The Art of the Helicopter John Watkinson - Karatunov.net

58 The Art of the Helicopter
Fig. 2.36 Servo error may be reduced by integration of the error. This is ideal for overcoming the effects of
static loads.
Fig. 2.37 Nested feedback loops. Here a flight director compares the actual aircraft location with the desired
location to produce a navigational error. This is supplied to the autopilot that corrects the course by changing
the attitude. The autopilot’s commands are executed by feedback servos that operate the controls until the new
attitude is correct.
The autopilot loop responds by producing an attitude error that keeps the machine in
the attitude needed to hold the new track. The attitude error is supplied to the powered
control system loop that actually controls the rotors.
The advantage of nested feedback is that each loop is stabilized in its own domain.
The powered control loop is compensated to prevent oscillation or instability in the
rotor systems, the autopilot loop is compensated to prevent instability in the attitude of
the machine and the navigation loop is compensated to prevent the machine following
a serpentine track to the waypoint. A further advantage of the nested loop is that
different parts can be changed. An inertial navigator could be replaced with a GPS
navigator, for example. Also in the case of a failure, only the affected loop needs to be