Glue joints

[iflypa28]

I'm a new builder and this is my first post here. I have done a ton of reading and research on the subject of aircraft woodworking and seem to still have a few questions not quite answered fully. I'm ready to start on the fuselage and want to make 100% sure I am doing this right! The aircraft is a Pietenpol Air Camper, I have ordered the wood kits from ACS an will be using T-88 Epoxy.

Question 1: In all my research I have read many things about not sanding the joints before gluing. Upon inspecting the wood I have noticed that the wood is not 100% smooth on the sides(the 1x1 longerons for example). Most of it is just slight variations in levelness due to the grain and a few light saw marks here and there. My question is, if you can't sand these areas smooth where the vertical braces are glued to the longerons, what is the appropriate method to assure a good glue joint. Am I just being to particular here?

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Question 2: I created a few test joints that mimic the joint between the longeron and a vertical brace. Again everything I read said the wood should break before the glue joint. This however didn't happen. On every test piece the glue stayed with the vertical brace (end grain side) and took the top "skin" of the longeron piece with it. So it doesn't appear that the glue is penetrating very far on the longeron side. I know these are simple butt joints that are not very strong and they will also have gussets on the final product. Is this the kind of failure I can expect or am I doing something wrong. On a side note I was very particular about mixing the epoxy properly and temp was 70F for the cure.

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Thanks in advance for any help!

[Sam Buchanan]

Welcome to the fascinating world of custom-built aircraft!

You have discovered why gussets or ply sheeting is used when joining wood stock. The butt joint is very weak because the end grain has absorbed much of the glue and the joint is starved. Fortunately this is not an issue because the strength of the joint is in the stock-to-gusset attachment which has far greater surface area, any glue at the end grain is largely inconsequential.

I've sanded many surfaces that were later glued but the T-88 is very capable of filling the small voids in your photo, the additional thickness penetrating the voids will actually add more strength to the joint. Keep in mind the strength of the structure is not due to any one glue joint but rather the totality of all the joints working together. This is not to encourage sloppiness but to prevent obsessive concern over each glue joint. Use good shop practices and your Piet will be a strong aircraft.

Another area in which to avoid unnecessary angst is mixing T-88. This stuff is quite forgiving, try to get a 50/50 ratio but don't go overboard with the process. I squeeze out two identical length beads on a flat board (paper plate), mix 'em together and get on with the business of aircraft fabrication. This glue is also quite tolerant of temperature variations, it just sets up quicker in warmer temps. As with many epoxies, T-88 has a longer pot life when spread out on a mixing board vs confined in a small cup.

Best wishes for a very enjoyable journey!

[DaleB]

Good luck and have fun with your Piet build. I too am in the early stages of building an all-wood plane. If you want to be a little picky about your T-88 mix ratio but don't want to waste a lot of time or effort doing it, you might like what I do with mine. I bought some 60cc catheter tip syringes (cheap on Amazon) and use those. I fill a pair from the larger T-88 bottles, then stand them on end for a day or so to get the bubbles out. You can then dispense exactly as much as you want, get the exact same amount of resin and hardener, and cap them so there is no mess. I have refilled my pair of syringes, but they're cheap enough to toss after use if you prefer.

I'm building wing ribs, two at a time. i have found that 12cc is just about perfect. It's enoigh for two ribs, a couple of test pieces from the cut-off scrap, and just enough left in the little mixing cup so I have some proof of how that batch worked. I'm really happy with this method.

Dale

[iflypa28]

Sam, thanks for the quick response! This info is super helpful and makes me feel quite a bit better. I tend to be very particular, which can be good and bad, and just wanted to be sure I had my ducks in a row before I got started.

I love the D.VII replica. Something like that just might have to be my next project!

[iflypa28]

Dale, that's funny that you mention the 60cc catheter syringes. My amazon package with the exact same thing just got here 10 minutes ago:) I figured they would be great for super small batches like that.

[martymayes]

Was the wood you purchases surfaced (planed) on 4 sides (S4S) or two sides (S2S)?

[iflypa28]

Well that's a really good question, I don't know. It was purchased as a kit with all the wood for the entire project and I don't recall seeing that specification anywhere. Ill look into that.

**Update: After looking around the ACS website they never give a specification for the Piet kit but other kits appear to be S2S so that's probably what it is.**

[martymayes]

I would say it was S2S as well that was just cut to size per the plans dimensions with a saw. That's why you have saw marks on some edges. In the end it just depends on your personal standards. As others have said the gusset carries the load in a joint. If you want it to look pretty you'll have to sand smooth.

FWIW, on your next project you can just buy the raw stock, cut to size and run through your surface planer (dream big with your workshop, lol) to exact dimension. You'll save money because built into the price of the precut wood "kit" you are paying some pimple faced kid to cut the stock to size. If they cut it slightly undersize (or oversize), it still goes in the kit. :(

[Bill Berson]

Try a sharp hand plane on scrap. If it tears both directions, not much you can do.

[FlyingRon]

While the end absorbing the glue is part of the reason end grain joints are weak, it's not the only reason. If that was the case, we'd just use more glue to seal up the end (in fact, glue "sizing" is a technique that's been known for centuries but it didn't solve the problem). The problem is that the fibers that the glue film sticks to on the engrain are much smaller than in the side grains.

If you're going to use epoxy, realize that it has different characteristics than then other woodworking glues (be it plane PVA, or hide glue, or resorcinol). The latter is applied very thinly on well-mated surfaces. Epoxy, on the other hand, has poor thin film character so if you try to use it the same way, you'll really be talking about a glue starved joint.

[iflypa28]

I have noticed that you can't use much clamping pressure. I built a jig for the fuselage sides so I wouldn't have to use clamps. What is everyone's experience with using a staple gun for the gussets with epoxy, does this apply to much pressure and squeeze the glue out? Is there a better way?

[cub builder]

I would suggest you look at some of the many KR sites (http://www.krnet.org/). They are built with the same wood stick and gusset framing with plywood shear webbing. You'll learn a lot about acceptable methods and techniques on their sites.

[martymayes]

What is everyone's experience with using a staple gun for the gussets with epoxy, does this apply to much pressure and squeeze the glue out? Is there a better way?

staples work, use caution because they will split the wood and if you staple through a strip of cardboard you can remove the staple after the glue dries. The staples will only rust anyway.

[bigdog]

What is everyone's experience with using a staple gun for the gussets with epoxy, does this apply to much pressure and squeeze the glue out?
I built a set of ribs for an L-2 restoration project and I got some good advice on the Biplane Forum. I used a pneumatic Senco stapler and SS staples. You need to adjust it so the staple sits flush on the gusset but doesn't crush the fibers. To adjust it, depress the trigger and grind off the plunger until the staples sit where you want them. With SS staples you can leave them in. The stapler makes it easy and quick. Well worth the money spent.

[rv7charlie]

To the OP,

I think you may be misinterpreting that 'wood breaks before the glue' premise. The premise doesn't mean that the entire stick of wood will break.

If you look closely at your test joint, you'll see wood fiber from the top of the 'T' adhered to the end grain of the leg of the 'T'. That is your evidence that the wood failed before the glue. What you're really seeing (as others hinted) is stressing a joint outside its design, and I don't mean overstress; I mean that the joint isn't designed to be stressed in the direction/angle you're stressing it.

I'm not an engineer, so I don't feel qualified to give a tutorial on it, but here are some starting points to see how to design joints.
https://www.masterbond.com/techtips/understanding-joint-design-optimize-adhesive-bonding

https://www.google.com/search?q=glue+joint+design&oq=glue+joint+design&aqs=chrome..69i57j0.4284j0j7&sourceid=chrome&ie=UTF-8

[mmarien]

To the OP,

If you look closely at your test joint, you'll see wood fiber from the top of the 'T' adhered to the end grain of the leg of the 'T'. That is your evidence that the wood failed before the glue.

I agree. The glue joint is perfectly acceptable. The wood failed before the glue. If you have gussets to hold the two pieces together then the glue between the two pieces is mute. The strength is in the gusset. On my wing I had one butt joint that didn't have gussets. The aileron spar butts up against the rib. While not called for in the plans, I used a couple of dowels to tie the pieces together.

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The other question about sanding. I used birch plywood for my rib gussets. The plywood is shiny smooth. I used 80 grit sandpaper to rough up the surface of the plywood so the glue had something to adhere to. A lot of people use plexiglas for the rib jig. The secret there is the shiny smooth surface of the plexiglas. T-88 sticks but not well and can be easily removed. Rough up the plexiglas surface (from multiple uses) and you'll start to have a difficult time cleaning off the glue.

You only need to use staples if you are going to pull the rib out of the jig before the glue is set. You can make multiple ribs in a day that way. You can staple through cardboard and remove the staples once the glue is set. The cardboard is so you don't mar the surface of the gusset. You can also leave the staples but they just add weight.

I made one rib a day in the jig and left it overnight for the glue to set. The next day I flipped it over, added the gussets on the other side and made a new rib in the jig. That way I had plenty of time to coffee with the locals when they showed up to critique.

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http://myhatz.blogspot.ca/

[rv7charlie]

I have noticed that you can't use much clamping pressure. I built a jig for the fuselage sides so I wouldn't have to use clamps. What is everyone's experience with using a staple gun for the gussets with epoxy, does this apply to much pressure and squeeze the glue out? Is there a better way?


Suggest research on technique vs glue type. IIRC, older glues like resorcinol require near perfect fitment, so that there is almost no glue gap, and significant clamping pressure, while others, like T-88, are much more tolerant of gaps. It would pay to read up on the proper technique for your glue of choice. The resorcinol glues are stronger than the wood, but require better technique to develop that strength, but can have an advantage over many epoxies when it comes to environmental properties.