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sub trim question

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Old 04-18-2004, 11:37 AM
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wyflyer
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Default sub trim question

I was thinking that I heard somewhere that too much sub trim is bad. For one of my airplanes on my 9c, I need -74 percent on the subtrim. I cannot adjust the linkages to make it work. Is it ok to use that much?
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Old 04-18-2004, 01:13 PM
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Phil Cole
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Default RE: sub trim question

Ideally your pushrods meed the arms on the servo and control surfaces at a right angle in the neutral position. Ideally you do this by adjusting the pushrod length and choosing the right spline on the servo. Sometimes the splines don't give you enough resolution, and then you can use sub-trim.

I suspect that in this case your rods are not the right length, and the servo has to be off-centre to get the control surface to the neutral position. This means you will have different response rates, depending on which direction you move the control stick. It also means that you could easily run out of servo travel in one direction. You will see that when the servo is near the end of its travel, it does a lot of moving for not very much movement at the control surface.

In other words, if the linkage isn't right, do what it takes to fix the linkage. If you feel that this is not possible for whatever reason, you can at least remove the arm from the servo and turn it round a few splices to make sure that you have all the motion from the servo.
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Old 04-18-2004, 05:46 PM
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wyflyer
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Default RE: sub trim question

It is not possible for me to change the linkage. I just want to know if that much subtrim is bad to have for some reason.
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Old 04-19-2004, 06:13 AM
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Default RE: sub trim question

ORIGINAL: wyflyer

It is not possible for me to change the linkage. I just want to know if that much subtrim is bad to have for some reason.
Yes, it is bad. On most radios, subtrim offsets the center of the servo from it's "mechanical" center. By having that much subtrim dialed in, and assuming that you have centered the servo arm mechanically in relationship to the neutral of the control surface, you don't have as much overall travel available to you. That's why it's SO important to redo your mechanical linkage to be close to servo-center.

The only exception to this "situation" when adjusting sub-trims that I'm aware of is with the Multiplex EVO radio which shifts the servos entire movement range and not just the servo center point.

These new generation computer radios are amazing in the things they allow us to do and it's easy to let the radio do a lot of the work for us when setting up a new aircraft. HOWEVER, to get the most out of these radios, it is important to START with properly centered and adjusted linkages. Once set up properly, radio features like sub-trim and end-point adjustments are great for tweaking out the last little inconsistancies.

I'm wondering why you say you can't adjust your linkage to mechanical center. Whatever the reason, it's really worth doing.

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