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TruTrak Pitch Servo Install

macrafic

Well Known Member
HomebuildingPics
Not sure why something that seems so simple should be so difficult!!! I've exhausted all the resources I can find on VAF and verified my installation against the simple diagrams that TruTrak provides. Yet, I still have an issue.

The BellCrank, as the top rotates toward the rear of the aircraft, contacts the jam nut on the rod end bearing. The pictures tell the story.

I could add washers/spacers between the rod end bearing and the bellcrank, but then then I would also have to add additional spacers on the other end (where the rod end bearing attaches to the servo control arm) to keep the pushrod straight. But, I don't have any more room to do that there, and leave enough room for the nut. I would have to use a longer attachment bolt, which would seem to make the attachment weaker and subject to movement back and forth as direction is changed.

I can't help but think I have something wrong here, since I haven't seen this problem on VAF. Can anybody see anything wrong from my pictures?

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Not sure why something that seems so simple should be so difficult!!! I've exhausted all the resources I can find on VAF and verified my installation against the simple diagrams that TruTrak provides. Yet, I still have an issue.

The BellCrank, as the top rotates toward the rear of the aircraft, contacts the jam nut on the rod end bearing. The pictures tell the story.

I could add washers/spacers between the rod end bearing and the bellcrank, but then then I would also have to add additional spacers on the other end (where the rod end bearing attaches to the servo control arm) to keep the pushrod straight. But, I don't have any more room to do that there, and leave enough room for the nut. I would have to use a longer attachment bolt, which would seem to make the attachment weaker and subject to movement back and forth as direction is changed.

I can't help but think I have something wrong here, since I haven't seen this problem on VAF. Can anybody see anything wrong from my pictures?

23i7x8o.jpg


2mgpowo.jpg


wuggm0.jpg
 
http://www.vansairforce.net/articles/ImagesInForums/images.htm

Basically copy the URL of the picture by right clicking on the photo, go to the "submit reply" box, click on the icon that looks like a mountain with a sun above it, past the URL and click "OK" Some web hosting sites let you copy URLs for smaller sized photos, which is nice. This one came out a little big.

Are the two threaded rods the same length? How much length is left on them? Can you lengthen one all the way (while still being safe) and then shorten the other so the tube shifts to the right?

DSC02153.JPG
 
The pushrod is set at the length specified by TruTrak. I can't lengthen them without leaving too little of the bearing in the pushrod. I can shorten them, but that won't solve the problem.
 
The TruTrak installation pdf for the RV-7 shows 2 #10 washers between
the Elevator Bellcrank and the A/P arm rod end. Does the arm have to be
parallel?
 
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Hmmm. I actually bought mine over a year ago. I'm looking at the materials that came with it and it has no information about bolts/washers/etc.. Just some pictures that show orientation.

I wonder if the documentation has been updated since I bought this. Anyway, I'll put them in there and see if that fixes the problem. Looks like a plausible solution, although it will not allow the pushrod to be exactly straight between the bellcrank and the servo. I have, however, seen reference to that and it appears to be normal.

If anybody has a pdf of the latest documentation from TruTrak, or at least the key page here, and could post it, I would appreciate it!

Thanks!
 
Woops! My mistake! I actually see a number 4 pointing to that location and, of course, #4 represents 2 #10 washers!

Sorry!
 
The rod ends look way to far out to me :eek:
Engagement should be minimum of 2.5 x the diameter of the tread shank (3/16 would be .47 minimum engagement)
 
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I just installed mine a couple of weeks ago. TruTrak's documentation shows an installation in an RV-6, which is a little different than the 7/9. The answer is spacers, either washers or pieces of aluminum tube, and longer bolts. Somewhere on the site is a thread or two showing just that, with corroboration from Lucas at TruTrak. I have photos, but they are on my camera which is at my hangar. I can post something tomorrow if it would help.
 
Also install a spacer in the bellcrank

I don't know why this is not mentioned in the instructions. I mentioned it in a previous post and got no replies. In your picture it is easy to see the bowing inward due to torquing the bolt to specs. You want to torque down most hardware onto a spacer/bearing/washer and not a 1/16" gap. It won't hold the torque and will loosen over time especially on something with so many load reversals even though it is a relatively small load.

On the issue of the nut hitting...on mine the hole was drilled about 1/4" from the centerline and has about a 5/16" spacer. Of course the RV-10 has the torque enhancer so slightly different design.

In any case, you could install equal length spacers made from 5/16" tubing and longer bolts.
 
I agree!

I think you've answered your main question, as the two washers should do the trick. But others have chimed in with two more concerns regarding the spacer between the bell crank halves and the thread engagement on the rod. I agree with their concerns.

Just a suggestion, but I replaced the Van's designed spacer with a slightly larger one that was essentially a triangular shape. That let the two original rivet holes hold the spacer in place, while allowing the bolt for the A/P arm to go through the tip of the triangle spacer sandwiched in there. Not sure if that makes sense, but picture a triangle pointing down. Just a suggestion.

Regarding the arm length, make a new longer aluminum arm if you don't have proper thread engagement. I would think TruTrak sent the proper length material, but there's always an outside chance that it wasn't measured correctly. Not trying to insult your building skills or TruTrak, but it's worth checking. Even though I've never cut something to the wrong length!:rolleyes:
 
some pix of an RV-7 installation

Not the greatest, but hopefully a bit of help. Link is set to TruTrak spec length. I spaced both ends to get it reasonably parallel to the longitudinal axis of the fuselage with minimal standoff distance.

1237727240_mDThB-L.jpg


1237727350_YwT3C-L.jpg
 
If I recall Dan's previous post on the subject, I believe his argument is that the bolt is being used as a cantilever beam and the strength of the bolt is better using the grip diameter than the minimum diameter of the thread.

Although he is correct that the bolt strength is best in the grip range, I don't think the concern is applicable in this case. As long as the bolt is tight, the bolt won't be in bending...it'll be in (single) shear.

Even if the nut were to loosen up, causing the bolt to be cantilevered, it'll still be cantilevered on the grip diameter by the edges of the holes in the bellcrank. Of course the aluminum would get wallowed out by the bolt, but I don't think the threaded portion could ever see anything but the tension due to tightening.

Maybe I misunderstood Dan's point, though. <shrug>
 
If I recall Dan's previous post on the subject, I believe his argument is that the bolt is being used as a cantilever beam and the strength of the bolt is better using the grip diameter than the minimum diameter of the thread.

Although he is correct that the bolt strength is best in the grip range, I don't think the concern is applicable in this case. As long as the bolt is tight, the bolt won't be in bending...it'll be in (single) shear.

Even if the nut were to loosen up, causing the bolt to be cantilevered, it'll still be cantilevered on the grip diameter by the edges of the holes in the bellcrank. Of course the aluminum would get wallowed out by the bolt, but I don't think the threaded portion could ever see anything but the tension due to tightening.

Maybe I misunderstood Dan's point, though. <shrug>

Makes sense, in principle, though I fear as much for the cantilevered shaft of the servo as for the bolt that's transmitting the load to the shaft. But I also have no sense of the loads involved, this being my first experience with an autopilot and related servos. I'll admit to looking at the whole thing with a somewhat jaundiced eye. Sure hope it works...

In my case I selected fasteners long enough that a washer was required under the nut to prevent it from bottoming on the thread. That doesn't quite prevent the thread, as opposed to the shank, from being subject to a bending load. I s'pose if that's the issue though, not hard (except on my feeble back) to turn them around.

How much torque can a servo such as these develop?
 
Post removed due to the issue already being raised in this thread. Computer glitch didn`t show page two.
 
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Regarding the AN 3 bolt attaching the servo arm to the elevator bell-crank shown in the above photo.
Another reason to turn the bolt end for end is in the rare case that the nut fell off, the bolt could back out slightly and bind on the vertical support structure restricting elevator movement.
 
Rod ends too far out....maybe ?

Might want to check. You do need the washers and as I remember it the connecting tube for the rod ends is a different lenght for the pitch and roll servos. Mix them up and they don't work. As I remember the install, it was exactly as TT recommends and everything looks vertical as it should.

The pushrod is at a slight angle because of the washers. Normal.

Bill S
7a Got the Grin
46 hours since Jan 2
 
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