Leading-edge flow reattachment and the lateral static stability of low-aspect-ratio rectangular wings

The relationship between lateral static stability derivative, Clβ , lift coefficient, CL, and angle of attack was investigated for rectangular wings of aspect ratioA= 0.75,1,1.5, and 3 using Stereo-Digital Particle Image Velocimetry (S-DPIV) and direct force and moment measurements. When the product ClβA is plotted with respect to CL, the lateral stability curves of each wing collapse to a single line for CL < 0.7. For CL > 0.7, the linearity and scaling of Clβ with respect to CL is lost. S-DPIV is used to elucidate the flow physics in this nonlinear regime. At α = 10◦, the leading-edge separation region emerges on the leeward portion of the sideslipped wing by means of vortex shedding. For the A 1.5 wings at α > 15◦, the tip vortex downwash is sufficient to restrict the shedding of leading-edge vorticity thereby sustaining the lift of the leading-edge separation region at high angles of attack. Concurrently, the windward tip vortex grows in size and strength with increasing angle of attack, displacing the leading-edge separation region further toward the leeward wing. This reorganization of lift-generating vorticity results in the initial nonlinearities between Clβ and CL at angles of attack for which CL is still increasing. At angles of attack near that of maximum lift for theA 1 wings, the windward tip vortex lifts off the wing, decreasing the lateral static stability of the wing prior to lift stall. For theA = 3 wing at α > 10◦, nonlinear trends in Clβ versus CL occur due to the spanwise evolution of stalled flow.