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Old 11-17-2019, 07:53 AM
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Centering servo arm issue

I am using 8711 servos with extreme flight 2Ē servo arms on my elevators. Well I center everything in my radio and I cannot get the servo arm onto the servo and keep it perfectly 90 degrees. Itís either one spline one way or another and it takes about 30-35 clicks of subtrim to center it. Is this normal or is there a trick to getting it on there? It just seems 30-35 clicks of subtrim is excessive.
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Old 11-17-2019, 08:00 AM
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Each servo and servo arm has a slightly different setup, depending on how the splines were cut. Besides programming or sub trim, the only other option is to try different brands of arms. Most of the newer servos now have "industry standard" (aka Futaba) 25 tooth splines to reduce that effect.
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Old 11-17-2019, 08:13 AM
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Do you get the same results with other servo arms like SWB, JR, etc. ?
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Old 11-17-2019, 09:03 AM
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Originally Posted by Bill Vargas View Post
Do you get the same results with other servo arms like SWB, JR, etc. ?
I havenít tried any other brand servo arms yet. I ordered all extreme flight arms for this build and tonight when I put together both elevator halves, thatís when I found this out.
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Old 11-17-2019, 09:06 AM
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Quote:
Originally Posted by ghoffman View Post
Each servo and servo arm has a slightly different setup, depending on how the splines were cut. Besides programming or sub trim, the only other option is to try different brands of arms. Most of the newer servos now have "industry standard" (aka Futaba) 25 tooth splines to reduce that effect.
The only way I see to fix from having that much sub-trim is to set the arm which will be closer to the elevator and then just run the turn buckle in as much as it can go.
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Old 11-17-2019, 09:22 AM
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I used to have the same problem but now use Futaba S-bus servos and adjust the centering in each servoís internal setting. I also use ball joint control pushrods and adjust the length for the surface to be level. Then I use a deflection meter to set equal throws.
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Old 11-17-2019, 10:01 AM
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Quote:
Originally Posted by extrabad3 View Post
I haven’t tried any other brand servo arms yet. I ordered all extreme flight arms for this build and tonight when I put together both elevator halves, that’s when I found this out.



Hmmm, ok.

If you were to remove the servo from the stab, return the sub trim back to Zero and put the arm on the other side of the servo,,, is it centered?
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Old 11-17-2019, 10:07 AM
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it's normal, and 35 steps of sub trim really isn't all that much. with a four arm horn (like what is provided with the servo) you can rotate it to find the optimum, but with a single arm you may have to add subtrim to achieve 90 degrees. this becomes very apparent when ganging servos on a single surface. what I have found on elevators is that one is close to ideal and the other needs subtrim, and it has nothing to do with mfr of the arm as I have used SWB, EF, DA, and H9 arms. what's important is the geometry of the servos are exact, and any adjustments to the end points can be made using travel.
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Old 11-17-2019, 11:02 AM
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Quote:
Originally Posted by extrabad3 View Post
The only way I see to fix from having that much sub-trim is to set the arm which will be closer to the elevator and then just run the turn buckle in as much as it can go.
The one thing you don't want to do is use the turnbuckle to even up the two elevators....the left needs to be the exact length as the right....
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Old 11-17-2019, 11:48 AM
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Quote:
Originally Posted by FGNewbie View Post
it's normal, and 35 steps of sub trim really isn't all that much. with a four arm horn (like what is provided with the servo) you can rotate it to find the optimum, but with a single arm you may have to add subtrim to achieve 90 degrees. this becomes very apparent when ganging servos on a single surface. what I have found on elevators is that one is close to ideal and the other needs subtrim, and it has nothing to do with mfr of the arm as I have used SWB, EF, DA, and H9 arms. what's important is the geometry of the servos are exact, and any adjustments to the end points can be made using travel.
This ^^^^
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Old 11-17-2019, 12:37 PM
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It is pretty normal to have some subtrim,but I try to have the least amount as possible. I over buy servo arms so I can try different arms to achieve that. But my process is lengthy because subtrim can affect max throw as being even. So I set subtrim to make the surface even and then measure full up and full down. If one side is more than the other, turn the turn buckle to reduce the highest max throw side. Mind you in the radio I have max adjust set the same around 120-130. But they are the same. Once I adjust the turn buckle down, I recenter with subtrim and the measure max throw. Adjust as necessary. Adjust subtrim as necessary. My goal is to have the turn buckle adjusted for even up and down. Once this is achieved, then I work on reducing subtrim. I try multiple arms to try and get the most minimal subtrim as possible. Less than 10 is my goal number. Once I find it, I remeasure max up and down. I normally have to adjust again. I do this all to find a subtrim under 10 and max throw even. I have swapped out servos before to get this to happen. When I do this on changed surfaces, I hardly have if any binding between the two and mid travel evenness is almost spot on. At rare times I have to adjust max throw in the radio 1 or 2 points to match max throw but that is is. Because if you have a larger swing of end point adjustments, mid travel does not match up and you end up with unequal elevator travel or binding in the rudder or aileron ganged servos.
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Old 11-17-2019, 12:49 PM
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Terry's custom did a great video on Servo arm set up on elevator and Rudder he's got a YouTube video and it was also on the PA you Viper build thread he leaves the Servo arm offset and adjust the pushrod and instill a cheese full throw both ways and that is how I set up all of my play with zero sub trim so that the servo moves the full distance both directions
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Old 11-17-2019, 07:47 PM
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One other thing to check; there is sometimes a difference between the spline placement on each arm. Try several different arms of the bunch you have and see if one might be closer to what you need.
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Old 11-17-2019, 10:05 PM
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Messing with subtrim can have a lot of unintended consequences, especially on ailerons. I found on my last model that bending the servo arm was a far better solution. I know it's a little unorthodox but I took the arm and put it in my vice. Then I took an adjustable wrench set up to grip the arm and tweaked it a VERY tiny amount. Then I pulled the arm out of the vice about a millimeter and did it again. I kept repeating that process until the correct bend was achieved. The bend was spread over approx. 14mm of servo arm length to not over stress any one point. This worked so well that I will do this from now on.
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Old 11-17-2019, 11:11 PM
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Quote:
Originally Posted by Bill Vargas View Post
Hmmm, ok.

If you were to remove the servo from the stab, return the sub trim back to Zero and put the arm on the other side of the servo,,, is it centered?
Hello. I did that today and it worked. I swapped the two servos around and they are now almost perfectly centered without any sub trim. Thank you all for your help.

Rs,

Craig
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