Fig. 13. Root locus plots showing the effect of partitioning the static stability derivative M into components due to pitch attitude (M) and aerodynamic incidence (M_w). Symbols and axes are as in Fig. 11. All additive combinations of M and M_w that are compatible with the measured values of M are shown. Note that the positive real root part of the unstable divergence mode becomes strikingly less positive (i.e. less unstable) as the ratio of M_w to M tends toward zero. (A) Locust `R'. (B) Locust `G'. (C) Locust `B'. In the case of locust `G' the two real roots eventually converge and split into a complex conjugate pair, corresponding to the phugoid mode of aircraft.