Progress, up to a point

During two working sessions today I got most of the assembly of the lower main spars complete. “Most of”, because when it came to laying out the plywood blocks for the landing wire attach points… well, after a while studying the parts and the prints and the spars, none of it makes any sense. The upper wing spars were not exactly straightforward; a few of the parts didn’t match the plans, but the end result looked correct in that the location and final layout of the blocks matches the print, even if the parts making up those blocks are a bit different. I’ll need to spend more time studying the plans for the spars, as well as the plans for the N-struts and landing wires to figure out what needs to be done.

I also ordered a gallon of T-88, hopefully I’ve got enough to last me until it gets here mid-week.

Edit: After looking at print #15 of the plans, things are quite a bit clearer. That drawing shows the flying wire, landing wire, and N-strut attachments, with part of the spars shown. Tomorrow I’ll be able to glue in the blocks for the lower main spars and, hopefully, get them finished. Well… “finished” is a relative term, I suppose. I haven’t started attaching the parts for the wingtip bow support, but I may just hold off on that for the time being.

Main spar progress

It’s been a busy week. The two upper main spars are essentially complete. Spar caps, stiffeners, and all of the filler and reinforcing blocks are assembled. The only thing not done are the outboard ends, where the tapered pieces for the wingtip bow attachment need to be assembled. The two lower main spars are under construction now. Both have the caps and web glued, and one has the stiffeners on one side. I’m planning to finish as much as I can today on the main spars, and hopefully get at least one of the rear spars glued up.

It’s been a real joy working in a place where there’s plenty of room to work. The availability of a large number of bar clamps – in addition to the 40 or so spring clamps I have – really helps as well. I took the day off work yesterday and was able to get over there once in the morning to set up a spar, then again in the afternoon to take that one off of the jig and glue up the next one.

Stu is anxious to get enough pieces built to start getting wings assembled so they look like wings — I can’t say I’m not in agreement. I was a little tempted to build a rear spar after the first main so we could assemble one complete wing, but I’m convinced that this way is a lot more efficient. For one thing, after the first spar we’ve both figured out enough ways to make the process better that each spar is taking less time than the one before. Just as an example, my process for getting epoxy into the grooves in the main spar caps has evolved and gotten much more efficient with each spar. It’s almost a shame there aren’t any more to do. On the last couple I would get some glue on the long edge of a popsicle stick and use that to spread it into the groove. After doing a foot or so, I’d slowly follow through with an acid brush with the bristles trimmed down to about 1/4″ or so, to make sure it was evenly distributed and get any excess out of the groove. The result was the cleanest glue joints of all the spars.

I’ve been going through T-88 like crazy. Gluing 9 and 10 foot long pieces of wood uses a lot of epoxy, and I know the laminations for the wingtip bows and tail surfaces will use a lot as well. Given the price increases since the last time I bought glue, I think I’ll just order a gallon.

Started on the second spar

This morning I went over to Stu’s and finished gluing the root attach blocks and the blocks for the N strut and flying wire brackets to the spar. Other than the tapered extension for the wing tip bow, that’s all of the wood for the first spar. Once the glue is cured, I can start locating and drilling the holes for the attachment bolts and bushings.

With that done, Stu and I started on the second upper wing spar. The bench has threaded inserts set into the edge, and he’s got a long steel rail that bolts on. It’s 2 x 2 inch square steel tubing with mounting brackets welded on. The rails were designed and built to bolt to the table, then he’s got a steel sled that sits on them that he can use to flatten a large wood slab using a router. We bolted one rail to the edge of the table, overhanging the work surface. Now there’s a long work bench with a straight, level steel rail on one edge.

We glued the groove in the spar cap, inserted the plywood web, then glued the other spar cap. One spar cap is clamped against the rail, then the assembly is clamped in place to hold it straight, flat, and properly spaced at 5-3/4″ total spar height along its entire length.

I think about 10 ml of epoxy is plenty to glue one spar cap groove. I mixed two batches today. I’ve been using 60 ml syringes ordered from Amazon to hold and measure the glue – it works wonderfully. I can very precisely measure out even very small quantities.

First spar, continued

It’s been a bit of a learning experience building this spar. The amount of glue required, of course, is one thing – about 45 ml or so, I think, for the whole thing. There are still a few pieces of plywood that need to be glued, most of them for the flying wire and N-strut attachment brackets. Yesterday Stu showed me his pneumatic pin nailer, which I plan to use. It’s not unusual to use staples or small brads to hold wood together while glue cures. This thing shoots ridiculously small wire pins — 22 or 24 gauge, I think — with no heads. They’re just enough to hold the wood in place, and won’t need to be removed.

I also found that the spar has about 1/8″ of gradual taper toward the tip end. That was a builder error; it’s supposed to be a constant 5-3/4″ across the entire length. When I assemble the other three they will be correct. It’s not a critical error; I can fix it with shims when the wing is assembled. It’s even noted in the plans that ribs may be shimmed where they attach to the spar. It’s just been an inconvenience. Before I found that taper we’d cut some reinforcing blocks that fit between the spar stiffeners to a constant length. Some needed to be sanded, some needed to be re-cut because they were too long. That’s when I measured the spar height more carefully. I had just measured one end and a spot partway down the spar before — I guess it didn’t occur to me that it would have a taper to it. Everything else is good, there’s no bend, twist, or warping, so it’s good to go.

Other minor things — the plans call out 120-1/4″ for the spar length, but the main caps are 120″. Obviously the quarter inch at the tip isn’t going to make a difference, I’ll just make sure both spars are exactly the same length.

I’ll go over tonight and finish the assembly of this spar, and hopefully get started on the next. I also need to find my box of 4130 steel tubing and see if I’ve got the stock I need to make the bushings for the wings. There are bolts through the wing that hold the attach brackets for the fuselage, struts, and flying & landing wires. All of the holes need steel bushings through them. Simple enough to cut and file to size; I just need the right size tubing. I bought a “grab bag” assortment of random tubing cut-offs from ACS a while back to see if I could learn to weld. If I’ve got a couple feet of the sizes I need, I can use them for this.

Gluing up a wing spar

Yesterday I headed over to Stu’s to glue up all of the pieces to the aft face of the upper right wing spar. There are two full length spruce spar stiffeners, four pieces of plywood at the wing root, spruce blocks placed at each rib location, and four plywood pieces for the flying wire attachment brackets. Then I’ll get to flip it over and do the same on the other side.

When I was building wing ribs, I found that a total of 8 ml or so was about right to make two ribs. Since I did that a lot, I got used to mixing up small batches of T-88 epoxy. Yesterday was the first time I have had to mix relatively large quantities — I went through two 20 ml batches, which is about the most I can do in the little medicine cups I bought for this. Even that left me slightly short. There are a couple of plywood attachment pieces that I didn’t get installed yesterday. System Three makes auto-mixing dispensing cartridges, but they’re substantially more expensive to use. As in, over six times the cost. I think I’ll be sticking to mixing cups. I may need to get some larger ones, though — mixing up 20 ml at a time isn’t going to work for a lot of this stuff.

Stacking them up

Nothing new, but last night I gathered up all 47 full ribs and stacked them up, carefully aligning the cross pieces for the main spar, aft spar, and nose spar.  I was a little concerned about the amount of variation I might find, considering the ribs came out of two jigs over a period of 20 months.  I was very happy to see that there’s no more variation than could be attributed to wood thickness.  Less than 1/64″, probably under .025″ anywhere along any of the ribs.  Then I stacked up the dozen false ribs I have done so far, same story. 

Wait – 47 full ribs?  Yes, it seems somewhere along the line I must have lost count and built one extra aileron rib.  I’ll either keep it around as a wall hanger/test piece, or maybe stick it under the wing walk for a little extra support.  I’m not sweating it.

This exercise has also reinforced my idea to build a sled for the table saw to lop off the ends of the ribs for the leading and trailing edge pieces.  They are very highly consistent, as they should be, so I think that will save time and make subsequent assembly steps easier.

The bad news is — my T88 epoxy is junk.  I refilled the dispensing syringes and the resin looked a little “off”, not perfectly smooth and clear as it usually is.  Maybe a little cloudy and with a different texture.  I mixed up a small amount and glued up some test pieces.  Where I glued the flat sides two sticks of capstrip together, the joint held well and pulled apart well into the wood.  However, where I had glued a couple pieces of geodetic brace into the slots, the glue joints failed allowing the braces to pop out without breaking.  The glue also was not clear and hard, as it should be, but had an almost grainy or mealy texture to it.  I don’t have any trust in the remaining glue in those bottles.  I’m certainly not going to risk using substandard adhesive to save about $20 worth of epoxy!  Into the trash it goes.  I have two new bottles here, and half a dozen new 60cc syringes on the way from Amazon.

Back to working in pairs

I finally went out and got the second rib jig from the garage and brought it down to the basement.  Ribs 24 & 25 are now curing, they will be ready to pull tomorrow evening.  My new routine:

  • Put the first jig on the bench.  Cut all the pieces fro one rib and set the geodetic pieces above and below the rib caps, ready to glue.
  • Stack the second jig on top of the first, cut all the pieces for it.
  • Mix up enough glue for both ribs (14cc seems about right)
  • Glue up the rib in the top jig and set it aside.
  • Glue up the rib in the bottom jig.

Double the completion rate.  I’m out of pre-cut geodetic pieces, so now I”m marking and cutting them individually with a razor saw.  It doesn’t take much longer, really, and each one is cut exactly to fit.

I’m almost out of pre-curved top capstrips.  The comments on the plans say you can bend the spruce dry and glue the ribs.  I don’t like the idea of having that much tension on the wood.  The nice thing about working indoors, though, is that I can keep hot water hot longer.  When I run out of pre-bent pieces, I’ll try the hot water bend method again.  In the garage the water cooled off far too quickly.  In the basement, I can get REALLY hot water in a capped length of PVC, and keep it hot long enough to soften the capstrip.  System 3 says T-88 works fine on damp wood…  I’ll try that and see how it goes.

Another rib

Just so I could feel like I’m not totally stalled, I built rib #15 this weekend.  6cc of glue turned out to be just about right, with no skimping and very little excess.  The new method of mixing and applying the epoxy with an acid brush worked out well.  I think I will go back to an earlier idea I had, of pre-wetting the groove with glue before putting the top and bottom capstrips into the jig.  It will be easy to do with the brush, and will save some assembly time.  I have a template with pencil marks defining where the glue should go; I think I’ll mark the glue lines along the bottom and top of the jigs to make it easier to apply the glue.

Mixing epoxy

I’ve been mixing epoxy in the little plastic medicine cups using “craft sticks” (think Popsicle stick).  I thought I was doing a pretty good job of stirring, spending some time scraping the sides and swiping along the bottom corners of the cup to ensure everything was mixed up.

Last night I mixed up about 2 cc of epoxy, mostly just to try out my new syringe dispensers.  I let it sit in the cup with the stick propped up in the center.  Today I popped the slug of glue out of the cup.  Lo and behold, there are sticky spots around the edge!  Nearly all of the glue is perfectly hard and cured, but there is just a tiny little bit of uncured epoxy right around the bottom edge.  Apparently the stir stick is not a good tool for making sure ALL of the material gets mixed together.

In light of that discovery, today I did another 2 cc sample.  This time I did the mixing with an acid brush with the bristles trimmed down to about 1/4″ to 3/8″.  After mixing I cleaned the brush out with a little MEK, just to see if that would be worth doing or not.  We’ll see how that one turns out after the epoxy has had a chance to cure.  Acid brushes are pretty cheap, so even if the MEK cleanup doesn’t work out it’s not a big deal.  You can buy the brushes by the gross for around $0.12 to $0.15 each.  But what will I do with about 950 craft sticks?  Good thing I have grandkids.


Prep work

I’ve been away from building for a while dealing with a remodeling project, the annual condition inspection on the RV-12, Oshkosh and few other odds & ends.  At Oshkosh I discovered that the wood shop guys had actually saved my miniature wing rib from last year (2016).  I brought it home with me, of course.  Last night I pulled the staples, cleaned it up and sanded it, and gave it a coat of clear polyurethane.  I figure it will make a nice wall decoration for my office.

Last night I also started cleaning up some of the enormous mess in the garage so I can get back to building.  I’m not quite there yet, but tools are slowly making their way back into drawers and onto pegs.  It may take a few more days, but it will be nice to get back out to a garage that’s not a complete disaster.  There are 14 wing ribs hanging on the wall, and the guy on the Biplane Forum is catching up with 11 finished.  He can work faster, since he can pull his ribs out of the jigs as soon as they are stapled.  I need to step up my game!

I did pick up a good idea from that discussion — using cut down acid brushes to mix & apply the epoxy.  They’re cheap, and it looks like I might be able to get more precise application of the glue where it’s needed, in the amount needed, with good penetration into the wood grain.  I’m anxious to try that, I have a handful of brushes here.  And, I got some of the epoxy  into 60 cc syringes so I can dispense exactly the same amount of each component.  I know it’s not terribly critical with T-88, but I still want to be as accurate as I can.