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11. 0 Automatic Trim
The automatic trim is an optional feature that allows the altitude hold system to automatically
adjust the aircraft electric trim motor to keep the elevator forces balanced. The pilot may then
change engine power settings and command climbs and descents without the necessity to
manually re-trim the aircraft.
The trim servo/motor is not provided by Trio as a part of the automatic trim system. It is usually
specified or provided by the aircraft builder / kit provider.
It is a common complaint that electric trim operation is often “too fast” and difficult to adjust
properly. To address this, the EZ series altitude control systems offer a very effective means to
adjust the speed at which the trim motor runs. Adjusting the trim speed is covered in paragraph
3.4 on page 8 and 9.
The drive to the trim motor originates within the EZ Gold Standard (GS) servo. It provides the
drive signal to the trim motor through the 6 pin connector on the GS servo. Pins 5 and 6 are
reserved for this purpose, as shown on the schematic on the previous page.
A means must be provided to allow the pilot to manually adjust the aircraft trim when the altitude
hold system is not engaged. It is also a necessary safety feature to allow the pilot to manually
override the servo trim function if required.
In aircraft that have the Auto-Trim feature, when the servo is engaged trim will automatically be
adjusted. If the servo is NOT engaged (i.e. in the READY mode) the electric trim may be
manually adjusted by rotating the encoder knob.
When the altitude control system is engaged
and commanding the Auto-Trim to run the
electric trim motor, it will display a small triangle
on the LCD display indicating the direction of
the trim correction.
When in the READY mode, and if the Auto-Trim option has been installed, rotation of the
ENCODER knob will allow manual adjustment
of the trim motor servo. “Clicking” the knob
position will result in a very fine adjustment of
the trim servo position, while “slewing” the
ENCODER knob rotation will result in larger
changes of the trim servo position. The LCD
will display as shown. The speed at which the
trim servo slew rate is set is managed in the
setup menus using the TRIM SPEED parameter (see paragraph 3.4.)
The diagrams on the following page show two methods that allow switching between manual and
automatic trim operation.
Option 1 is a manually operated toggle switch that can be positioned in either the “Manual” or
“Auto” position. In the Manual position, there will be no automatic trim available, and in the “Auto”
position there will be no manual trim available. It will require that the pilot remember to reposition
the switch when the altitude hold system is not engaged.
Option 2 shows a method that disengages the automatic trim circuit from the GS servo whenever
the manual trim switch is operated.
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