There are several types of glue that are commonly used in balsa building. It’s tempting to think of one glue as “the best” but they all have their advantages and disadvantages. Different glues appeal to different people for various reasons. Try a few types and see which ones you like using.
Cyanoacrylate adhesive is commonly referred to as CA glue. CA intended for RC plane building is available in thin, medium and thick formulas. In my experience, the types of CA glue sold in hardware stores tend to set slowly, they don’t soak into the wood, and they just don’t seem to make as good of a bond. Buy the brands sold by hobby dealers and you’ll be a lot happier.
CA is good for general building. It’s a great way to stick balsa to hardwood, plywood, or other balsa. CA penetrates balsa very well and makes strong joints. In fact, it is stronger than the balsa it is joining. It doesn’t soak into hard wood, so you’ll want to use a thicker grade for joining plywood, spruce, bass, and other hard woods. Or choose a different type of glue such as Titebond.
A lot of folks say that CA is brittle, which I suppose is true if you define brittle as not being flexible. The conventional wisdom says that firewalls, landing gear, wing bolt blocks and tail-to-fuselage joints should be made with Titebond or epoxy. But I have built entire planes using nothing but CA, as have many other people, and I’ve never had a CA joint that failed due to brittleness. However, when building larger planes you may want to go ahead and follow the conventional wisdom and glue your firewalls and landing gear with epoxy or Titebond, if only to save some money. These other adhesives can handle the stress, so you’re OK in any case.
Epoxy is very strong and should therefore be used on high stress parts such as wing attachment points, landing gear blocks, and as previously mentioned, firewall joints. It’s also a good choice for gluing wood skins to styrene foam parts. It’s available in various formulations denoted by cure time, such as 6 minute epoxy, 15 minute epoxy, or 30 minute epoxy. The longer cure times require clamping or pinning. Fast curing epoxy may be held by hand if you’re patient enough. Slow curing epoxy makes stronger joints because it has more time to soak into the wood. It also has more time to drip, so watch out for that.
Another very important use for epoxy is fuel proofing. One of the great benefits of epoxy is that it can be thinned with alcohol and applied with a brush to the engine bay, the fuel tank compartment, wood struts, and anywhere else you find exposed wood. If you choose to thin epoxy, make sure you use alcohol with no water in it. Bathroom alcohol has about 30% water and will produce poor results. The stuff called denatured alcohol, found at hardware stores, is free of water. Use a few drops, just enough to get it to spread easily. Epoxy can also be thinned with heat. Spread it around, hit it with a hair dryer, and let it soak into the wood. Don’t get it too hot or it will boil and set immediately, leaving a lumpy finish.
The yellow glues such as Elmer’s wood glue, Titebond, etc, are commonly used for model airplanes. They tend to make very strong joints. Clamping or pinning are required because of the long set time. It’s a good idea to apply glue to the parts to be joined, let it soak in for a short time, then reapply glue and assemble the joint. These adhesives are water based and can cause wood sheets to warp, so are not recommended for laminating large balsa sheets. They produce strong joints and can be used in high stress areas.
Back in the old days model airplane builders used a lot of cement. The notes on old plans and construction articles refer to “cementing” pieces together. These glues are made of plastic dissolved in a solvent. They soak into balsa wood, and they generally require pinning and clamping. These glues will not cause wood sheets to warp, because there is no water.
The classic model cement was Ambroid, but it is no longer available. The old fashioned hobby cement that is still available is Sig-Ment. Another type of cement is nitrocellulose cement from Aircraft Spruce. This is supposedly the same as Ambroid. It’s an excellent cement, and it goes by the name Fab-Tac.
I like to think of cement as being very similar to CA. It soaks into balsa, creating a strong joint. It is also very effective for attaching balsa to spruce and plywood. But I can think of better alternatives for joining hard woods when no balsa is involved.
Cement is my favorite adhesive for edge gluing sheets of balsa.
The most famous brand is Gorilla Glue. Polyurethane is good for large laminations, foam wing skins, and low stress stuff. It tends to foam as it sets, so the pieces will be pushed apart unless you use lots of weight. Don’t use polyurethane for high stress joints because it isn’t strong enough. I tried it myself on structural parts a long time ago, and crashed an airplane.
We can also address different situations and name which types of glue are suitable.
If you’re laminating fuselage doublers or balsa sheet parts, use cement or thin-spread epoxy, and clamp the parts or press with weights. Or use medium CA and press with your hands for a few seconds, but you have only one chance to get it right.
Don’t laminate with thin CA because if you apply it from the edges it won’t get all the way to the middle. If you apply thin CA to the face of one sheet before joining it will set before you attach the other side, or it will go through the balsa and stick itself to the table. Don’t use water based adhesives such as Titebond, wood glue or white glue, because they will definitely warp your wood, unless you leave it weighted or clamped for a week.
To laminate a wood skin onto a foam core such as a wing core, spread glue thin and assemble the parts, and press with weights. Use 30 minute epoxy or Gorilla Glue. This is one of the rare cases when water based adhesives such as white glue or yellow wood glue is appropriate for laminating, because the curve of the airfoil will not allow warping.
I had a friend a long time ago who attached wing skins to foam cores using 3M 77 spray cement, but I couldn’t make it work right and it crashed two airplanes. So if you can figure it out, have at it.
Do not use CA or cement because they will dissolve foam.
Edge joining sheets
Place the sheets together and tack every few inches with thin CA. Hold the pieces up and join with thin CA, allowing the glue to run down along the crack. The disadvantage to this method is the hard ridge that will show through the covering film.
Or use Fab-Tac, Sig-Ment, white glue, or yellow wood glue according to the method described in the edge joining article.
General construction of balsa sheets and sticks, and plywood
Use white glue, yellow wood glue, or CA. Epoxy can be used in high stress locations, but it’s easy to get carried away and add an extra ounce or two to your plane when using epoxy, so most guys use it only when necessary. Epoxy doesn’t evaporate or shrink, so whatever amount you put on your plane stays there. Yellow glue such as Titebond can be used in high stress locations as well as all general gluing. Gorilla glue is not recommended for large pieces with small joints because of its low strength. Save it for laminating. Use cement for cap strips, wing tip laminations, wing skins on wing ribs, laminating patches into holes if you make a mistake, or gluing longerons and blocks onto the outside of a fuselage.
Stick cowl blocks together with cement or well-clamped Gorilla Glue, and I mean you’d better really clamp it to keep it from foaming. These adhesives are easy to sand. CA in the laminations will show as raised lines after sanding. The various formulations of Titebond have varying degrees of sandability, so choose wisely after you experiment with them. Epoxy seems like overkill in this situation, and it can be hard to sand. This is one of those cases where you’ll want to experiment to figure out your best choice. I’ll admit that I generally use CA because it’s easy, and I can usually see the lines when it’s finished. I have limits to my patience I guess.
For attaching a plastic canopy to a film covered airplane, use Pacer Formula 560 and hold in place with masking tape, clamps, or whatever wizardry it takes to keep it from moving while the glue sets.
You can also use E6000 and the same clamping methods. In either case, watch for drips and clean them off before the glue sets completely. Don’t expect the plane to be in service for several days after your last application of canopy glue.