The effect of the direction of application and magnitude of loads on the stability and natural frequency of flexible rotors is analyzed, when the rotors are subject to non-conservative torque and force. The stability criterion derived from the energy and variational principle is discussed and a general Galerkin method in which admissible functions are used is employed for numerical analysis. Illustrative examples are treated to demonstrate the analytical developments and the practical relevance of the work is discussed. (C) 1996 Academic Press Limited