Got a couple more false ribs glued up, now I’m waiting for the epoxy to cure. I also unrolled the plans to get a good look at the wing layouts, and re-read the instructions for building wing ribs. The instructions say to make 42 main ribs (I have 43) and 37 false ribs. These two make 20, or it could be 21 if I cut down the extra main rib. I’ll just keep making them and see what happens. I really need to get that second jig back on the bench so I can do 3 at a time.
False ribs = true :)
Well, I set up the second jig for three false ribs. I’m only making two at a time on that one, though, plus one on the first jig, so three at a time. One of the three setups on #2 is just not quite “there” enough to suit me. To be honest, I don’t really know how much difference it makes if there’s a 1 to 2 mm variance in the shape of the lower or upper surface of the airfoil. I’m getting the ribs all as close to identical as I possibly can, but I know I’ll need to do some minor true-up work once they are all ready for assembly to the wing spars. What I think I’ll do is stack them all together on some scrap wood cut to match the size and shape of the spars, then use a long sanding block to true everything up. That’s a while of, still.
In the mean time, I’ve got around a dozen or so of the false ribs done, and am cranking out 3 per day on the days I get any work done. Now, however, I’ll have to take a day off to hot-soak and bend some more capstrip. I’m out of the pre-bent pieces.
Minor milestone – the last full rib
Sitting in the rib jigs right now are the last aileron rib (#16 of 16), and the sixth false rib. After the glue cures on these, I’ll modify the second jig to fit as many false ribs as possible. I think I can fit three on it for sure, possibly four – but that’s doubtful. I’d like to start cranking out four false ribs at a time.
So I guess it’s time to go start shopping for the best plank or two of white pine I can find to get started on the laminations for the tail surfaces. Pretty sure Menard’s will be my source for those; the stuff I find at the other big box stores is more suited for a dog house or the pulp mill than anything else.
Ready for ribs
Well, the work bench is cleared off — well, at least enough to get both jigs on it. I’ve got a couple dozen sticks of geodetic brace stock shaved down. The epoxy syringes are filled. All I need now is a razor saw and a steam setup.
Yes, I have a razor saw. Somewhere. I pulled it out during the kitchen remodel, and I remember seeing it in a box of tools we were using, but now I can’t find it. It’s a Zona, good quality but inexpensive. A new one is on the way. Two, in fact; one medium and one fine tooth. I also have a steam box that I built for the capstrips, but I’m unsure whether I’ll use it in the basement. It takes bench space (which I now don’t have down there), and drips water out the end by design. I’m thinking about ways I can use it vertically. I had looked at rigging up a piece of pipe or something with a heating element for hot water, but it looked like an awful lot of extra work considering I’m over halfway through the ribs. I’ll probably need it for the tail and wingtip bows as well, but I’m not entirely sure yet how I’m going to steam 6 to 9 foot long strips of wood. Tonight I tried soaking half a dozen capstrips in a bucket of hot water for an hour — we’ll see how well that worked.
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.
Back at it – more ribs
Got the basement workbench cleared off enough to move rib production indoors. So far I’m only using one jig, but I’ve knocked out two ribs now. I need to get the second jig set up. One rib per day isn’t going to cut it. Sure is nice working in the basement instead of the garage though.
It’s damn cold outside!
I’ve been wanting to get back to building ribs. The garage bench has been piled too deep to get anything done, and now it’s been below zero for over a week solid. Insulation or no, it’s damned cold out in the garage. Too cold for epoxy to cure, and too cold to work.
So… there’s a perfectly good work bench down in the basement. 8′ long and rock solid, built by Dad back in the 1960s. It’s been pretty much completely covered up for the past several years with a collection of parts, partially-disassembled or -assembled prototype projects and assorted debris from the side business I was running, plus a CNC router that took up about 3′ of it. I have cleared most of it off (much of it into a trash can). The CNC machine is shelved for now; I’ll maybe resurrect it when needed at a later date. It will probably come in handy for cutting the instrument panel and/or nose ribs.
The next issue is containers for the geodetic rib braces. There are 24 braces used in each rib, all of which are of course slightly different. Just enough so that none are interchangeable. I was using paper cups to hold them in the garage, but it’s a completely unsatisfactory solution. In the basement I think I’m going to try stapling taller plastic cups along the back edge of the bench and see how that works.
I’m not moving power tools down there from the garage, so new geodetics will be cut with a razor saw. We’ll see how that works out… it’s one of those jobs I wish I could set up a machine or fixture to do. It’s 24 different lengths with over 40 different angle cuts, so I’m not sure how I’d make that work. Even cutting them out in batches on the band saw meant fine-tuning each one with a sanding block before assembly. It’s fiddly work and tedious as hell. Classic gusseted ribs would be a whole lot faster and easier to build. If I were starting this over I’d probably just depart from the plans and build them all that way; I’d probably have them all done by now.
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.
Trying out the steam box
The other night I finished up the steam box and made a set of little standoffs for the bottom. I wanted to keep the wood off the bottom of the box to allow air circulation, but also allow water to run back to the low point at the rear. I got a little lazy and didn’t put in dowels like most plans show.
Tonight I loaded it up with eight cap strips and let it run for 45 minutes while I went out and hung some outdoor lights on the deck. When I came back I found that the door had developed a pretty good curl — concave from the outside – and was venting a lot of the steam past the door seal. The spruce inside was fairly pliable, so I clamped three pieces in my bending form and put four more in the rib jigs. They would only fit partway down in the jigs, since the wood is swollen. While the wood cooled down I got all of the geodetic braces ready to install. I’ll glue these two ribs up tomorrow night.
I did email System 3 about their epoxy. Some of the ribs were assembled with the capstrips not pre-bent. They’re fine, but the wood will spring out of shape if it gets hot enough for the epoxy to soften. I don’t want that to happen. so I was thinking about clamping them in their current shape and steaming them to relieve some of the stress in the capstrip. Of course that will soften the epoxy, and I wanted to know what that would do. Their answer is that T-88 will begin to soften at around 120 degrees, but once returned to room temperature will be at full strength. That’s great news.
I’m definitely not in love with the geodetic rib bracing design. 24 braces, no two alike, and I’m having to hand-fit them all. It takes for-freaking-ever. I have to wonder how much heavier 3/32 plywood would be with suitable lightening holes. A guy could pop sets of them out on a CNC router in no time, save a ton of time and effort on the ribs, and probably end up with much stronger ribs too. But, I don’t think I’m going to re-engineer the ribs right now.
Second jig built
Yesterday I did a little shop cleanup. Not a lot, just enough to get done what I wanted to do. I cut a length of 3/4″ x 3/4″ pine and cut a bunch of 1-1/2″ blocks for the second rib jig. I got the work bench partially cleared off. The box from the R/C Cub went underneath, and the plans sheet and wings were moved to one end of the bench. I now have roughly half the bench covered by model airplane wings, while the other half is being used to build the real thing. ‘Murica, as my kids would say.
So, I built the second wing rib jig. I used the first rib from the first jig and blocked everything up. I used some plastic sheet Lisa had picked up underneath it to prevent the glue sticking things together. When it was all finished, I mixed up 20cc of epoxy and glued up two ribs. That’s going to work as long as it stays cold, but the glue was gelling by the time I was finished. If it warms up at all there won’t be time to do two at a time, unless I can find a way to pre-glue the capstrip grooves a lot faster. I wonder if thinning out a portion of the T-88 and brushing it into the grooves would work. For that matter, I wonder if pre-gluing the grooves is even necessary.
This morning I pulled both ribs from the jigs. You can’t tell them apart, and you can’t tell them from the other four I had done. Any differences between the six ribs I have done are slight enough they can be removed by sanding. So, now I can double my production rate.