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prkaye

Well Known Member
Perhaps I should have been more diligent about making my center-section spacers more exactly. After rolling the canoe, I took out the spacer blocks and measured the gap with a digital caliper. My spar (clamped together) measures .442. The exact readings of the center-section opening depended on where I took them, but in the worse case at the top of the opening is 1.486 (too wide by .044), and at the bottom the opening is 1.415 (too narrow by .027).

I emailed Vans about this, and apparently "this may make things difficult". They have asked their engineering department about it and I should hear back from them in a couple of days.

Has anyone here experienced this much of a discrepancy in the carry-through spacing? Did it work out OK in the end?
 
After riveting the bottom fuselage skins to the center section and then riveting the fuse side skins to the center section uprights (ALL HOLES PREPUNCHED), I wondered why the 4 spacers had to be 1.4375 thick. Maybe for the control column thru bolts? Certainly not so the wing spar can easily slide in and out. I'm thinking if the gap between the front & rear center sections is too narrow, there's something else going on.

Steve
 
Yeah, with it being pre-punched I didn't think there was a need to be so precise about making the spacer... maybe there's just that much variability in the match-drilling process.
 
Steve said:
I wondered why the 4 spacers had to be 1.4375 thick. Maybe for the control column thru bolts?
Click to expand...
Hi Steve - I'm at the point where I need to make the spacers and I was wondering this also. What tolerance would be acceptable? 1.438 +/- 0.002? or maybe 0.005?

It looks like the forward bottom skin (F-972 in my case) will located both halves of the center section fore & aft, and the center section side plate (F-905) will do the same for up & down. It's a pretty stiff piece of metal.

I guess I'm wondering why I even need spacers? Is it because the skins / spacers aren't always on there to hold everything in alignment? Or is it because over the sizes involved the pre-punched holes aren't accurate enough?

Just wondering... I may have a machine shop build my spacers for me.

dave
 
Another local builder machined alum spacers that are the exact thickness of my spar. I'm going to borrow them briefly and see if I can get them in my center section. If not, I suppose the best thing is to drill out the skin-center section rivets in that area, stuff the spacer in, and re-rivet with the spacer in there. Hope that isn't required though.
 
false spar/spacer thickness

Phil,
prior to the prepunched kit, builders (like me) had to make a false spar/spacer to set this distance.
On the Matronics e-mail list you will find a debate about acceptable tolerances (search false spar and thickness). A Van's employee contributing to the list at that time offered the advice that (on an RV-8) in his view .010 - .015 greater than actual spar thickness would be OK, but .040 would be too much given the nature of the joint.
I made a spacer of oak, planed down to exact spar thickness - verified by measuring the spar, and confirmed by adding up the thicknesses of the component parts of the spar.
For an alternative method, see Sam Buchanan's excellent web site.
I think that for tolerances of this scale, one cannot safely rely on the "pre-punched" clecoed holes to keep things close enough.
Bill Brooks
Ottawa, ON
Canada
 
I used spacers and if my memory is correct, when I inserted the clecoes there was a slight misalignment of the bottom skins pre-punched holes with the holes on the centre section spar flange. In this instance I had first started putting the clecos in at the aft part of the bottom skin and worked forward. I decided to take all the clecos out and this time started clecoing at the spar. This seemed to work much better and assured the pre-punched holes lined up at the spar with the spacers installed. So I am assuming that if I had not used spacers and just relied on the pre-punched holes then the gap may have been incorrect??

Finley
9A
 
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no spacer

I did not use a spacer and the wing spars fit fine... snug, but fine.

Jason
 
I made my spacers from nylon blocks that I milled to 1.4375 +/- .005 and using all of it. While both wing spars are a slip fit into the center section, one wing does fit a bit tighter than the other.

Steve
 
Yesterday I borrowed some alum spacers machine by another local builder, that are exatly 1.442 (+- 0.0005). When he took the spacers out of his center section (he built the fuse with these in-place), the gap sort of pulled itself closer together as he pulled the spacer out. To get it back in he had to push, but it could be done with one hand and not too much effort.

So I took these spacers home and tried them in mine. At the bottom of the center secton, on one side the spacer fit quite well... one handed slip-fit (a bit of resistance, but not hard to push in). On the other side at the bottom the fit was a fair bit tighter. So on that side I removed about 30 rivets around the area, put the spacer in and re-riveted with the spacer and close-tol. bolts in place. Now the fit in that one is similar to the other side... snug but i can push the spacer in pretty easily with one hand.

The tops of my center section are quite loose... the spacers will not stay in place by friction. However, along the top bars of the center-section, there is considerable play and flex - the gap can be closed with one hand gripping the bars of hte center section, and squeezing together. Certainly after I put the close tolerance bolts through, it closed up and everything is snug. Looking closely at it, this does not transfer shear loads to the skin-to-center-section rivets up on the top part... there is considerable flex between the thick center section bars and those relatively thin vertical plates on the outboard sides of the center section halves.

So - in a nutshell, I believe the spacing is more critical along the bottom of the center section, where the rivets along the bottom skin lock things pretty much in-place. But there is a lot more natural flex and play in along the top, where it's only the side-skin rivets holding things in place (and eventually the wing bolts of course).

As Ken from Vans pointed out to me, one of the potential problems with sloppy spacers is that the wing bolts may not fit properly. I've verified that my close tolerance bolts go through both halves with no problems (except getting these close tolerance bolts through each hole requires some persuasion with a hammer).

At this point I know my wings will fit without applying any serious force, and my bolts will go through, so I'm going to build-on.
 
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