The S-70 flight control system can be explained in 4 parts.
1. The S-70 incorporates an irreversible hydraulic flight control system. As such there is no direct mechanical control of the rotor systems without hydraulics. The system incorporates a mechanical mixing unit that provides collective to yaw coupling. I believe that it also provides collective to pitch and roll coupling as well. “John and Nick help me here”!
2. Since the basic system is dynamically unstable, a Stability Augmentation System (SAS) is provided to provide basic rate referenced limited dynamic stability. Control is limited to plus or minus 10% of total flight control authority by what Sikorsky terms inner loop control. Inner loop control is provided in the AFCS without moving the cockpit controls. As such it is transparent to the pilot.
3. On top of this, the S-70 is equipped with varying levels of autopilot through the trim system. Basic S-70s have attitude and heading retention as functions of the trim system. This works as an outer loop function and as such provides control by moving the flight controls. Trim control movement rate of input is limited to 10% of total control per second but can move the controls full throw. Trim can be easily overridden by the pilot. Advanced models of the S-70 also incorporate Airpeed hold, altitude hold, hover position hold and automatic approach departure functions.
4. The S-70 also incorporates a digitally controlled Stabilator that provides for improved maneuver stability and a more level fuselage attitude during approach to hover.
I hope this helps. I am sure John D and Nick N may be able clarify any discrepancies and add more details and background.
