Evaluation of Standard Gear Metrics in Helicopter Flight Operation

Six well known gear metrics were computed from vibration signals previously recorded from an OH-58C helicopter under controlled flight conditions. For the pinion gear studied, an optimal number of rotations was found for the average. Stationarity decreased as the number of rotations increased. It is conjectured that the dynamic environment of the helicopter is the primary contributor to the decrease in stationarity that was observed during long averaging periods. The six metrics exhibited a complex relationship with torque and rotor rpm. The relationships of metrics to torque differ from that found in the literature on test rigs. Using thresholds derived from earlier test rig results, several metrics exceeded their nominal thresholds a significant number of times, and most of these occurred during descending forward flight maneuvers.