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...

DX6i Transmitter Flap Mix

...tail-heavy no more

I took the Phoenix out for a fly this afternoon, not having flown it for a while, and took the opportunity to try the flap mix which was something I'd overlooked when I first got the transmitter.

I'd set the Phoenix up for flaps when I'd first got it, but have never particularly liked flying with flaps as it makes the Phoenix fly tail-heavy, dampening what are generally immaculate flying manners.

This is because the wings are in a high-lift configuration but the tail is not, causing it to dip at low speeds associated with flaps. The other day, flipping through the menu's when setting up the Stinger, I came across the flap mix menu and suddenly it made sense.

The mix is very simple, allowing you to dial in a dose of elevator with the flaps. Now the trick is, to cure tail-heaviness as in the case of the Phoenix, you don't want positive elevator, as this would just push the tail down more and increase the angle of incidence of the wings.

What resolves the issue remarkable well is negative elevator, effectively working as a flap for the tail. On the Phoenix I set negative elevator at 20 for a flap setting of 10, which I tried out this afternoon and which works brilliantly well.

Highly recommended on the Phoenix...

DX5e Disassembly

...to see if I can sort the aileron issue

I had a go at trying to fix my other transmitters issue with chatter on the aileron control. After having seen instructions on how to disassemble the DX6i while I was reading through that controller's instruction manual, I bit the bullet and decided to have a go with the other.

There are six screws at the rear of the unit which need to be undone, then you simply pivot the back down at the bottom and lay it down flat. There is sufficient cable length to make this easy, and it's obviously how they assemble it.

What was immediately clear, however, was that replacing the potentiometer on the aileron control would not be easy - it's well integrated and unfortunately not easy to disassemble. That's why there doesn't seem to be any spares on offer, at least for this particular unit, with the exception of a replacement antenna, which simply slots into place.