4: SIVA Unmanned Aerial Vehicle (UAV). Figure 3: Designed IWT in the refrigerated chamber. forces, and therefore the same boundary layer detachment and stall performance. [PERSON] et al. [40] (p. 64) recommend a minimum Reynolds number of between \(5\times 10^{5}\) and \(8\times 10^{5}\) based on wing chord for low-speed tunnels to be used for aeronautical development testing. Another important factor to take into account is the blockage due to the tunnel walls, especially important in a closed test section. The blockage due to the presence of the model (solid blockage) and the wake blockage causes the effective velocity to increase around the model. This increases lift and drag, and this effective speed must be taken into account in the calculation of aerodynamic coefficients when the blockage ratio exceeds 5%. In any case, the ratio of the model \"frontal area\" to the test section total area should not exceed 10% [40] (p. 65). [PERSON] et al. [58] also recommend not to exceed this 10% blockage limit. For tests with bluff bodies or with models that exceed this maximum recommended area, the blockage will be greater, so it will be necessary to perform prior CFD simulations to estimate its influence. Defining a test section height of 0.4 m and a scale of 1:5 for a wing section of the SIVA UAV (and therefore a chord of 181 mm and a maximum thickness of 29.1 mm), the value for the blocked area is 7.3%, an acceptable value that represents an increase in effective speed of 0.42% (from 70.0 m/s to 70.3 m/s), calculated as the sum of the effect due to solid blockage [59] and the effect due to wake blockage [60]. In addition, a scale of 1:5 means a very convenient \(Re\) of 8.9 \(\times\) 10\({}^{5}\). Smaller scales would reduce the Reynolds number and larger scales would increase the blockage (see Figure 5), so it is a good compromise point. Since the ratio between the width and height of the test section should not exceed a ratio of 2:3 [40] (p. 65), a test section height of 40 cm implies that the width of the test section will be 27 cm. Therefore, the tunnel will have a test area of 0.1 m\({}^{2}\) (40 \(\times\) 27 cm\({}^{2}\)) and will reach a speed of 70 m/s. An optimum blockage of 5% for an accurate measurement of the aerodynamic coefficients will be obtained with profiles with a maximum thickness of 20 mm, which corresponds, for example, to a NACA 0012 airfoil with a chord of 167 mm.