Winglet Dihedral Effect on Flow Behavior and Aerodynamic Performance of NACA0012 Wings

This study investigates the effects of Reynolds number, angle of attack, and winglet dihedral (δ) on the smoke-streak flow patterns, surface oil-flow configurations, and aerodynamic performance of the wingleted wings. The airfoil is NACA 0012 and the winglet dihedral varies from −30° to 135°. The smoke-wire technique was utilized to visualize the three-dimensional flow structures. Furthermore, the effect of δ on the wingtip surface vortex was examined using the surface oil-flow scheme. The wingtip surface vortex was observed on a baseline wing using the smoke-streak flow and surface-oil flow visualization schemes. Moreover, the length of wingtip surface vortex (L_b) decreases with increasing δ for δ > 15° where L_b denotes the major axis of wingtip surface vortex. The maximum L_b/C of 1.2 occurs at δ = 15° which is about 42% higher than that of a baseline wing, where C represents the wing chord length. The high flow momentum expands the wingtip surface vortex toward the winglet when δ < 15°. However, the minimum L_b/C of 0.55 occurs at δ = 90° which is about 34% lower than that of a baseline wing because the wingtip surface vortex is squeezed intensely at high δ. The aerodynamic performance was measured using a force-moment balance. The experimental data indicates that the lift-drag ratio at stalling (C_L/C_D)_stall and maximum lift-drag ratio (C_L/C_D)_max occurs at δ = 90°.