Tuesday 11 January 2022

Supertrim - a better interface for trimming crow brakes

In this article, I describe an improved user interface for trimming out crow brakes. It uses the standard features of OpenTX - no Lua script is required.

The two components of pitch trim

Crow brakes are great for losing height. However they affect the trim of the model. Normally this manifests itself as a nose-up pitching moment which increases as crow is deployed. 

To counteract this, we employ a crow-to-elevator mix, commonly know as compensation. So, on a model with crow brakes, there will be two components to pitch trim:

  • A constant offset, adjusted with the brakes off, using the elevator trim. 
  • A crow-dependent compensation, adjusted with brakes fully deployed. The mix is typically adjusted using the throttle trim. 

The problem is that we have two trim levers (elevator, throttle), both affecting the models pitch trim. It's easy to forget which one to use.

A better way: Supertrim 

With 'Supertrim only the elevator trim is used

The function of the trim changes automatically:

  • When brakes are off, adjusts the base trim
  • When brakes are deployed, it adjusts the crow compensation.

The procedure for trimming is then very simple:

  1. Zero crow, adjust trim
  2. Full crow, adjust trim

Final tweaks can be made by editing a compensation curve.

How it works

The OpenTX code is described, together with a demo file, in OpenTX Clinic.

Inclusion in Pro templates

Supertrim has been flight tested on my Sting using a modified version of my F3F setup, and it works very well.  It's not as flexible as my Crow aware adaptive trim system, however the latter involves installing a Lua script.

Supertrim is implemented in my latest DLG, DLG6S and F3F templates for OpenTX. Where possible, it will also be implemented in future versions of my other OpenTX templates.

[edit 19 Feb 2022: I'm working on an F5K template, with Supertrim for both motor and brake compensation. It should be very easy to trim!]

After flight test of Supertrim, at Ivinghoe Beacon

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