THE RH WING SKINS
When I was at a fly in, probably Sun ‘N Fun in 2003, I saw an RV with one piece upper wing skins. If you have read my narrative on assembling the left wing you’ll know that I successfully made and installed a one case upper skin on that wing. I believe that Van’s made this skin in two pieces for two reasons. The first reason was ease in handling. Once the one piece skin was fabricated it was not as easy to handle as either of the two sections of the skin provided in the kit. The next reason was probably weight savings. The inboard skin is .032” and the outboard skin is .025” so I would assume that you may be saving a pound or two in weight on of the airplane. Since one of my goals is to have this airplane look as good as I can make it I will eat this particular weight savings in favor of cosmetics, although I am not sure if I can make these two upper skins look as good as the ones that I saw at that show. Had I been smart enough to take a picture of the plane with those skins I would have put a copy here.
The first step towards installing the wings skins was to make sure that the original skins were going to fit after the ribs and spars we’re assembled. In the photo above left, the lower outboard wing skin has been cleco’d to the wing assembly to make sure that I will not have any problems attaching the skins for final assembly. As with all of the parts that are pre-drilled in this kit, this wing skins fit like a glove. At that point I placed the wing in the stand and started to work on the one piece skin. The photo above right shows the factory’s upper skins P/N W-703 (outboard) and W-702 (inboard) sitting on the sheet of .032” aluminum which stretches across two work benches. I have made a total of three of these 3’ X 6 ‘ rolling workbenches in our shop in Tennessee. It was nice to have two of them to use at the same time for this project because the sheet metal is 12’ long. The length of this metal caused a problem with shipping. The metal was damaged and I had to be careful what portion I was going to use so that the damage did not appear in my skin. I think I will give this skin a P/N; W-702/703.
In the right photo at the start of this narrative, you can see that I carefully aligned the outboard upper skin along its outboard and forward edges with the sheet metal and clamped it in place. In the photo above left I have drilled with a #41 drill bit all of the holes on three edges of that wing skin. I then added the second skin along the inboard edge of the outboard skin. Care has to be taken because there are two rows of fasteners at this seam. Only the fastener holes at the rib should be drilled. The photo at right above shows the inboard factory skin cleco’d to my W-702/703 which has now been trimmed to its final size. The way that I trimmed it was that while both factory skins were still cleco’d to the sheet metal, I drew a line along the two edges that needed to be trimmed. I then placed masking tape on each side of the line so that I wouldn’t damage the protective coating that is on the sheet metal. I then cut just to the outside of the line with a jigsaw that had a blade about halfway between a metal cutting blade and it wood cutting blade. If the cutting blade is too fine it takes forever to cut and the blade gets plugged up with aluminum. If the blade is too coarse then the risk is that it will catch the edge of the aluminum and you might have some small creases along the edge which will defeat the purpose of improving the cosmetics. After the edge was trimmed I clamped my W-702/703 along the edge of the two tables with the freshly trimmed edges of the sheet metal overhanging the tables slightly. I then filed the remaining material away to the line. I should have taken pictures of the trimming process but I didn’t.
All of the fastener holes in the inboard portion of the skin have been drilled at this point with a #41 drill bit. I use a #41 bit because this is the approximate size of the fastener holes that come in most of the parts in the kit. I have gotten into the habit of de-burring the #41 holes and dimpling them without enlarging to #40. This saves time and the dimpled holes are a little tighter around the rivet shank. This helps reduce a rivet’s tendency to bend if it is slightly too long.
The photo above left shows the outboard portion of the “W-702/703” wing skin being match drilled to the outboard factory skin. The fastener holes along the aft edge have been picked up at this point and the fastener holes along the ribs are being drilled. After the skin was drilled it was time to dimple it. I removed the coating from the inside of the skin and then I decided to try something different. I left the protective coating intact on the outside of the skin and dimpled the skin. I used a piece of scrap as a test case first. I removed the plastic coating from one side, drilled several holes and then dimpled them. I removed coating from that dimpled side and the dimple looked OK. What did I gain? There were no scratches caused by the dimple pilot.
After all these steps were taken to make and drill the skin it was time to check to see if it actually fit. In the photo above left you can see that the skin fit like a glove. The next step was to etch alodine and prime the skin. The photo above right shows the primed skin hanging in the “paint shop.” This pant booth has now given me 2 ½ years of faithful service.
The wing walk doubler and aileron attach bracket parts were etched, alodined and primed at the same time (photo above left). See the “Aileron and Controls” and the “Ribs and Spar” sections for details of the aileron parts. After the primer had dried I removed the protective coating along the rivet lines on the outside of the skin. The dimples looked the same as they always do, except there were far fewer scratches. I am sure that I will use this process from this point forward.
The next step was to cleco the “W-702/703” upper skin in place on the wing sub-assembly and install the rivets. In the photos above the “W-702/703” upper wing skin is installed. I had help riveting the skin even though Mary was not available for this one. I think it looks very good. I will not know for sure until it is painted, however. If it is wavy or lumpy, glossy paint will accentuate it. At any rate, I feel comfortable at this point that the two wings are going to look pretty good when painted.
One of the things that must be done while riveting the upper skin to the wing is a constant check that the wing is not twisted. I performed this check several times throughout the course of the wing skin installation starting with a check performed prior to placing the skin on the wing. The check is simple. The spar must be level from top skin to bottom skin at each end. A plumb bob is hung from the leading edge about 1” to 4” from each end of the wing. A measurement is taken from each plumb bob string to the rear spar. These two measurements should be approximately the same. The photos above show my final measurements after the skin riveting was complete. The two measurements were less than .050” apart. I can live with that.
The picture above left shows that the two plumb bobs still attached to the wing. As I said before, I measured for twist several times during the wing assembly. I left the two plumb bobs attached to the wing throughout the riveting process. The picture above right shows the wing from the bottom with the upper skin attached. There is no particular reason for this photo except I just thought that this assembly looks good and shortly it will be completely enclosed.
It was now time to remove the wing from the stand. As a matter of fact, I was now finished with the wing assembly stand so I gave it to Mike Stansbury who is building an RV 7 also. I placed the wing on a work bench upside down (below left) to allow me to install the wiring and the bottom skins. See the section titled “Wiring” for details on electrical wires needed in the wing. I also decided to cleco the bottom skins to check for fit one last time (below right). The photo shows the outboard skin cleco’d in place. I had to trim some material along the forward edge because in overlapped the leading edge. I had the same problem on the left wing if I recall.
After the lower skins were trimmed as necessary it was time to dimple them. In the photo above left, I am dimpling the holes along the edge with a pneumatic squeeze. The rest of the holes got dimpled with the "C" frame tool. As with the upper skin, I only removed the protective coating from the inside of the skins at this time. I removed the protective coating along the rivet lines just prior to installation of each skin. I wish I had used this procedure throughout the assembly process. Etched and alodined the two skins and hung them one at a time in the paint booth for priming. See the picture above right.
After the primer was dry I cleco’d the inboard skin to the wing. I initially installed clecos along the spar with one cleco in each rib. I then placed a two by four between the upper skin and the lower skin to raise the lower skin to give me access to buck the rivets by myself. In the photo above left you can see that I am riveting the inboard skin to the spar. If I had this to do over again I would rivet from the rear spar forward. It would have been easier. The right hand photo shows the two by four I used to prop the skin up. This actually wasn’t necessary because I could have and did shoot these rivets through the lightning holes and the access holes. When I got close to the rear spar I had a difficult time getting my hand and a bucking bar to the rivets the same time, but I managed.
The photo above left shows me installing rivets in the ribs through the access holes. The farther aft I went the more difficult it became. I could have used arms about a foot longer by the time I got to the rear spar. Had I started at the rear spar instead of the front spar, I could have braced the skin up and gained access to the rivets on the rear spar much easier. Oh well! Live and learn as they say. The photo above right shows the lower inboard skin P/N W-704-R installed as far as it can go at this time. The row of clecos is where the flap hinge goes.
The outboard skin was a little more straightforward. I learned from the inboard skin installation. I cleco’d the skin to the first two ribs from the inboard end (Upper LH photo). I then gently pulled the skin back over itself and taped it in place (Upper RH photo). Care must be taken using this procedure because it would be very easy to put the crease in the skin at the most outboard clecos on the front and rear spars. Using this procedure allowed me to get to the rivets on the rear spar much easier than before.
After I managed to install all of the rivets in the first two ribs of the lower outboard wing skin, I put clecos in a rib two bays further outboard and pulled the skin back over itself again and taped it. Again I was careful not to crease the skin at the cleco line. I put all of the rivets in these two bays. I then lowered the skin and put clecos in a rib one more bay outboard. Again I pulled and taped the skin. The two photos above show the last bay where I taped the skin up to shoot rivets. In the second to the last bay I simply held the skin up while shooting rivets into the spars. In the last bay, I was able to shoot all of the rivets by reaching through lightning holes in the end rib.
The photo at left above was taken after both lower wing skins were installed. The photo above right was taken after the nutplates for the access panels were installed on the two lower skins. The wing skin installation is now complete. We will now move on to the wiring of the wing.