TruTrak Digiflight Install Manual - Manual (Page 4)

DigiFlight Autopilot Installation Manual TruTrak Flight Systems
March 2002 Printing 2
Pitot and Static Connections ( DigiFlight 200 / 200 VS Only )
All multi-servo TruTrak autopilots require connections to the pitot and static lines. The preferred method of this connection
would be tee fittings near the aircraft’s altimeter. The static line for the autopilot requires due care in its construction, as
excessive lag or insufficient static orifices can cause the autopilot to oscillate (hunt) in pitch. Although there is compensation
within the autopilot sufficient to handle moderate amounts of lag, the importance of a good static port and line cannot be
overstated. In some cases problems can be caused by having a large number of devices (including the autopilot) connected to a
single, insufficient, static port. In other cases, the static line itself is adequate but there are one or more devices connected to the
same line, one of which has a large static reservoir. A simple remedy for this problem if it occurs is a tee-fitting near the static
port, and a dedicated line to the autopilot only. Obviously, an insufficiently-large orifice coupled with large static reservoirs
can aggravate the problems associated with lag.
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 might be generated by certain electrical devices. One known 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 such problems 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 disturbance. Shielding has no effect on this problem;
the only solution is to keep strobe light wiring as far away as possible from any electronics which can be affected by pulsating
magnetic fields.
RFI/EMI considerations
The autopilot programmer is shielded and does not generate any appreciable level of electromagnetic interference. Moreover,
the servo lines (except for power and ground) are low-current and cannot contribute to RF interference. The servo power and
ground lines do have switching currents through them, but so long as there are no parallel runs of servo power and ground lines
with such things as poorly-shielded antenna lines or strobe light power lines, there is no need to shield the servo harnesses.
The autopilot itself has been internally protected from RF interference and has been tested under fairly extreme conditions,
such as close proximity to transmitting antennas. However, it is always good practice to insure that such antennas are properly
shielded and not routed directly over or under sensitive panel-mounted electronic equipment. Most problems in this area are the
result of improper RF shielding on transmitting antennas, microphone cables, and the like. The most sensitive input to the
autopilot is the Control Wheel Switch input. This line should not be routed in parallel with transmitting antennas or other
sources of known RF interference. If necessary, it can be shielded with the shield connection to pin 19 of the autopilot
connector.