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- Aerodynamic Analysis of a Forward–Backward Facing Step Pair on the Upper Surface of a Low-Speed AirfoilPublication . Freitas, Luís Gonçalo Azevedo; Gamboa, Pedro VieiraThe Long Endurance Electric Unmanned Air Vehicle (LEEUAV) is a project of a green, low-cost, small footprint electric solar UAV which was designed for civilian surveillance applications, such as coast, forest, or border patrol. Therefore, long endurance is desired, which is accomplished by a lightweight airframe design and an electric propulsion system assisted with solar cell arrays. The LEEUAV has an approximated mass of 5 kg, 4.5 m of wingspan and an 8-hour flight endurance. To ease the construction, its wing, in addition to its aerodynamic function, must accommodate the solar cells on its upper surface. Since the solar cells have a finite thickness, they create an offset, with a forward facing step at the beginning of the solar cell array and a backward facing step at its end. These two steps affect the aerodynamic performance of the wing mainly because the forward facing step forces the transition of the flow from laminar to turbulent. The aim of this thesis is to study the influence of these steps in the aerodynamic coefficients of the LEEUAV’s airfoil, having as variables the offset’s position, length, and thickness. To do this, a numerical analysis was performed initially using XFOIL, an interactive program for the analysis of subsonic airfoils, and then ANSYS Fluent, a commercial computational fluid dynamics (CFD) software. First, in XFOIL, a total of 444 combinations of offsets were tested, in order to understand the range of positions that could be more appropriate for placing the solar cells. After this initial study, another analysis was performed to better understand their most appropriate positions, considering their precise thickness and length. Afterwards, a small amount of combinations was chosen to perform a CFD analysis that served to validate and refine the results obtained from XFOIL. Since there was no experimental data regarding the LEEUAV’s airfoil, several turbulence models were initially tested to see which best resembled the XFOIL’s solution. Subsequently, an analysis of the selected combinations was performed and the best position for placing the solar cells, based on the performed analysis, is between 21% and 23% of the airfoil’s chord.