Mechanical Considerations
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
.
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 the servo travel should be very nearly the same in both directions. In most cases this
means that the servo arm needs to be perpendicular to the push rod but there are exceptions such as the RV-4 and
RV-8 installations.
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 the servo drives the push rod the same distance in both directions,
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 nicopress 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).
Magnetic Considerations
Because the autopilot contains a built-in magnetometer for a backup source of heading in the event of GPS loss, it is important
to try to locate the programmer away from known sources of magnetic disturbance. The calibration procedure can account for a
moderate amount of fixed disturbance (for example, nearby iron objects) but it cannot adjust for changing magnetic fields such
as would be generated by aircraft compasses or certain electrical devices. One such source of such problems is the “Flag”
mechanism in some older DG or HSI devices. These units use a solenoid to hold the flag out of sight, and the magnetic field
will then change when the flags come and go. If at all possible, place the autopilot so as to be as far as possible from such
devices. A hand-held compass can be used to assist in finding fixed or variable disturbances prior to installation of the
autopilot. Even a few inches can make an appreciable difference in the magnetic disturbance level. It should be noted also that
strobe light controls generate very strong currents in their wiring, thus they will create a periodically pulsating magnetic field
TruTrak Flight System Digitrak Installation & User Guide
2 October 2002 Printing
