Trim Differences Manufacturer to Manufacturer - RC Groups
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Jan 27, 2018, 10:49 AM
Registered User

Trim Differences Manufacturer to Manufacturer

For the longest time, I've used a Hitec Aurora 9 and compatible receivers for all my models. Last winter I got a Spektrum DX6 because I wanted to try indoor flying and that seemed to be the predominant (if only) option for the receiver/servo/ESC bricks for indoor use.

Since the DX6 came with a full-sized receiver I went ahead and put that into a motor glider design I'd been working on and finally got that plane into the air recently. It's been a real pain, though, getting that model trimmed out.

I sat the model back on the bench and went full one way and then the other with my elevator trims. The bottom line is that I couldn't get much deflection with the trim tabs even with the maximum trim inputs.

I just put a Hitec receiver into this model this morning and range of trim available is much more like I've been used to.


Is this just a difference between manufacturers? Is there something I'm missing about setting up the DX6? I found a "trim step" adjustment in the DX6's menu, but that seems to be how many notched a single push on the trim lever gets you, not how much throw plus or minus 100% on the trims translates to.
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Jan 27, 2018, 11:45 AM
AndyKunz's Avatar
Yes. The trims on a Spektrum radio are intended to be used as trims. As such, they give you +/-30% of normal travel from stick deflection.

You are correct about how Trim Setup works.

It sounds like you are using trims for something beside in-flight trimming. Here's the proper technique (works with all brands):

1) Set trim, sub-trims, and anything else that might affect center of travel to 0. If you look on the Monitor screen, you should see them centered, and with the value 0 or nearly so. Adjust your Travel values to 100% for all surfaces.

2) Disconnect the servo output arms from the servos.

3) Power up the receiver and servos. All servos should now be positioned at the center.

4) Move the control surfaces to center, and attach the servo arm to the servo. You may need to adjust your control linkage lengths to achieve this. Do that. Proper mechanical setup is extremely important.

5) Adjust the clevis positions on servo horn and/or surface control horn to provide the full-stick travel required by your model. Get it as close as possible mechanically.

6) Now you may adjust the transmitter's Travel setting for each servo to provide the exact amount of travel required for that surface. You got it as close as you could mechanically, now you use the transmitter to tweak it. You should still be in the 90-110% range for this - if not, go back and change mechanical components so that you are.

At this time, going end-to-end with the trims, you will find that you have full control of the trim as expected. Note that if you are above 95% on Travel, the trim won't do anything the last few clicks as you have exceeded the electronic range possible.

Jan 27, 2018, 03:19 PM
Registered User

I'm pretty fussy, actually...

...about getting things squared up mechanically. I use the DuBro do-dads on my servo arms so I can adjust the pushrods and then screw those into place, for example.

I don't adjust the throws, though, mechanically. Rather, since most of my models are roughly the same size and use either mylar or tape hinges, I've pretty much got the control arm/control horn dimensions down that match the torque of the servos I typically use (HS-55 or equivalent) to the resistance of the control surfaces.

The throws I'll set with the end point adjustment in the transmitter, and what I end up with changes after I see what the flying characteristics are. Sometimes I'll even change those after a period of time just because I decide I don't like the "feel" of the plane in the air.

I definitely wan't getting 30% of the full stick deflection, though, with my Spektrum transmitter. That's about what the Hitec is giving me now, which is why I was wondering if different manufacturers have programmed in different percentages in the setting for their trims. (This doesn't seem to be an adjustable parameter, which I wish it were.)
Jan 27, 2018, 03:39 PM
Registered Taranis User
Miami Mike's Avatar
You should adjust your linkage to get the control throw you need with as much servo rotation as possible. This minimizes the effect of gear slop in the servo, gives you the highest possible resolution and precision of control position, the greatest torque, and the lowest stress on the servo.
Jan 27, 2018, 03:46 PM
AndyKunz's Avatar
Originally Posted by Lon Enloe
I definitely wan't getting 30% of the full stick deflection, though, with my Spektrum transmitter.
Look at your Monitor screen. When you move the stick you will see it travel +/-100%. Now adjust the trims. You will see it go +/-30%.

Absolutely guaranteed. I wrote the code. That's what it does (the actual amount is 29.something%)

That's about what the Hitec is giving me now, which is why I was wondering if different manufacturers have programmed in different percentages in the setting for their trims. (This doesn't seem to be an adjustable parameter, which I wish it were.)
Another point to keep in mind is that different manufacturers define 100% differently.

For Spektrum and JR, 100% is 60 degrees of rotation (with Spektrum/JR servos, but it's pretty standard), and the pulse width ranges from 1.1ms to 1.9ms. You can crank this up to 150% to get 0.9ms to 2.1ms pulses.

My bet is that what you're seeing is the difference between what Hitec calls 100% and what Spektrum calls 100%.

Jan 28, 2018, 09:23 PM
Registered User

So, I did an experiment...

...with a new airframe that I have to decide which electronics to use on.

Tried both transmitter/receiver combinations in the same mechanical setup, both transmitters set to 100% end point adjustment, 0 subtrim, and trim tabs initially zeroed out as well.

The two manufacturers seem to agree on what 100% is, since I got the same amount of throw on both moving the sticks.

The maximum trim settings are considerably different, though. The Spektrum trim settings are just as you describe (I certainly won't argue with the person who wrote the code!) but it looks like Hitec allows trims up to almost 50%. So, I don't think I'm crazy (at least not based on this).

Any chance the amount of trim could be a user-defined value in a future software revision?

I'll admit that "Well, you should be more careful when building and outfitting" would be entirely appropriate but having a backup plan, especially on a maiden flight, would be comforting.

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