Abstract: The gas exciting force caused by blade tip clearance may cause rotors unsteady under certain condition. The theory and the characteristics of the force caused by the blade tip clearance are analyzed. Taking the Jeffcott rotor as an example, the influence factors of rotor system with gas exciting force caused by blade tip clearance are discussed through Routh-Hurwitz rule. It is found that the asymmetric stiffness and large damping ratio will both enhance the threshold of stability. A method that can be used to calculate the threshold of instability is put forward and an optimized model is set up in the paper. Threshold of the instability is improved by optimizing bearing stiffness and diameter of rotor system. It is indicated that this method has good results in improving systematic stability by an example.

Key words: gas exciting, Alford force, stability, whirling motion, Jeffcott rotor