TruTrak EFIS AP (8300-057C) - Manual (Page 5)

Servo Installation
The installation information in this section is extremely important and must be clearly
understood by the installer. Improper servo installation or failure to observe and diagnose
installation problems prior to flight can result in extremely serious consequences, including
loss of ability to control the aircraft. If there are any questions on the part of the installer it
is mandatory to resolve these questions
prior to flight of the aircraft.
Most modern experimental aircraft use push-pull tubes to drive the primary controls. These tubes
generally have a total travel of 3” or less; therefore, it is best to connect the autopilot servo to the primary
control by the same method. This connection consists of an arm on the servo connected by a push-pull
rod to the primary control. Rod-end bearings are required on each end of the push-pull rod. The servo
arm must not rotate even near to the point called OVER CENTER, the point at which the primary aircraft
control would lock up. Some aircrafts mechanical primary control installations will not allow this to occur
and do not need the servo stops. This is a condition that would result from the servo being back driven
when the pilot operates the controls, or from the servo itself driving the controls to a stop. To protect
against this mechanical stops are supplied with the servos. These stops are drilled so that they can be
mounted at different angles as required (18
° intervals)
In addition to the proper use of the stop it is important to know the amount of travel on the primary
control that the servo can handle. With the push rod connected to the outermost hole (1 ½”) the travel on
the primary cannot exceed 2 ½”, the intermediate hole 2 1/16”, and the inner hole 1 5/8”. It is important
to note that at the neutral point of the control the SERVO ARM must be PERPENDICULAR to the push
rod, and that the stop must be mounted so as to limit travel as near as possible to equal amounts in both
directions. In certain factory-designed installations there may be well-proven exceptions. There will be
installations in which space does not permit the use of the stop. When this is done the aircraft’s primary
control stops must be positive and care must be taken to be sure that at the neutral point the servo arm is
perpendicular to the push rod, and that the travel limits of the servo arm are not exceeded. There are
installations in which the travel of the push-pull tube exceeds the allowable 2 ½”. For such installations,
the drive can be applied to a bell crank at a radius point that moves the desired 2 ½” of maximum allowed
travel in the outermost hole of the arm.
When there is no way to have a drive point of less than 2 ½” or when the primary control is cable-driven
it is necessary to use the capstan-cable servo drive. When this is done the servo should be mounted so
that the 1/16” diameter cable which wraps around the capstan when extended parallel to the primary cable
is approximately 3/16” from the primary cable. If the primary control travel does not exceed 5” the cable
locking pin will be 180
° away from the point at which the cable leaves the capstan. When the primary
control is at the neutral point this means the total cable wrap around the capstan is 360
°. If the primary
control travel is greater than 5” the cable wrap is 720
°and the pin is adjacent to the output point when the
primary control is at the neutral point.
The cable clamps when properly installed will not slip and thus get loose, but it is desirable to NICO press
or swedge a fitting on to the cable so as to provide added assurance that the cable will not become slack.
If the bridle cable is not sufficiently tight there will be lost motion in the autopilot drive. This will result
in hunting (oscillation).
TruTrak Flight Systems 3 EFIS Autopilot Installation Manual
April 2009 8300-057 Rev D