Planform and flexibility on lift characteristics for flow over low-aspect ratio wings

Force measurement and surface oil flow visualization were carried out in wind tunnel to investigate the effects of wing's planform and flexibility on aerodynamics for flow over low-aspect ratio wings. It is found that the empirical formulas proposed by Polhamus can present remarkably accurate predictions before stall for both rigid and flexible wings. The spiral vortex is responsible for rigid and flexible wing abrupt drop of lift coefficient after stall when the sweep angle is below 35°. The maximum lift coefficient and the stall angle for flexible wings are smaller than those for rigid ones when the sweep angle is below 40°, and the wing flexibility nearly does not influence the aerodynamics as the sweep angle is beyond 40°. Moreover, lift coefficient curves exist hysteresis loop in the following range of sweep angle: 10°-35° for the flexible wings and 5°-50° for the rigid wings. It can be deduced from oil visualization experiment that the sudden disappearance of spiral vortex flow structure may be responsible for the hysteresis.