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  #31  
Old 09-28-2014, 07:05 AM
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aarvig aarvig is offline
 
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Default Seriously you guys...

We can argue this till we are all blue in the face. You can't rivet two test pieces together, wiggle them and say it feels tight and call it good while assuming what you just did compares to the stress loads experienced in flight.
ARE WE FORGETTING THAT VANS RECOMMENDED REPLACEMENT??? Don't confuse the builder, the precedent has been set. Replace the d@%m spar and move on.
Its only a couple thousand bucks, it was an expensive mistake that we can all learn from.
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  #32  
Old 09-28-2014, 07:34 AM
brad walton brad walton is offline
 
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I think it is useful and educational for us all to understand why this is unacceptable. It is not, in my mind an issue of questioning Van's engineering staff, rather an issue of knowledge to be applied here and elsewhere in the construction process
  #33  
Old 09-28-2014, 07:39 AM
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aarvig aarvig is offline
 
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Quote:
Originally Posted by brad walton View Post
I think it is useful and educational for us all to understand why this is unacceptable. It is not, in my mind an issue of questioning Van's engineering staff, rather an issue of knowledge to be applied here and elsewhere in the construction process
Yeah, you're right and I do agree that it is beneficial but if you re-read some of the posts it may cause someone to believe they DON'T have to replace the spar. That maybe there is some wiggle room. While in some areas of the aircraft wiggle room is perfectly acceptable...the spar is not one of those areas.
I know from my own experience that I can take what I read on this forum as solid gold advice (there are certainly people on here that I now know who I would take their advice as solid gold). Other people may just be "thinking out loud." I think we need to carefully clarify that is our intent so builders don't unwittingly take advice from this forum that may do them or someone else harm. The OP clearly states that he is having a hard time believing that .009 over is a deal breaker which tells me his intent is try to garner enough advice on here to give him confidence to move forward with not replacing the spar.
I'm just saying that we need to clarify our position on something this serious. If Vans has recommended replacement that is all the advice any of us need. Beyond that we can "think out loud" but we need to be clear that is what we are doing. The Vans staff are the foremost experts on the aircraft. I certainly don't believe they are recommending replacement to generate the sale of a spar.
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Last edited by aarvig : 09-28-2014 at 07:46 AM.
  #34  
Old 09-28-2014, 08:32 AM
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Mark Dickens Mark Dickens is offline
 
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I can't believe this thread. How can there be any reasonable discussion about not following the factory's advice? I have yet to see any accredited aerospace structural engineer (and I know there are several here) pop up and tell you to ignore the factory.

I know I've spent more than the $1600 and change over the years on various replacement pieces. I can't imagine that I'd contemplate taking a chance on the WING SPAR (for cripes sake!) just to save a few bucks. If you don't have the dollars to replace the spar, then you need to sell the kit, because believe me, you don't have the money to complete it anyway.

Make the right decision.
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  #35  
Old 09-28-2014, 09:42 AM
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RV7A Flyer RV7A Flyer is offline
 
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Quote:
Originally Posted by N427EF View Post
Quote:
Yes I have a dimpled test piece which nests nicely into the countersink with no play.
Maybe all this is based on miscommunications.
My question, as well. How many people built their planes by countersinking *exactly this way* instead of using micrometers on every countersink???

The OP also said none of the holes are knife-edged.

Sounds to me like he did it exactly the way thousands of other people have done it, and likely the construction manual says (don't recall, don't have it handy, but I don't remember seeing anything about putting a micrometer on a countersink...but I built a -7 started in 2007, so my plans may say something different).

It's worth communicating again with Van's, though...just to make sure *and* understand why they're advising replacement.
  #36  
Old 09-28-2014, 11:07 AM
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Snowflake Snowflake is offline
 
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The reason a dimpled skin will sit in a slightly too large hole without slop is that every dimple has a slight radius at the bend, because it's a bend. Every countersink has a sharp corner at the top, because it's machined. In order for the skin to truly sit flush against the surrounding metal, the diameter of the countersink at the top of the hole would have to be larger than the diameter of the dimple in the skin... At the outermost point of the radius of that bend.

That means that a skin that sits *completely* flush will have a dimple that does not contact the taper on the countersink. Drive a rivet in there, and it won't do much to spread the dimple out against the taper of the countersink... It'll just hold the assembly together, with a couple of thou clearance around the dimple. That gap, in time, will lead to the smoking Scott refers to.

In contrast, if your dimple perfectly matches your countersink, the skin will sit a few thou high off the mating surface. More if you're not using spring-back dies, less if you are (spring-back dies bend a sharper corner). During riveting everything will snug up that last couple of thou as the riveting action will form a sharper corner in the skin using the countersink as a form (actually, as they're both aluminum, they probably both deform and just create a tighter radius).
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  #37  
Old 09-28-2014, 11:11 AM
BillL BillL is offline
 
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Quote:
Originally Posted by rvbuilder2002 View Post
From my understanding the OP is aware (now anyway, possibly not when the countersinking was done) that when countersinking for insertion of a dimple instead of a rivet, that you go approx. .006" - .007 deeper (approx. because it varys slightly depending on the rivet size and the thickness of the skin).

He has stated that with a rivet inserted in the hole (as shown in his attached photo), the measurement to the top of a rivet head, below the surface of the spar web is .015". This means that the countersink is about .009" too deep (2.5 times what it should be).
Quote:
Originally Posted by RV7A Flyer View Post
My question, as well. How many people built their planes by countersinking *exactly this way* instead of using micrometers on every countersink???

The OP also said none of the holes are knife-edged.

Sounds to me like he did it exactly the way thousands of other people have done it, and likely the construction manual says (don't recall, don't have it handy, but I don't remember seeing anything about putting a micrometer on a countersink...but I built a -7 started in 2007, so my plans may say something different).

It's worth communicating again with Van's, though...just to make sure *and* understand why they're advising replacement.
Reactionary posts and decisions are best avoided and confronted with facts. NO ONE HAS REFUTED VANS RECOMMENDATIONS. Scott has said why (using the OP's dimensions) the dimensions are important from a structural stand point.

The real question (at least for me) is around the original measurements. I have tried to measure these danged countersinks and wished many times for a proper depth gage - I found one on ebay(with calibration ring). I tried many times to measure countersinks with diametral and protrusion and it resulted in many different dimensions. The best one is a rivet and a very straight edge to see if it is proud or sunken. Flush is easy to quantify, larger/smaller is not.



If the OP is 100% satisfied with the measured dimensions, then the case is closed. This is important, as it is a significant undertaking to replace the spar and the opportunity for additional issues is increased.
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and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.”
  #38  
Old 09-28-2014, 11:21 AM
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Sam Buchanan Sam Buchanan is offline
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Quote:
Originally Posted by aarvig View Post
If Vans has recommended replacement that is all the advice any of us need.
Well...maybe that is all the advice some of us need, but I wouldn't say that is the case for all of us.

I'm not making a statement on the validity of replacing the spar, just reminding us that there is a very wide range of expertise within our ranks. For the builder who has no prior experience constructing an aircraft, then Vans's recommendation should not be taken lightly.

But we have individuals in our community who have built multiple aircraft, some from kits, and others scratch-built from plans, and maybe even some on the gov't's nickel. These very experienced builders who are accustomed to making not only decisions about whether or not to scrap a part, but also how to fabricate the part, are going to be more open to discussions about why a component should be replaced.

Are those who are so strident about following Vans's guidance doing exactly the same when it comes to avionics and firewall-forward decisions? We need to be consistent, and cognizant of the wide range of expertise and risk management tolerance in our community.

Just thinking out loud....
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  #39  
Old 09-28-2014, 11:37 AM
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apkp777 apkp777 is offline
 
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I didn't like countersinking the spar on my -9. I thought it was a questionable way to assemble the skins to the wing. I would guess there are a lot of RV's flying that have a least some degree of knife-edged holes similar to yours. One thing for sure, I'd make sure that "properly" CS holes are significantly better than your existing ones before I would replace the spar. I hate to see you get a new spar(s) only to find that they turn out the same as the old. Also, I'd get some pics of some other RV's spars (preferably flying versions) and see if others have perfectly airborne spars that looked much like your at that stage.
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  #40  
Old 09-28-2014, 11:44 AM
AlinNS AlinNS is offline
 
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What an absolutely timely post.

As a new builder, this touches on my biggest concern with building my new kit. When is a rivet/countersink/dimple too deep or not deep enough? Easy formulas and ways to measure the width/depth of a shop head but what about the measurement we are talking about here. I can't seem to measure it consistently even with my micrometer. I have started just putting a straight edge on the rivet head after riveting and seeing if its flush. This doesn't tell me if the tolerances on the countersink are accurate, or does it? So far I'm just playing with scrap and sneaking up on what seems flush to me.

I'm not at all interested in debating what Vans has to say. With my obvious lack of experience, I would not question them.

I'm betting I'm not the only one reading this thread that would like to know how some of you real experienced folks approach measuring countersink depths and what exactly you want to see in the finished joint. I certainly haven't found much info on the topic.

Anybody help out with this? Should this be a new thread?

Thanks All!
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