The dependence of the vibration characteristics of gas turbine engines on the rotor speeds becomes highly complicated in engines with two and three rotors, both because of the simultaneous dynamic action of the multiple rotors and the ambiguous relationships between their speeds. In this paper, the gas turbine engine is analyzed in the context of the theory of non-linear oscillation — as a complex system comprising a large number of non-linear elements and multiple periodical forces of different frequencies (defined by the rotor speeds). This paper presents results, which indicate that the level of vibration can obtain critical values at certain relationships between the rotor speeds. As a practical application of this phenomena it is shown that the number of three-spool engines 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.

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