  Flutter of an elastic body of revolution spinning about its axis of symmetry
is prohibited in the subcritical spinning speed range by the Krein theorem for
the Hamiltonian perturbations. Indefinite damping creates conical domains of
the subcritical flutter (subcritical parametric resonance) bifurcating into the
pockets of two Whitney's umbrellas when non-conservative positional forces are
additionally taken into account. This explains why in contrast to the common
intuition, but in agreement with experience, symmetry-breaking stiffness
variation can promote subcritical friction-induced oscillations of the rotor
rather than inhibit them.
