Spar web update

For anyone else who might be in the same predicament, the fine folks at Homebuilt Airplanes came up with a couple more sources. B&D International carries 5′ x 5′ sheets of Finnish birch aircraft plywood, and will cut and ship UPS. Boulter doesn’t carry as wide a variety, but might be good if you’re closer to the east coast than the west.

The price at B&D is higher than Aircraft Spruce. If you price it by the square foot, it’s not terribly higher. 13%, on the 1/8″ sheet I need. Not terrible.

Spar webs

The plans and BOM call for plywood spar webs, cross-cut, 60″ long. The webs are butt-spliced mid-wing, with what seem to me to be fairly narrow blocks of spruce on either side.

Problem is, you can’t buy 60″ wide sheets of aircraft plywood in the US. Aircraft Spruce has ONE size, and not a size I need. Everything else I’ve found is in 48″ wide sheets. And since the web needs to be crosscut, it’s not as simple as just buying a 4′ x 8′ sheet. I’ve contacted Fisher Flying Products as well as another supplier in Canada. Fisher’s tab will be roughly $340, the other place was pretty reasonable for the plywood but wanted over $500 for crating and shipping. To put that in perspective, ACS will sell me the plywood I need for about $100, delivered.

The question is, can I make 48″ long spar webs work in a way that is at least as good, or preferably stonger and better, than the original design? I am no engineer. Especially a mechanical engineer. I’m certainly not qualified to make such a call. Fortunately, I have someone local who is an engineer, and is qualified to help make that decision and recommend another way to do it — if there is one. I’ll be taking the wing plans to the February EAA chapter meeting to go over them with him and see what we can figure out.

If that doesn’t work out, Spruce does sell 5/32 ply in 61″ x 61″ sheets. The spar webs are specified as 1/8″ for the main spar, and 1/16″ for the rear spar. I could just make both webs 5/32 with a slight weight penalty. The plywood would weigh 3 to 5 pounds more, but I’d probably also need to trim 1/32 to 3/32 from the spruce stiffeners, so that might make up for some of it. Again, I’ll need to consult with a real engineer to see if that wold work or not. Ending up with a plane that’s a couple pounds heavier but significantly stronger is OK. Ending up with a plane that is in any way less strong or less safe is absolutely not. And if the ribs need to be modified a little to clear a beefier spar — I’m OK with that.

Tempus fugit

Another year draws quickly to a close, with what feels like not a lot of progress on this project. I have a tall stack of ribs, but really nothing else. I had hoped to have a good start on the tail surfaces by now, but haven’t even started on them. We’ll need the basement for a family Christmas party, so no laying a 4×8 sheet of MDF over the island down there to start laminating pine. No worries, though. I have located some nice clear pine boards at Menard’s that will rip down nicely into 10′ long lamination strips. I have an order of Finnish birch ply queued up on Aircraft Spruce’s web site, ready to place the order once I’m sure I have all the stuff on there that’s going to be expensive to ship. As soon as we’re finished with the post-party cleanup, I’ll have the basement available to work in through the rest of the winter since the garage will just be a bit too cold for epoxy.

I also got a great idea from one of the guys on the Biplane Forum. I’ve been pondering how to store the wing ribs. I think I’ll build short pieces of main and rear spars out of pine and non-certified plywood, just to assemble all the ribs and clamp them together. That way I can surface sand them all together, and maybe if I’m lucky use that assembly to run them through the table saw for trimming as well. Maybe.

Just making sure…

Reluctant to toss a fair amount of epoxy, I mixed up a larger batch of about 8 CC, just to make sure I got a good mix ratio.  The first batch I had mixed to test the suspect glue was very small, and I figured I may have just muffed it.  No such luck.

I mixed 8cc, then glued two pieces of capstrip together with a totally unscientific scarf joint.  I then glued in two short pieces of geodetic brace stock, ends cut at roughly the angle of the braces in the ribs.  After 24 hours of cure time, all three joints came apart at the glue line, not the wood.  Into the bin it went, all of it.  I’m glad I didn’t use any of it for construction.  Fortunately I have another quart of epoxy here, unopened, and some new syringes.  I’m now set up with fresh glue in new dispensers. 

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.

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. 

Bending wood and building ribs

I got two false ribs glued up last night and pulled them from the forms today.  The new razor saw arrived yesterday afternoon, so that was nice to have.  I’m leaving all of the ends of the top and bottom capstrips a little long.  I am planning to just build a sled for the table saw out of MDF, with dowel pins or wood blocks to locate the ribs for the four cuts I’ll need, referenced to the main spar and rear spar.  That’s one cut for the nose, one for the trailing edge, and two for the tail ends of the false ribs.  That can wait until all of the ribs are finished, maybe later depending on how cold & crowded it is in the garage.  They really don’t NEED to be trimmed until I’m ready to assemble the wings, so that may be a while yet.

I figure I’ll get two, maybe three false ribs out of a pair of capstrips that would make one regular rib, so I’ll have plenty of opportunity to use whatever method I do finally end up using to pre-bend the wood.  Over the past couple of nights I’ve tried a couple of methods of bending capstrip.  The other night I ran a bucket of the hottest water I could get from the tap.  It was probably 140 F or so, coming from a bar sink just a few feet from the water heater.  I put half a dozen capstrip pieces in there, let them soak a couple of hours, and put them on the bending forms.  That seemed to work well, with no broken pieces.

Last night I added a gallon or so of hot water to the bucket, just to top it off some more.  The water would have been lukewarm at best.  I soaked another half dozen strips for 12 hours or so.  Of the six, one snapped as I was bending it in the form.  That’s about what happened the last time I tried soaking in cold water, so obviously that’s a waste of time and good spruce.  Won’t make that mistake again.

Well, that was dumb.

I was filling the 60cc syringes I use for dispensing T-88 epoxy in an exact 1:1 ratio.  Typically I overfill them a little, then stand them on end tips up to let the bubbles rise to the top, then squeeze a little back into the bottles to get the amounts in each syringe evened out.  Yeah, it’s a little picky, but it lets me easily keep the ratios exactly the same.  Anyway, I got a little distracted, and accidentally squirted about 4cc of hardener right into the half-full bottle of resin.  Oops.  All I could do without making things worse was let it sit.  I thought about trying to scoop it out with a long handled spoon, but decided all I’d really do was mix it in.  I figured either I’d end up with a hardened mass on top that I could scoop out later, or it would ruin the entire remaining half bottle of resin.  That would be about $22 worth of epoxy…  not the end of the world, but it would kinda suck.  I was hoping the hardener would not slowly sink to the bottom, ruining resin as it went.

Well, I got lucky (or science worked in my favor).  Today there was a gooey layer of semi-cured epoxy sitting on top of the resin.  I was able to scoop it out using a chunk of scrap spruce.  Assuming things don’t go south in the next few weeks until I need to refill the syringes again, it looks like things will be OK.