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  #1  
Old 01-09-2012, 10:48 PM
Flyin'Bryan's Avatar
Flyin'Bryan Flyin'Bryan is offline
 
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Default Trim Servo Pushrod not centered through access hole

So I am almost done with the trim tab on the left elevator. All of the hinges are drilled and the trim tab edges are aligned nicely with the rest of the elevator. Only problem is that when I attach the threaded pushrod to the control horn, it is slightly off center with the exit hole on the bottom of the elevator. I followed the plans to mark the outboard hinge hole 3/8 of an inch from the edge of the hinge.

The only other thing I have noticed after looking at other folks pics of their trim tab hinge alignment is that my eyelet order is reversed. I don't think this matters much, as long as the trim tab is in proper alignment with the elevator, but perhaps I am wrong.

I think the alignment, or in this case the mis-alignment of the pushrod, has much more to do with where I drilled that first hole in the trim tab hinge. That, combined with the final placement of the hinge on the elevator side ultimately dictates where the pushrod will end up. Can some one tell me what stupid thing I did here that might have caused this?

I have no problem ordering a new hinge or digging out a sufficient amount of the elevator skin to ensure proper clearance of the pushrod. My main concern is how far over this forces me to move the trim servo mounting brackets on the cover plate to mantain a straight line to the control horn.

Thanks in advance for your comments/answers/thoughts.



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Last edited by Flyin'Bryan : 01-09-2012 at 11:00 PM.
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  #2  
Old 01-09-2012, 10:57 PM
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rv6rick rv6rick is offline
 
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Default

Simply joggle the all-thread.

You are having way too much fun .
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  #3  
Old 01-10-2012, 06:40 AM
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You're getting ahead of yourself. The servo actuator arm is offset, that arm does not go through the spar hole, it attaches to the servo. The servo jack screw is what goes through that hole.
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  #4  
Old 01-10-2012, 08:39 AM
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I agree with Walt, looking at my old preview plans on sheet 5PP. The way to check this is to put the servo on the cover plate, attach the threaded rod to it (instead of the trim tab), install the plate (with the trim tab moved out of the way), and then move the trim tab until the horn meets the rod. It should be pretty close.

DON'T bend all-thread. Ever. The bend creates stress at the threads, which will crack in the area of the bend. The stuff supplied with the servos is thick enough that I wouldn't particularly worry about it in straight runs but if you must bend a pushrod, use music wire. It's been awhile but I believe you can get threaded rod ends of the appropriate size from a hobby supply that caters to giant-scale R/C aircraft which are then silver-soldered to the music wire. Then, of course, your weak link is the solder joints. Better not to bend pushrods if you can help it.
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  #5  
Old 01-10-2012, 09:50 AM
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Default Clarifying

Thanks for the responses guys. Rick, I hope you are doing well, and it is good to see you posting, especially when it involves helping me out! I love your avatar!

Walt and Patrick, first, to clarify, I am not talking about alignment of the rod as it relates to the jack screw hole in the front elevator spar. Sorry if I was missleading... I should have titled this post more appropriately as "Trim Servo pushrod not centered through tapered exit hole."

I am referring to the alignment of the rod as it relates to the tapered exit hole in the elevator skin. Even if the trim servo lines up OK with the rod in its "as is" straight state, the rod is still slightly off to one side of the tapered exit hole. I have a procedure to determine the necessary trim servo location using a template, and I will of course be trimming the rod down to proper size after all that is done.

So in hopes of not sparking another debate about bending or not bending the threaded rod, the offeset is not too terribly extreme, but it is just enough to bug me. My first inclination will be to measure this out and see if I can trim some of the edge of the exit area in the skin and still get the trim servo to fit correctly. If not, I have the bending tools necessary to slightly joggle the threaded rod as Rick suggests, so I am not too worried about that either, but agree that stress concentrations are the concern. The resulting bends would be minimal.

In a past life I was an avid RC scale modeler, so working with pushrods is not new to me. I will admit, however, that I have not had the "pleasure" of working with full threaded rod before.
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Last edited by Flyin'Bryan : 01-10-2012 at 10:17 AM.
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  #6  
Old 01-10-2012, 10:55 AM
aerhed aerhed is offline
 
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I'm not seeing it. Can't you just tip the horn inboard a little and the rod will lay right down the slot? Then mount the servo to match.
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  #7  
Old 01-10-2012, 10:58 AM
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I just saw the same thing on a friends RV7, I told him to just joggle the actuator arm on the trim tab, to bring it into alignment.
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  #8  
Old 01-10-2012, 11:47 AM
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My 8A was like this. I enlarged the slot. Here's some more info:
http://websites.expercraft.com/gepre...ry&log_id=9098
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  #9  
Old 01-10-2012, 02:38 PM
brucegray brucegray is offline
 
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Default Sleepless Nights

Bryan, I feel your pain. On the 1st go around everything had to be perfect! If not a order was in to Van's/ACS. Now on the 2nd build I realize there are 2 sides to the airplane, "Top & Bottom". Everytime I approach the project,"can it be seen easily attitude"?
The location in question is hard to see at best especially on a tail dragger. If your nose wheel still someone is bent over or on their knee's checking out a oops!

If I can remember and not there yet again, 2 sets of trim horns in the kit. I need to look back at construction photo's to see which one's I used. My 1st one came out dead nuts! Probably jingsted myself on # II.

Keep building it will come to you. I put a lot of things off until I was able to look, see, and talk to other builder. Weigh it all and go with what's comfortable.

Good Luck and remember to have fun building. Too many sleepless nights over nothing.

Best,

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  #10  
Old 01-10-2012, 04:37 PM
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Default Responses to more replies

Thanks again for the responses all.

AERHED (Dave): Nope. The one thing that I will NOT do is touch, bend, or alter the control horn in any way. Bending that will ruin the strength and the integrity of the horn, and put it out of its prependicular alignment with the hinge line. The control horn on the trim tab has to stay "as is."
--
Appears that some of you with lots of building experience are recommending to joggle the rod (thanks Rick and Jon), and, respectfully, some of you are recommendng not to do this, but to trim the skin away for clearance and keep the rod straight. (Thanks Patrick and Guy).

I will measure everything up tonight to see if I can trim the skin and still keep the rod straight. I am also concerned about maintaining proper flush rivet edge distance if I have to remove too much material from the one side in order to clear the rod and clevis.

Bruce - Hehe. I gave up on perfect a long time ago. Just trying to settle for safe now. Yup, I got the stock version of the E717 and E718, and then I also got the pre-trimmed version of the same parts with the Trim Servo intallation kit. The pre-trimmed ones are what I used.

Sorry to hear that you have had to repeat the process. Problem is, with recent events at Reno, and other VAFrs posting additional messages about ensuring that your trim tab installation is sound to avoid flutter problems (See today's VAF front page toward the bottom), this is an area where I am willing to sweat and worry a little more than normal.

I'll have a solution tonight and update accordingly with more pics.
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Building RV8
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SportAir Workshop Graduate x 4: Sheet Metal, RV Building, Electrical, and Composite classes.
Tail Wheel Endorsement Completed
Empennage done, Wings in progress, N462AK reserved.
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