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Mixing throttle and rudder with a spectrum 7 series tx
by mike roach
I discovered the hard way that water rudders on seaplanes really do not work terribly well. At times particularly if the wind increases, it’s just impossible to turn downwind and the plane merely weathercocks and laughs at you. But having the ability to steer using the throttles just like the real thing is a far easier and safer option. You set up a mix so that pushing the rudder stick to port increases the revs of the starboard motor and the plane spins round merrily to port until you release the rudder. This option is controlled by a switch on the TX so that you don’t accidentally perform a spiral dive instead of a gentle turn when actually flying.
My old Multiplex TX had a number of mixing options that were fork-bendingly difficult to understand but which Trevor was pretty good at, so once he had programmed one of my models it was simple enough to copy that mix to any other that needed it. That TX died earlier in the year and it’s replacement is the Spectrum DX7. Things are not quite so straightforward and no-one in the Club knew what to do, nor was there much on RC Groups, to be honest..
I enlisted the help of Frank Jaerschky, a modeller friend from Canada, who had met the same problem with his Twin Otter and indeed I saw it working at Chilliwack in 2009. He posted photos of the TX screen at each stage and I optimistically set about programming one of the model memories on my TX to the same standard. Could I get it to work? Never in hours and hours of fiddling about, including one near-destruction of a model as a motor started up unexpectedly (always do your checks with the props removed!) and span the wing though 180 degrees before I managed to catch it without my fingers being “Burred” (or “Martined” if you prefer).
Eventually I emailed Frank and he said “you are mixing Throttle with Aux 2? Do you have Aux 2 on your RX?” The penny dropped. I had assumed that Aux on my 6 Ch RX was the same as Aux 2, but no, Spectrum have cleverly made the only other proportional channel Ch 7, so you have to spend even more money to get a bigger RX.
I did try mixing Throttle with Gear and Flap but they are essentially bang-bang channels and will not give a linear response.
So a quick virtual trip to Hong Kong and the wonderful Mr Hobby King got me an Orange 9 Ch RX (see my review of Orange RXs later in the magazine) and first try I got a mix on unlabelled Channel 7 (Aux 2, if you have skipped to this point!).
I had a working mix, but the motors did not spool up together. The servo travel screen on the TX enabled me to match the travel on Aux 2 to the Throttle channel, and wonder of wonders, both motors started at the same point and throttled up perfectly.
Here’s how to do it.
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