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

How to program crow brakes

Introduction

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 can be any stick or slider of your choosing. The default 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. We're going to use the throttle stick to control the amount of crow. The first task, therefore, is to assign the throttle stick to the Input field.

Butterfly mix with input curve

Adding a curve

There's still a bit of work to do on the input side. When the stick is fully forward, it carries a value of +100% for a crow value of 0%. Similarly, with the stick fully back, it carries a value of −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%), and this must be done before applying any mixer weights. 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. Easy mode is disabled.

Curve editor - scrolled to end, showing curve points.

Configuring the ailerons and elevator compensation

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

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

Aileron outputs

Configuring the flaps

The next task is to adjust the flap travel, and this requires a little more thought. In fact, this section merits an article on its own, but I'm including it here because of the need for an extra mix.

The problem with flaps is that 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 aerodynamic neutral. Instead, with the servo in the centre, the flaps naturally fall at around 20 or 30 degrees below neutral.

Don't be tempted to fix this by adjusting Subtrim - it will build in a large amount of differential into the servo response. The correct way is to add an offset mix. The purpose of the mix is solely to bring the flaps to the neutral position. The parameters for the offset mix are as follows:

The offset is adjusted so that the flaps are at the aerodynamic neutral when all other mixers affecting the flaps - including the Butterfly mix - are disabled. The offset mix must be 'always on', and it must be adjusted before we finalise the crow setup, so that the crow movements can be adjusted relative to the flap neutral position.

Flap offset mix, with offset set to 15%

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

Refinements you might want to add:

Both are beyond the capabilities of the stock Butterfly mix, and for the time being I leave these as an exercise for the reader.