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Old 02-10-2011, 12:19 AM   #1
DanWard
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Question servo zero, angle of the dangle, and flight surface equal travel

Group,

The new guy needs help. 17+ years ago I just built a plan, put servos in, and went and had fun. But now that I'm back in the hobby radios are computer (I have a DX6I), planes have guages hat allow you to measure flight surface travel in both directions and spec with suggested flight surface deflection.

Problem is mechnically speaking with trim and sub trim at zero I don't get equal travel in both directions. I've read, I've studied, and I'm just not sure so I'm coming to the group for help.

I've attached a picture of an a aileron servo in the wing of an ARF that I'm assembling. In the picture you will see the servo is mounted in premade hole, is not parralel to ribs and is not parralel to eileron pushrod.

After a lot of observations it would seem to me that to get equivilent travel from a flight surface in either direction, with the servo at zero (middle, zero trim, however I should say it) the relationship between center line of servo are and pushrod should be 90 degrees. 90 degrees would result in equal pushrod travel in both directions as the servo transitions through its arc of travel. Anything other than 90 degrees means that I won't end up with equal travel. How do I word this? Am I making any sense. If at 90 the same movement of the servo in either direction produces the most X travel and minimum Y travel but as it traverses through its range of motion some X travel is lost and Y travel increases. X travel produces flight surface movement, Y does not, because the servo movement is an arc.

OK so if that is right then let me move along. I observe that splines in all 4 of my servos seem to be at differnt places when compared to servo zero position. It is as if splines are not indexed to zero when Heitec makes the servo. Compund that with Dubro servo arm not being indexed...well there is no place when splines on servo matches splines in servo arm that produces a true 90 degree relationship between servo arm and pushrod.

Couple all of this together and I'm going crazy because mechically speaking between servo positioning in relationship to pushrod, variation in servos, and lack of coordination between Dubro servo arms...well 90 degree relationship just doesn't seem to be possible and the result is I get more travel in one way than the other on the flight surface.

So then I dive off into this world of computer radios. I find my DX6I has something called sub trim and what I have read seems to indicate it could (not sure if it should) be used to compensate for this mechanical mismatch. In the attached picture I have used sub trim to adjust zero position on the servo sutch that at neutral the relationship between pushrod and center line of servo arm is as close to 90 degrees as I can come up with.

I think I'm headed in the right sirection with this because I now have equal movement of the flight surface in both directions. While I did not include a picture of the aileron servo in the other wing I did note that to produce the same 90 degree relationship the sub trim setting is different.

So please help me. Am I all screwed up or have I actually got this right. I acknowledge that sometimes what my eyes read, and mind reckons, can get me way off path.

Thanks,


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Old 02-10-2011, 01:41 AM   #2
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Your use of the subtrim, to obtain a 90 degree relationship between the radius of the servo arm and the pushrod, is correct.
Note, however, that the location of the hinge-line can also contribute to unequal travel. If the hinge-line is offset towards the top surface of the movable control element, there will be more "up" than "down" travel. If the hinge-line is offset towards the bottom surface, there will be more "down" than "up" travel. If the hinge-line is exactly in the center of the thickness of the moveable control element, there will be equal amounts of "up" and "down" travel.
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Old 02-10-2011, 01:42 AM   #3
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Originally Posted by DanWard View Post
Group,

Problem is mechnically speaking with trim and sub trim at zero I don't get equal travel in both directions. I've read, I've studied, and I'm just not sure so I'm coming to the group for help.

I've attached a picture of an a aileron servo in the wing of an ARF that I'm assembling. In the picture you will see the servo is mounted in premade hole, is not parralel to ribs and is not parralel to eileron pushrod.
One thing noted in your photos is the aileron control horn. Normally, the pushrod holes should be located directly over the hinge line. Putting those holes off center from the hinge line will give you a bit of differential aileron throw from full up, to full down.

Another issue, IMHO, I don't like those simple set screw fittings on your servo arm. I've seen several of those things come loose in flight, resulting in loss of the model.

At the very least, put a small "Groove" in the pushrod under the set screw with a Dremel or file. Then install the set screw into that groove with thread locker such as LocTite.

Unless you are dealing with a very fast model though, you might not even notice the different up and down throws on your ailerons, provided both sides are identical in their up and travel.
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Old 02-10-2011, 02:33 AM   #4
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Thank you both for your replies. Glad to hear on on the right track with the sub trim.

The control horns came with the kit. It is a 3DHS and they use some sort of fiberglass control horn. 1st time I've seen or used a control horn that only has one hole it it (no adjustment holes up and down the horn). They glue into a prefabed slot in the aileron which positions them for you.

I'm also with you on the simple screw fittings. Don't much care for them. One loose screw away from failure. That is why you see the aditional setscrew collars. One I get the adjustment right I'll tighten and locktight the setscrew provided by 3DHS and then I'll move a collar up againt each side and locktight them. They wanted a similar setup on pull - pull rudder but I bailed out and when ith a standard clevas.

Again I thank you both for the time you took to reply. Your input is appreciated!
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Old 02-10-2011, 04:57 AM   #5
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Originally Posted by DanWard View Post
Thank you both for your replies. Glad to hear on on the right track with the sub trim.

The control horns came with the kit. It is a 3DHS and they use some sort of fiberglass control horn. 1st time I've seen or used a control horn that only has one hole it it (no adjustment holes up and down the horn). They glue into a prefabed slot in the aileron which positions them for you.

I'm also with you on the simple screw fittings. Don't much care for them. One loose screw away from failure. That is why you see the aditional setscrew collars. One I get the adjustment right I'll tighten and locktight the setscrew provided by 3DHS and then I'll move a collar up againt each side and locktight them. They wanted a similar setup on pull - pull rudder but I bailed out and when ith a standard clevas.

Again I thank you both for the time you took to reply. Your input is appreciated!
Let us know how it flys! (Can't do any flying around here, we have two feet of snow on our field!)
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