Variations Observed in the AC Generator Signal Period of a Sea King Helicopter

摘要

The AC generator in the Sea King helicopter can be used as a convenient source ofa tachometer signal from the main rotor gearbox, thus enabling synchronous vibration signal averages to be calculated for every shaft in the gearbox. The signal averaging software developed by ARL determines the shaft speeds by measuring the tachometer signal period. Variations in the tachometer signal period not caused by changes in gearbox speed will affect the accuracy of the vibration signal averages. An examination has been made of the AC generator signal of a Sea King helicopter recorded on a Bruel Kjaer 4 channel FM tape recorder (model 7003). The results show that the signal averaging software measures short term period variations (from one period to the next) in the AC generator signal of up to -2% of the mean period. It was found that while most of this variation probably arises from noise present in the recording environment, a considerable proportion comes from noise introduced through the tape recording and filtering processes.