TWF Techniques: Sparless Wing Joiner Tube Installation (Part 2 of 4)
Posted by J.S. Decker on Dec 30, 2007, 15:02
This is the second part of a series of articles on installing joiner tubes in sparless TWF wings.
4. Cut the root ribs. This is mostly a band saw and bench sander exercise, but there are a couple other considerations. Printout a template that locates the joiner tube hole, the chord line, the airfoil highpoint and the outline of the rib. Attach the template to the root rib material with spray adhesive. Drill the joiner tube hole, do this before cutting rib out to eliminate balsa tear out. I use a hole cutter made from brass tubing or a Forstner bit to get a clean hold. You can drill this hold a little over size if you do not have a Forstner bit the exact size as the joiner tube, or if you do not want to make a hole cutter, and fill any annular space with resin later.
I use hole cutters made from brass tubing for a lot of tasks when building composite planes and think they are worth the time to make if you scratch build. I make them by cutting three lengths of brass tube. The brass tube that is the diameter of cutter is about an inch longer than the other too pieces, which are brass tube sizes that are one size smaller and one size larger than the cutter. Typically I make my cutters 3-inches long and the nesting smaller and larger pieces are 2-inches long. I lightly sand the inside and outside mating surfaces and glue them together at one with sleeve adhesive (Loctite 680) or CA. The purpose of tripling the brass thickness is to strengthen the end of the cutter that is chucked in the drill so that the chuck does not deform the cutter tube. Chuck the cutter in a drill when the adhesive cures and taper/sharpen the inside of the cutter with a small file with the cutter turning in the drill. You can a triangular file to file teeth into the cutter if you want a cutter with saw teeth.
The top is the root rib for a JARTeron.
Brass tube cutter used to cut the holes through root ribs.
5. Trim the cores for the root ribs. I use a #11 knife blade and a flexible straight edge to trim the root end of the core by the thickness of the root rib at the correct root rib angle necessary to mate with the fuse. Square the core up with a sanding block as necessary, a Perma-Grit sanding block works great for me. Do not worry about cutting into the core beds, but do not trim the end of the core bed off. If you have not trimmed foam before with a knife, here are some tips, 1) practice on piece of ½-inch thick foam, 2) use a new knife blade, and 3) make the cut by making a smooth shallow first cut and then making multiple repeat passes without lifting the straight edge. With just a little practice you will be amazed at how smooth and square you can cut foam.
Trim the cores to remove the foam at the wing root.
6. Attach the root ribs to the cores. I use 3M77 like Phil Barnes demonstrates in his vacuum bagging techniques video, or if I am in a big hurry start sanding, a very thin layer of 5-minute epoxy mixed with microballoons.
Freshly attached JARTeron root ribs prior to sanding to match the cores.
7. Sand the root ribs to the cores. Is use 5-1/2-inch and 11-inch Perma-Grit sanding blocks with 2-inch masking tape on end to sand the root rib to the cores. I do most of the work with the coarse side and then just a couple strokes with the fine grit side. Move the block parallel to the wing surface so that you do not cut into the core.
Perma-Grit sanding block and conventional sanding blocks with siliconecarbide paper.
8. Making the joiner tubes. Cut a 12-inch long piece of 13/32-inch brass tubing in half (I use a small, cheap miter saw from Harbor Freight), de-bur the ends, roughen the outside surface with 60 grit sandpaper, and plug one end of the tube with a 1/16-inch thick balsa disk. I cut the balsa disk with the hole cutter described above and CA the balsa disk into the end of the tube.
Wing tubes pluged with 1/16-inch balsa.
9. Install the end grain balsa sub-rib. This sub-rib is a two-inch wide piece of end grain balsa. The sub-rib is installed at the end of the joiner tube. The purpose of the sub-rib is to transfer stress at the end of the jointer tube into a larger area of the wing skin. I install the sub-rib by marking the end of the joiner tube when held on the core, mark the perimeter of a two-inch wide piece of balsa centered on the joiner tube centerline, cutout the foam with a #11 knife blade, glue a rough cut piece end grain balsa into the slot with Titebond (aliphatic resin), and when the glue has dried, sand to profile with a Perma-Grit sanding block.
Sub-rib cutouts, and sub-rib balsa.
Sanding the sub-rib to profile.
10. Cut joiner tube slots. Cut a 5/8-inch wide slot between the root rib and the sub-rib centered on the joiner tube center line. Cut along the root and sub-rib and remove the foam to create the 5/8-inch while slot that the joiner tube will pass through.
