Boundary-Layer Flow Effects on Aerodynamic Performance of Forward-Swept Wings

The effects of a forward-sweep angle ($\phi$), Reynolds number ($\mathsf{R}$), and angle of attack ($\alpha$) on the boundary-layer flow and aerodynamic performance of a finite forward-swept wing (NACA 0012 airfoil) were experimentally investigated. The Reynolds number and the forward-sweep angle were $2.5\times {10}^4<\mathsf{R}<{10}^5$ and $0°\le \phi \le 45°$, respectively. The chord length was 6 cm, and the semiwing span was 30 cm; therefore, the semiwing aspect ratio was 5. The boundary-layer flow structures were visualized using the surface-oil-flow technique. Seven boundary-layer flow modes were identified by changing $\mathsf{R}$, $\alpha$, and $\phi$: separation, separation bubble, secondary separation, leading-edge bubble, three-dimensional ($\phi =0°$)/bubble-extension flow, and bluff-body wake modes. Furthermore, a six-component balance measured the aerodynamic loadings. The aerodynamic performance related closely to the boundary-layer flow modes. The dimensionless bubble length ($L_b/C$) indicated that a high forward-sweep angle conducted a longer bubble length than that at a low forward-sweep angle. The pseudostall occurred in the bubble-extension mode, while $\phi =30°$, and ${\alpha }_{\mathit{stall}}$ of 19.5° at $\phi =45°$ was about 1.16-fold larger than that of 9° at $\phi =0°$ as $\mathsf{R}=8.0\times {10}^4$. Namely, the forward-sweep angle retards the stall.