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Aerodynamics of an airfoil with plain flap in presence of momentum injection Triplett, Benjamin I.

Abstract

The concept of Moving Surface Boundary-layer Control (MSBC), as applied to a NASA LS(1)-0417 airfoil with plain flap, is investigated through a planned wind tunnel test-program at a subcritical Reynolds number of 10⁵. The airfoil carries two rotating cylinders for momentum injection. One is located at the leading edge of the wing and the other a the leading edge of the flap. For conciseness, they are referred to as wing cylinder and flap cylinder, respectively. High speed rotating cylinders controlled the key momentum injection parameters, Uw/U and Uf/U. Here Uw and Uf are the surface velocities of the wing and flap cylinders, respectively, and U is the free stream wind speed. Experiments are conducted to characterize the performance of both the two-dimensional (i.e. infinite aspect ratio, 2-D), and of the three-dimensional (i.e. finite aspect ratio, 3-D) cases. The 2-D model airfoil with leading edge momentum injection demonstrated the effectiveness of the concept. The maximum lift of the airfoil was increased by 177% with a delay in stall from 10° to 50°. In general, the airfoil performance improves with an increase in Uw/U. The momentum injection at the flap enhanced the effect of flap deflection. With the flap deflected to 45°, the flap cylinder rotation further shifted the lift curve to the left, and increased the maximum lift coefficient from 1.65 to 2.25. On the other hand, there is no apparent change in the stall angle of attack. Furthermore, the flap cylinder rotation increased the magnitude of the suction pressure peak and decreased the suction pressure at the trailing edge, suggesting a narrowed wake. For the 2-D model with a flap deflection of 45°, the combined wing and flap cylinder rotations showed significant improvement on the airfoil performance. The maximum lift of the airfoil in this configuration was 3.6, a 300% increase over the basic airfoil. This is 213% higher lift than that for the airfoil with flap deflection but without momentum injection. The leading edge cylinder delays separation of the boundary-layer, allowing the airfoil to operate at high angles of incidence without stall. The effective wing-camber, introduced by flap deflection, is further increased through any combination of the momentum injection at the wing and the flap. The three-dimensional study indicated that the relative effectiveness of momentum injection at the wing and flap, is the same, i.e. it is independent of the effect of aspect ratio. As well, the effect of aspect ratio is essentially the same with and without the MSBC. These facts should help aircraft designers in implementation of the MSBC in prototype designs. The fundamental information presented in the thesis should serve as a reference and prove useful in further study aimed at aerodynamic design of a wing.

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