During the operation of a turbofan engine at part speed, near stall, and elevated inlet pressure and temperature, several vibratory instabilities were excited simultaneously on the first fan rotor. The torsional and bending contributions to the main flutter mode were resolved by using casing-mounted optical displacement sensors. Strain-gage spectra were used to identify other instabilities in the blade-deflection spectra. The characteristics of optical-displacement spectra and their role of monitoring rotor-blade vibrations are discussed.

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