## 3 Wind Tunnel Instrumentation and UAV S45  
The Open Return Subsonic Wind Tunnel, the unmanned aerial vehicle, and the aerodynamic balance, as well as the instruments used during wind tunnel tests, are described in this section. The LARCASE laboratory at ETS owns the Price-Paidoussis Twelve-Meter Open Return Subsonic Wind Tunnel, shown in Figure 1. This research apparatus, which is used to test wings and diverse types of prototypes, makes it possible to safely perform studies at various flow conditions. The LARCSE wind tunnel consists of a centrifugal fan, a diffusing section, a settling chamber, a contraction section, and a working section. The airflow can develop a maximum speed of 0.18 Mach thanks to the engine and the double impeller centrifugal fan. The two inlets at the opposite side of the rotor allow the air supply to increase the pressure flow, and the 24 small propellers inside each inlet allow the fan to turn at a much higher speed than fans with large blades. The engine and the centrifugal fan are located inside the soundproof mechanical room that protects the wind tunnel from dust particles. The working section is the most critical part of a wind tunnel as that is where the effects of the flow of the wing prototypes are measured. An important step before testing is the calibration of the test chamber that contains the studied model so that the flow conditions inside it can be  
Figure 1: (**a**) The Price–Paidoussis Open Return Subsonic Wind Tunnel. (**b**) Pitot tube location (1) and aerodynamic scale location (2). monitored [21]. The main parameters to describe the flow in a wind tunnel are the total pressure, the static pressure, the dynamic pressure, the temperature, the flow speeds, and the Reynolds number [22]. These flow conditions can be monitored from the instruments inside the test section. The diffusing section consists of a wide angle diffuser, a large settling chamber, a contraction section, and a test section. The flow goes through a series of five filters; the first is a honeycomb-shaped filter, and the other four are nylon square-shaped filters positioned 0.5 m from each other, as shown in Figure 2. The settling section makes it possible to straighten and to reduce the turbulence in the flow to a turbulence intensity value of 1%. Figure 3 shows the different dimensions of the wind tunnel sections. Figure 4 shows the design of the wind tunnel sections with the ANSYS ICEM CFD software. Figure 4 shows (a) the settling chamber section, (b) the inlet location, (c) the inner fluid where the wind model is located, (d) the test section and diffuser, and (e) the outlet section. Figure 4: Geometries of the wind tunnel sections in 3D with coordinates: (**a**) the settling chamber section, (**b**) the inlet location, (**c**) the inner fluid where the wind model is located, (**d**) the test section and diffuser, and (**e**) the outlet section. Figure 3: The wind tunnel dimensions in cm. Figure 2: The wind tunnel sections with coordinates. The S45 is an unmanned aerial vehicle (UAV) manufactured by Hydra Technologies, a company based in Mexico. It serves in Mexico as a surveillance platform for the military, law enforcement, and police agencies. The UAV S45 incorporates a monoplane design with twin tail booms. The airframe is made of lightweight materials, such as carbon fiber, S-glass, and Kevlar. The UAV S45 has the following dimensions: wingspan: 6.11 m; half wingspan: 2.70 m; length from the frontal rotor to the rear rotor: 1.93 m; and tail span: 1.36 m. Its length from the frontal rotor to the tail is 3.094 m, and its wing surface is 2.72 m\({}^{2}\). The UAV's materials are aluminum, titanium, and a composite material (carbon fiber, S-glass, and Kevlar), and its weights are as follows: the operating empty weight (OEW) is 55 Kg; the maximum takeoff weight (MTOW) is 69.4 Kg; the maximum landing weight is 66.50 Kg; and its maximum payload, which is its carrying capacity, is 3.89 Kg.