In this post I describe an adaptive trim system which makes it easy to achieve perfect pitch trim at all crow settings.
Using the adaptive trim, the elevator trim is repurposed so that it bends the crow compensation curve. The process is completely transparent - no special action is required during the flight. 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.
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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 achieve an accurate landing.
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.
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| Crow/elevator 'compensation' curve on author's Stribog |
All fine, except that traditionally curves can only be 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 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. And not just to bend it, but to bend it intelligently.
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 control!
The second element is the Lua script. Its job is to monitor the state of the trim buttons. When a click is detected, the script reads the crow value, identifies the nearest point(s) on the compensation curve, and moves them up or down. (In terms of the Lua code, the script reads the curve, modifies it, and writes it back.)
The rest is simple: the compensation curve is applied to a regular crow-to-elevator mix, the output of the mix being the required elevator delta.
With minimal crow (up to around 4%) the system emulates a regular trim lever
(limits = +/-25% of full travel). Beyond that, 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 'Curves' menu, in real time.
In addition, the script generates a custom telemetry screen which displays (a) the compensation curve, (b) the crow value and (c) the active points. The screen is just for information - there is no need to look at it whilst flying.
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| Telemetry screen (X9D, X9D+, X9E). Vertical bar is crow value. |
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OpenTx Widget (X10, X12S) |
Using the system
To optimise the curve, the pilot applies varying amounts of crow and adjusts the trim as needed. A couple of passes through the entire crow range will normally be required in order to fully optimise the curve.
With slope soarers and F5J models, optimisation can be completed 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 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.