Sparis P., Vachtsevanos G.

Abstract: The effects of engine torque variations on the power spectrum of a helicopter transmission are examined using data from tests on the ground and on the test bed. The measurements were performed on healthy and faulty transmissions for a particular serious type of fault namely a cracked planetary gear plate. The analysis also includes preliminary experimental data from a seeded crack that was introduced to investigate crack growth rates, stress-strain distribution, fault diagnosis and prognosis. For the simplification of the mathematics a novel technique for frequency mode analysis is introduced that can considerably simplify the corresponding codes for diagnosis and prognosis. The power spectrum analysis techniques are first applied in the case of the healthy rotor operating under different engine torques. Then it is further extended to include time varying effects due to the presence of a rapidly developing crack that tends to modify the vibration energy distribution at various frequencies. Both FFT transform and wavelet mathematical tools are utilized. Finally, the effect of the presence of the rotor is examined in comparison to the ground tests, which also has a considerable effect on the power spectrum of the helicopter transmission. The preliminary results of the analysis show there are considerable differences between the various cases of power spectrums of the transmission operating under different conditions and crack dimensions, which complicate fault detection and require the development of specialized features for this type of catastrophic helicopter transmission fault.