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Trim reversal in OpenTx

Be careful with your input weights!

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Tim Kearsley05/12/2019 08:59:28
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620 forum posts
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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.

Ron Gray05/12/2019 09:13:01
1597 forum posts
391 photos

Personal never change the + to - on the inputs, I always do that either in mixes or in outputs, never had an issue doing it that way.

Peter Christy05/12/2019 09:35:38
1640 forum posts

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

Manish Chandrayan05/12/2019 11:07:20
610 forum posts
71 photos

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

Tim Kearsley05/12/2019 11:29:42
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620 forum posts
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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.

Frank Skilbeck05/12/2019 12:28:04
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4550 forum posts
101 photos
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.................

Peter Christy05/12/2019 12:47:53
1640 forum posts
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

MattyB05/12/2019 13:16:12
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1987 forum posts
30 photos

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.

Manish Chandrayan05/12/2019 15:15:44
610 forum posts
71 photos

Pete it was all in lighter vein. Yes a good understanding of computers help and coming from Futaba background is no help.

I don't have any OpenTx but have a Graupner set that has been a steep learning curve. A manual originally written in German, that is translated to English by some one in far east further muddies the water

Tim Kearsley05/12/2019 15:56:15
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620 forum posts
3 photos

Matty - thanks for that, I'll have a look. Something to slow down the Excalibur for landing would be handy!

Tim.

Peter Christy05/12/2019 16:30:51
1640 forum posts

Manish: Don't worry, no offense taken, I knew what you were getting at. I'd hoped my reply was similarly light-hearted, but perhaps should have added an extra cheeky for good measure ! wink

And yes, I know what you mean about Chinglish instructions!

--

Pete

Martin McIntosh05/12/2019 18:13:40
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3021 forum posts
1112 photos

I too got caught out like that. Taranis is not for me, anyone want to buy it?

Sheepish05/12/2019 20:20:07
12 forum posts
1 photos

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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