Carlson Skunk Works

November 11th, 2013

AMF 14H MARANDA SN:1026 Making Sawdust November 11, 2013

Posted by Roger in AMF 14H Maranda SN1026, news

I had a really good weekend! The leading edge skins have been installed on the left wing, except for the small inboard section where the leading edge makes a break. The wing has sort of a gull wing look like some Stinsons that I have seen.

Friday afternoon I got the chance to work on the pitot-static tubes. Saturday morning I cleaned the carpet in the dining area. Once the carpet was done it was starting to get quite nice and my wife gave me the go ahead to spend the rest of the day working on the plane. I was careful to take a bunch of pictures, so here is what I got done.

Friday afternoon I gathered materials and tools to try to make the pitot-static tubes. I had done quite a bit of research about the cost of purchasing these and decided that trying to do it myself would probably cost about as much, but would deliver a great deal of satisfaction. So I bought a ten foot roll of 1/4″ copper tubing, some solder, a #60 drill, and two #10 x 1/2″ belt rivets. Tony Bingelis provided instructions and a basic design in one of his books and this information is also available in the homebuilder section of the EAA web site. I went to the EAA web site and grabbed the image of the drawing and printed it. I also have the book, but wanted a copy of Tony’s drawing that I could make some notes on.

I dug around in my plumbing box and found a tin of flux and my tubing cutter. I also had a propane torch from when we built the house in Minnesota and I plumbed in the hot water heating system. At this point I was ready to go.

My biggest concern was how to plug the end of the tubing for the static tube. The tubing cutter made it easy to cut the tube. Tony only said to seal the tube and suggested crimping it, putting a screw in the end of it, filling it with solder, etc. I wanted something that looks fairly nice, so I started hunting around and came across some belt rivets at my local Ace Hardware store. These were #10 x 1/2″ long and had a large, nearly flat head on them. I got two just in case and they were $0.32 each. These were copper, so they worked quite nicely.

I started by figuring the length of tubing that I would need for my tubes. I calculated that a two foot length would provide plenty of tubing, but would still be small enough to be relatively easy to work with.

The plans call for eight holes and show them as being positioned in two radial groups of four holes about 1/2″ apart. I measured where I wanted the holes to be and chucked the #60 drill into my drill press. Then I used my tubing jig for positioning the tubing in the drill press. I position the bottom of the V so that the drill hits the bottom of the V and clamp the jig in place, then I position the tubing and drill the holes. It is not fancy, but it works pretty well. The hard part with soft copper tubing is that it is pretty difficult to get the tubing to be perfectly straight. I decided to drill these holes before I put the plug in the end of the tube just in case things got messed up. That way I could be sure that these holes were done correctly before I committed one of my plugs.

The next thing was to cut some small pieces of solder. The order of assembly is to use a pop-cycle stick to stuff some flux into the end of the tube, push the rivet into the end of the tube, then drop the pieces of solder into the tube from the open end and have them end up at the rivet end. Once all the parts are in their assigned locations I just needed to heat the end of the tube by the rivet and let the solder pieces melt. Unfortunately, the matches that I had in the garage had gotten soggy, so I had to go find some other matches before I could heat things up.

I don’t know if the tubing had some protective coating, or if my torch wasn’t burning cleanly, but the tube seemed to get pretty sooty. I was careful to hold the rivet end down while I heated the tube so that the solder would melt. The flux bubbled up nicely and some bubbled out of the holes that I had drilled in the tube. At least I was sure that I had heated things enough to be sure that the parts were soldered together. Next I needed to grind the rivet down to the same size as the copper tubing. I tried using a hand file and that worked reasonably well, but I found that my bench sander worked really well.

I found some paper towels and a Scotch Bright pad and got the static tube cleaned up pretty well. Then I took it back to the drill press and made sure that the holes were open. I am able to blow through the tube without difficulty, so I think I am ok. The rivet shaft is 1/2″ long, so I should be able to safely file the end to a nice rounded shape without compromising the finished product. However, I don’t think that is really needed, so I am leaving it as is for now.

The next thing to so was bend the tube. I happened to have a set of tube benders that I had bought for a plumbing job. These are basically coils of wire that the tubing fits inside of. With the tubing in the bender one gently bends the tube while making sure that the bender continues to slip along its length. Of course, I needed both a static tube and a pitot tube, so I needed to cut another two foot piece of tubing and bend it appropriately. The pitot tube is a lot easier to make because it is just a tube. I didn’t need to plug the end or drill holes crosswise through it. Just cut it to length and bend the end. Then I soldered the two tube together so that they became a single unit.

The final task will be to make a mounting bracket for the unit. Once again my local Ace came to the rescue with a piece of copper sheet. This is fairly thin, but yet thick enough to make a durable mount. I still need to complete this, but the intent is to make a U channel just wide enough for the two tube to lay in. Then I will solder the tubes to the bracket and bend the copper plate to make mounting ears. The plate is about four inches wide and a foot long, so it gives me enough material to make a couple of mistakes if I need to.

I took Randy’s lead and made a mounting place for these tubes on a rib just behind the front spar. The rib I chose is between the strut mount and the jury strut mount and is about 66 inches from the root end of the wing. This will be mounted on my left wing, so that the length of tubing to the instruments is as short as possible. Having the pitot tube located in the triangle formed by the strut, jury strut and wing should help to protect it from people accidentally running into it.

Saturday afternoon was very nice and provided a good time to finish applying sealer to the areas of the wing that would be covered by the leading edge skins. Then I needed to scarf and bend the skins. Once again I used the heat gun to apply heat to the plywood in the areas where I wanted to bend it. This works really well! Heat the side of the plywood that will be on the inside of the bend. Gently bend the plywood as it heats, but be sure to spread the heat over the entire length of the bend. When the plywood is bent about right I grabbed some of my clamps and held the bend in place while the plywood cooled. Using this method allows me to bend a sheet of 1.5mm plywood to a 1″ to 1 1/2″ diameter without any problems.

The next thing was to seal the inside of the nose skins. I held the plywood in place and marked where the ribs would go, then taped the areas where I would apply glue when it was time to attach the skins. I put on a good solid coat of sealer, reapplied the clamps to retain the curve and called it a day.

Sunday was colder, so I moved my wife’s car out of the garage, positioned the wing where I could work on it and closed the garage door. I removed the tape from the areas where I now needed to apply glue and went over the surfaces to be sure that everything looked right. I mixed up a batch of T-88 and started smearing it over the inside areas of the skins that did not have sealer on them (where the masking tape was). Then I held the skins in place (one at a time) and stapled them in place.

This time I used 3/4″ wide scarf joints and took a bit more time in applying the skins and the joints where the skins met turned out much nicer than on the right wing.

Today and tomorrow are supposed to be a bit cool, but after that things are supposed to be in the mid to upper 50s. I am very hopeful that I will be able to finish this wing before the winter weather sets in.

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