Aerodynamic characteristic comparison of the forward and backward-swept wings

Swept angle can increase positively the drag divergence Mach number of wing which results in dramatic improvement of the maximum speed of modern aircraft. Forward swept wings in fact totally are not utilized at all for its potential aeroelastic divergence issues. However, knowledge on the aerodynamic characteristic of forward swept wings is greatly different among researchers. Simplified models are constructed to compare the influence of swept angle on aerodynamic characteristic of the forward and backward swept wings. For both forward- and backward-swept wing models, the wing planform has a leading- and trailing-edge sweep of -45/45 degree and was untapered. The wing span of 4 meter, in conjunction with a chord length of 1 meter, yielded an aspect ratio of 4. Here flow phenomenon on the forward- and back swept wings are reproduced by solving Reynolds-averaged Navier-Stokes Equations through Computational Fluid Dynamic (CFD) technique in the paper. The numerical calculation used half models of the wings to reduce the computational load efficiently. Based on the same structural grid CFD computation were conducted by FLUENT CFD software at Mach number 0.2 and Reynolds number 4.6E6 based on the mean aerodynamic chord length of the wings. Comparisons of the force coefficients between forward- and backward-swept wings show the back-swept wing generates a higher lift coefficient and a higher lift slope than forward-swept wing before the stalling angle, which means that back swept wing has a higher maximum lift coefficient while in fact a smaller stalling angle than forward swept wings as can be predicted. Meanwhile the recorded data actually shows lift curve of forward-swept wing gradually changed at the stalling angle which reflects its better stall performance than the backward. The drag curves demonstrate that forward-swept wing has a lower drag coefficient than the backward at small angle of attack which benefits from the elliptical lift distribution. However situation changes to totally opposite after AOA 8°mainly caused by the separation at the forward swept wing root which leads to dramatic drop of the z direction momentum flux and the increase of the pressure drag shown in the Cdp figure. The integrals of the forces/momentum flux was carried out to explain how the aerodynamic characteristic presents for both wings. Spanwise lift distribution shows that the forward swept wing is very similar to the ideal elliptical distribution which generate better aerodynamic performance. However with the increase of angle of attack, Y direction momentum flux became bigger and bigger which leads to flow obstruction at the forward swept wing root resulting in significantly drop of the wing root Z direction momentum flux, which means less lift and more drag are produced.