Vibration Health or Alternative Monitoring Technologies for Helicopters

This research project has aimed to take a fresh look at helicopter main rotor gearbox (MGB) condition monitoring, in light of recent advances in sensing and wireless technology. A review of previous accidents and a failure modes analysis showed no clear patterns of failure in helicopter transmissions and rotors. As a result, the accident to G-REDL was considered as the key case study to address, not least because the main rotor gearbox represents possibly the most challenging environment in which to achieve condition monitoring. A review of existing condition monitoring techniques across aviation and other industries has shown that there are a number of promising approaches available including fibre-optic strain sensors, torque rate sensors and others. However, when considering the specific case of real-time
monitoring of rotating components inside a main rotor gearbox, the range of available technologies, which have been shown to be effective, is limited. Lab-scale testing on a ‘single planet’ type configuration showed that close monitoring allowed outer race bearing damage to be detected, with AE showing a detection advantage over vibration in that configuration. The analysis of these results used adaptive filters, enveloping and spectral
kurtosis to extract defect frequencies from the signals, a more advanced technique than is typically used in existing HUMS systems. In order to support the use of AE inside the gearbox, an analogue, nearfield, wireless transmission system was developed capable of operating in that extremely challenging environment. The system is able to transmit a signal from the sensor with sufficient bandwidth to allow AE analysis to take place, and enough power to condition the signal and run the associated electronics. The phase response is virtually linear, meaning that time signals are correctly represented. A broadband sensor was identified which was able to operate at both typical AE and typical
vibration frequencies, and which was able to withstand the temperature and oil present within the gearbox. The sensor is small and frangible and presents little risk to the gearbox were it to be released into the gears. The wireless system and the sensor were fitted to the planet gear of an operational gearbox and
tested at operational speeds, temperatures and loads. Damage was introduced into the planets gear bearing outer races, in the form of cut-out sections of two different lengths. Analysis of the system output showed an apparent saturation of the sensor, possibly due to the high energy levels at the gear mesh frequency of the epicyclic stage, which cause periods of null response from the system. Despite this saturation, analysis of the signals for the two damage conditions, at three power settings, showed that for all power settings the outer race defect frequencies were clearly visible in the enveloped spectrum when compared with the no damage case. The research programme has shown that in-situ condition monitoring for helicopter main rotor gearboxes is feasible and that it is able to offer detection of incipient damage when traditional external vibration measurements cannot.