The relationship between lateral static stability derivative, $C_{l_{\beta }}$, lift coefficient, $C_L$, and angle of attack was investigated for rectangular wings of aspect ratio $A\mathrm{R}$ $=0.75,1,1.5,$ and 3 using Stereo-Digital Particle Image Velocimetry (S-DPIV) and direct force and moment measurements. When the product $C_{l_{\beta }}A\mathrm{R}$ is plotted with respect to $C_L$, the lateral stability curves of each wing collapse to a single line for $C_L<0.7$. For $C_L>0.7$, the linearity and scaling of $C_{l_{\beta }}$ with respect to $C_L$ is lost. S-DPIV is used to elucidate the flow physics in this nonlinear regime. At $\alpha ={10}^{\circ }$, the leading-edge separation region emerges on the leeward portion of the sideslipped wing by means of vortex shedding. For the $A\mathrm{R}$ $\le 1.5$ wings at $\alpha >{15}^{\circ }$, 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 $C_{l_{\beta }}$ and $C_L$ at angles of attack for which $C_L$ is still increasing. At angles of attack near that of maximum lift for the $A\mathrm{R}$ $\le 1$ wings, the windward tip vortex lifts off the wing, decreasing the lateral static stability of the wing prior to lift stall. For the $A\mathrm{R}=3$ wing at $\alpha >{10}^{\circ }$, nonlinear trends in $C_{l_{\beta }}$ versus $C_L$ occur due to the spanwise evolution of stalled flow.

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