Synchronous averaging requires an accurate phase reference (tachometer) signal. In helicopter transmissions, such a signal can usually be obtained or derived from an engine or main rotor gearbox accessory, but is not as readily available for the tail rotor gearbox. This report examines whether a separate tail rotor gearbox phase reference signal is necessary by investigating the relative jitter, due to dynamic tail drive shaft twist, between phase reference signals obtained from the main and tail rotor gearboxes in a S-70A-9 Black Hawk helicopter
