Stability and Control of the YUH-61A

The design of the hingeless rotor YUH-61A helicopter is described from a flying qualities standpoint. The dominant characteristic of the hingeless rotor is its large hub moment capability, which produces an inherent high control power and high angular velocity damping. The design objective for the YUH-61A was to produce an unaugmented helicopter whose stability and control characteristics are satisfactory for IFR flight. A stability and control augmentation system was then added to optimize flying qualities and attain a second design objective which was an overall pilot rating of 2.0. Contribution of various aircraft components to longitudinal stability in cruise flight are presented. The longitudinal short period maneuver parameter, and static longitudinal stability are examined since these represent the largest influence on pitch stability. The beneficial aeroelastic effects included in the YUH-61A hingeless rotor are blade sweep, c.g.-a.c. offset, and control system stiffness. The influence of these parameters is shown. Sizing of the horizontal tail and its use as a controllable surface was based on wind tunnel testing which showed the high dynamic pressure and angle of attack due to rotor downwash at the tail which occurs in transition. By properly controlling the incidence of the stabilizer, hub moments and control trim change can be controlled throughout the forward flight envelope.