Sub Trim?
12-10-2009, 01:37 PM
#1
Sub Trim?
John Glezellis' "Let's Talk Giant Scale" column in this month's MAN mentions that in setting up our aircraft, we should not use Sub Trim to center the servos/surfaces; doing so will change the travel of one servo in relation to the other.
This may be true, but then how do we compensate for the center angle difference between two servos set up opposite each other to perform the same tasks (i.e. aileron or elevator)?
When using after market servo arms (like SWB) you will find that they do not slip onto the output gear at a perfect 90 degree (perpendicular to servo) position when servos are centered with zero sub trim.
If John expects us to leave sub trim at zero and adjust surface centering via mechanical pushrod length, I understand that concept, but doing so results in servos opposite one another traveling through different arcs, thus, we still do not achieve equal travel.
I am not sure which is worse - sub trimming servos, at neutral, so that servo arms are exactly the same in relation to pushrod lenth and surface centering OR adjusting surface centering using pushrod lengths and leaving neutral servo arms at different angles in relation to each other?
This may be true, but then how do we compensate for the center angle difference between two servos set up opposite each other to perform the same tasks (i.e. aileron or elevator)?
When using after market servo arms (like SWB) you will find that they do not slip onto the output gear at a perfect 90 degree (perpendicular to servo) position when servos are centered with zero sub trim.
If John expects us to leave sub trim at zero and adjust surface centering via mechanical pushrod length, I understand that concept, but doing so results in servos opposite one another traveling through different arcs, thus, we still do not achieve equal travel.
I am not sure which is worse - sub trimming servos, at neutral, so that servo arms are exactly the same in relation to pushrod lenth and surface centering OR adjusting surface centering using pushrod lengths and leaving neutral servo arms at different angles in relation to each other?
12-10-2009, 02:13 PM
#3
RE: Sub Trim?
In reading the article more closely, John is referring to aileron differential specifically - do not use sub trim to achieve aileron differential. My BAD[
].
However, as long as I have this post up, my question still stands - is there any problem with using sub trim to achieve servo centering?
].However, as long as I have this post up, my question still stands - is there any problem with using sub trim to achieve servo centering?
12-10-2009, 07:48 PM
#4
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RE: Sub Trim?
I don't see any other option to using sub trim to achieve the correct arm position on the servo at neutral. Normally I set up each servo horn at 90 degrees to the servo for neutral. I use a set square and sub trim it to get it spot on. Then I know my servos are the same both sides.
As long as you don't use end points that allow one servo to reach the end of avaliable travel in any configuration, what's the difference?
michael.
As long as you don't use end points that allow one servo to reach the end of avaliable travel in any configuration, what's the difference?
michael.
12-10-2009, 10:06 PM
#6
RE: Sub Trim?
I agree, saying that sub trim is bad is sort of a myth IMO. It's only bad if it's large enough to prevent your servo from going to an extreme end point that you need for your setup. I use sub trim all the time and use protractors to make sure the servos and surfaces have same up/down degrees (adjusting end points). I only have problems when the sub trim gets too high. That limits the amount the servo will go in the direction of the added sub trim. And I think it's more the tx limiting it than the servo.
12-11-2009, 09:50 AM
#7
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RE: Sub Trim?
I used to use a fair amount of subtrim in order to make sure I had things 90 degrees to the surface. I have since switched to not using subtrim and have found things overall match up better and matching travel / end points is easier.
The best thing to do is to either use Hitecs and program your center or set all of your servos on the table at neutral and use the ones that match the closest on the same surfaces. On 8711s I have been very lucky to be able to find pairs easily.
Without subtrim (or using less than 10%) I have found my setups to be more evenly matched throughout travel. The only time you will notice a difference with the arm not being 90 to the surfaces is in extreme 3D throws. You are going to loose a few degrees on one side and have to tone down the other too match.
The best thing to do is to either use Hitecs and program your center or set all of your servos on the table at neutral and use the ones that match the closest on the same surfaces. On 8711s I have been very lucky to be able to find pairs easily.
Without subtrim (or using less than 10%) I have found my setups to be more evenly matched throughout travel. The only time you will notice a difference with the arm not being 90 to the surfaces is in extreme 3D throws. You are going to loose a few degrees on one side and have to tone down the other too match.
