Crow brakes in action (Freestyler VI, pilot Mark Redsell)

How to program crow brakes


Crow brakes (also known as 'butterfly') are a really cool feature on full house sailplanes. In this article, I'll explain what they are, and how to program them.

Why crow brakes?

Crow brakes are used to control a glider's rate of descent. At their heart they're very simple: the flaps go down and the ailerons go up, adding drag and making the glide path steeper. Unlike elevator, crow brakes have a minimal impact on airspeed - this makes them perfect for controlling the landing approach.

Crow brakes do, however, come with an unwelcome companion, namely a nose-up pitching moment. Fortunately there's an easy remedy, which is to mix in some down elevator or 'compensation'. The more crow is deployed, the more down elevator is applied.

A crow mix therefore requires one input (your crow lever), and five outputs - two for the ailerons, two for the flaps, and one (or two if it's a V-tail) for compensation.

The Butterfly mix

Ethos offers a mixer for crow brakes, and it's called 'Butterfly'. The input source can be any stick or slider. The outputs are the flap, aileron and elevator channels.

The Butterfly mix provides a solid foundation for your crow setup, but there's still some work to do. The following sections explain what's needed…

Butterfly mix with five outputs

Configuring the input

When configuring the Butterfly mix, it's helpful to consider the input and outputs separately. Let's deal with the input first.

For our example, we'll use the throttle stick as the input source:

Butterfly mix with throttle source.

Adding a curve

This section applies only to Ethos versions 1.2.10 and prior. Ethos 1.2.11 applies a curve internally, saving you the trouble of going through the steps described below. However, the new behaviour is not ideal (the curve is hidden and cannot be edited), and further changes are scheduled for a future release.

There's still some work to do for the input. This is because of the way the crow stick works. When the stick is fully forward, the input has a value of +100% for a crow value of 0%. Similarly, with the stick fully back, the input value is −100% for a crow value of +100%.

The first task, therefore, is to transform the stick range (-100% to +100%) to the crow range (100% to 0%). Impossible task? Well no - that's what mixer curves are for! The simplest curve would have two points (-100,100) and (100,0). But hey, this is Ethos - why not refine this curve, to prevent the brakes from deploying if the stick is nudged accidentally! To do this we insert an intermediate point at (85,0). Only when we pull the stick below the 85% position will crow start to deploy.

So the finished curve looks like this:

Curve editor - scrolled to end, showing curve points. Note: easy mode is disabled to allow custom x-values.

Configuring the ailerons and elevator compensation

With the input done, we can turn our attention to the output channels.

The ailerons and elevator outputs are straightforward to adjust. We apply full crow and adjust the weights of the aileron and elevator outputs to achieve the required movements.

Aileron outputs

Configuring the flaps

The flaps require a little more effort. This is because of their travel is grossly asymmetric, with a large downward deflection and little, if any, upward travel. This means that the servo centre will not correspond to the airfoil neutral. Instead, with the servo in the centre, the flaps naturally fall at around 20 or 30 degrees below neutral.

In theory, you can of course re-map the servo centre position so that it coincides with the airfoil neutral by adjusting Subtrim - but don't! It will build in a large amount of differential into the servo response.

The correct way is to add an Offset mix (new in V1.3.3). It's purpose is solely to bias the servo arm position so that with all other flap mixers disabled, the servo is offset to the airfoil neutral. This offset mix must be permanently enabled since it's a geometry issue, and not specifically for the Butterfly mix.

The parameters of the Offset mix will be as follows:

The weight is adjusted so that the flaps are at neutral when all other mixers affecting the flaps - including the Butterfly mix - are disabled. The offset must be finalised before we continue the crow setup, so that the crow movements can be adjusted relative to the flap neutral position.

Offset mix, with offset set to 27%

Offset mix showing flap outputs

Finalising the flap travel

With the offset mix finalised and adjusted, we can return to the Butterfly mix for the final task, which is to adjust the flap travel. This is done by adjusting the relevant weights:

Butterfly mix showing flap outputs

De-activating crow brakes

By default, the crow mix is 'always on'. However many pilots like to disable the crow function during normal flying. This is easily done by setting the active condition to a switch. With crow disabled, the flaps will go to the correct flap neutral position, thanks to the neutral offset mix which we created earlier.


Refinements you might want to add:

For now, I leave these as an exercise for the reader.