TruTrak Digiflight Install Manual - Manual (Page 10)

DigiFlight Autopilot Installation Manual TruTrak Flight Systems
March 2002 Printing 8
The next step in the check-out procedure is to verify that the serial input from the GPS receiver is being properly received and
interpreted. With the aircraft outside of any building, power up the GPS panel-mount receiver and the autopilot. After the GPS
receiver acquires its position, the autopilot will begin to flash the “*” character once per message from the GPS unit showing
that valid position data is available. The display will still show MAG followed by a flashing “*” character, followed by the
present approximate magnetic heading. If no “*” is displayed even after it is known that the GPS unit has a position fix, the
problem must be diagnosed. Possible reasons for such a problem are,
" Pin 25 on the connector is not wired to a source of RS-232 serial data
" The GPS receiver’s baud rate disagrees with that selected within the autopilot
" The GPS receiver’s serial output port has not been properly configured to provide the information
The remaining adjustments relate to the dynamics of flight and compensation of the magnetic backup system in the autopilot.
DigiFlight 100 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 activity adjustment (LAT ACTIVITY) determines how briskly the autopilot responds to roll disturbances. The setting can
be adjusted, in flight, over a wide range; thus the autopilot can be tailored to adapt to any aircraft installation.
The activity adjustment 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.
On the first flight, manually fly the aircraft to a suitable area for testing. Engage the autopilot by pressing ENTER. Observe
that the SEL field now shows the captured present ground track (shown after TRK on the display).
Press and hold the knob for about eight 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. Continue to press ENTER until the main
flight screen is back on the display.
Once satisfactory results are obtained in flight dynamic settings, the back-up magnetometer of the autopilot should be
calibrated. Proceed to “Magnetic Calibration” for instructions on this operation.