Gap in upper spar splice wedges

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Kevin Kimball

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Mar 25, 2012, 8:38:06 AM3/25/12

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Darin

Further, see if the slope of the wedge blocks matches the slope on each end of the cs spar. Pinned with 2 bolts, the slope should be continuous and smooth from cs spar through to end of wedge block.

Sent from my iPhone

Kevin

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Darin Bishop

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Mar 25, 2012, 8:58:27 AM3/25/12

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Kevin,

Here are pics of the angle cuts on the -24's. I ran a pencil line down the edge of the cut to show it better.

I'm wondering looking at the last two pictures if I need to instead re-work the angle cuts on the center section spar, it doesn't feather down to a fine point at the outer edge and there seems to be a tiny gap created at the inner edge of the -3 spar too. I'm thinking that maybe the CS spar cut is just a tiny bit less sweep angle than needed?

Thanks for the help and looking forward to seeing you next weekend!

Darin

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Darin Bishop

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Mar 27, 2012, 4:21:27 AM3/27/12

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Kevin,

I figured the best thing to do was get out the calipers and machinst rule, measure all the parts and draw them in autocad so I could see their relationships and figure out where the changes need to be made.

FINDINGS:

Center spar left is .878 thick. The holes and overall length are spot on, and the left angle cut is 7.02 degrees while the right angle cut is 6.98 degrees. I found the very outboard 28.25" edges don't feather to a fine point like the other pieces, both edges are .030 thick.

The left spar is .878 thick and its cut is at 7.50 degrees. The left wedge block is cut at 7.25 degrees and its taper begins exactly on the 28.25"hole center.

The right spar is .874 thick and its cut is 7.4 degrees. The right wedge block is cut at 6.98 degrees and its taper starts .077 outboard of the 28.25"hole center.

POSSIBLE SOLUTIONS:

Assuming the wedge blocks are ok, I drew up a center section with trimmed wedge cuts, having exact 7degree cuts making a sharp knife edge at the 28.25" on both sides instead of the .030 edges. This made the wedge blocks essentially fit with no gap to the spars, but then it resulted in mis-fitting front faces of the center section and outer spars, with a .040 difference, which makes the -26 spice plates bridge uneven surfaces. I deem this unacceptable since the center spar would have to be thinned slightly to allow the -26 to fit.

The next thing I tried was adjusting the angle cuts on the center section spar, basically trimming the .030 edge to a knife edge, but tapering it so not taking anything off where it meets the front face of the spar. I can then slide adjust the outer spars a little bit more inboard to eliminate the problem wedge block gap. This would have the left spar joint at 7.25 dgrees and the right one at 7.1 degrees. The only problem with this method is that I would have to oblong the two holes I've already drilled in the outer spars. The new holes would be offset by only .050". Assuming that the 7.25 & 7.10 degree joints are within tolerance, if having first two holes drilled in the outer spar a little oval is ok, then this would probably be my best solution. I assume those two holes would be filled with epoxy, plus supported by all the other wedges and doublers etc.

If not, then last option is to get blank -24 wedge blocks and I'll put them in the bridgeport, taper them myself and put the holes in. I just need the taper to start .249 & .224 outboard of the 28.25"hole centers, instead of on the hole centerlines as the ones here do.

Thanks,

Darin

Kevin Kimball

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Mar 27, 2012, 6:47:07 AM3/27/12

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Darin,

Timing being what it is and that I am at snf, this will need to be on hold until I get back to the shop next week. The thicknesses are well within the drawing tolerances. Actually, I think the tapers are too but still I would like to have my info in front of me so I can sort things and then get back to you. It is not impossible for this to happen. However, it is the first instance brought to me by a builder out of 125 or more wing kits. I realize you didn't buy a wing kit from us having purchased some partially built wings from another person and these spars are replacement parts you recently purchased.

So, I would like you to hold everything and wait for me to get back and get with my wood vendor before doing more to these parts. Once you modify them, the cannot be returned.

Kevin

Sent from my iPad

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Darin Bishop

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Apr 5, 2012, 1:37:47 AM4/5/12

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In case you didn't get any.....

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Darin Bishop

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Apr 8, 2012, 12:35:30 AM4/8/12

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Kevin,

I thought I'd send this update here since we talked about this at SNF etc. As you suspected the wood was cut fine, and all it took was some sanding of the angle face on the center section spar. Just sanding the taper down to a fine knife edge worked good and the joints fit pretty well.

The right one still has a tiny gap, but I think will go away with a little bit more work when I get back from Boston on monday. I'm wondering how perfect to try to get it, is any gap allowed, if so how many thousandths? I know the relatively large gap I had previously isn't, but looking at my other spar I can see fine glue lines.

Thanks,

Darin

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kevin kimball

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Apr 8, 2012, 9:37:23 AM4/8/12

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Darin,

You need to fit the parts to have no visible gap. Then you can glue them up. Glad you got it sorted out.

Sincerely,

Kevin Kimball, Vice President

Jim Kimball Enterprises, Inc.

PO Box 849

5354 Cemetery Road

Zellwood, FL 32798-0849

407.889.3451 phone

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Darin Bishop

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Apr 8, 2012, 10:32:10 AM4/8/12

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Thanks will do.

Darin Bishop

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Apr 9, 2012, 4:43:27 PM4/9/12

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Turns out that tiny gap only goes about 1/2" deep because that outer spar already has taken a very slight cup. Its nothing like the spar that it replaced, but I am glad I'm going to be sliding ribs on asap. I guess wood from back east doesn't like drying out here in California.

Queston: should I sand a matching concavity into the wedge block, sand some flatness into that spar in the glue joint area, or glue it up and straighten it out with clamping pressure, our some combination thereof???

Now I know why I prefer metal work!

Darin

kevin kimball

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Apr 9, 2012, 8:45:33 PM4/9/12

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Clamp it up.

Sincerely,

Kevin Kimball, Vice President

Jim Kimball Enterprises, Inc.

PO Box 849

5354 Cemetery Road

Zellwood, FL 32798-0849

407.889.3451 phone

Darin Bishop

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Apr 9, 2012, 11:16:54 PM4/9/12

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I took that joint apart, and set that spar blank out on the lawn in the sun, cupped side down. That lawn trick didn't accomplish anything with the old spar, but it was very old, hard and dried out. I was curious if it would have any effect on fresher wood.

While it sat out there all afternoon, I took another look at the CS spar & wedge block and found that I had left the CS spar edge a little fat at that top side which probably added to that little gap shown in the last pics. I must have let it slip out of position when I was sanding it clamped to the wedge block. This time I left the bolts in, clamped around them and sanded off another few thousandths.

When I brought the spar back in and put a straight edge on it, it actually had lost most of the cupping. I don't know how long it will last but the joint clamps up real nice now without having to crank on the clamping pressure. I think until I get to sliding ribs on it, I'll put it on the lawn as needed to keep it straight while it dries out and finds its final moisture content.

Thanks for all your help as usual!

Darin

Andrew Budek-Schmeisser

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Apr 10, 2012, 12:54:28 AM4/10/12

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I think we've all learned something!

Darin Bishop

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Apr 10, 2012, 4:27:22 AM4/10/12

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Yep, when you get your wing kit, get er done! Don't let it sit around.

Andrew Budek-Schmeisser

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Apr 10, 2012, 9:37:32 AM4/10/12

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That's usually a good idea...however...

Wood hadswhat's called an equilibrium moisture content (EMC), which is specific to the species and the environment in which it's used. When it's cut it's dried to reach EMC - for the environment at the mill. After it's shipped it'll want to reach a whole new EMC. During the time in which it reaches EMC, dimensional changes are going to occur. Or they're going to try to happen. Cross-grain shrinkage in a softwood 2 x 4 shipped from Washington to Arizona can exceed 1/8" on the wide face.

Case in point, a few years ago I rebuilt the centersection for a KR-34 (think Waco 10) in south Texas, and then moved to Lubbock. The area around the root ribs is complex; lots of 'boxing'.

I didn't get back too the project for a few months after the move, and when I did, I found a crack running from the front spar root to one of the bolt holes for the root fitting. I almost fainted. It was repairable (with ply doublers and a new set of root fittings) but it was not easy. The spar had shrunk across the grain, and since the structure was glued up and the bolts were installed, it was restrained, and the shrinking process built up internal stresses that exceeded the cross-grain tensile strength. (You can see the same thing if you have a wooden deck - if the deck boards are held down with two nails, it's probably split at the nail holes, if it's gone through a few good wet/dry seasons.)

Was it avoidable? Maybe - if I hadn't installed the bolts, the structure may have remained flexible enough to absorb and dissipate the stresses without failure.

Kevin Kimball

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Apr 10, 2012, 9:48:56 AM4/10/12

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Our wood kits use all kiln dried lumber both Sitka spruce for cap strips and Douglas fir for spar material. This wood comes from the NW to Wisconsin where it is stored and machined into finished parts.

Agreed, the wood can and will change in dimension or shape as the moisture content changes. In very dry climates or very cold and dry heated work places, it is a good idea to humidify the air around the wood to minimize the changes. Once assembled and epoxy coated, the wood is more stable yet can still change size or shape if relocated to the other extreme of climate, wet or dry.

Here in FL, we have no issues of the wood drying. We used to make certified wooden aircraft propellers and had to use dry storage to keep the moisture content down to the FAA reg of 12%. We had a moisture meter to test the wood for every prop.

Sent from my iPhone

Kevin

Andrew Budek-Schmeisser

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Apr 10, 2012, 10:14:28 AM4/10/12

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And that says a lot for the quality of your kits...you do it right!

Phillip Palmer

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Apr 14, 2012, 4:37:50 PM4/14/12

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Kevin,

With the 7° sweep of the upper wing the ribs need to be "cut" so they fit on the spar properly. When I made my ailerons it was pretty easy since the ribs were small and there was only a few ribs. With the wing ribs its a little more of a big deal.

How do you guys cut/saw/file/etc. the 7° chamfer(?) in the front and reat slots of the ribs? I was thinking of making jig of some kind, but figured I'd ask the expert before I follow some crazy idea.

Phil

Phillip Palmer, PhD

Pitts Model 12 builder

www.m12builder.com

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603-321-1051

kevin kimball

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Apr 14, 2012, 8:29:26 PM4/14/12

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Phil,

the HP wing requires the ribs to be sanded 1/2 way thru not all the way across the spar slot verticals. So, the inboard edge of the front sticks and the outboard edge of the rear sticks (front and rest spar slots) get sanded half way. Draw a pencil line right down the center on the vertical sticks in the spar opening. Sand to that line. So 1/8" on truss, 5/16" on compression. We make a check tool that is 7 degree to see that we can get about 8 degrees of angle between rib and spar. That way, we have some wiggle room for sliding the ribs on.

Sincerely,

Kevin Kimball, Vice President

Jim Kimball Enterprises, Inc.

PO Box 849

5354 Cemetery Road

Zellwood, FL 32798-0849

407.889.3451 phone