Preparing to Cut the Outer LE skin

I'll start this section with a huge disclaimer. I am not, nor have I ever been - a certified aeronautical engineer or aviation mechanic. Any changes that I am applying to my aircraft are entirely of my own design. I do try my best to research all aspects of making substantial design changes to an aircraft, and evaluate and address all safety concerns. I try to apply recommended procedures to any modification that I make that may differ from the original kit designer's plans.

If anyone decides to utilize any part of the information I provide in this blog to apply similar changes to their aircraft, they do so at their own risk. Each builder must evaluate their own research, skills, abilities, and experience, and must reach their own decisions as to the safety and applicability of any change applied to a tried and tested aircraft design.

I am embarking on a modification that, if it should go wrong while in flight, can have serious consequences. Having said that, I have done my best to ensure that all safety precautions are being taken, and that standard or recommended procedures are being followed for all modifications I am making. So, now you have been warned!

With the LE and fuel tank skins finally butting up to each other properly, it is now time to figure out once and for all just exactly where the new rivet holes, screw holes, and holes for the nut plates will be located for the removable LE. While most other builders would simply view what I am about to do as something very similar to installing a Duckworks support bracket assembly through a small opening in the LE skin for installing a landing light, they would only be partially correct.

There are some key differences in what I am about to do and the duckworks landing light bracket assembly:

1. For the duckworks installation, this takes place in the last or next-to-last bay in the leading edge, closest to the wing tip. For my installation I am using the opposite or most inboard bay of the LE. Higher loads are experienced in this inboard area of the LE than those that occur out at the wingtip.

2. The opening in the outer LE skin for the duckworks installation for the landing light is not as wide as the entire bay, whereas my removable LE assembly will span almost the entire width of the bay - somewhere in the neighborhood of 7 or 8 inches in a bay that spans 10 inches from one rib rivet hole to another rib rivet hole.

3. I don't think that the duckworks mod cuts the skin back as far as I am going to cut mine. Again the key difference here is that I will have a large section of LE skin that will be completely removable. All you are doing with the duckworks mod is cutting a hole that is big enough for the landing light support, mounting bracket, and the light itself to fit through.

4. The only "subskin" that the duckworks mod utilizes is the flanges of the support spar that rest against each rib web. I don't even think that the top or bottom flanges of the support bracket even touch the LE skin at all. My subskin rests against both the top and bottom of the LE skin and inserts between the ribs and the outer LE skin.

So while there are some similarities to the duckworks LE/Landing light mod and what I am doing, there are still some significant differences.

Here are some pics from tonight that show the joints between the LE, Fuel tank, and top main wing skins where they all butt up to each other. The seam between the LE and fuel tank skins is barely visible running vertically in the center of the pics, and the top wing skin seam is on the bottom.

The first several pics above are of the top side, and the last one on the bottom. Apparently I did not take any pics of the gap issues I mentioned in my previous posts, but lets just say it was extremely noticeable. In the above pics, it simply does not exist - exactly the way it is supposed to be.

For my final "before cut" measurements, I think I may have hashed at least some of this out in a previous post from several months ago, but doing it again here to make sure I have captured all the latest detail. Here is what I am dealing with:

Each 408 rib has a top and bottom flange where the LE skin is attached to the LE rib.

Those rib flanges are about 5/8 of an inch wide, or 10/16th of an inch

The rivet holes in each rib are pretty much centered in the rib flange, at about 5/16ths of an inch.

This leaves the remaining rib flange edge width of 5/16ths of an inch that I must  account for in my removable skin measurements.

While discussing this with my tech counselor, he agreed that I will need to leave a certain amount of LE just past each rib flange on both ends, and I will then need to add new rivets on this additional portion of the skin that will hold down the edges of the outer LE skin very close to the cut line for the removable section of the skin.AC 43.13 is almost no help with the details of how to apply rivets in this situation, however I learned from my tech counselor that the overall objective of a mode like this is to try to ensure that any loads imposed on the structure are allowed to pass through the structure in a similar manner that they would have if the repair or mod was not needed or applied. So you can't build up a set of rivets that will cause load paths to shift to other weaker areas of the structure, so this requires some careful planning and understanding how these loads were originally designed to be handled by the kit manufacturer.

After toiling over all this stuff again, I decided to follow a particular paragraph in AC 43.13 on page 4-26, paragraph 4-58, SubPart e, that describes how "repairs" to "damaged" skin should be made, which states that
" The repair seams are to lie along stiffening members, bulkheads, etc.; and each seam must be made exactly the same in regard to rivet size, splicing, and rivet pattern as the manufactured seams at the edges of the original sheet."

Then it references figure 4-16, which shows several examples of skin patches and repairs and how the rivets are to be applied.

Since I am actually the "manufacturer" of this plane, and Vans is actually the "designer" of the kit, this loosely translates to ensuring that any major skin "repair" matches the original designer's rivet layout as much as possible. To that end, I now know that I will add another row of rivets  in parallel with all the rivets already attaching the skin to each LE ribs. The details of this are:

Top side:
The first 3 rivets in top flange of each LE rib are 1 inch a part. Then all remaining holes are 1.25 inches apart.

The outer LE skin starts to truly flatten out (not as curved) after about the 8th rivet hole from the front of the LE. This is almost 2/3 of the total distance from the front to the rear of the LE skin. This is a lot further back than I was planning to go to make the cut line for the top side of the removable LE. So I need to ponder this a bit more.

Bottom side:
Only the first 2 rivets in each rib are 1 inch apart.This is because the bottom of the LE is obviously flatter than the top and is not quite as stressed as the curvature on the top part of the LE is. The remaining rivets after this point are once again spaced about 1.25 inches apart.

The LE bottom skin starts to flatten out after the 4th rivet.

SO I will be creating a parallel set if new rivet holes that will mirror the existing rivet holes in each rib. As far as where how far back to cut the LE skin is concerned, I have to decide if I will stagger this so that the span-wise cut line will be the same on both top and bottom (5th rivet hole from the tip) or go to the 5th rivet hole on the top and the 4th rivet hole on the bottom. I'll have to eyeball this a bit more.

This post is more than long enough, so I'll pick up where I left off tomorrow.

KPR