Fig. 11. Root locus plots showing the effect of reducing the speed derivative M_u to zero for each of the three locusts upon the roots of the longitudinal equations of motion. The plots are in Argand diagram form, i.e. the real part of the root (n) is plotted along the x-axis and the imaginary part of the root () is plotted along the y-axis. Roots to the left of the vertical black line are stable. The filled circles denote the position of the roots of the system matrices defined in Equations 16; the centres of the triangles denote the position of the roots of the system matrices when M_u=0. (A) Locust `R'. (B) Locust `G'. (C) Locust `B'. Note that reducing the value of M_u causes the divergence mode to move towards stability.