Nevertheless, if the aspect ratio (AR) between the chord and the span of the wing decreases up to a certain limit value, three-dimensional effects on the wingtip dynamics play a key role. It was also observed by Sheldahl and Klimas that the slope decreases for the AoA greater than 5º - 6º. Their results showed a reduction of the maximum lift coefficient (C Lmax = 0.9 - 1.0) and the stall angle (α stall = 10º - 12º) in comparison to the study of Abbot and von Doenhoff, though the slope of the lift curves as function of α for small AoA were in agreement. Two decades later, Sheldahl and Klimas tested a 2D NACA0012 model for chord based Reynolds numbers between 3.6♱0 5 ≤ Re c ≤ 7♱0 5 and AoA in the range 0º < α< 180º. The minimum C D depends on the roughness of the wing surface. For lower angles than stall, the lift coefficient increases linearly with a constant slope ΔC L/Δα. The maximum C L observed by these authors was 1.1 - 1.6 for chord based Reynolds numbers ranging from 3♱0 6 to 9♱0 6, and with stall angles between 12º and 16º. Abbot and von Doenhoff presented a large experimental data summary for different 2D airfoils. The NACA0012 airfoil has been extensively studied and its aerodynamic features are well known as well as the comparison between numerical and experiments by using Large Eddy Simulation. Hence, C D and C L coefficients involve a whole fluid-structure scenario, giving us an overview of the interaction between the wing and the flow that passes over it. Most of these numerical or experimental investigations were performed analysing specifically the flow behaviour. In finite wings at low Reynolds numbers, drag and lift coefficient variations are mainly due to three mechanisms: Wingtip vortex laminar boundary layer separation leading to the formation of a laminar separation bubble (LSB) and the subsequent turbulent separated shear layer and finally the vortex shedding in the wake behind the wing. The relationship between both forces is determined by the wing cross section aerodynamic features. A wing profile is a surface that might be designed to provide lift force and the minimum drag. This is the motivation of our experimental study: to analyse the aerodynamic characteristics at low-to-moderate Reynolds numbers in order to compare our results with other works with different aspect ratios. There is also a great interest to study 3D shape-change and optimization frameworks where the aspect ratio plays a significant role. Typically, UAVs operate in the range between 50,000 and 150,000 chord Reynolds number. It is also of great interest the aerodynamic performance regarding the maximum lift, lift curve slope, and polar curves. Introduction Recent research in unmanned aerial vehicles (UAVs) deals with experimental measurements in which the wings present different aerodynamic characteristics as the chord Reynolds number or the aspect ratio are varied.
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