Avionics News July 2022 - 51

amplifier and servo. Also, like
the previous systems, the yaw
damper will have an adjustment
to compensate for small errors
in the reference. Unlike the systems
discussed previously, the
reference for the yaw damper
can only be a rate gyro or rate
sensor. In some systems, the
rate gyro is included in the same
box as the amplifier. In many
installations, the yaw damper is
operated by a separate switch
in the control panel, which will
allow the pilot to use the yaw
damper without activating the
rest of the autopilot.
Figure 1 was adapted from
Pennsylvania College of Technology's
computer-based training
unit on basic autopilots. In the
figure is a representation of the
reference, shown here as a turn
and bank indicator. The sumRate
Gyro
Summing
Junction
Amplifier
Servo
and
Rigging
Trim adjust
potentiometer
Figure 2: If the aircraft slips or skids, an error voltage is produced, which is amplified to a value
sufficient to rotate the servo and deflect the rudder.
ming junction, trim adjustment, amplifier and servo are all
shown. Unlike the figures published in previous months,
this figure shows a relatively accurate representation of
the rigging between the yaw servo and the rudder. Shown
in the lower left is a simple representation of an airplane
to help the reader visualize the condition of flight. Figure
1 depicts a condition where the aircraft is in coordinated
flight. In other words, it is neither slipping nor skidding.
Figure 2 depicts an aircraft in a slip or a skid. For our
purposes, whether the aircraft is slipping or skidding is not
important. What is important in this case is the left rudder
needs to be applied to correct the condition. As Figure 2
shows, the yaw damper is moving the rudder to the left.
Momentarily, the aircraft condition will revert to that
shown in Figure 1. Flight instructors teach pilots to " step
on the ball. " This is what is happening in Figure 2, but instead
of the pilot moving the rudder, the yaw damper is doing
so. When a yaw damper is in use, the pilot may never
see the ball move out of center. The autopilot corrects the
condition before it is observable.
For ease of explanation, the systems described in the
previous Theory & Practice columns have used analog
examples. In a modern system, most will be digital. As a
result, reference outputs and their trim corrections will all
be a digital setting rather than a potentiometer adjustment.
Each make and model of aircraft has different flight
characteristics. Rigging configurations, the amount of
pressure required to move the control surfaces, handling
characteristics, and more are all different. As a result,
autopilots cannot be generic. Each autopilot must be
designed, configured and installed specifically for a
make and model of aircraft. An autopilot may be included
in the type design or, in the case of a retrofit, installed
under the authority of a supplemental type certificate.
Engineers have approached customization in different
ways. Autopilots may have different servo part numbers
for different aircraft. Some may turn faster, slower,
or apply more or less torque. The computers may be
different with a different part number for each make and
model. Another customization technique is to build a
standard computer with a special circuit board that customized
the computer for the particular aircraft in which
it is to be installed. Some aircraft may need an additional
accelerometer installed. In today's digital world, some
of the customization can come through variations in the
software used by the system.
Unlike navigation systems, where the pilot interprets
the navigational commands and steers the aircraft appropriately,
an autopilot is taking over the pilot's role of inavionics
news * july 2022 51
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Avionics News July 2022

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