Marked and Drilled the Remaining LE Skin Rivets for the LE Mod

It was last March when I had finally managed enough courage to mark and drill the additional inboard and outboard rivet holes that will secure the edges of the LE skin next to each rib flange. I left off at the point where I needed to determine the number of rivet holes to make and drill for rear-most rivet holes on the top and bottom sides of the LE.

I specifically left off with a bit of a dilemma, because I was not sure about the rear rivet hole placement in the corner. If I used the marks from the lines I had drawn for the rivet holes, then the rivet pitch (distance between rivets) between the corner rear rivet hole and the first edge rivet along the edge near the same corner was a little bit wider than it is supposed to be when setting a section of multiple lines or rows of rivets next to each other.

SO I made a decision to at least mark and drill rear corner rivets, realizing that if I needed to later I could always add one more rivet between that one and one along the side to fill the gap. Again, the goal here is to prevent having to reassemble and disassemble the entire LE yet again for a couple of lousy rivet holes. I want all the holes to be marked and drilled so that I only have to disassemble the LE one last time. It's time to finish this beast once and for all.

Step one for locating the rear-most rivet holes was to use my rivet fan by drilling the four marked rear corners (2 corners each for the top and bottom side of the LE skin) with a #40 drill, and then clecoing the edges of the fan to each corner hole. I started with the bottom of the LE because it is flatter than the top side and a bit easier to work with.

With both corners of the fan clecoed in place, I then took a #40 drill bit and manually turned it inside each of the holes of the now equally-spaced rivet fan to mark and start the drill bit in each of the holes to prevent the drill bit from slipping off target. Once that was done I removed the rivet fan to view my handy work:

Then  was able to finish drilling each of the new holes on the rear side. This next pic shows the left side corner and the amount of space between the rear corner rivet and the first side rivet, which is at least 7/8 of an inch I think. it also shows the intersecting lines where I could place one more rivet in between these two to fill the gap. The problem with using that mark is that it creates a situation where that rivet's edge distance to the curve of the removable skin it right on the allowed limit, and so if I do add another rivet to this location I will probably set it back from the corner just a bit to present more of a slightly rounded rivet pattern around each corner, which would match the contour of the removeable section of the skin. And of course it also shows the rear rivet holes all clecoed together.

 Here is another closeup of the other corner on the top skin. I should also note here that the original plan was to use the same distance between rivets to adhere to the same standard practices for making patches to LE surfaces by matching the same rivet spacing used where the LE skin attaches to the main wing spar, but I felt that using the rivet fan to locate the holes also just about matched this same spacing, and it made it much easier to ensure that I would have a row of equally spaced rivets along that line.

Working on the curved surfaces always has many challenges, and frankly I have found this to be the most difficult aspect of the build so far, especially as it pertains to this LE mod that I am doing. Here is  a much better shot of how I positioned the rivet fan on the top side of the LE skin, which of course is the most curved side.

And the full row of rear rivet holes drilled and clecoed on the bottom side:

With all the LE skin to subskin rivet holes marked and drilled around the perimeter of the removable skin section, now it was time to determine what screw size nut plate size, the number of each along each side and rear of the removeable section, and the spacing in between each one. This was the most frustrating part of the mod so far, because I am unable to find any written source of info whatsoever that contains a formula or rule of thumb to use to determine how many screws or bolts need to be used in a certain area. So I contacted my Technical counselor and he consulted with a certified A&P mechanic. Basically I was sited the same rules that are established for rivet spacing, so I did not find this to be particularly helpful. I know that aircraft screws and bolts each have very specific ratings in terms of tensil, shear, and torsion ratings, and each nut plate also has ratings for this, but the one missing link to all of this is that I do not know how much of these forces are being applied to the areas where they will be used on the LE. 

So my next best method was to look at several areas of the airplane where Van's is applying similar screws and nutplates, and copy the spacing as much as possible. I plan to use K1100-8 nut plates with AN509-8R8 screws to attach the removeable section. I noticed that in various areas of the kit Van's uses spacing for these nutplates and screws that is anywhere from 1 7/8 inches to 2.5 inches. Based on on that, I set out to determine how many nut plates and screws need to be along each inboard and outboard top edge, and how many should be applied to the rear edge. this next pic was a first attempt at laying these up, but later on this changed a bit, after my tech counselor confirmed a suspicion that I had that you should not try to attach nut plates or screws directly on a corner of any part. 

This is because the corners of anything are where the largest stress concentrations tend to occur, and so if you want things to start cracking just start putting fasteners directly on corners of various parts. Instead, you need to offset the fasteners so that they are on either side of the corner. IN the next post I'll show the corrected/new layup for the nut plates on both the top and bottom sides of the LE. There are about a million measurements involved in all this, which is also why I will address all that in a new, separate post. Bottom line is that I think I now have the layout determined for the screw holes on the top and bottom of the LE, and it amounts to about 20 of them (10 on the bottom and 10 on the top. The screw holes have to be drilled before I disassemble the LE for the last time, so that I can then drill the corresponding holes for the rivets that attach each nut plate to the subskin. I plan to drill these holes to #30 first, and then to #19. Once the LE is disassembled I will be able to finish drilling and sizing all the screw holes in the subskin, dimple them,  and FINALLY cut out the section of the LE skin. Almost there.....

And one last pic of the rivet fan on the top side of the LE. Notice how it is positioned differently that how it was on the bottom side. this is because of the difference in flatness on the bottom vs. the top of the LE: