Dimpled the Top Outer Main WIng Skin Tonight

With just about all of wing spar flange holes for the fuel tank, LE, and main wing skins countersunk, the next logical step was to dimple the main wing skins, starting with the top ones, since they will be riveted to the wing first, when the time comes.

I was a little apprehensive about this at first, for the following reasons:

1. These are the longest and probably the widest skins of the entire airplane, so keeping them clear of obstructions or things that could put a ding in them is a challenge
2. The C-Frame is only so deep, and it is not deep enough to dimple all the rib holes in the skin without turning the skin around, but it does get most of them in one pass.
3. I needed to make sure that the skin would not get caught on my bench vise. I used the blankets to prevent this from happening.
4. Ever since I decided where to place the wing stands, I was concerned about the amount of space between the wings and the work bench, specifically for this situation where I would have to dimple the wing skins. It turns out that there is "just enough" space here to be able to work with the skin by turning it slightly in one direction or the other as you dimple the holes furthest to the rear. This brings the front end of the skin outward toward the wing frame. SO you have get creative, but not near as creative as I had to get with the pre-bent LE skin.
5. You have to have almost double the length of the skin available as room on a bench or a table to allow the sliding of the skin from one end to the other as you dimple all of the holes.

As it turned out, everything worked out fine. Unfortunately, I missed match drilling a couple of holes out of hundred or so that are drilled, so that wasn't too bad. I'll get those taken care of just as I did for the ones I missed on the LE skin after I cleco it back on the wing frame. ow you determine that, as I said before, is you pay very careful attention to how the male dimple die fits into the hole. If it does not freely adn easily insert itself into the hole, then most likely it has not been match drilled. Whatever you do, do NOT try to dimple a hole like that, or you will end up with a ding that will need to flattened, or worse yet, a badly formed hole in the skin that needs to be addressed.

I also used the pneumatic squeezer to dimple as many holes as possible around the perimeter of the wing skin, so that I did not have bang as many of them with the C-Frame. You also need to be careful  not to try ot dimple two sets of holes in this skin yet. the first set are the farthest holes in the outer portion of the skin. These are the attach holes for the wing tip, and you do not address these until the wing is essentially finished. The other set of holes that you do not touch yet are the holes for the aileron gap seal and the flap support bracket. These are both on the trailing edge of the wing and are mounted after the row of holes that attach the skin to the rear wing spar flange.

I turned the skin over before I started dimpling to debur the edges on the other side of the skin, and to apply the small bend in the top of the skin edge where it attaches to the wing spar flange, to prevent it from curling up when riveted. I used the edge forming tool for that, and it went rather well once I got the  depth adjusted correctly. I still need to round the corners just a bit and get the back side of this skin by filing those bumps and deburring that edge.

I guess that I should also mention that I temporarily took the fuel tank assembly and put it back on the wing spar. I needed the space on the second work bench for clearance to dimple the skin, and with the LE in the cradle, The only spot for the fuel tank is back on the spar.

Here is a pic of the skin on the dimpling table

Next is the blanket over the bench vise

Next is the skin with some of the holes dimpled

Next is pic of the lack of space you have as the skin comes outward. At its worst points, I had to turn the corners of the skin so I could get close enough to the ram while the male dimple die was held in the hole on the table. IN another case when the ends of the skin start to interfere with objects near the work bench, such as refrigerator, you can curve the skin up against those surfaces while the rest of the skin stays flat on the table, this works just fine.

The last pic shows several holes on the inboard upper corner of the wing skin that I did not dimple yet. The reason for this is that you are supposed to leave an area of about 3-4 inches in this corner to form the scarf joint where the inner an outer wing skins overlap. All that simply means that you need to taper the thickness of the corners of both the inner and outer wing skins on both the top and bottom of each wing so that they flow into and match the thickness of the .032 fuel tank skin. This requires grinding, filing, or sanding the skins in this area to reduce the thickness of each one. So there is no point in dimpling these holes yet until that joint is scarfed.

This skin now needs to be clecoed back on the wing frame after removing the top inner wing skin to counterskink the remaining wing spar flange holes adn dimple the holes in that skin, as well as the wing walk doubler, which frankly I have not seen for a very, very long time since both of them were clecoed to each wing frame. I do know that I never primed that doubler, and the inside of each wing skin also need to get scuffed, cleaned and primed, so there will be some more painting in  the near future,

After I dimpled all the holes in the skin, I turned over the skin to check the top side to see how well the dimples were formed. I have to say I was very happy with these, because they are very crisp and clean and appear to be very well formed. I recently saw a fellow builders RV-7A and was looking at his wing skins, and his rivets and dimples looked very nice as well. This means that the height of my dimpling table is set just right, where the male dimple die is in the set holder and it sits just proud of the rest of the carpeted table, so that the dimple dies are contacting the metal skin when the dies are hammered together.

After I get the top skins dimpled, it will be time to address the remaining work on the LE mod. I also came up with a plan for making a mock up of it using some old aluminum angle and some 1/16th inch aluminum from HD to fab the wing spar web, and I can attach some additional angle to use as a clamping platform. Then I can rivet the two ribs to the mocked up wing spar, fab another subskin and outer skin, and make the cutout, just like I am doing on the real part.

I think I will still need to do this when I finally start fabbing the gun nacelles for the lights that I plan to use. I do need to make sure that the gun ports are strongly attached, and also blend into the front of the LE very well. That will involve some foam and fiberglass experimentation with making a mold for the light mounts. I can do all that on the mock up until I know that I have all the kinks worked out of that process.


KPR