Supertrim

Mike Shellim 9 June 2022

Supertrim is a programming technique which supercharges the elevator trim lever. It provides a smarter, more intuitive way of trimming out crow brakes.

While not as flexible as my Crow aware trim script, Supertrim is programmed using the standard features of OpenTX - no Lua scripts needed.

Why Supertrim?

Traditionally, two separate trims are needed if using crow brakes: first, the regular elevator trim is used to adjust the base trim; and a second (spare) trim adjusts the amount of crow compensation.

However there's a problem with this: the pilot must decide which trimmer to use, depending on whether the brakes are retracted or deployed. It's easy to make a mistake and move the wrong trim. Worse, the error may not be apparent until it's too late.

With Supertrim, only the elevator trim is used, its function changing automatically:

The result is a safer, more intuitive user interface.

Requirements

In order to implement Supertrim, the following are required.

Code

Supertrim depends on a neat feature of OpenTX, whereby a trim can be repurposed in real time to update a GVAR. While the GVAR is updated, the base trim is held at its last value.

Here's an example implementation. The throttle stick is used for crow brakes, pull down to deploy.

GVARS

GV1 - min=0%, max=40% -- stores compensation value

 

CH2:elevator

+= Source=I2:Ele Weight=-100% -- elevator input + base trim
+= Source=Thr Weight=+GV1 Offset=−GV1 -- compensation

 

Logical Switches

L1 Thr < 100 -- true when brakes deployed

 

Special Functions

SF1 switch=L1 - Adjust GV1 Source=TrmE -- trim updates GV1

Here's how it works:

Adding a compensation curve

The amount of compensation is normally non-linear with respect to crow, so a curve will usually be included in the compensation mix. Nothing special here.

Use with flight modes

The example code above assumes a single flight mode. The technique can also be used in setups with multiple flight modes including a dedicated 'crow' mode. In this case, the base trim in crow mode should be shared from another flight mode. Sharing of trim settings between flight modes is configured in the Flight Modes menu. For more info see Advanced trims.

Demo file

A demo can be dowloaded here (X9D profile). If using the Companion sim, note that dragging the trim handle doesn't work when adjusting compensation. Instead, click on the arrowheads at each end of the trim sliders.

Integration in Pro templates

At the time of writing, Supertrim is integrated in the latest DLG, DLG6S and F3F templates. The plan is to include it future versions of the remaining templates, where applicable.

Links

For more info, see Advanced trims, in particular the section "Using a trim to update a GVAR".