An experimental investigation of the wake of an Unmanned Aerial Vehicle (UAV) model using flow visualization techniques and a 3D Laser Doppler Anemometry (LDA) system is presented in this work. Emphasis is given on the flow field at the wingtip and the investigation of the tip vortices. A comparison of the velocity field is made with and without winglet devices installed at the wingtips. The experiments are carried out in a closed-circuit subsonic wind tunnel. The flow visualization techniques

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... nclude smoke-wire and smoke-probe experiments to identify the flow phenomena, whereas for accurately measuring the velocity field point measurements are conducted using the LDA system. Apart from the measured velocities, vorticity and circulation quantities are also calculated and compared for the two cases. The results help to provide a more detailed view of the flow field around the UAV and indicate the winglets' significant contribution to the deconstruction of wing-tip vortex structures. Aerospace 2017, 4, 53 2 of 17 appearance of the induced drag term [8] . Due to the importance of the induced drag to the total drag force budget, a lot of effort has been made in the previous decades to minimize this component and pertain the wingtip vortex. The most notable achievement is probably the winglet concept, introduced by Whitcomb [9]. Having been studied by various researchers over the years, for issues that have to do with aerodynamic and structural issues, using both experimental and computational tools for design and optimization purposes [10] [11] [12] [13] [14] , it is essentially a sophisticated type of wingtip fence that alters the flow at the tip in a manner that increases aerodynamic efficiency and reduces the size of the tip vortex. Aerospace 2017, 4, 53 2 of 17 drag to the total drag force budget, a lot of effort has been made in the previous decades to minimize this component and pertain the wingtip vortex. The most notable achievement is probably the winglet concept, introduced by Whitcomb [9]. Having been studied by various researchers over the years, for issues that have to do with aerodynamic and structural issues, using both experimental and computational tools for design and optimization purposes [10] [11] [12] [13] [14] , it is essentially a sophisticated type of wingtip fence that alters the flow at the tip in a manner that increases aerodynamic efficiency and reduces the size of the tip vortex.