Nonlinear Effects of Vibration Increase in Multi-Rotor Engines

The dependence of the vibration characteristics of gas turbine engines on rotor speeds becomes highly complicated in multi-rotor engines, because of the simultaneous dynamic action of the multiple rotors and the ambiguous relationships between their speeds. In this paper, two and three rotor gas turbine engines are analyzed in the context of the theory of non-linear oscillation as a complex system comprising a large number of nonlinear elements and multiple periodical forces of different frequencies (defined by the rotor speeds). This paper presents theoretical and experimental results, which indicate that the level of vibration by some engines can obtain critical values with climb at certain relationships between the rotor speeds (1:2, 2:3, 3:4, etc.). As a practical application of this phenomena it is shown that the number of three-spool engine returns from the aircraft to the engine manufacturer, due to different kinds of malfunctions, for example due to activation of the "intensified vibration" alarm, may be approximately three times that of returns of analogous two-rotor engines (if the specific constructive and technological precautions are not taken). K e y w o r d s : individual vibration characteristics, rotation speed, two rotor gas turbine engine, three rotor gas turbine engine, relationship between the rotor speeds