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.

Simply joggle the all-thread.

You are having way too much fun .

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.

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.

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.

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.

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.

My 8A was like this. I enlarged the slot. Here's some more info:
http://websites.expercraft.com/geprevo/index.php?q=log_entry&log_id=9098

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,

Bruce Gray
RV8 Tail Kit #82332

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."
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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.

I think I see what you are saying. In the picture, that is where the pushrod lies naturally when the clevis end is attached to the horn. Is there no play? I'd wait until the servo is attached and then if the line between the servo connection and horn is down the center of the opening, I'd do as aerhed suggests and use seaming pliers to twist the end of the horn slightly to line it up. Before you do that, though, move the surface through it's full motion to be sure the horn ends up centered at both extremes of motion. Much safer to tweak the horn slightly than the threaded rod.

flion said:

I think I see what you are saying. In the picture, that is where the pushrod lies naturally when the clevis end is attached to the horn. Is there no play? I'd wait until the servo is attached and then if the line between the servo connection and horn is down the center of the opening, I'd do as aerhed suggests and use seaming pliers to twist the end of the horn slightly to line it up. Before you do that, though, move the surface through it's full motion to be sure the horn ends up centered at both extremes of motion. Much safer to tweak the horn slightly than the threaded rod.

Click to expand...

Yup, you've got it Patrick. So as suggested, I went ahead and measured everything based on maintaining the straight line of the rod as it extends from the control horn on the trim tab. I figured if the alignment did not work out, I would only need to replace the E616PP cover plate, which I can easily do. The first two pics show the rod attached to the trim tab servo, with the armature of the servo all the way out. The round cotter pin head just overlaps the skin of the support bracket E615PP, and the other end of the pin just overlaps the other side ever so slightly. This is why I could not close the cover plate all the way. Maybe if I reverse the jack screw just a bit I can get to fit correctly. I will try that tomorrow.




This last pic shows that the alignment between the servo and the control horn is pretty much spot on. The template process I used to measure and align everything worked out quite well. Thanks goes to Steve Riffe for that.


So this is where things are at if I leave the positioning of the servo mounting brackets as currently drilled.

Again comments/feedback are most welcome.

Start over?
Keep as is and move on by trimming the necessary areas of the cover plate, support bracket, and skin?

Walt said:

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.

Click to expand...

What Walt said.

Perspective is always difficult in these kind of pictures but it looks (in the second picture) like the servo arm is not on center with the slot. Don't worry about offsetting it for the cotter pin. When the plate is in place, the pin will remain beneath the skin. So it looks like you need to buy a new plate and move the servo over just a bit. Easy and cheap fix. It looks like your control horn is aligned pretty well but you can just 'lean' it inboard a bit without affecting the height and it should line up pretty well with the centered servo arm.

Eyeballing the centerline on that slot in the plate was difficult for me. What I ended up doing for the similar ones on the -10 was to put the plate, outside skin down, on a piece of paper and draw the outline. Then I could measure between points on the side and draw a centerline down the slot. Finally, I replaced the plate on the paper, lined a ruler up with the centerline on the paper and continued it onto the plate. Since the -10 uses a cable bracket (similar to the manual cable for your model) I just found it's distance and marked a perpendicular line there and used that to align the bracket for drilling. (Then I did it again when I replaced the Van's bracket with billet ones.) You could use a similar process to locate the servo.

That's exactly what I did, except...

Only difference is that instead of marking a reference line down the "center" of the slot, I marked the position of the mounting brackets so that the rod still aligns perfectly straight with the control horn. So yes, the offset is quite noticeable. I will tinker with it some more today to see if I can at least get the cover plate to lay flat.

From these next pics you can see that the overall offset equates to about 1/8 of an inch or so. In the last pic, the small mark just to the left of the longer vertical mark is the location of the inside edge of the left mounting bracket based on Vans measurement given in the plans. That is almost 1/4 inch off from the current edge of the left mounting bracket, and is way too far over in the other direction, just as many others have noted in other related posts. So if I shift the brackets over about 1/8 of an inch that should center it up.