Tutorials : Setting Up Crow Brakes
The 4000 provides excellent crow brake facilities, with support for 2, 4, 6 or even 8-servo wings! However there are some tricks and gotchas to be aware of when setting them up. If you're not familiar with mixer concepts please read Mixers Overview first.
The easiest way to implement Crow is to use the built-in Butterfly mixer. The Butterfly mixer has the following control inputs, which are normally set as follows:
- Aileron input - causes control surfaces to move in opposite directions
- Elevator input - control surfaces droop in response to elevator input. Often called 'snapflap'.
- Spoiler input - inner surfaces droop, outer surfaces rise.
- Flap input - camber control on inner and/or outer wing surfaces
So the first task is to assign all the wing servos to the Butterfly mixer in the Assign Servo screen. Please note, for diff to work properly, you must observe the channel numbering order rule.
Cancel Aileron Diff
Set the Differential option to OFF in the Adjust Control -> Aileron. Initially we'll set up aileron travel to be give equal up and down. Diff will be dialled in last of all, this makes set up a lot easier as there is one less thing to worry about.
We can now set up some curves in the Butterfly mixer. The curves are set up in the Servo Travel screen. The curves below are a guide. If the servo rotates the wrong way in response to an input, reverse the curve by pressing the R key in the Servo Travel screen.
Flap Servo Curves
Let's set up the curves for the flap servo.
Spoiler Control to Flap Servo
The first curve will set the response of the inner flap servos to a Spoiler control input. We will assume that the flap linkage geometry has been adjusted to allow full servo travel for full flap deflection, i.e. one end of servo travel corresponds to flap surfaces at neutral, the other end is flap surfaces fully down.
As usual when setting up a curve, we consider the one input and ignore all other inputs. Let's look in detail:
- Spoiler forward (-100). This is the spoiler off position. The flap surface is at neutral. We arrange it so that the servo goes to nearly one end of travel (-80%). This allows 20% headroom allowed for reflex due to aileron input.
- Spoiler Stick back (+100). This is full spoiler, the servo goes to +100, corresponding to control surface fully down, no further deflection needs to be allowed for.
This results in the following curve which must be set up:
Aileron Control to Flap Servo
Here's the curve showing the movement of the inner ('flap') control surfaces, due to Aileron control input. Again, ignore all other inputs when setting a curve. We arrange for the flaps to act as ailerons, but with reduced movement. Note that we provide for equal deflection either side, any diff is dialled in later.
Elevator Control to Flap Servo
The snapflap curve: We arrange it so that feeding in down elevator has no effect, but pulling up elevator causes the inner control surfaces to deflect a small amount. (as a refinement, we could use a 5,7,9 or 13 point curve, to provide dead band and/or a maximum deflection, but this is best done using a Multimix.)
Aileron Servo Setup
Below is a guide to the sort of curves you will enter for the Aileron servo.
Aileron Control to Aileron servo
Set up for full and equal deflection each way (ignore Differential for now).
Elevator Control to Aileron servo
This is similar to the Elevator to Flap mix described above. Normally the ailerons will be set up for less snapflap volume to prevent tip stalling in turns.
Spoiler Control to Aileron servo
Adjust so that both ailerons go up in response to Spoiler.
This completes basic setup up of the wing servos. Let's now look at a few gotchas and tweaks.
Flap Servo Centring problem
When using the Servo Centre screen to adjust the servo centre, the servo always jumps to its true centre position, i.e. all stick positions, mixers, trims etc are cancelled. By arranging for control surface to line up with an edge when the servo is at centre, it is easy to see the effects of temperature or voltage drift, or bent linkages.
However, in the Spoiler to Flap curve (see above), servo centre corresponds to the stick almost in the middle, i.e. flaps surfaces will be approximately 40 degrees down. Consequently, it will not be possible to check for servo drift easily as the flap will be half down with the servo in the centre. This is a bit of a problem if - like me - you use Volz Micromaxx servos which are notorious for drifting with temp!
However, there is a solution if you don't mind using only half the servo travel for the spoiler function, from servo centre to one end point. See the curve below.
Note that the forward stick position (spoiler off) now corresponds to servo centre. However, you only get servo travel of 0 to 100 instead of -100 to +100, i.e. half travel. To get the same control surface movement, the flap control horn will need to be halved which will increase the load on the servo gears. But at least the control surface edges line up when you go to Servo Centre!
As usual there is a trade off involved.
Set Aileron Diff
You can now dial in some diff in the Aileron adjustment screen. This will be applied automatically any servo which has Aileron an input - in practice that means the flap servos as well as the aileron servos.
To avoid binding the hinges, you can either
- Restrict the movement of the controls so that when both aileron and spoiler are applied together, the sum of their movement does not exceed the hinge limits.
- Set the individual controls to their maximum, and just let the Servo Limit program take care of the max servo movement.
I prefer the latter. In fact the LIMIT function is a great feature, which for some reason other manufacturers don't routinely include.
Aileron Differential Suppression
Dial in aileron differential from the Adjust Control -> Aileron screen. The figure you dial in will apply to all the control surfaces.
If differential is enabled when full crow is selected, roll control may be inadequate because
Photo 3b shows the effect, note there is very little roll couple from the ailerons.
Photo 3a shows the effect of aileron differential suppression (ADF). As crow brake is applied, differential is reduced to zero, thus restoring full downward movement to the aileron and improving roll control.
To implement ADF,
Some pilots disable crow in certain flight modes, in order to prevent accidental deployment. There are is only one foolproof way of doing it, which is to leave the Spoiler input On in all flight modes, and to set a FIX VAL on the Spoiler control for those flight modes where the spoiler must be disabled.
- For flight modes where the spoiler is active, set FIX VAL = OFF, i.e. spoiler stick is active.
- For flight modes where the spoiler is to be disabled, set FIX VAL = -100 (note the minus sign). This emulates spoiler stick forward.
The way NOT to disable crow
Don't try disabling the spoiler inputs to the wing servos - apart from requiring lots of button pressing to set up, you may get problems with servo centring. If you set up differential suppression on the spoiler stick that won't work properly either.