The troll speaks.
When Sikorsky first developed a stability control system it was originally installed in HSS-1 and eventually into the H-34. The automatic stabilization system was comprised of an autopilot originally installed in an F-86 fighter. There were some minor modifications necessary. The aux servo was equipped with electric motors that received signals from rotary variable differential transformers, which were attached to the flight controls (Collective stick, the cyclic stick and the rudder pedals. If any of these controls were used the RVDT would send an input to the auto pilot which in turn would command one or more electrical motors in order to displace the pilot valve(s) on the aux servo which would modify the disc attitude. The cyclic controls would be locked in place by magnetic brakes, which were in turn connected to the controls by force gradient springs. The pilot could make minor adjustments to the cyclic by moving the stick and compressing the springs. When the pilot removed his input force the cyclic would return to the original position.
If the pilot wanted to change the attitude or direction he would press a disconnect switch which cut out the autopilot and energize the magnetic brakes which allowed unfettered movement of the cyclic. By releasing the cutout the auto pilot would reengage and the new course would be maintained. There were similar cutouts on the rudder pedals. When the pilot wanted to set up a new direction (heading) he would move the pedals and in the process cut out the auto pilot allowing the directional gyro to assume and hold the new heading. By removing his feet from the pedals the course would be set.
If the autopilot detected a deviation from the established heading or the attitude of the helicopter it would send a signal to the aux servo returning the helicopter to the preset conditions. When the autopilot made a correction the flight controls would not move.
A similar situation existed in the collective as it had a barometric hold provision. If the pilot wanted to rise or lower he would disconnect the bar alt hold move to the new altitude and reengage the bar alt sensor.
Granted the system was by today’s standards antiquated. It had a lot of problems mostly human induced but the design philosophy was good. Why then have there been so many deviations from this design philosophy. The apache flight control system is similar but with more modern hardware and the EH-101 is a step backward in their selection of the means to maintain stability through the autopilot.
I anyone finds fault with my description it must be understood that I went to Sikorsky ASE school 48 years ago.
