A generalized dynamic balancing procedure for the AH-64 tail rotor

The tail rotors on the AH-64A Apache and AH-64D Longbow Apache incorporate a unique design, which includes two, two-bladed teetering rotors that have an azimuth spacing of 55°, instead of the more usual 90°. Maintainers have observed that some Apache tail rotors can be extraordinarily difficult to balance dynamically. This investigation uses RCAS (Rotorcraft Comprehensive Analysis System) numerical simulations of tail rotor response when mass is added to the tips of single and adjacent blades to investigate possible causes for this balancing difficulty. The simulations show that the 1/rev, vertical, vibratory force response due to added tip mass varies as a function of the mass distribution between two adjacent blades, and the azimuth spacing between the two blades. As a result, the tail rotor balance sensitivity coefficients, if used as for a single blade, will be inaccurate; and may be a prime contributor to the problems observed while balancing tail rotors. An analytical model of the AH-64D tail rotor, with characteristics similar to the RCAS model, and which incorporates the influence of structural impedance through the balance sensitivity coefficients and phase angles, is used to develop a method for accurately determining the amount of tip mass required to reduce the 1/rev vibrations to acceptable levels.