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Trim reversal in OpenTx


Tim Kearsley
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Recently, I've set up a model with a V-tail on a Taranis radio (a Durafly Excalibur). I've always found V-tails can be a bit fiddly to set up - you get the servo travel directions correct for the elevator and then find one is wrong for rudder, or vice-versa! Anyway, I ended up with both elevator servos going the wrong way, so , in my infinite wisdom, I set the weight of the elevator input from +100 to -100. Great, all works as it should.

On Tuesday, we maidened the model. All good, just a few clicks of right aileron plus a bit of down elevator needed. Except..... every time I put in some down elevator trim the situation got worse! It quickly dawned on me that the elevator trim was reversed! A swift bit of Googling revealed that this is expected behaviour if you've set a negative weight on an input. The remedy was simple - put the input weight back to +100 and switch the two mixer values for the elevators from + to -.

The thing is, I don't think that OpenTx is working as it ought really. To me, a trim is simply a way of making a small, fixed movement of the stick, and OpenTx should recognise that if you've effectively reversed the stick, by changing the weight from + to -, then the trim should be reversed too.

What do you think?

Tim.

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One of the beauties of OpenTx is that it is almost infinitely flexible - but with that flexibility come a few "gotchas"!

You are thinking of trims the way they work on a "conventional" system, but in OpenTx, they are a separate control in their own right.

Because of this, it is possible to have "crossed" trims, ie: the LH trims operating on the RH stick and vice versa. Some people like this, as it stops you having to do an awkward reach across the TX if you find yourself with a very out-of-trim model.

It also means that the "trim" levers can be assigned to a completely separate function, and not trim at all. For example, a friend of mine uses OpenTx professionally for controlling cameras remotely, and uses the "trim" levers for zoom and focus functions, while the sticks are used for pan and tilt. Very useful and flexible, and something a conventional transmitter is incapable of achieving.

Incidentally, I have a number of Ace Micrpro 8000 transmitters from the mid-90s, one of the first attempts at a computer controlled transmitter. This features crossed-trims as standard, but when you reverse the stick functions (done by setting the end points, rather than a simple "reverse" toggle) you have to make sure you also reverse the "rate" settings as well, otherwise the "rate" switch becomes a "reverse" switch! Gotcha!

Like many features of modern transmitters, its only an issue if you are unaware of it. And if its any consolation, I too have been "got" by an unexpected occurrence when I programmed an IC power helicopter for the first time in OpenTx , and this resulted in a hot start. It certainly made me jump, but luckily for me, years of experience have taught me to be ready for a hot start, and I was able to shut everything down quickly and safely while I investigated.

As I said, infinite flexibility can also provide an infinite number of ways to mess things up, but luckily most people rarely come across them!

Chalk it down to experience! You won't make the same mistake again, and others who read this thread will now also be aware as well.

Cheers,

--

Pete

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I think you hit the nail right on the head Pete, when you say that I'm looking at OpenTx from a "conventional" viewpoint. I'm a relative newcomer (a few months) to Taranis and OpenTx, having been a Spektrum user prior to that.

I can certainly see the flexibility of having the trims as controls in their own right. It hadn't entered my head that they wouldn't follow the "direction" of the main stick, but, as you rightly say, I'm aware now, and definitely won't forget it!

I must say that I love the OpenTx system generally, and the flexibility it gives you in achieving what you want to do. Just watch out for the "gotchas"!

Tim.

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Posted by Peter Christy on 05/12/2019 09:35:38:

It also means that the "trim" levers can be assigned to a completely separate function, and not trim at all. For example, a friend of mine uses OpenTx professionally for controlling cameras remotely, and uses the "trim" levers for zoom and focus functions, while the sticks are used for pan and tilt. Very useful and flexible, and something a conventional transmitter is incapable of achieving.

Not all, Multiplex Royal Pro and Profi have ability to separate the trims and use them as separate channels.

BTW had exactly the same with a club mates model on a Taranis Q7 he asked me to maiden for him, I couldn't work out why trimming the elevator made it worse! got it down safety and discovered that the trim was reversed. Note to self check any future club mates models that trim works as expected.................

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Posted by Manish Chandrayan on 05/12/2019 11:07:20:

In my opinion the Open Tx and some other brands as well have been programmed in a way a computer programmer/code writer thinks and not necessarily a pilot thinks. Especially pilots of certain vintage (read age) wink 2

Well, I've been flying R/C since the mid-60's, and am rapidly approaching my 70th birthday, so I'm not convinced its purely an age thing.

Having said that, I have been working in electronics all my working life, and learned about computers (mostly mainframes and mini-computers) in the early 70s. So perhaps I do have an unfair advantage. But even that didn't help with the heli, as I followed standard computing practice, and it caught me out!

Having said all that, I do understand why pilots whose previous experience has been Spektrum / JR / Futaba / etc can get confused by OpenTx. At a basic level, OpenTx is no more difficult than any other system. Its when you try and do something slightly out of the ordinary that it can catch you out!

Frank: I never claimed that Multiplex was "conventional"! wink

I know of only one local modeller who ever used Multiplex. Sadly he passed away earlier this year, so my experience of recent Multiplex systems is nearly zero!

--

Pete

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I doubt there is anyone who hasn't been "gotcha'd" by this issue when starting out in OpenTX; I know I was, and I had moved from a Multiplex Evo that had very similar logic in the main. As Peter points out though, for me the flexibility the system gives more than makes up for these kind of additional complexities (especially given 95% of my models are now created off of previous templates making them very easy to setup).

PS - Tim K, if you want to look at an alternative setup for the Excalibur I posted one on RCSettings with some relatively advanced functionality, including a "go-around" mode that allows the top half of the throttle stick to act as a throttle, and the bottom half as spoilerons.

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Another way to end up with unexpected trim reversal is to replace the standard stick units with Hall effect ones and forget the change the down load for the TX firmware to hours sticks or similar.
I’ve had this twice now, once on my QX7 that I changed the sticks on and once on a second hand X9e that had had the sticks changed but not the firmware before I owned it.

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  • 2 years later...

Taranis X9D+, OpenTx 2.3.14, XJT V2. Over the past few years I've also experienced odd behaviour with the trims. Last week I performed some detailed testing and discovered that with all servo directions normal i.e. positive, the elevator trim direction is opposite to the stick direction on the actual Tx (not in OpenTx). I have made the trims move in the correct direction by using negative weights in INPUTS on the elevator channel and it probably has been like this since setting up the model. On the screen on the Tx which shows the two stick positions in square boxes, the stick dot also moves in the opposite direction to the elevator stick movement. The other control sticks move in the correct directions. Everything is correct in OpenTx. Have I got a wiring problem in the Tx or have I missed something obvious here?

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