TruTrak Digiflight Install Manual - Manual (Page 14)

DigiFlight Autopilot Installation Manual TruTrak Flight Systems
March 2002 Printing 12
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 holding the knob for three seconds, then releasing it, 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.
The BAUD RATE setting has already been done before this flight. Press ENTER to skip to the next screen.
The MAG CALIBRATE function will be done after all other operations. Press ENTER to skip to the next screen.
The screen now displays VRT ACTIVITY along with an underlined value. In the same manner as was done for the roll axis,
use the knob to find a setting which results in the appropriate response. Again, too high a value will be jittery or oscillatory
and too low a setting will be sluggish and unresponsive. Having found the desired VRT ACTIVITY setting, press ENTER to
store the value and move to the VRT TORQUE field.
In a manner similar to the lateral axis, rotate the knob to choose a torque setting sufficient to fly the aircraft in light to moderate
turbulence without slipping the servo, yet not so high as to be difficult to override manually. Having selected this value, press
ENTER to store the value.
The MIN AIRSPD setting has already been done before this flight. Press ENTER to skip to the next screen.
The next screen is the STATIC LAG field. It is set to 0 at the factory but can vary between 0 and 2 to suit a particular static
system. The value 0 assumes a static system with very little “lag”; the value 2 assumes a fairly large amount of lag. To
diagnose the lag of a particular system, it is necessary to be in the altitude hold mode of the autopilot, in very still air, in
straight-and-level flight. If these conditions cannot be found during the first flight, leave STATIC LAG set to zero. Press
ENTER to restore the main flight screen to the display, and perform the operation on a later flight.
To determine the correct setting for STATIC LAG, fly in still air in straight and level flight, in altitude hold mode. Observe
whether the altitude appears to oscillate, or “hunt” up and down. If this is the case, it may be caused by several factors, one of
which is the amount of lag in the static system. Increasing the STATIC LAG value to a 1 or a 2 may cure the problem; however
this should be set to the smallest value that satisfactorily flies the aircraft in the pitch axis, as the larger the value the less
responsive the autopilot will be to vertical commands or altitude error. Other possible causes of hunting in altitude hold are
“lost motion” in the servo or control linkages or too low a level of vertical activity setting. If there is hunting in altitude, but the
lag setting does not seem to make any difference, leave it set at zero. Excessive lag in the static system itself can be caused by
undersized static ports, improperly placed ports, long static lines, or especially by attached equipment with large static
reservoirs. The autopilot can be adapted to cover a wide range of static systems, but in truly extreme cases it may be necessary
to provide a separate static line for the autopilot so that other equipment attached to the port does not degrade the autopilot’s
performance.
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.