Manual Electric Trim (MET)

How it works

The Electric Trim control is a "split" switch located on the control wheel. This allows the plane to be trimmed by your thumb, not using the trim wheel, located on the pedestal. Moving the switch up or down activates pitch trim servo which also results in movement of the trim wheel.

When the autopilot is engaged it controls the pitch trim servo, when is not engaged, electric trim may be freely used to command the pitch trim servo motor instead of autoplilot. If trim switch is moved while autopilot was engaged, it automatically disengages the AP (roll and pitch), not affecting the FD.

Why it's split switch

You always should move two switches to control the trim servo, moving just one side of the switch shouldn't actuate the trim. If you hold one side of the split switch down for more than several seconds (depending on aircraft), the trim switch will automatically become disabled.

Trim switch are split by two parts for safety. If it was a single switch you could accidentally bump the trim switch with your thumb at the very wrong time (and disengage AP). Also, a switch could getting stuck and lead to fast dangerous overtrim.

The left switch is the "ARM" contact, it can be a simple safety contact or electromagnetic clutch control switch for trim motor, the right switch controls the UP and DN contacts to control the DC motor direction.

RSC/SimVimX MET functionality and configuration

Custom RSC/SimVimX functions allows you to make ET in your cockpit the same way as in real aircraft.

  • With two-way trim switch you can control the DC motor that rotates the pitch trim wheel.
  • If the Trim Motor outputs are assigned, the Encoder rotated by the motor controls the virtual trim.
  • Else, if you don't use the Trim Motor the ET switch controls the virtual trim commands directly.
  • As soon as you slide the trim switch while autopilot is on, the autopilot disengages.
  • The same DC motor is used for autopilot pich trim commands to rotate the pitch trim wheel.
  • When the trim motor is controlled by autopilot the encoder input connected to the wheel is deactivated to prevent the "control loop".

A simple and obvious trim switch implementation is using the right switch as up/down trim commands actuator, and the left "ARM" switch as a circuit interrupter for the "command" switch, to prevent controlling the trim with right switch only:

As you can see, to make these two-way switches you can use either a pair of momentary buttons (left circuit on the picture) or spring-loaded 3-position rocker switch (right circuit).

To assign the electric trim function in the configurator find the "Electric Trim UP / Dn" parameter (Elec_Trim) for 3-position (or two button) switch.

Trim Wheel, DC Motor Control

To control DC motor to rotate trim wheel, two output SimVimX signals are used in RSC Interface . You need to assign two parameters in the configurator (the "Trim_Motor_Up" and "Trim_Motor_Dn") to any couple of output pins or driver/register outputs.

These two outputs you need to connect to any suitable DC motor driver (H-Bridge circuit board, such as L298N etc). You need to add a gear reducer to get desired rotation speed for your wheel.

For example, this board can drive two DC motors. You need only one side (Motor A for example). Open the layout Map "Flight Controls" in the configurator and assign the digital outputs "Trim_Motor_Up" and "Trim_Motor_Dn"

Connect the "Trim_Motor_Up" output to Input1 and "Trim_Motor_Dn" output - to the Input2. Use appropriate voltage source for your DC motor (12v or either from 5up to 35v depending on the motor). No need to connect 5v input, but connect the GND of course.

To be able to rotate the wheel by hand, you need to make a simple friction clutch (discs with some friction material) that does not prevent you from turning the wheel when the motor is off, but has enough friction to allow the motor to rotate the encoder.

In more "advanced" implementation (see the note above for the "Electric Trim ARM" parameter ) you can have some electro-mechanical clutch that activates only when electric trim (using the left switch) or AP pitch trim is working (using the AP On signal).

RealSimControl / SimVim / SimVimX 2012 - 2022