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  #1  
Old 06-07-2012, 03:59 PM
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N546RV N546RV is offline
 
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Location: Brookshire, TX
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Default Rudder stiffeners, a dumb mistake, and a sanity check

First, allow me to show this photo from last night, which in retrospect is quite illustrative:



Eagle eyes will notice that the two topmost stiffeners are oriented differently than all the others. Those were the very first ones I cut, and I found it rather odd that all the other stiffeners I cut seemed to be oriented. But I sallied forth, spending the rest of the evening cleaning up the stiffeners and finally clecoing them to the rudder skin before going to bed.

During a lull in the day's work, I strolled out to the garage and was admiring my work, when I noticed that the edge distance on the leading edge of the two "backwards" stiffeners was quite a bit more than all the others. A quick reference fit check with the spar confirmed that there was too much material there. Huh, guess I'll have to trim those down a bit more.

I'm mildly ashamed to admit that it took a further four or so hours before a rather plausible explanation for all these oddities became clear. Yep, I cut the taper for those two stiffeners on the wrong end.

My off-the-bat feeling is that these pieces should be salvageable, provided I ensure the taper is correct and trim the extra material as needed to allow the spar to fit. The only thing that gives me pause is the way the other stiffeners are designed-they have a lot more edge distance at the trailing edge/tapered end than at the opposite end. Modifying my miscut stiffeners would leave with pieces that don't have this extra bit of material at the tapered end.

Is this a problem? Is there a reason for that extra material?

For reference, here's a comparison of the tapered ends of a correct stiffener (top) and a custom Philip unit (bottom):


If I have to rebuild these, I'll be particularly miffed, since I just placed an order for some replacement parts yesterday...

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  #2  
Old 06-07-2012, 04:17 PM
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Default

You need a minimum of 2D edge distance, looks like it may be a little short but hard to be sure based on a picture.
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  #3  
Old 06-07-2012, 04:26 PM
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FresnoR FresnoR is offline
 
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Default

It doesn't look too bad

Do you think he would be ok if he added an additional hole and rivet between the two existing holes, for added security?

I seem to remember having to trim those things a bit to fit the curve and other parts anyway. however, that was a long time ago.
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  #4  
Old 06-07-2012, 04:44 PM
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N546RV N546RV is offline
 
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Default

Thanks...but I think I was unclear about my question. The comments on minimum edge distance are correct, and that is something I should take a second look at, but that's actually not what I'm asking about.

Looking at the bottom stiffener, the end that's shown should be the leading edge/non-tapered end. As Walt pointed out, there's pretty much minimum edge distance there and little more. All of the correctly-cut stiffeners have this edge distance at the leading edge.

However, the trailing edge of the correctly-cut stiffeners has significantly more material. The top stiffener is correctly cut, and has probably 5-6D edge distance.

What I'm wondering is if there's a particular reason for that extra material on the trailing edge. If I modify the miscut stiffeners the way I'm thinking, then I'll end up with 2D edge distance at the trailing edge. So if that extra material on the other stiffeners exists for a reason, then that could be a problem. But if that extra material doesn't serve any structural purpose, then I think I'm in the clear with my plan.

My WAG is that the extra material is just there because there's room for it, but that's just a guess. I'm just looking for some thoughts from the experienced folks here. I want to make sure my gut feeling isn't fooling me, yanno?

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  #5  
Old 06-07-2012, 05:12 PM
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Default Trust me I know

Stiffner material is cheap! Heck I even now have a few extra..
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  #6  
Old 06-08-2012, 11:02 AM
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Flyin'Bryan Flyin'Bryan is offline
 
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Default Ditto what Randy said

Quote:
Originally Posted by randyintejas View Post
Stiffner material is cheap! Heck I even now have a few extra..
So I understand you questing the impact of having a slightly shorter trailing edge length for the stiffener, but I guess my suggestion still falls under the category of "if the plans say it should be this length, then it should be that length."

The key is in the name of the part - "stiffener." If you reduce or remove part of the "stiffener" then you are not stiffening up part of the area on the skin, leaving it susceptable to vibration, movement, etc.

Once they are installed you will also be putting some RTV sealant way down on those trailing edges to help prevent cracking that results from the two stiffeners rubbing against each other. To properly stiffen the skin the stiffeners should be cut to the correct length.

My take on this is that if the trailing edge of the rudder is subject to significant stresses that can lead to potential cracking, it probably needs to be as stiff as possible to help prevent that from happening. I had to re-order some additional stiffeners for my elevators after the back riveting messed up some dimples, so I now have some extra elevator stiffeners as a result.

Go ahead and place the order, cut them to the correct dimensions - you will sleep better knowing that they are correct.

Have I had to place back to back orders before? - yup. That part stinks, but I guess I formed my own sense of quality control for the build a long time ago, and resigned myself to ensuring that all the parts needed to be fabricated to spec. Funny thing is, I have even preordered some extra parts in advance of working on the original ones, because I figured I would probably mess them up in the future, based on experiences from other builders. The trim tab skin is one example.

As another heads up - if you are working on the Rudder - watch out for the marked cut lines for the rudder support bracket on the bottom. I forget the part number (R405 maybe). Vans give you similar notches on both sides of that part that need to be trimmed to the correct size, just like the stiffeners, but if you cut using those lines as the reference, you won't have enough edge distance for the rivets. Lots of posts about that here on VAF - but figured I would give you some advance warning. Maybe you can order an extra one of those with the stiffeners just in case......
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Last edited by Flyin'Bryan : 06-08-2012 at 11:10 AM.
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  #7  
Old 06-08-2012, 08:43 PM
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Subwaybob Subwaybob is offline
 
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Phillip,

I am no structual engineer but I can tell you right now that if that knocks your plane out of the sky or if even 10 areas like that do it, then I'm dead. I have rivets that are not perfect, stiffeners that may not be scratch free, and skins that have been repaired FROM THE FACTORY on my quick build. Point being that Rv's are OVER BUILT to the Nth degree.

Now, with that said the material is cheap and if your going to be flying along wondering if the tail is going to zoom past you then order it. Van's is pretty fast at getting you stuff. (Go ahead and get some more piano hinge rod too), your going to need it on the trim tab. The one they send isn't long enough and they know it!!
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  #8  
Old 06-08-2012, 08:50 PM
MJarreau MJarreau is offline
 
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Default Newbie Thoughts/Questions

I am under the impression from my build manual (RV-10) that the sealant on the rudder is not for structural support, rather to assist in building a straight trailing edge, "One way to help keep the trailing edge straight is to bond the components together before setting rivets." (Section 5H, paragraph 2.)

Another point I recently read (don't recall where and don't know its validity) was that all stresses transfer at the rivets. My question is: if the rivets carry the stresses, what good is the material in question? Is it, perhaps, to dampen vibration? Perhaps it's for adding strength to the stiffener itself? (This is for my education. I have no real knowledge of such things.)

My inclination would be to replace the parts, mostly because I have so little knowledge yet.

Quote:
Funny thing is, I have even preordered some extra parts in advance of working on the original ones, because I figured I would probably mess them up in the future, based on experiences from other builders.
HA!

Michael
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  #9  
Old 06-09-2012, 11:22 AM
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Default

That is nothing compared to the rudder mistake I made.
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  #10  
Old 06-09-2012, 11:31 AM
Rupester Rupester is offline
 
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Default No sweat ...

THis is an easy mistake to make, and a delightfully easy mistake to fix. Later on you'll smile at the thought of this minor hiccup. Wait til you get to trimming the canopy, and you realize that a significant error will cost you well over $1000.
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