Supertrim

Supertrim provides an easier way of adjusting crow compensation. While not as flexible as my Crow aware adaptive trim, it does not require a Lua script.

Background

Crow brakes are cool! However, they tend to generate a nose-up pitching moment which, if left uncorrected, can make landings tricky.

The standard fix is a crow-to-elevator mix, commonly called crow compensation. The more crow, the more down elevator is added on top of the 'base' (regular) trim.

The rate of compensation is typically adjusted using a spare trim. However many radios don't have a spare trim. Worse, the pilot has to remember which trim to use (the regular 'base' trim, or the compensation trim). Wouldn't it be nice if both the base trim and compensation mix could both be adjusted with the regular elevator trim?

Fortunately, it's quite easy to do in EdgeTX. Welcome to Supertrim!

Supertrim

We saw above that there are two trim components: the base trim (the native trim function) and the compensation rate (for the crow->elevator mix).

Supertrim dynamically repurposes the elevator trimmer, so that it adjusts one or other component depending on the position of the crow stick.

• With the brakes off, the base trim is active and adjustable. Compensation is neither active nor adjustable. This corresponds to the default behaviour we know and love.
  Total trim = 'Base' trim (adjustable)
• With the brakes deployed, the base trim is active, but not adjustable (it's frozen at its last value). Compensation is active and adjustable.
  Total trim = 'Base' trim (held) + Compensation (adjustable via trim, and increasing with crow).

In practice, the pilot simply hits the elevator trim when needed. Pretty simple, eh?

How it works

The magic of AdjustGV

Supertrim depends on a cool feature of OpenTX whereby a trim can be repurposed dynamically via the AdjustGV special function.

• When the sf is active, normal behaviour of the trimmer is suspended. The trimmer updates the GVAR instead of the native trim value.
• When the sf is inactive, the GVAR value is held. Normal trim behaviour is restored.

With Supertrim, GV1 is used to store the compensation rate.

Using AdjustGV to implement Supertrim

Here's a simple implementation within a single flight mode. The trim behaviour is controlled directly via the crow stick.

How it works :

• L1 detects if crow is active, and is used to activate special function SF1.
• SF1 controls the behaviour of the elevator trim:
  • When crow deployed (L1=True), SF1 is enabled: trim updates GV1. Native trim is held at its last value.
  • When crow idle (L1=False), SF1 is disabled: trim updates native trim. GV1 (compensation) is held at its last value.
• GV1 stores the compensation value. The range is defined by Min and Max (set in the GVARs menu). Suggested range 0% to 40%.
• CH2 is the elevator channel. It has two mixes:
  • 1st line: elevator input + base trim.
  • 2nd line: crow compensation. The range is twice that of GV1, that is 0 - 80%. When crow is off (throttle stick fully up), the output is zero.

During flight tests, the brakes should be fully deployed before adjusting the compensation.

Adding a compensation curve

Compensation is usually non-linear, so a curve should be included in the compensation mix. Nothing special here.

Use with flight modes

The code above assumes uses a single flight mode. However, the technique is well suited for multiple flight modes.

A typical solution will have separate Crow and Cruise modes, with the base (native) trim shared between the two. In Cruise mode the elevator trimmer will adjust the base trim, while in Crow mode it'll adjust the compensation. For an example see the demo file below.

Note: For more info on sharing trims see Advanced trims.

Integration in RC-Soar templates

At the time of writing, Supertrim is integrated in the latest DLG, DLG6S and F3F templates.

Download

Links

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