Carlson Skunk Works

January 7th, 2014

AMF 14H MARANDA SN:1026 Making Sawdust January 7, 2014

Posted by Roger in AMF 14H Maranda SN1026, news

Happy New Year!

I was wondering what to do for the first post of the new year. Then one of the guys on the Maranda Yahoo group asked for some details about the process that I used to prepare and attach the leading edge skins to the wings. After putting together my explanation I thought it might be useful to have it in my building blog just in case anyone happened to read it and it would be a way to sort of archive this in case I ever needed to remember how I did it. Therefore, what follows is the overly long explanation of the process that I used:

What I did was scarf the plywood before I bent it. However, I did not glue the scarf joints together until I fastened the plywood to the wing. So the overall process went something like this:

1. Cut the plywood to 2′ x 4′ pieces.

2. Mark the plywood pieces so that you know what order they will be in when secured to the wing. This is also important when scarfing.

3. Scarf the plywood. I applied the plywood from the root to the tip. The first sheet of plywood used the first full sized rib (the 14-H has sort of a gull wing and the root rib is smaller) as the starting point. The root end of the first sheet should not be scarfed. The tip end of each sheet should be scarfed so that the scarfed area is outside of the wing. This makes it easier to mate the next sheet to this one.

4. Mark the location where the nose strip should meet the plywood. I used a sewing tape measure determine the distance from the lower back edge of the spar to the center of the nose strip. I marked that distance on each sheet of plywood. That was where I heated the plywood and is the center of the bend. The result is that the portion of the plywood that covers the underside of the leading edge is about two inches longer than the portion that covers the top of the leading edge.

5. Mark the location of the ribs on the plywood as well as the location of the top and bottom of the front spar. I clamped the plywood to the bottom edge of the spar and marked the ribs on the inside of the plywood from the spar to the nose strip, then I marked the rib locations on the top edge of the plywood on the outside, but close to the edge of the plywood. This allowed me to finish marking the inside of the plywood after it was removed from the wing and the outside rib location marks helped with keeping things aligned during the gluing process. While you are at it mark where the plywood should end on the top of each rib and where each sheet of plywood will end on the spar. Be sure to allow for the scarf joints.

6. Heat and bend the plywood pieces. I clamped them at the nose to help maintain the bend while they cooled.

7. Put masking tape over the inside areas of the plywood where the ribs, spar and nose strip will contact the plywood. I used 3/4″ frog tape which worked really well. I wanted to be sure to keep the sealer from preventing the glue to make good contact. I also put tape over the scarfed area. I also masked the areas of the ribs where the plywood needed to be glued and the top and bottom edges of the spar. While I was at it I masked the top and bottom edges of any ribs where plywood would be attached later. In my case this was the root two ribs and the tip two ribs.

8. Apply sealer or spar varnish to the inside of the plywood and to the areas of the wing that will be inside the area covered by the leading edge skins. I only sealed the areas that would be on the inside. I did not seal the portion of the plywood sheets that would be behind the front spar. If you want to and if you have everything masked that needs to avoid being sealed, you can go ahead and seal the whole wing. I only did the first coat of sealer on the whole wing at this time.

9. Remove the tape once the sealer has completely dried.

10. Apply glue to the areas of the inside of the plywood where the tape was removed. The glue not only secures the plywood to the wing leading edge, it also is the sealer for the areas where the tape kept the sealer from covering, so be sure to cover the entire area where the tape was.

11. Secure the glued up plywood to the wing leading edge. I used LOTS of 1/4″ staples for this. I began with the wing tip end and stapled the plywood to the underside of the leading spar so that the plywood used the spar as sort of a straight edge to guide it. Then I stapled the plywood to the underside of the ribs and continued around the leading edge until everything was secured. Work from side to side to keep things even and straight. Watch the markings that you made earlier to be sure that the glue lines are actually mating with the ribs in the correct places. As stated above, I started with the plywood sheet at the wing root end and worked my way to the wing tip. I was sure to apply plenty of glue to the scarf joint, but didn’t staple the scarf joints themselves. If you were careful with the marking and scarfing, the scarf joints should pretty much take care of themselves. Imperfections can be touched up later.

12. After the glue has cured completely, remove the staples and inspect the joints.

13. The plywood for the wing root area between the root rib and the next one where I started gluing now needs to be marked, bent sealed, etc. The first sheet of plywood will now need to have its inboard edge sanded to match the angle that this new piece of plywood will meet it. This sanding was only done on the top of the wing and around the nose to the place where the angle is such that the new plywood should be sanded on the inside. I think you will see what I mean when you get that far.

14. If there are any gaps in the scarf joints mix some sawdust (or sanding dust) with some glue and work it into the gap. On my first wing I had to do this a couple of times to close one gap. Once the glue is cured it can be sanded smooth. When the wing is covered, you will be the only one to know where the imperfection is. I used a palm sander to go over the whole wing leading edge before I applied sealer to the whole wing. I paid special attention to the scarf joints, but found that they were not a problem at all. There will be a bit of a ridge of glue at these joints that can be easily sanded down to make a nice surface for your wing.

An extra pair of hands (or maybe more) can be a lot of help for holding and handing you things. I was able to do my wings mostly by myself, but having another set of hands to hold the plywood in place while I put the first staples in would have helped a lot. I had several occasions where I found that I had to pull a skin off and reposition it. Having more hands to hold the skins in place while they are being stapled will help the scarf joints to come out much better as well. Extra eyes can be a big help as well. You could have someone watch the alignment of each piece as you apply staples.

Speaking of having an extra pair of hands …

For the next two Sunday evenings the youth from our church are going to come over and “help” me work on the plane. This winter I am wanting to finish the elevator and build the ailerons and doors. These are fairly simple tasks and the kids from church should be able to help with quite a few of the tasks involved.

I have been told to expect about eight teenagers, so I need to have tasks for them. Therefore, I don’t want to have too much done ahead of time.

There are several things that will need to be glued, so they will learn how to mix T88 and use the staple gun.

I am going to have a team work on laying out the aileron spars and another team work on laying out the aileron ribs.

Some of the others can be cutting out parts, including wood pieces for the ailerons and elevator, and metal parts like the aileron control horns.

I hope to get some pictures of the activity for the next post.