TruTrak DFC Install Manual - Manual (Page 11)

TruTrak Flight Systems DFC Autopilot Installation Manual
9 May 2006 Printing
Having verified the correct direction of response to the tilt sensor, secure the yaw damper module to the bulkhead. Re-engage
the yaw damper and adjust the leveling potentiometer (which protrudes from the face of the yaw damper module) to stop the
movement of the servo. The aircraft should be on a level surface (with its “ball” centered) for this adjustment. Once the proper
adjustment is done, press the MODE key to disengage the yaw damper, re-enter the lateral setup using the MODE button, and
set YD ACTIVITY value to zero. This insures that the yaw damper adjustments do not complicate the first test flight of the
autopilot. Holding down the ALT button to enter the setup mode, and repeatedly pressing ENTER to advance to the MIN
AIRSPD field, set the minimum airspeed to the desired value for actual flight. This should be an indicated airspeed (in knots)
which is safely above the stall but not below normal approach or climbout speeds.
The remaining adjustments relate to the dynamics of flight and compensation of the magnetic backup system in the autopilot.
First Flight
The first flight should be done after having completed all the setup and testing on the ground. For the first flight, it is important
that the GPS unit is properly functioning with the autopilot, so that the dynamics of flight can be set without consideration of
the calibration of the magnetic backup system. As discussed earlier, when there is proper connection to the serial input of the
autopilot, the display will show a flashing asterisk “*” in the display to the right of the word MAG; once taxi speed exceeds 10
knots, the display will change from MAG to TRK if the GPS unit has achieved a position fix and sufficient groundspeed. If this
does not occur on fast taxi speeds, it is best to diagnose the problem prior to first flight of the autopilot.
The two activity adjustments (LAT ACTIVITY and VRT ACTIVITY) determine how briskly the autopilot responds to roll and
pitch disturbances. They can be adjusted, in flight, over a wide range; thus the autopilot can be tailored to adapt to any aircraft
installation.
Each of the two activity adjustments covers a numeric range of 0 to 12. Unless the value for a particular aircraft is provided by
TruTrak, it is advisable to start with a setting of zero and work up from there. Most installations would ultimately require
somewhat higher settings.
Prior to takeoff on the first flight, synchronize the autopilot’s altimeter to the aircraft’s primary altimeter value. With the
autopilot off press the ALT button once, to show the ALTIMETER SYNC screen (take care not to accidentally enter the ALT
SELECT screen by mistake). Use the encoder knob to adjust the altitude reading to agree. Each click of the knob gives 100
foot increments; to get ten-foot increments, push in on the knob and rotate it. Having set the autopilot’s altimeter to agree with
the primary altimeter, press ENTER to record this value.
On the first flight, manually fly the aircraft to a suitable area for testing. Engage the autopilot using the ON OFF switch.
Observe that the SEL field now shows the captured present ground track (shown after TRK on the display) and the SVS
(selected vertical speed) field shows the approximate present rate of climb or descent in feet per minute. Use the VS UP or VS
DN buttons to set the selected vertical speed to zero. Press and hold the MODE button for a few seconds until LAT ACTIVITY
is shown on the display, along with an underlined value. Rotate the knob to select the value zero (0), and observe the resulting
control movement. Increase the value one setting at a time, taking time to observe an increasing level of control response. At
some point, if too high a setting is chosen, the autopilot will be jittery and over-active. Back the setting down until the autopilot
is responsive but not over-active. It is best if these adjustments are made in conditions of moderate turbulence (the TruTrak
loves turbulence) so as to make it easy to observe the response of the autopilot to disturbances. It will be noted that a fairly
limited range of activity setting will be acceptable; too low a value will result in sluggish response, while too high a value will
result in nervous, inappropriate response. Within this acceptable range there is room for individual preference; some people
prefer a more aggressive autopilot than others. It should be noted that any builder can accomplish this adjustment procedure
and no professional is required.
Once the desired LAT ACTIVITY level is established, press ENTER to store the value.
Next, the LAT TORQUE field is adjusted. Again, it is best that this be done in light to moderate turbulence. The reason is that
more torque is required of the autopilot in turbulence than is the case in still air, because the velocity of the servo is greater as
turbulence requires more rapid servo movement. This means that when activity is set to the high end of the acceptable range, a
higher torque setting will be required.
The reason for setting LAT TORQUE to a setting less than its maximum (250) is to reduce the current draw of the servo and to
make it easier to override the autopilot should the need arise. Manual override is not normally required, as using the control-
wheel switch or the ON OFF button will disengage the autopilot, but it is best to have a setting of torque which can be
comfortably overridden if necessary.
Once the desired LAT TORQUE level is established, press ENTER to store the value.