The Art of the Helicopter John Watkinson - Karatunov.net

260 The Art of the Helicopter
Once powered controls are available, it is then a small step to make them accept signals
from an automatic stabilizing system that can reduce the pilot’s mental workload. The
stabilizing action of the pilot is augmented or even replaced by a signal processor
programmed to respond to disturbances in the way a skilled pilot would. However, the
signal processor has no eyes and needs information about the flight conditions in the
form of input signals. Figure 7.1(b) shows that the aircraft instruments no longer just
have a visual indication, but also output signals representing the current condition. It
will also be seen in Figure 7.1(b) that the outputs of the signal processor can act upon
the powered controls.
In such a machine there are two parallel control paths. Figure 7.2(a) shows that the
control of the machine can be entirely by the pilot, entirely by the signal processor, or by
a combination of both, with the possibility that the amount of contribution from each
control path may also change throughout the flight. This must be the source of further
complexity. It is fundamental to negative feedback that there can be only one overall
feedback loop in a system. Two negative feedback loops around the same process will
fight each other. For example, if the outputs of the pilot’s stick and the processor are
simply added, by fitting what is known as a series actuator in the pushrod, the pilot’s
controls cease to function. Figure 7.2(b) shows why. It is the goal of the processor to
maintain the attitude of the machine. If the pilot applies, say, left cyclic, the machine
will begin to bank left, but the processor will sense this as an error and apply right
bank. The machine will continue to fly straight, with the processor precisely opposing
everything the pilot does. If the pilot releases the stick, the series actuator may succeed
in moving the stick instead of the controls.
In early systems using series actuators, engaging the autopilot must lock the stick in
the neutral position to give the series actuator a mechanical reference against which to
react. The pilot has to disengage the processor if he wants to resume control.
In the parallel actuator system shown in Figure 7.2(c) the actuator moves
the mechanical reference of the stick centring springs. With the signal processor off,
the actuator may perform the trim function. With the signal processor on, the pilot
releases the stick and the actuator moves both stick and controls. This gives visual
confirmation that the system is working, but more importantly the pilot can override
just by moving the stick against the centring springs.
In modern systems there is a more convenient way. Figure 7.2(d) shows that the
processor is permanently on, but the pilot’s stick is connected differently. If the stick
is released, it has no effect and the processor controls the course. However, in order
to override the processor, the pilot simply moves the stick as normal. This generates a
false error that is fed into the processor. For example, if the pilot wishes to bank left,
he moves the cyclic stick left, and this generates a signal that is added to the processor’s
bank angle error to indicate falsely that there is a right bank condition. The processor
acts to cancels that condition by performing a left bank.
Thus it will be seen that the way for the pilot to override a feedback system is not to
attempt to oppose the output, which cannot work, but to modify the reference value
or the parameter the system is trying to hold constant. In this way the system still
stabilizes the override manoeuvre. For example, in the example of the left bank above,
the system now acts to hold the bank angle constant. In the event of gusts disturbing
the bank angle, the system would correct for them. In this way the combination of
control system and real pilot is close to the ideal.
Figure 7.1(b) showed a simple stability augmentation system in which the goal is to
reduce the naturally divergent behaviour of the helicopter as well as external disturbances
from gusts using a parallel actuator. The reduced pilot workload may make
single pilot instrument flying possible. Clearly the stability augmentation system must