mickeymouse23

My Feedback: (1)

Okay so I keep having problems. My firwalls keep coming loose at the joints. It started last year with a plane and the guys at the feild deemed it was just a bad batch of epoxy. so i bought different epoxy and reworked the plane in question.it lasted about 4 flights and the motor fell of in the air. airplane didnt survive. so i put this cub together and the firwall came loose after running the motor once. Any ideas?im using dexcron 15 min. epoxy. been using it for as long as i can remember. onther thing is that it is hot were im using it at,,, sometimes 90 degrees. any comments are welcome. thatnks,mike

octane-link

its probably not getting into the wood to bond. You can try putting a little bit of epoxy onto the wood, and work it into the piece, then apply the gluing layer. Or, you could use Hysol to glue it, that stuff is really strong. Wood glue is another option too.

shd3920

First: I don't know if the working-time of epoxies make much of a difference but I always use the 30-minute epoxy. Some say, and I believe it, that the longer working-time of the epoxy the stronger it is.
Second: How much epoxy do you use to install the firewall? You should use enough epoxy that it squeezes out of the joint where it attaches. Then just before it is fully cured I wipe off the excess epoxy with rubbing alcohol.
Third: Are you installing tri-stock at the corners of the firewall where it attaches to the fuselage? Each corner of the firewall needs to have a 1/2" tri-stock support epoxied in place.
Fourth: Are there gaps where the firewall joins? In some instances I have had to epoxy tri-stock to the FRONT corners of the firewall as well as the rear because of gaps caused by ill-fitting firewalls.
Fifth: Be sure that you are mixing equal parts A & B of epoxy.

And NO, I do not agree that wood glue works just as good. It is not a strong enough bond for the vibrations caused by the engine and should never be used for high-stress areas.

EDIT: Oh yeah, and when you install anything with epoxy you must make sure there is no oil or greasy residues from anything on the items you are gluing as epoxy won't stick. Maybe you might wanna rub the firewall edges with alcohol before gluing and letting the alcohol dry before applying the epoxy to make sure there are no residues on it.

mickeymouse23

My Feedback: (1)

yeah i put tri stock in it when it origonly came apart,, second hand plane, i always make sure of equl parts, maybe im not using enough, it didnt oze out afterwards. ive been building for 15 years and have never had a problem. thats why im stumped

shd3920

Are you clamping the firewall in place or just letting it dry unclamped? I used to use tape but I had an instance where the tape came undone and shifted the firewall, so now I only use clamps for firewalls and formers.

BillS

When fixing a firewall on an airplane with many flights I often use toothpick dowels for additional strength. Drilling for the dowels also provides new wood for the glue to grip. Drum sanding the edges also removes old glue and residue and provides new wood for the glue to grip. The epoxy will not grip very well if someone before you used a lot of CA.

Bill

Stickbuilder

Get your hands on some K-2R. Use this product to clean the oil and oily residue from the surfaces to be glued. Cross drill the firewall and install with toothpick dowels, and use triangle stock. Add some fiberglass cloth behind the firewall and to the fuselage sides. Use Epoxy (30 minute only) to bond the cloth and the wood. Clamp everything together and leave it alone for 24 hours.

BALANCE YOUR PROPS. (Probably the most important suggestion)

Are you using an engine that is within the reccomendations of the model manufacturer?

Bill, AMA 4720
WACO Brotherhood #1

Deadeye

Or you could skip the epoxy and go with polyurethane glue. Much stronger bond, based on my experience. Clamp tightly, and mist your wood with water for the ultimate bond. After you get the hang of polyurethane glue, your epoxy bottles will collect dust.

octane-link

Agreed that the longer curing time gives more strength, try 30 minute. These tips are all very good.

I have used wood glue on a plane that flew at about 20 lbs, the plane crashed due to an elevator failure. I would recommend wood glue as a last resort however, there are a lot of options to exhaust first.

MinnFlyer

My Feedback: (4)

If this thing has come off this many times, just gluing it may not be enough. Try drilling 1/8" holes through the fuse and into the firewall and filling the holes with 1/8" wood dowels thereby pinning the firewall in place

mickeymouse23

My Feedback: (1)

wow never thought of all those ideas, thanks guys. what i was thinking is just start from scratch and put a new firewall in with some new tri stock. i will pin it this time with some dowel stock. the plane in ? is a cub from sig. its the 40 size plane and for engine i put on a la 40. i will go buy some 30 minute and try that also. again thanks for all the sugetions they are greatly appreciated, mike

da Rock

I mix my epoxy on note cards. The 15minute batches don't penetrate the top note card at all. The 30minute penetrates and sticks the top card to the next one down. That suggests how it works in the model.

One thing that also happens with the quick epoxies. The first epoxy you apply has some time to penetrate, not much but some. The second dab you apply has less time because it's been starting to setup while you been screwing around adjusting that first dab. So you got some in the mixed glob that can still be picked up with your applicator and you don't want to waste it, so you smear it on. How far into the wood you figure that stuff is going to get?

Epoxy actually isn't at it's best for reinforcing firewall-to-fuselage joints by itself. Spread on some 30 minute and then stick in the wedges suggested. And better yet, use some glass cloth.

Campgems

The quicker the epoxy hardens, the more rubbery it is when hard and the holding power recuces. Having said that, I use 5 minute epoxy for firewals and have never lost one, except for the nose first into the ground from 50 ft up under full power. Even then, most of the failure was balsa and or light ply failure, not the joint.

One thing I always do when using epoxy though is to wet both parts before joining them. Epoxy will trap air when maashing two parts together, and it also has a bunch of trapped air from mixing. I use a stick of flat balsa and "paint" both pieces where they join, the add some extra and pull them together. Also, heat accelerats harding, and also makes the epoxy thinner before it starts to kick. Always mix in a fresh cup, I pick up 1 oz medicne cups at a local medical supply for $2.50 a hundred. I always mix a bit more than I think I'll need. I hold the cup in one hand when applying, and as soon as I feel some heat starting to build, I know it is going to go stringy in a short time and it is time to make sure the parts are together and where you want them. You are out of time aplyying at that point.

I did have a mix of epoxy go south on me a few weeks back. I had repaired a wing hold down lug and the repair snapped right off. The epoxy looked like it was crystalized. I expect I got to much hardner in it. To much will cause overheating during the cure and give bad results.

Don

mickeymouse23

My Feedback: (1)

well thank you very much,, i do apreciate all the comments, like i said before i do plan on pinning it but the firewall is only a 1/16 of a inch. should i double it up? the fuse sides are balsa without a ply doubler. i dont think fiberglass would be to bad of an idea for extra strenght. thanks mike

Campgems

1/8" firewall? Using an LA 40 on it? IF, and it is a big IF, the firewal was interlocked with a network of other ply, including the sides, It MAY hold up. 1/4" aircraft ply for a filewall is my minimum standard for a 40+ engine. Even a 1/4" will require at least 1/2" HARD tri stock as a back up and I would use 3/8 Hard tri stock in front of it. I am not susprised a 1/8" firewall would not hold with the engine, not mater what you glued it in with.

Don

mickeymouse23

My Feedback: (1)

im using 3/8 tri and will make a new firewall from 1/4 inch ply. i took out my dremel and sanded all the old glue off. im getting ready to install the tri stock i think i will pin that into place also. the fuse is not interlocked into the firewall. there is no ply doubler in the fuse either. i am also gonna glass the firewall in once it is installed. thanks again,mike

jetmech05

I use 5 min epoxy from wal mart you know the 2 tubes 1 plunger kind. I've never had a problem with my fiirewalls.
I mix the epoxy on a paper plate install the part and keep the paper plate after the cure time i check the epoxy on the plate to ensure it hardened