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Old 12-06-2017, 09:54 PM   #326
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I've finished rib #33 of 44, but got derailed by a phone call from Wicks (detailed elsewhere): they're getting out of the Spruce spar business, and my order for spars was canceled. Confusion ensued, but I've just placed an order for solid spars from Aircraft Spruce. As a potential alternative, I got a quote from Steen which ended up being $2795 to build up laminated spars -- I'm certain they'd be the nicest spars ever, but that price is over-the-top expensive. If ACS has any problem fulfilling this latest order, I'll look into laminating my own Spruce spars.

In other news, an O-320-E2A came up in the classifieds here for a paltry $3000. A quick phone call revealed that it was IRAN'd (as I'd call it) an unknown number of years ago for airboat use, replacing the pistons, rings, bearings and a "seal kit." No logs. It's been stored in dry conditions since then, and was pickled with dryer plugs. The person I spoke with said that he did check the crank for runout, but didn't Magnaflux anything. I'm inclined to pass on this one as being too many unknowns (reflected in the price), but it did cause a flurry of what-ifs to percolate through my brain and distract me from work.


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Old 12-06-2017, 11:43 PM   #327
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I don't know if you'll do better than 3K for a core. If the crank is straight you could do a field OH and enjoy a "new" engine.


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Old 12-06-2017, 11:52 PM   #328
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Back when airboats commonly used aircraft engines here in Florida they were built almost exclusively from unairworthy parts. You could assume any airboat engine to be totally worthless as a source of aircraft parts. There were exceptions like the O-360 on our airboat which came from Pitts N6CQ and was overhauled by qualified aircraft guy. But even that engine had no records following it around.....
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Old 12-07-2017, 12:12 AM   #329
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I don't know if you'll do better than 3K for a core. If the crank is straight you could do a field OH and enjoy a "new" engine.
Based on a conversation I had with my A&P, I decided that I'd rather not take the risk. The seller is an A&P with an active maintenance business, so he doubtless priced it according to its value. Yes, it's cheaper than a core, but if there's a crack in the crank or cam, it's $3k that's been tossed in the toilet. If I were actually at the point of needing an engine, it would be a different question, too. But I'm at least 5 years from wanting an engine, probably more at the rate I'm going.
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Old 12-07-2017, 12:59 AM   #330
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Won't be the first 3k you throw in the toilet in the aircraft ownership/building world!
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Old 12-07-2017, 03:56 AM   #331
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He'll need that three AMUs for spars, right?
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Old 12-07-2017, 08:16 AM   #332
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Yeah, I'd like to maintain that best-effort attempt to not make bad $3k decisions, at least.

So: new wrinkle. I did a batch of destructive tests on sample glue joints tonight, and one of them failed. It's the batch from the little stub noses that go in front of the fuel tanks on the top wings (pictured below) -- they're mostly a plywood rib nose, with a couple sticks of capstrip to stretch them as far as the spar.

The failure manifested as the test joint coming apart with a mushy feeling, as if the glue never cured and remained a bit wet. It had some stick to it, but the sample definitely parted in the glue and not in the wood, with almost no penetration into the wood. I'm guessing it was just a bad mix, as a subsequent rib's test batch was perfect. I'm making test batches every 5-10 ribs, though I think I'm going to step that up, with this failure.

The question now is, what do I do about it? I have to assume that there have been other bad batches. My log doesn't make any mention of the glue for those ribs, so I thought it was a normal batch at the time. They were done very quickly, being so small. I'm not heartbroken that these little guys have to be redone, and I'll be curious to test them and see how bad they actually are.

Does anyone know a way to test ribs for bad glue mixes without just destroying them? Check the glue with a pointed tool to see if it's malleable? Try to lift a gusset off the capstrip? Load the rib with 211 lbs and see if it breaks? (The max load of the design is 1550 lbs, times 6g is 9300 lbs divided by 44 ribs is 211 and change.)

Thoughts and advice welcome. I don't want to redo all the ribs (and I demonstrably don't have to, for the good test glue batches), but I'd like to identify any that I do have to remake before I thread them on my spars and commit to them.
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Old 12-07-2017, 10:24 AM   #333
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Besides making test joints, I just kept my batch I mixed and checked it after each rib came out of the jig.
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Old 12-07-2017, 01:55 PM   #334
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If you keep track of which parts were made from the same batch as the test joints you would have a short list of questionable parts to inspect/replace. That's about the best you can do. If I didn't know which parts came from that batch of glue and the problem was an incomplete cure I would check the finished parts as you described to find any soft glue. But you may not find soft glue on the parts if the problem was poor mixing. The poorly mixed glue might have come from the bottom of the cup after the parts were finished. Tough call......
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Old 12-07-2017, 02:05 PM   #335
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Besides making test joints, I just kept my batch I mixed and checked it after each rib came out of the jig.
I did the same thing.
What ever I mixed the T88 in, after use let it dry with the rest of the rib. Get back to it at sometime and inspect the remains at the bottom of the cup and yank at the wooden stirring paddle. It was hard as a glue can be and that satisfied me.

Yes, I did some destructive testing before I started on the real ribs.

As with anything, consistency is the key to reliable success. That's what I like about the micro scale for mixing.
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Old 12-07-2017, 03:37 PM   #336
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I did the same thing.
What ever I mixed the T88 in, after use let it dry with the rest of the rib. Get back to it at sometime and inspect the remains at the bottom of the cup and yank at the wooden stirring paddle. It was hard as a glue can be and that satisfied me.
Ditto here on checking the glue after curing. I mix the glue in disposable medicine cups, from which I can easily pop the cured slug of glue the next morning. It was from doing that I determined that no amount of mixing with a wooden stick was getting 100% of the glue mixed well; I'd always have tiny bits in the corners that were sticky. Since I switched to mixing and applying the glue with a trimmed-down acid brush, that is no longer happening. Perfectly cured glue everywhere. I'm sure my grandkids will find a use for the remaining 950-plus wooden mixing sticks.
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Old 12-07-2017, 06:10 PM   #337
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Good thought on checking the left-over glue. I've probably got the cup from the failed batch in the trash can, and I might be able to identify glue batch to rib correlations at least roughly using an archaeological strata approach, which will point at the range of ribs to check.

I've been mixing a new batch of glue for every half-rib, so the chances of having a completely failed rib are low. The worrying thing is that I've been reasonably careful with using weight-based mixing with the proper 100:83 ratio, though I know I've missed it by a few percent on some occasions. I'm going to take some steps to tighten up my process; mostly getting a more precise scale, and being more patient for the epoxy to warm up before mixing. I'm surprised that 0.1g precision isn't enough, but I think it's been leading me astray.

Bill: fortunately, I'm mixing minuscule batches, so a bad batch doesn't have much effect. The particular batch I identified as a problem was unusual in that it was used to build the two tiny ribs pictured, since one batch was enough for both of them.
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Old 12-07-2017, 08:02 PM   #338
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I'm surprised that 0.1g precision isn't enough, but I think it's been leading me astray.
Ian, I've had no problems at all when mixing by 1:1 volume ratio. I have a couple of 60cc syringes I use now to dispense small amounts accurately. when gluing ribs (remember, my ribs are built a lot differently than yours and probably don't use as much glue) I'll typically squirt about 3 to 4 cc of each component - getting the amounts equal id the important part, of course. Mix with an acid brush trimmed to about 3/8", apply with the same brush. I'll usually glue a couple pieces of scrap together for destructive testing after it cures. Then I just let the cup with any leftover glue sit, so the next day I can pop out the little quarter-sized slug of cured epoxy to make sure it's good.

So far so good. Even when I had sticky spots off the bottom of the mixing cup, I have had no bad glue joints. The sticky spots were tiny, tiny amounts of un-mixed or poorly mixed glue in the bottom corners, not enough to materially affect the strength of what went on the wood.
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Old 12-07-2017, 09:01 PM   #339
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Scales? Heck Iíd bet most squirt out a eyeballs equal amount of each and go on. Youíd have to be off a lot for the glue to not dry. Now Iím not saying it will be as strong as properly mixed would be. But it would have to be off a lot for it to stay sticky. Iíd bet itís more of a mixing issue since you are already using scales etc. donít over think this to much. Iíd bet you either made a mistake on the scales on that batch or just didnít mix it up good enough. Just my arm chair quarterback opinion. Do your best but if you are already using scales, it sounds like just a simple, minor issue you caught because of your existing QC program. No need to ramp up to crazy level.
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Old 12-07-2017, 09:15 PM   #340
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I mix on the plastic top of a cottage cheese container. Using both bottles at once lay out two equal length lines and mix together. Line length determines large or small batch. Never had a failed glue joint. Just lucky I guess.
Per lotahp1, mixing ratio not super critical.
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Old 12-07-2017, 09:30 PM   #341
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Scales? Heck Iíd bet most squirt out a eyeballs equal amount of each and go on. Youíd have to be off a lot for the glue to not dry. Now Iím not saying it will be as strong as properly mixed would be. But it would have to be off a lot for it to stay sticky. Iíd bet itís more of a mixing issue since you are already using scales etc. donít over think this to much. Iíd bet you either made a mistake on the scales on that batch or just didnít mix it up good enough. Just my arm chair quarterback opinion. Do your best but if you are already using scales, it sounds like just a simple, minor issue you caught because of your existing QC program. No need to ramp up to crazy level.
Guilty! I used either the equal stripes on the bottom of a pop can or for bigger doses, the graduated medicine cups then stir it until consistent in color. No test failures.......yet
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Old 12-07-2017, 10:18 PM   #342
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So, a new theory just occurred to me: the failing test joint was glued up when the temperature was around 50 F. It was never warmed up beyond whatever I did in the shop (so never above 55, and that for 2 hours or less at a time). I'm going to do a few more test joints to check the theory, but I suspect that if I warm up my stack of ribs for a couple days, I would solve the whole problem.

Obvious question: will T-88 that's been left in the 40s and 50s for a month and a half fail with a mushy release? Or is that definitely a mix problem? I'll know based on my own tests in a couple days, but I'm curious if anyone else has experience with this.
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Old 12-07-2017, 10:42 PM   #343
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From the T-88 data sheet:
Quote:
At 70įF, T-88 will harden in 6-8 hours and will reach functional strength in 24 hours. T-88 has been specifically formulated to cure as low as 35įF without reduction in strength; this cure will require approximately one week.
I learned the hard way that it will take a couple days for it to really harden up at 45-50 degrees. I started bring the rib jigs in for the night when the garage is cold, else the epoxy will not be fully cured by the time I come out to do the next pair of ribs. But... a month or more? Bad mix for sure.
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Old 12-07-2017, 11:16 PM   #344
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I wonder if a bad mix plus low temperatures might be a poor cure while the same mix would cure alright at higher temperatures. It's worth testing.
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Old 12-08-2017, 12:57 AM   #345
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Quote:
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Obvious question: will T-88 that's been left in the 40s and 50s for a month and a half fail with a mushy release? Or is that definitely a mix problem? I'll know based on my own tests in a couple days, but I'm curious if anyone else has experience with this.
For an accurate answer to this question or any question that you may have on your rib issues, best to contact System Three.

My opinion is, if T88 has been left in the 40 and 50's for a month and 1/2. This should not deteriorate the chemistry of the product.
Probably better for shelf life of product at this temperate than storing at 90+ degrees F

I would get the product and working temperature out of the 40 and 50s and up to the 70 and 75s for mixing the components and working though.

Fact if not: tough to squeeze product out spout of the containers - tough (thick) to stir (mix) together - tough to spread with brush - tough to penetrate the wood cells - will take long time to cure out when cold.
You probably know this already.
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Old 12-08-2017, 03:28 AM   #346
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I build my model airplanes in a detached shed. I keep the glue in the house now days, only bringing it out for building. Keep them in a small cardboard box. Some is T-88. I had some joints pop, glued up cold after 24 hrs. I wont use the stuff if it cant be 65-70 for a few hours. Build a hot box; cardboard box lined with foil and a lightbulb for heat. Let them bake for a couple of hours. All my real airplane gluing has been summer time.
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Old 12-08-2017, 05:10 AM   #347
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Thereís a plus side to being a bachelor...my dining room is currently my wing building room.
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Old 12-08-2017, 05:40 PM   #348
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I did some testing last night, and came up with the following: among the 30 glue mixing cups I unearthed, only the 5th one I pulled out was noticeably tacky. It doesn't correspond to the failed glue batch, which would have been around cup #20-25. The rest of the cups felt like well-cured epoxy: slightly flexible, hard surface, slightest hint of tackiness (vs. a commercial plastic surface like a PTFE milk jug). So there's a questionable rib among my last ~4 ribs, although it's only the gussets on one side.

I also checked the glue tackiness on completed ribs, and discovered that all the ribs made since October 29 have a consistent slight tackiness to them when compared to earlier ribs, which don't feel tacky at all. These later ribs wouldn't give me any pause at all except for my new hyper-awareness of glue cure.

I also prepared some sample joints to show what happens with joints that come inside, joints that stay outside for a short period, and joints that stay outside for a month and a half. The mix on those joints was 1g resin to 0.9g hardener (where I discovered that my scale jumps from 0.7g to 0.9g -- great), and so is representative of the worst of my mixing mis-measurements. That assumes I can trust this scale at this level, which I kind of doubt now.

Now to wait and see what happens. Since I've already had a very good test joint with my "cold then warm" cure regimen, I feel safe enough continuing with building ribs. I carefully checked the bottles last night, and the remaining liquid in each is within 1mm of each other, so any mis-mixing I've done has been pretty subtle, and has no definite pattern either direction. Given that all the ribs since Oct 29 feel a little different, I wonder if my glue is changing, either due to cold storage followed by sudden reheating in a hot-water bath, or age (these bottles are about a year and a half old).
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Old 12-08-2017, 06:51 PM   #349
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Temperature sounds more like the culprit than mixing issues. The unmixed product in the corners of the cup isn't going to be spread on the joint is it. That's why it's unmixed, you can't get to it, how did you get to it to put on the joint?

Two trains of thought about gluing in cold temperatures. One is that the stuff will never go off and the other is that once brought up to temperature it will. I've seen too much evidence of the former to take the risk.

Another factor is that the viscosity of the product will be a lot greater at those low temperatures making mixing more difficult, increasing the chances of an incomplete mix. (which wouldn't be restricted to a corners of the cup thing)

Epoxy viscosity is greatly affected by temperature. If you have thick glue, it is going to penetrate the timber less isn't it, also affecting the strength of the joint.

Warmth is your friend when gluing….

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Old 12-08-2017, 07:32 PM   #350
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Iíve heard of people microwaving T-88? I assume do this before itís mixed.


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