Process Improvements for the Ah-64 Tail Rotor Vibration Analysis

The Apache was the first helicopter of its kind to fly with the tail rotor blade offset by 55 degrees as opposed to the traditional 90 degrees. Current balancing methods approximate the rotor system as a traditional 4 bladed, same plane system with different weight sensitivity coefficients for each set of blades. The Apache tail rotor computer model was built using the Rotorcraft Comprehensive Analysis System (RCAS) program. The objective was not to develop solutions for the sensitivity coefficients, but to identify predictive trends that result from the tail rotor blade location. Currently, sensitivity coefficients applied to the adjustment algorithms of the Aviation Vibration Analyzer (AVA) processor are broken down into an outer blade coefficient and an inner blade coefficient. Evidence from the computer simulation results computed in this research showed that the sensitivities depended on the quadrant the phase fell into, not the relationship between the inner and outer blades. That would mean every blade would have two sensitivity coefficients, one for the adjacent 55 degree quadrant and one for the adjacent 125 degree quadrant. Information collected from RCAS provides enough validation that changes should be made in the process and the next steps should be taken to determining more accurate sensitivity coefficients for the Apache tail rotor balancing procedures.