The Art of the Helicopter John Watkinson - Karatunov.net

Fig. 7.50 With the autopilot disengaged, the pushrod from the pilot’s controls operates the spool valve upper
right. With the autopilot engaged, the solenoid valve admits pressure to the engage piston which moves up,
locking the pilot’s pushrod to the actuator and connecting the EHV outputs to the hydraulic ram.
possible. The second opens the autopilot select valve connecting the EHV spool to the
main ram.
Now the autopilot can move the control by supplying a signal to the EHV and can
measure the control position using the feedback signal from the LVDT in the ram.
Note that as the pilot’s controls are locked to the actuator body by the engage cylinder,
they will move as the autopilot operates, giving the pilot confirmation that the machine
is literally flying itself.
Two nested feedback loops are involved here. The outer loop consists of the attitude
gyro to produce an error if the machine turns around the stabilized axis for any reason.
The attitude error will compensate for factors such as the moment of inertia of the
helicopter about the controlled axis and will then be fed to the actuator loop. The
actuator loop converts the attitude error into a control movement in such a sense that
the error is cancelled. The actuator feedback loop stabilizes the controls and gives the
servo stiffness against flight loads. The machine will maintain the attitude dictated by
the gyro indefinitely.
When the autopilot is engaged it has full authority. If the pilot tries to move the
controls of a parallel system, the stiffness of the hydraulic actuator will oppose him.
The LVDT will sense an unwanted movement and the autopilot will operate the EHV
to oppose it. Instead the autopilot control panel carries knobs the pilot can turn to give
the autopilot a new reference. Effectively the pilot’s control knob adds an offset to the
output of the reference sensor. For example, if the pilot wished to make a constant rate
turn, he would set the turn control knob to an appropriate angle. This would add an
offset to the roll axis gyro output that the autopilot would sense as an attitude error. The
autopilot would roll the helicopter until the attitude error was apparently cancelled.
Thus only a 5 ◦ bank angle signal from the gyro would cancel a −5 ◦ bank input from
the control knob. The helicopter would indefinitely remain at a bank angle of 5 ◦ and
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