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( 2.9 / 94 )I glued in the leading edge stringer as well as an additional lamination on the wing tip bow. The reason for the extra lamination was that after shaping the nose area of the tip bow, there wasn't enough area for the leading edge skin to attach. The extra 6 inches of lamination along the underside of the leading edge gave 1/4-inch of glue area for the skin after shaping. That should be sufficient for the low stress joint.
I am trying a new (actually old) adhesive - Weldwood Plastic Resin. I really like T-88, but it isn't available locally. I can get it in St. Paul, MN at Rockler, but they only carry the very expensive 8-oz. kit. I can currently get WPR at the local Ace Hardware store, and it is MUCH cheaper. I think the plastic resin works fine, though I can see where it would be unforgiving of gaps.
Speaking of locally (well, kind of local) available products, I stopped at McCormick's Lumber in Madison, WI this weekend to get some 1/16" birch plywood. They were out of the plywood, but they did have a very nice selection of Sitka Spruce. The quarter-sawn rough-cut boards were 1"x9" in lengths from 14' to 18'. I didn't have to go into the stack more than a few boards to find one that will make some nice longerons.
Yes, the spruce is more expensive than the lumber at the local lumber yard (we do still have a lumber yard in Eau Claire, in addition to Menards) - but I haven't had much luck finding wood that I would make a spar or longeron out of. The structural grade douglas fir at Lyman Lumber was one exception - I found a few 2x10s that I could have used. Each timber could have yielded a couple one-piece longerons or caps, maybe double that with scarfing and laminating. That douglas fir was better than half the cost of the nearly perfect spruce - not worth my time to save so little.
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( 2.8 / 10 )I sprayed the leading edge ply with near-boiling hot water to soften it, then used a dozen cargo straps to pull it into position around the leading edge ribs on the wing.
The radius is pretty sharp for the 1/16" bias-ply birch. It took a fair amount of coaxing to get the skin tight to the ribs, especially from 2" to 6" back from the nose on the top. In the image above you can see where I needed to use some oak boards under the straps for extra pressure in that area.
On the strut-braced wing with two struts per wing, the leading edge is not normally load carrying - the chordwise moment is directly carried by the struts. So why use the expensive 45-degree plywood? The primary reason is because the bias-ply holds shape better between the ribs with less tendency for warpage. The other reason is that it does offer some degree of structural redundancy. Even though the skin cannot resist the moment from all the design flight loads, there is a good chance that it would be sufficient to resist 1G loads in the event of a rear strut failure.
1 hr.
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( 3.1 / 9 )I cut and fit the nose stringer. The 12-foot stock I had was just a few inches short of extending from the root rib out to the wing tip bow, so I scarfed on a short extension.
I also ripped the 45-degree birch ply into wide strips for the leading edge. These strips were scarfed end-to-end to make one large piece for the leading edge skin. The plans call for two butt joints in the leading edge skin over heavier nose ribs with cap strips. Aside from the PITA scarf joints, I feel that the one piece leading edge skin will be easier to build and will form a smoother leading edge.
It is interesting to note that the 45-degree birch ply has a very pronounced sheen that the 90-degree ply does not. The 45-degree ply is mil-spec, whereas the 90-degree stuff is GL-2. I'd wager that the mil-spec material is more sensitive to needing the surface scuffed. Testing that I did with the GL-2 ply and epoxy didn't show much difference in bond between scuffed and un-scuffed.
2 hr
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( 3.1 / 9 )Cut and fit the lower aileron diagonals today.
1 hr
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( 2.6 / 73 )I attached five of the noseribs today. I also glued in a few more of the aileron diagonals. All the top diagonals are finished. When the bottom diagonals are all in, it will be time to cut the ailerons free.
2 hours
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( 3.4 / 63 )I cut out the eleven nose ribs today. They were roughed out from 1/4" birch plywood with a bandsaw then matched to my noserib template using a laminate trimmer router bit that has the bearing. These were all clamped together and stack sanded. While they were clamped, I also cut the notch in the nose for the leading edge moulding.
2 hours
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( 2.5 / 53 )I scarfed together 4 strips of 1/16" plywood to form the gusset strip covering the top of the rear spar. This gusset attaches the center and rear section of ribs to the spar. It isn't a structural necessity to scarf this part, it just makes a nicer joint.
I had a little extra epoxy, so I also whipped up a couple of gussets that hold the wing tip bow on.
2 hr
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( 2.9 / 29 )I made a jig to help shape the top of the wing tip bow to match the ribs. It is simply a straight 1"x6" that is long enough to bridge several ribs. The end is fashioned to attach a Dremel tool. I have a 1/4" collet in the Dremel with a 3/4" straight cut router bit. The bit is set flush with the bottom of the 1x6. By laying the jig perpendicularly across the inner ribs, I can follow the rib profile with the cutter.
I set the bit high at first and profiled the tip bow in multiple passes. A little follow-up with the sanding block, and the tip bow looks better than I had hoped. I profiled the bow from the front spar back to the trailing edge. I only had one spot where I wasn't careful enough with the profiling jig, and the router cut a little deep on the inside edge of the bow. It's a small nick that will eventually be beneath a gusset, so no worries.
I also made the final glue joint where the bow attaches to the front of the front spar. The bow had a fair amount of spring in it to force it into the proper shape, so I added a small filler block to reinforce the joint. Eventually the leading edge skin will act as a gusset at this joint, but it will probably take some abuse from sanding and shaping before the L.E. skin is attached.
2 hr
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( 2.7 / 36 )When I first laminated the wing tip bow, I was disappointed that it didn't look like it would fit correctly. I thought I had layed it out in CAD correctly, but when I put the bow on the wing, it looked like it most of it would be trimmed away in the profiling. I was going to fix this by adding additional laminations to the inside of the bow so that there was sufficient material remaining for the desired stiffness after shaping.
A second look at the situation determined that the additional laminations would be unneeded. I trimmed the ends of the spars to match the 3/4" thickness of the bow. With minimal forcing, I could get the bow to fit the way I wanted, except possibly for the leading edge area. I trimmed the bow and glued it in place. The resulting cross section of the bow should be roughly trapezoidal in shape - 3/4" wide with the height tapering from 3/4" high on the inside to 3/8 of an inch at the outside.
1 hr
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