AH-64 Tail Rotor Balance Influenced by Stabilator Natural Frequency

The U.S. Army AH-64 Apache tail rotor can sometimes be difficult to balance. At certain tail rotor balance 1/rev vibration amplitudes the tail rotor balance will not respond in accordance with the amplitude and phase coefficients that are programmed into either the Modernized Signal Processing Unit (MSPU) or the Aviation Vibration Analyzer (AVA). This results in the applied corrections not producing the expected results. Instead, when plotted on a polar plot the tail rotor 1/rev vibrations "spiral" in toward the center after multiple balance adjustments. Modal analysis testing of Apache stabilators has identified a rigid body roll-yaw response near the tail rotor 1/rev of 23.6Hz on some aircraft. This paper documents how stabilators with a natural frequency at or near 1/rev can influence tail rotor balance. This paper also shows a simple method to alleviate this problem and perform a successful tail rotor balance.