Crow-aware adaptive elevator trim

In this post I describe an adaptive trim system which makes it easy to achieve perfect pitch trim at all crow settings.

With this scheme, the elevator trim is repurposed so that it bends the crow compensation curve. The trimming procedure is completely transparent - no special action is required, the pilot simply uses the trim as required.

I'm now using this system across all my F3F models. It allows me to optimise the compensation very quickly - even before the first landing of a new model.

[edit: although this article is primarily for EdgeTX and OpenTX, adaptive trim can also be implemented in Ethos v1.5, without the need for a Lua script]

Quick revision: crow compensation

Crow brakes are at their heart very simple: the flaps go down and the ailerons go up, increasing the rate of descent. However crow brakes have an unwelcome secondary effect, in the form of a non-linear pitching moment. Left uncorrected, the pilot has to juggle both the crow and elevator controls, making it difficult to keep the model on a steady glide path during the landing approach.

The 'compensation' mix

The standard remedy is a crow-to-elevator mix, plus a compensation curve to handle the non-linear behaviour.  Typically the curve starts steep, tailing off at mid-crow and maybe increasing again at full crow.

Crow/elevator 'compensation' curve on author's Stribog

All fine, except that traditionally the curve is adjusted manually. This means landing the model each time an adjustment is required, and it can take several flights to get right.

But hey, it's really just about pitch trim... wouldn’t it be nice if we could simply use the elevator trimmer to bend the curve? That way, we would be able to optimise the curve whilst flying the model.

Turns out it is indeed possible.

The Adaptive Trim system - how it works

I've called my solution the 'crow-aware adaptive trim' system. The basic idea is to use the elevator trim to bend the compensation curve.

There are three elements to the implementation. The first is to repurpose the elevator trim lever as a dumb two-sided momentary switch - easily done in OpenTX. Once freed from its normal duties, the trim becomes a highly flexible device.

The second element is the Lua script. Its job is to monitor the trim lever, waiting for clicks. When a click is detected, the script fetches the crow value, identifies the nearest point(s) on the compensation curve, and moves it up or down. (In terms of the Lua code, the script reads the curve, modifies it, and writes it back.)

The rest is straightforward: the compensation curve is applied to a regular crow-to-elevator mix.

With minimal crow (up to around 4%) the system emulates a regular trim lever with range of +/-25% of full travel. With > 4%, the limits are relaxed allowing for aggressive compensation. Beeps and repeats are handled in the script.

Telemetry screen

Changes to the compensation curve are reflected in the regular Curves screen. In addition, the script generates a telemetry screen showing (a) the compensation curve, (b) the crow value and (c) the active curve points. The screen is just for information  - there is no need to look at it whilst flying.

Telemetry screen (X9D, X9D+, X9E). Vertical bar is crow value.

OpenTx Widget (X10, X12S)

Optimising the curve

To optimise the compensation curve, the pilot deploys crow incrementally, adjusting the trim as needed. Two or three cycles through the entire range will normally be required. 

With slope soarers and F5J models, optimisation can be accomplished in a single flight. Thermal soarers may require a couple of flights due to reduced flight time.

Video 

Below is a video showing how the system works, and the result of a flight test in a 40 mph wind. Just to make things difficult, the compensation curve was zeroed before the flight.

Download the script

The Lua script can be downloaded here. [23 Sept 2020 - version 3 supports all transmitters from the X9-Lite to the X12S, and clones].

Acknowledgements

My thanks to Lothar Thole for the initial port to the X10 and X12S. Also to MiamiMike, L Shems, and hisroyaldudeness on RC Groups for their invaluable input.