On the left, the stability region of the energy |$E_s(\alpha )$|⁠, while on the right the stability region of the dimensionless stiffness |$K(\alpha , E_s)$|⁠, determined by (3.16) and (3.17), with |$\theta _d^*=1.5$| for varying angles of attack |$\alpha $| from |$10^{\circ }(=\pi /18)$| to |$30^{\circ }(=\pi /6)$|⁠. The upper and lower limits are, respectively, |$E_s^+$| and |$E_s^-$| on the left, and |$K^+=K(\alpha , E_s^+)$| and |$K^-=K(\alpha , E_s^-)$| on the right. The region of stable fixed points, with eigenvalues |$-1<f^{\prime}(Y^*) < 1$|⁠, is marked in grey. The region of unstable fixed points with eigenvalues |$f ^{\prime}(Y^*)>1$| is marked in blue. |$f^{\prime}$| is the eigenvalue notation used in the legend.