Carlson Skunk Works

Well, I made a bit more progress this morning. Yup, this MORNING! My son had to be at the hospital at 4:00 AM to do a job shadow thing and, since my wife works close to the hospital, she went with. By the time they left I was awake and the coffee was ready.

What to do? Grab a cup of coffee and build airplanes!!!

All the half spars are basically done, but there are a couple that still need a few parts to be glued on. That is where I started. I pulled the finished half spar from the jig and replaced it with another one that still needed the spacer blocks for the compression struts and the aleron pully mount. I fitted those parts, then turned my attention to making my first ever scarf joints.

I had already cut the 2mm plywood strips for the spar webs, but they needed to be scarf joined to turn them into a single piece of plywood for the spar web. Scarfing is simply tapering the ends of the plywood at about a 12:1 taper. Since the plywood is 2mm thick, my scarf joints will be aobut 24mm wide.

On each piece I measured back from the edge of the plywood 25mm and drew a line to show where I was going to do the scarfing. Then I grouped the pieces by which end and angle the scarf would have to be made. I had two groups of four pieces of plywood each. The pieces were stacked such that they were back 25mm from the edge of the piece immediately below. Once in position I clamped them together and proceded to make the taper.

I started with a block plane and roughed them down at least half way. Then I switched to a sanding block and 60 grit sandpaper and proceded to make dust until the taper looked pretty good (to me anyway).

Each sheet of 2mm plywood is comprised of three layers of wood. The middle layer has its grain running perpendicular to the two outside layers. As the sanding removes the wood the layters show up as different colors. With each joint being 24mm wide, each layer of wood will show up as a color band about 8mm wide. The surface of the scarf ideally should have all parallel lines. I think I came reasonably close.

Here are a couple of pictures of my efforts:

The only problem that I had was that one of the pieces got inserted into its group backwards and I ended up scarfing the wrong end of that one piece. This is not a big problem because I can just scarf the other end of this piece and re-cut the end that I scarfed by mistake.

Once I shook all the dust off of the pieces and myself, I selected a couple of pieces to be my first ever attempt at gluing up a scarf joint.

The final step for the morning was to put wax paper underneith all the joints that I am going to glue. Mix up a batch of T88 glue and start smearing it on. The parts on the half spar are pretty easy to do ( I have now done quite a few of these). Then I lay a piece of wax paper over each joint and drive in a staple or two to hold things in place until the glue cures. I also laid a piece of wax paper under the scarf joint and proceded to smear the joint with glue.

Getting the plywood to lay still while I adjust it to be straight was a bit tricky. I finally put another piece of wax paper over the joint and put a staple through the joint near one edge. Then I took my long straight edge and made sure the two pieces of plywood were straight with each other and put a staple through the joint near the other edge. Then I added a couple more staples in the middle to make sure that everything was being held securely and evenly. I also did a second scarf joint.

Later today (lunch time) I will put the last piece of fir on the last of the half spars and glue up the other two scarf joints.