Crow-aware adaptive elevator trim

Crow brakes are a fantastic aid for achieving accurate landings. However, they also generate an unwanted pitch change and - just to complicate matters - the effect is non-linear. The traditional remedy is to make adjustments to a mixer curve, and it can take several flights to get it right.

In this post, I describe a smart way to adjust the curve, using the regular elevator trim. This means that the pilot can achieve perfect trim in just a single flight.

Quick revision: crow compensation

Crow brakes are at their heart very simple: the flaps go down and the ailerons go up, increasing drag and therefore the rate of descent. 

However, crow brakes also generate a non-linear upward pitching moment. Left uncorrected, the pilot has to juggle the elevator while deploying the brakes, making it difficult to maintain a steady glide path angle during the landing approach. 

The 'compensation' mix

The standard remedy is a crow-to-elevator mix, with a compensation curve to deal with the non linear behaviour. Typically, the compensation curve starts steep and then tails off:

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

Traditionally the curve is adjusted manually. This means landing the model each time, and it can take several flights to get right. Needless to say, it's a tiresome process.

But hey, it's really about pitch trim... wouldn’t it be nice if, intead of adjusting the curve manually, we could bend it in real time, using the regular elevator trimmer? That way, we would be able to achieve perfect trim while flying the model!

It turns out that this is indeed possible.

The Adaptive Trim system - how it works

There are three parts to the adaptive trim system. 

The first part is to repurpose the elevator trim lever as a dumb two-sided momentary switch - easily done by disabling the trim function. Once freed of its normal duties, the trimmer becomes a highly flexible control.

The second element is the trim handler. Its job is to wait for the trim to be clicked. When a click is detected, it fetches the crow value, identifies the nearest point(s) on the compensation curve, and moves the point appropriately. With EdgeTX and OpenTX, a Lua script is needed. With Ethos, this step can be done using logical switches and VAR actions, no Lua required. 

Finally the compensation curve is applied to a regular crow-to-elevator mix. Nothing special here.

With small amounts of crow (up to around 4%) the system emulates a regular trim lever (+/-25% travel) including beeps and repeats. Beyond 4% the limits are relaxed allowing for aggressive compensation.

Telemetry screen

The Lua script solution (for EdgeTX/OpenTX) generates a custom telemetry screen showing the curve and crow value. This is just for pre-flight checks - there is no need to look at it whilst flying. 

Telemetry screen (X9D, X9D+, X9E). Vertical shows the crow value.

OpenTx Widget (X10, X12S)

Optimising the trim

The procedure for optimising the trim is straightforward. The pilot deploys crow incrementally, adjusting the trim as needed. 

Two or three cycles through the full range of crow 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 for OpenTX and EdgeTX is available here.

For Ethos, no script is needed thanks its powerful curve features. Most of my Ethos templates have adaptive trim built in. The Excel documentation provided with each template explains how it works.

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.