Abstract: In this paper the troubleshooting is performed for a certain type of turboprop engine starting under "hot state", in which the blades in one side of the compressor rotor seriously rub the case at a speed near and lower than the idling speed. The measurements of vibration characteristics for both trouble free and trouble engines starting under both "hot state" and "cold state" were made. The measured vibration signals were analyzed with a frequency spectrum analyzer. Based on the comparison between the experimental results and the calculated dynamic characteristics it is believed that the rotor shaft is deformed and bent due to the uneven temperature distribution around the shaft as starting under "hot state . and there exists a critical speed near and lower than the idling speed of the engine . The nonlinear response of the bent rotor appears due to the nonlinear stiffness of the flexible front support of the compressor rotor under the relatively high load, the amplitude rises continually after the rotor passes the conventional critical speed, and then leads to the fault described above. Some suggestions for remedying the frouble were have been given. Such failures are reduced significantly after some of the remedial measures were taken.

Key words: rotor, hot state, critical speed, nonlinear response