Scarfing the wing skins - more info

Wow. It's been a week and a half since my last post. In that time there has been a winter storm, large tree branches falling off the trees that threaten the roof of the house, severe winds, cold temps, and a lot of other things that I had to get done to meet other obligations that had nothing to do with work or with building the plane. I was way too over-extended last week, and I finally got a chance to recover a bit on Sunday. I was pretty much running on empty all last week.

I had to finish editing a video that I produced for my EAA chapter with footage that I shot at Oshkosh when B-29s FIFI and DOC flew together, making this the first time in 50 years that more than one flying B-29 flew in formation with other B-29s. Turned out pretty good - but as always video editing-doing it properly anyway - is an awful lot of work.

Then I promised to do a presentation for the meeting as well - so I had a double whammy for last Friday.

I finally had some time this evening to get back to the scarf joint. In between the last post and this one, I did some more research on VAF about this whole scarfing process. Turns out that, as usual, I should have done the research BEFORE I started the process when I mistakenly thought I had it all figured out.

NEWS FLASH for builders trying to scarf this joint with power tools. DON'T DO IT! I found a 3 page post on VAF with folks basically asking the same questions that I was. The basic advice was NOT to use power tools, as there is a risk of a sanding disc grabbing the skin and sliding away from the desired work area. TOO LATE FOR ME! I already did this on the outer surface of my inboard wing skin, you know - the part that everybody looks at on the top of the wing when your airplane sits on the ground. I took off too much metal over too wide an area. Now I am going to have to have an ugly primer patch on that section of the inboard skin. So, it's not too big of a deal since I plan to paint the plane anyway. But had I wanted to polish my airplane, this skin would probably need to be replaced.

So once again I have proven that for every step that I take where I do something that I have never done before, I manage to screw something up at least once. So my advice to anyone following this blog is frankly to follow somebody else's (Venting). Anyway, following the advice on this VAF post that I found, I drew lines on the inner and outer section of the skin where the joints overlap so that I would know what the borders are supposed to be. Then I took some electrical tape and placed it along side the marks to provide a sort of protective barrier for the next step.

Then you take a standard tooth vixen file and start tapering the skins using the hand held file - and NOT a power tool. Here is pretty good shot of what things looked like before I started to remove more material with the file:

While this is hard to see, the fuel tank skin is on top, and the inner and outer top wing skins are on the bottom half of this pic. There is no longer a large triangular gap where the outer skin rides over the top of the inner skin, so that is progress. However, there is just enough of a ridge sticking up a bit higher than the fuel tanks skin edge, so it still needs more work. I was quite discouraged to read that some builders had managed to complete this whole task in about 15 minutes. Sucks to be me I guess.

Here is shot that shows the damage that a runaway sanding disk can do, as well as the sharpee line that I drew on one side of the skin. So I am overextended with my skin removal by about another 1.25 inches or so. I drew another line from the bottom side of the wing on bottom of the outer skin so that this line would also be present on the other skin when I separated them again.

And here is the tape applied along each line on both skins

Then you take the file and start systematically taking even amounts of metal off by running it across the area of the scarf joint on both skins

You need to keep slightly angling the file so that it takes more material off at the upper corner of each skin, and thickens up the further away from the corner that you are. My original plan included using a small scotch brite wheel on my drill, but I used it for the wrong purpose. My plan was to use it to remove fine amounts of material, just like it does when you use it to debur the edges on various sheet metal parts. However, for this task, it does not do so well, because the wheel gets gunked up with aluminum as you run it over the flat skin. It is not meant for removing large quantities of aluminum across large flat surfaces. So after I tried this on both skins, I could see where it left some peaks and valleys in the surface of each skin.

Using the straight file allows you to remove an exact amount of material much more controlled and precise than sanding by hand or using a power tool, and you can do it a little at a time and repeatedly check your work as often as you wish. In my case, using the file allowed me to knock down the peaks and even the valleys out so that the skins would lay flat against each other when clecoed together. Here is shot after I filed some more material away on both skins:

So this was looking better, but there was still a pretty good ridge between the tank skin edge and the wing skins that you can feel when your finger nail runs down along the tank skin toward the edges of the top wing skins. At this point I was satisfied with the fit from using the file. But now it was time to start smoothing out the scratches, and doing some final material removal by switching to smaller grit aluminum oxide sand paper. The sanding disk that I used was 180 grit (quite coarse). Next I used some 220 grit sandpaper to take out the larger scratches from the disk and the file. I also had to smooth out the areas where the disk ran away from me a little. These are the areas where primer will unfortunately be visible on the upper surface of the wing, but it has to be done.

I took a couple more shots after sanding, but they came out really blurry, so I'll have to take some more tomorrow. The end result after sanding was encouraging. The rise in the skin is almost gone. I got tired of re-clecoeing every single bay of the wing skins each time I wanted to check the fit, so as I got closer to the finished edge that I wanted, I ran clecoes along each bay, closest to the overlap joint, to secure the wing skin edges against the wing spar flange. Then I clecoed the overlapping skins together, and checked the fit.This seemed to work pretty well without having to re-cleco ALL of the ribs and skin holes on the more outer and inner sections of each skin.

The current state of the joint is that it is almost flush with the tank skin. I still have about 2 more stages of sanding to do. Most builders recommend final sanding of the scarf joint area with 600 Alum Oxide sand paper.I am going to start with 320 sand paper first, then clean with acetone very thoroughly, and then finish with 600 grit paper, and finally some primer.

Here is a parting pic of my hands full of aluminum dust after sanding the skins with 220 grit paper:

I decided that I am going to leave a small ridge on the skins for now. When the time comes for paint, There will be a lot of additional prep that will involve sanding, filling, priming, and sealing. This finishing process should being the edges flush with each other, so I am happy with where the joint is at right now.  I also do not want to remove any more metal, because I pretty much messed up the whole "scarfing" process, and managed to remove a hell of a lot more skin than I should have. So now I get to worry about the skin being too thin in this area, and being prone to cracking later on as the wing flexes in this area during flight. FANTASTIC!

Anyway, the joint looks a heck of a lot better than when I started. When I do the right wing skins and the bottom skins on the left wing I will exclusively use the file from now on, and I will pay close attention to keeping the scarf joint within the confines of the overlapped section of each skin.

I do still like the decision I made to make the joint with the skins and the fuel tank still attached to the frame, since Vans implies that you do this step with the skins off the wing, and they certainly do not have you check the fit with the fuel tank in their crappy instructions.  As I said before I started this step, how in the hell are you supposed to verify that the fit is correct if you don't check it against the fuel tank skin? But I digress - again.

All for now.

KPR