TruTrak Altrak VS - Manual (Page 3)

TruTrak Flight System 1 Altrak Installation & User Guide
January 2009 8300-014 Rev B
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.
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 outer most hole (1 1/2") the
travel on the primary can not exceed 2 1/2", 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 1/2". For such
installations, the drive can be applied to a bell crank at a radius point that moves the desired 2 1/2" of
maximum allowed travel in the outer most hole of the arm.
When there is no way to have a drive point of less than 2 1/2" 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 onto 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).