DX6i Differential Thrust

...who needs a rudder?

Searching for a solution to the Canadair's pathetic rudder, I happened across some forums discussing differential thrust as a solution to replace rudders, with posts indicating that this thrust solution in fact outperformed conventional rudders.

The idea is that, instead of having two motors always producing the same thrust, when you turn you programme your radio so that the "inside motor" reduces power which assists with the turn. This is particularly desirable in the case of water planes where the plane rudder alone is usually insufficient.

This immediately piqued my interest as a potential software solution to the hardware issues I was facing. If successfully implemented would also allow the rather prone rudder to be removed to the parts box.

There was a minor glitch, however, in the case on the DX6i (and perhaps DX7 as well), as I will explain. The full solution requires three mixes to be used, one to allocate a new channel (usually auxiliary/flap channel) as a second throttle channel, and then two additional mixes which link the two throttle channels to the rudder, thereby allowing the appropriate engine output to be reduced to effect turning (turning right reduces right engine power and vice versa). The issue is that the DX6i has only two mixes available.

Fortunately I came across a forum, including a post by constantmotion on this specific issue, offering a novel solution working within the limits of the DX6i as quoted below:

Mix 1:
Thro -> Flap
Rate D -100% U -100%

Mix 2:
Rudd -> Flap
Rate D +30% U +30%

"If you go into the Monitor mode you can see how the mixes affect the channel outputs. The left motor ESC goes into the throttle channel, and the right motor ESC goes into the Flap/Aux channel. This is important; if you plug the ESC's in reverse your differential throttle will be backwards. 

The first mix makes the flap channel parallel the throttle channel for throttle control, and the second mix causes the flap channel to vary depending on rudder position. Changing the rate on mix #2 will affect how much differential throttle you get while turning."

I've tried this on the Canadair and can happily confirm that it works. The only thing I would add, as I discovered, is that the ESC from which the red wire has been disconnected (as apparently you must with multi-engined aircraft) must be connected to the auxiliary channel, while the full lead is connected to the throttle channel.

I happened to have it the other way around and ended up with one lifeless ESC bleeping plaintively until I figured out what was the problem. As my motors were then swapped, I fixed the issue by making the percentages in Mix 2 negative.

The difference between constantmotion's solution and the 3-mix version is that only one motor gets variable speed, increasing or decreasing as per the percentage set in Mix 2. The only time this would not function is if you were turning away from the aux motor side on full throttle, in which case the aux motor would, of course, not be able to speed up.

I have conducted a bench test of this system and it shows potential, but am hoping to do a water trial soon to confirm just how effective it is. Hopefully it will work sufficiently well to allow me to dump the rather underwhelming original rudder...