  The invariant manifold approach is used to explore the dynamics of a
nonlinear rotor, by determining the nonlinear normal modes, constructing a
reduced order model and evaluating its performance in the case of response to
an initial condition. The procedure to determine the approximation of the
invariant manifolds is discussed and a strategy to retain the speed dependent
effects on the manifolds without solving the eigenvalue problem for each spin
speed is presented. The performance of the reduced system is analysed in
function of the spin speed.
