THE YAH-64 EMPENNAGE AND TAIL ROTOR - A TECHNICAL HISTORY

Since the original design in 1973, several configuration changes have been made to the horizontal and vertical stabilizers and to the tail rotor of the YAH-64. These changes have been made to solve various technical problems which were not evident until the aircraft entered its flight test program. By incorporation of these changes, the YAH-64 meets all the requirements of the flying qualities section of its System Specification. The horizontal stabilizer progressed from a low, fixed position which produced a large, nose-up trim shift at Low speeds to a T-tail which produced a large nose-up fuselage attitude in climb to a movable stabilator which solved both problems at the price of some increased complexity. The empennage change to the stabilator configuration required raising the tail rotor gearbox. This change permitted the diameter of the tail rotor to be increased to provide better performance in right sideward flight (including high altitude operations) where unexpectedly high tail boom side loads were encountered. The maximum pitch of the tail rotor was chosen to satisfy the sideward flight requirement without encountering excessive drive system loads during reversals of hover turns. The vertical stabilizer was cambered to unload the tail rotor at high speed to prevent high stresses. The analysis of the various primary and secondary problems and their successful solutions represent a body of experience which should be of value to those developing the next generation of helicopters.