Analysis to predict whirl instability of a whole engine, including flexible bladed disks, due to both linear and nonlinear destabilizing mechanisms at any speed, not only the critical, is advanced. The basis is the previously published, and now extended, equivalent two-dimensional mechanism (E2dM). Proven through physical logic are the principles: (1) a deflection or slope with a phase angle greater than 90 deg lead in response to an external disturbing force or moment respectively signals instability; (2) response of a system absorbing energy is modulated at speeds below onset of instability. Tactics to exploit these principles through fundamental analyses of response are devised.

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