Tutorials : How Trims Work

Mike Shellim 13 Mar 2003

updated 12 May 2008

This section describes

How trims work
How to use the undocumented TRIM->CENT menu to centre your trim levers quickly.

Background: how trims work

The trim position of any control is determined by (a) the position of the trim lever and (b) by an offset which is programmable by the user. Both parameters are displayed in the Control Adjustments screen.

bottom left value (LEVER): shows the position of the trim lever. Zero corresponds to lever at centre.
bottom right value (OFFSET): offset, can be set by the user using the digi-adjuster or +/- keys.

The actual trim is the SUM of the two values. In the example above, the actual trim is -10 + 0 = -10.

Flightmodes

While the LEVER parameter depends only on the trim lever position, the OFFSET value can be programmed. Moreover, you can set different values for different flight modes.

Re-centring the Trim Lever

Afterwards trim flights, you'll probably want to re-centre the trim lever.

Centring the trim lever will cause the value of the LEVER component to change back to zero. In order to maintain the overall trim value, you must adjust OFFSET to compensate. In the example above, OFFSET would be changed to -10. Now when you centre the lever, the applied trim value will be the same as before this operation.

The adjustment of OFFSET can be done manually through the previous screen. Alternatively you can use the TRIM-CENT menu:

The TRIM-CENT menu shows the applied trim value (the sum of LEVER and OFFSET) for each control TA, TB, TC, TD. To centre a trim lever,

Select the flight mode using your flight-mode switch (only necessary if you have set different flightmodes).
Enter the TRIM-CENT menu
Select the control you wish to centre
Press the key again. A confirmatory beep sounds, and OFFSET is adjusted so that the trim lever can be moved to centre.

You can now centre the trim lever.

Note that if you have mulitple flight modes, the procedure is quite complicated. For each flight mode, you have to move the trim to where it was during flight tests, and then repeat the steps above. There is some scope for error here, and personally I favour setting the value of OFFSET manually via the CENTRE menu.

Acknowledgements

With thanks to Peter Bailey.