Work continues on the spars. Over the past few days I’ve put the wingtip bow supports on all but the upper main spars. Hopefully I’ll do those today or tomorrow.
I’ve been looking for the aluminum stock to make all of the wing attachment fittings. I need a mix of 6061-T6 and 2024-T4, in two different thicknesses. The 6061 isn’t too difficult to find, but as yet I haven’t found anyone that has the 2024 in stock. In fact, Wicks is the only place that even claims to carry the size I need, and they’re out of stock. I’m ordering the 6061 and bushing material. Irritatingly enough, the instructions call out the size of tubing used for the bushings but not the material. I’m fairly sure it’s 6061 AL tubing, and fairly sure it doesn’t matter if it’s AL or 4130 steel.
Rear spars built, and tip supports added. Now building the tip supports for the main spars. At some point I need to get some AL stock and see if I can find my box of 4130 tubing to see if I’ve already got bushing material. Then I will need to drill the holes for the attachment and strut/wire fittings before starting wing assembly.
The spars don’t look terribly complex on the plans, but there really is a lot of detail work involved. It’s dragging on longer than I had hoped since it’s a trip over to Stu’s every time I want to get a little work done. I think I may need to take a little more structured approach to this — study the plans, then make a punch list for each trip so I don’t waste time while I’m there trying to figure out the next step.
Last night I did some work to get the parts ready for the wingtip bow supports on the rear spars. I ended up not gluing them on, since I’ve got to think a little more about how I’m going to clamp the pieces together. I did get the stiffeners glued to the rear face of the upper rear spars.
Up until now everything I’ve done with the spars has resulted in parts that were still interchangeable. Main spars are identical and fore/aft symmetrical, so there’s no right or left side. The rear spars are not fore/aft symmetrical, so now I have a left and a right upper rear spar. I had to take just a few minutes extra to make sure everything was properly oriented before attaching those stiffeners, or I could end up with two left or two right spars. Needless to say, there are some extra pencil markings on them…
I’ve seen all kinds of methods people use to mix up their T-88 epoxy. The manufacturer says to mix the resin and hardener (Part A and Part B) 1:1 by volume, or 100:83 by weight – which are the same thing. Plenty of videos on YouTube and EAA Hints for Homebuilders show various methods, and I’ve read some pretty involved threads on various homebuilding message boards. Some squeeze out equal lines of the two parts. Some cut the bottle tips to different sizes and squeeze out for a measured amount of time. Some have built scales to measure out by weight. Some do it in graduated mixing cups, like the little medicine cups that are easy to find. Some just use “TLAR” – or, “That looks about right”. You can buy T-88 in twin dispensing cartridges that automatically dispense and mix the glue — but it’s something like six times the cost of buying it in bottles, and of course you’re going to waste some each time when you throw out the mixing tube.
I have a bit of an aversion to not measuring epoxy accurately, and estimating and hoping for the best didn’t seem to me like a good plan when building an airplane. I know the precise ratio isn’t super critical; even System Three says that. Still, though, why guesstimate or get “close enough” when it’s so easy to get an exact amount?
I bought some 60cc catheter tip syringes through Amazon. They’re cheap and disposable, though I’ve reused mine a number of times. I fill one with Part A and one with Part B, and usually use a permanent marker to mark the cap for the Part B (darker) syringe, just so I don’t accidentally mix the caps up and ruin some epoxy. I’ll fill them just past the 60CC mark, then stand them tips-up overnight to let all the trapped air bubble up to the top, then squeeze a little back into the bottle or jug until the plunger is exactly on the 60cc mark. Pull the plunger back just a bit to get a little air in the tip, and cap it.
By doing this you can very accurately measure both parts to mix up any desired quantity of glue, from 2 cc up to 120 cc. Gluing plywood spar webs to spar caps? I’ll mix up 20 cc at a time, 10cc from each syringe. Building ribs? About 8 or 10 cc of glue will do two wing ribs, so 4 or 5 cc from each syringe. When the syringes are empty, you can either be a cheapskate like me and refill them, or just toss them in the trash if they’re too grungy to re-use or if you’re Daddy Warbucks and don’t care about the cost. I’ve got two pair of syringes that I use, and each has been refilled probably six or eight times. I just ordered some more syringes, since these are getting a little sticky and I want to keep fresh ones on hand. I use them for other things as well, so it’s never a bad idea to have some around. You also don’t want to run out of glue before you’re done building for the day, so I try to always have at least two sets filled.
Note: Pay attention to the quality of the syringes you buy. The first few I bought were made by B-D (Beckton-Dickinson) or Brandzig, are nice heavy duty plastic, are marked with 1 cc graduations, have good caps, and are generally high quality. The next batch I bought are thin, lightweight, shorter, marked with 2 cc lines, and are generally cheap Chinese crap. I’ll toss these after the first use and chalk it up to a lesson learned. As is almost always the case, it’s better to spend a little extra for good quality.
Happy birthday to me! 🎂 This morning I glued the two lower rear spars up – webs to caps only, and set them aside to cure. That’s all four rear spars now ready for the addition of the plywood blocks for attachment brackets, flying wires, landing wires, and N struts. The cut-off pieces of 1/16 ply will make shims for the lower main spar strut/landing wire attachment points that need to be 1/8″ above the spar caps.
In the afternoon I went back over and glued in the blocks and plywood plates on the root end of the rear spars. There are other blocks and structure to be installed outboard, where the struts and wires attach, but after taking the time to figure out the plans and exactly what needs to be done it was getting too late to glue those bits up. The plans could benefit from an oblique view to better show how things go together, or at least two views of the area… but they’re old and drawn by hand, and there’s only one view. One has to hope there’s a reason that parts of it are built the way it’s shown. The blocks in the kit are oversized and will need to be trimmed, and I want to make sure I have plenty of time to make sure everything is in the right place.
All four main spars substantially built; exceptions are noted below. Two upper rear spars have been started, with plywood web glued to the upper and lower caps – not really caps, but for lack of a better term…
Remaining work: Main spars need to be trimmed to length and dihedral clearance cut made on the lower spars. All holes need to be drilled, bushings fabricated and glued in place. I’ll need to fabricate and glue in shims for the landing wire/N-strut attach brackets on the lower main spars.
