Hi. The past weekend I used subtrim to adjust the idle on a TwinStar instead of using the mix-rate. It didn't work very well.

So what is subtrim and how should it be used?

Subtrim is sort of a "hidden" trim setting. This allows you to center your servos without changing the main trim settings. I use subtrim to level all of my control surfaces visually on the bench, and then use the regular trim to level out the plane in flight. When setting up a plane, I usually center the subtrim & trim, then adjust the linkages to get the control surfaces where I want them, then use subtrim if I'm a little off at the end.

Ditto pkh,

I use mine to center the control surfaces. Then on the maiden flight, I trim the model out using trim on the stick, then work THAT trim back out using the subtrim. This gives me centered trim at the stick and I can adjust it a click or two depending on the wind/whatever on that particular day.

So, once a model is trimmed out, I move all that trim into the subtrim so it "looks" centered on my transmitter.

Hope that makes sense.

Oh. So if I'm using a single servo to operate the ailerons and I install the servo arm but it's not parallel to the aileron because the notches on the servo knob are slightly off, I can use the sub-trim to adjust the knob so the servo arm can be installed so it's perfectly parallel? (sorry about the run-on sentence)

I have a 6exap futaba. Does that have subtrim on it?

Sub-trim should be used to center the servo wheel in relationship to the pushrod, not the control surface. This way you geometrically get the same amount of throw on the rod from the servo in both directions as opposed to setting the servo up with differential. You should first try to achieve this mechanically and then resort to using sub-trim for minute adjustments. An exception to this might be the throttle where you are attempting to center a mechanical trim lever and achieve a proper idle speed. Control surfaces should be centered using the clevis at each particular horn.

goirish - I'm pretty sure it does but check your manual for sure.

foose & goirish- Futaba arms are staggered. Look at you manual. By rotating them 90 degrees you shift them 1 degree relative to the servo. Pick the correct arm/wheel position first and the use sub-trim if needed. This works best with 4 & 6 arm & wheel outputs but also with 2 arms. Again, read your manuals.

spiral_72 - close, but after the maiden you should be readjusting your push-rod lengths to recenter your TX trim levers. By using sub-trim you are de-centering the servos themselves.

Thanks Bruce. I guess I looking for sub-trim and on the 6exap it is listed just as trim. That is what shows up on the screen and then you can adjust the trim next to sticks separately

http://manuals.hobbico.com/fut/6exas-manual.pdf
I just looked at your manual and I was wrong, you don't have sub-trim. I did say I wasn't sure didn't I? Sorry. Look at page 5 though for what I was saying about the servo arms.

Before starting to set up a plane you want to center the trims in the TX for a starting point.

Got page 5. What is the trim for one the tx--not the one beside the sticks. Page 13--isn't that the same thing as sub-trim?

Nope, that just shows the position of your "regular" trim. This should be at "0" before you start to set up your plane. Then do the maiden and trim as needed. This will then give you a digital "number" for the amount of trim you have given the plane. Adjust pushrod lengths or linkages so that you can return these numbers to "0" in cruise configuration. It doesn't have to be perfect but the closer you get, the more trim you will have available if you NEED it in flight. This is why you always start at "0" for a maiden.

Ya know what goirish? I'm confusing both of us. You are right. Pg 13 is the sub trim after all. Wish I had one of those sitting here to play with, it would make this easier. You do still want to start at "0" before set-up though. ARRRGGHHH

Whew!!!! we worked through that. Have a great day.

Personally, I never use Sub Trim - never saw the need.

As far as Page 5 of the manual, it's a little vague.

What they are telling you is this:

First, set all of your trims to the center.

Then, place the servo arm on the servo so that it is 90° to the servo***


*** what a lot of people don't realize is that with Futaba servos, the servo arms are designed so that whenever you rotate to a different arm you will get a different angle (See illustration)

I could be wrong (like my disclaimer? ) but I believe that the Trim Setting is to adjust how much deflection one gets at any particular control surface when it's trim lever is adjusted/moved, i.e., if the trim setting is too sensitive (moving too much), one can lessen the sensitivity. At least, that is how it is on the 7C and 9C. An easy way to tell is for goirish to enter that mode and try it. If the control surface does not move when adjusting the values, it is not acting like sub-trim.

Wrong.

Maybe. Key point from the 6EXAS Manual, page 13:

"Adjusting the trims with the trim levers changes the servo’s position in increments of “4.� If finer adjustments are required, land the
model, then enter the program as described below to adjust the trims in increments of “1.�

That is definitely NOT how it works on the 9C... control surfaces do move when using sub-trim. Here's the definition of sub-trim from page 101 of the 9C manual:

SUB-TRIM: used to fine tune the center or neutral point of each servo. Allows full trim function from the trim sliders for
flight trimming

The 9C does both. And then some.

That's bizzare - my older 6XAs has sub-trims. Why would they introduce a NEW radio to REPLACE an older model and take features away?

Well, with the DX7 TX, I used the subtrim to adjust the throttle setting so one engine would idle that same as the second engine. The carb definitely moved open wider when I adjusted the subtrim.

I had the subtrim moved almost to its max to get the right idle. Here were the effects:

1) When my throttle was at idle, both engines idled at the same RPM

2) When I moved to half-throttle, the subtrimmed engine's carb would be nearly wide open, therefore almost full throttle. The non-subtrimmed throttle was where it should have been.

3) When I pushed the throttle to full, both carbs were wide open.

4) I'm pretty sure I lined up the both servo arms on the throttle so they were perpendicular to the servo (90 degrees) before I messed around with the subtrim. So they looked the same. When I opened up the nacelle after messing around with the subtrim, the subtrimmed servo arm was deflected at least 25 degrees compared to the non-subtrimmed servo.

So if I'm thinking correctly, full-subtrim would definitely show up in the control surfaces.

You failed to read the later post where I re-corrected this post. There is indeed sub-trim.

The levers (or buttons) on your 6EXA below and to the side of the sticks are your digital trims. My older 6XAs has analog trims where a wheel rotates to the right/left or up/down to adjust the control surface (in flight) then it stays there. The digital trims on the 6EXA always return to center when you nudge them to "blip" in small amounts of trim.

The advantage of digital trims is they can't accidentally be knocked off their last setting. The advantage of analog trims is you can quickly move the lever a bunch. Makes it handy to use the analog trim on your idle to cut off the engine - the throttle trim doesn't have any affect on the high side. On the 6XAs, you may not remember where the trim lever is supposed to be when you have multiple models in memory - each with their different trim settings. The 6XAs has a trim memory where after you have your airplane trimmed out (and the levers all over the place), hit trim memory which memorizes those trim settings for that model memory. Then you move your trim levers back to neutral. The control surfaces are still trimmed where you last trimmed it but you start again with a trim lever in it's center detent. Then if a trim lever gets "bumped" accidentally it's easy to find where it should be by moving the trim lever back to it's neutral setting where you can feel the center detent.

So to summarize,
The obsolete 6XAs has analog trims, subtrims (STRM) and trim memory (TMEM) which you have to set manually.
The current 6EXAs has digital trims but no subtrims. Digital trims get memorized automatically.
The 9CAPs I have has digital trims, subtrims and automatic trim memory.

I still don't see it. My 9C has digital trims and subtrims. I can also adjust the digital trim granularity with the trim function just as the 6EXA has. That is not subtrim.