To: Graviman
The gyro control on the Cheyenne was mounted above the main rotor and was connected to the swashplate via control rods that ran through the hollow main rotor shaft. The swashplat, springs and the power control unit (servo) were mounted underneath the gearbox. The springs you mentioned were there to provide a force proportional to the movement of the servo piston rods.
Although the pilot would cause the gyro to displace inputting a control force to the respective rotor blades the pilot would return the cyclic stick to the neutral position and the gyro due to rigidity would maintain the position commanded by the pilot. The springs would return to their static position and the nutating force to the gyro would return to “0” This is similar to the control input to that of a fixed wing aircraft.
The original design of the Cheyenne incorporated a 90-degree lead angle on the position of the servo in relation to the swashplate. However this caused problems when the rotor blades were redesigned to compensate for a significant weight increase in the airframe. The Cheyenne had a serious problem with rotor blade divergence resulting in the fatal crash of one Cheyenne and another that flew apart in a wind tunnel.
Parker Bertea the designers of the Cheyenne hydraulic system were charged with finding a solution to the problem of divergence. It took well over a year and they came up with a system of push pull rods and electrical sensors running from the rotorhead down to the servo. The mechanical linkage would sense a divergence and signal the servo to modify the pilots input. (**) The system worked beautifully and provided a vibration free ride. However, there were so many single point failure modes that would cause loss of control and by that time the program was cancelled.
(**) The Lynx has a similar system but it is electronic. If this system were turned off in flight the helicopter would fly to the left with forward cyclic. Did someone say 18-degree offset. The Lynx has a 15-degree offset during the rigging procedure.
The gyro was mounted to the rotorhead using a constant velocity joint. This eliminated leading and lagging of the gyro arms.
