Splicing wood and other construction techniques

Model Airplane News, Dec 2001 by Yarrish, Gerry

I was sitting under a shade tent at a giant-scale fun fly recently when someone asked me where I got the longer-than-standard-size materials I use to build my big airplanes. I didn't know what the fellow meant. He told me that his local hobby shop carried only 36-inch lengths of balsa and spruce, and that 48-inch-long sticks were available only by special order. I said that I regularly splice my stringers and spars and think nothing of it. "But how do you do it safely?" he asked.

For this column, let's discuss some common construction techniques and how wood splicing should be done.

STRONG SPLICES

When you want to turn two pieces of wood into one, you can't simply glue them together. Some cutting and fitting is necessary so that the splice will bear as much stress as the original wood pieces. To accomplish this, you have to increase the gluing area for the splice by cutting the two mating surfaces in a diagonal line.

The weakest, least supported joint you can make is a simple butt joint. Gluing two pieces of wood together end to end provides very little surface area for the glue. Overlapping the two pieces is a much stronger way to join them, but if you want to use the part as a stringer or a spar, the overlap isn't practical because the pieces are not attached in a straight line. A diagonal splice keeps both pieces in alignment. As a general rule, I make the length of a splice at least six times the thickness of the material being glued together-roughly 3 inches across for a 1/2-inch-square spar.

To make the two pieces match precisely, I first tack-glue the two parts on top of each other with a few drops of thick CA while making sure the edges of each stick are flush. I then draw a straight diagonal line at the ends to be joined (see photos). I use my band saw to make the cut, and I make certain it is square to the top edge. I then sand the cut surfaces (still glued together) smooth with a belt sander. When the two parts are separated, the two angled surfaces match perfectly.

To keep the two pieces aligned when I glue them together, I use a simple, wooden alignment jig. The jig is made with a flat base and two guide pieces (rails) glued on top that form a space between them that's the same as the thickness of the pieces being glued together. For the jig shown here, I used pieces cut from an old yardstick to act as rails. I also use a piece of Great Planes' Plans Protector material to prevent the parts from being glued to the jig when the adhesive oozes out of the joint.

To form the glue joint, I place one piece in the jig and spray it with a light mist of kicker. I then apply CA to the second piece and slide it into the jig and up against the first piece. After the glue has set, I use a sanding block to sand the face of the joint smooth. I then turn the part over and sand the opposite face smooth as well. That's it. No magic-just a very strong joint.

JOINT REINFORCEMENT

For most model structures, the diagonal joint splice will be more than strong enough all by itself. But if you don't trust it, you can easily place the joint in an area where surrounding sheeting or other structure members will help to support it. Typically, with a fuselage longeron, you can place it so that the joint is glued against a flat fuselage side. In stick construction, you can place the joint at a junction cluster where vertical and diagonal members meet (see illustration). It is also a common practice to strengthen these areas with thin plywood gusset plates. In a wing spar, the LE or TE sheeting can add gluing area, or you can attach front and back gusset plates around the glue joint. So the next time you need a 70-inch-long fuselage longeron, don't sweat it-splice it!

GUSSET PLATES

Gusset plates add much strength to glue joints by increasing the surface area for the glue to penetrate. It is often the case that when a wooden structure is damaged in a crash, it is the wood itself that breaks, not the glue joints. The gussets don't have to be very thick to do their job. For airplanes as large as 1/3 scale, 1/32-inch-- thick ply will be more than adequate. Gussets made from 1/32-inch ply can easily be cut with a pair of scissors, and they can be attached to the structure with quick-setting CA glue. Even though the gussets are light, I don't make them much larger than three or four times the thickness of the parts I am gluing together.

Cut the gussets to shape, position them on the structure and use a pencil to mark guidelines for applying the glue around them. Apply glue to the framework and then mist the gusset with kicker. Placing the gusset into position over the joint then "kicks" off the CA glue and greatly speeds construction. The photos show several gusseted joints that were used in my 1/3-scale Pietenpol Aircamper project.

SPRUCE GLUE

Spruce is often used for stringers, longerons and spars in giant-scale planes. It's easy to cut and shape, readily available and can often be used in place of more expensive balsa stock if you reduce its dimensions to save weight. I have used CA to glue spruce, but its tight grain often doesn't allow CA to penetrate completely. For this reason, I use slow-- setting epoxy and aliphatic resin. Some relatively new aliphatic glue I have found is Elmer's exterior-grade carpenter's glue. This glue is darker than interior-grade glue and is much thicker. The label says it has a "gel" formula, and I think it is very good for gluing spruce as well as other hardwoods and plywood for model use. You do have to lightly clamp together the parts being bonded, but the glue is very inexpensive and is readily available at most hardware stores. Give it a try and save some construction dollars.