Prep for rib flange mod

Since I don't remember if I already documented the process that one has to go through to determine how to size, fabricate, and rivet a custom part, I figured I would do it here.

As I previously mentioned, I have to fabricate a total of 4 small flanges, 2 per 408 rib. They are only going to measure about 2.5 inches x 1 inch, with a half inch flange on the end to match the one already on the rib, so they are not very big. Regardless, you basically have to treat this like you are doing a repair, and in that case you need to refer to AC 43.13.2B, chapter 4, Sheet metal repairs, and use the techniques and procedures outlined in that reference to determine exactly how to construct and rivet the parts together. Here are the details of how I figured this out:

1. I took measurements of the area on the rib web that is available for me to apply the additional flange parts. The goal is to make the rear flange match the same dimension as the one on the 709 rib as closely as possible. On one side of the existing flange the width needs to be about 1 and 7/32 inches, and on the other it measured about 1 and 3/8 inches wide. Both parts can be about 2 inches long before I run into the lightening hole. I may not need to use the full 2 inches but I am starting there.

2. The process for this is:
 a. Determine the required sheet thickness for the part
 b. Determine the rivet size to use for the part
 c. Determine the number of rivets to use
 d. Determine the rivet spacing/layout

The thickness needs to be the same for both parts. reference page 4-26 in AC 43.13. I will be using 2024-T3 aluminum that is .032 inches thick, which is the same material used for the 408 rib. So the total thickness will be .064. Here is where I found something interesting when comparing the info provided on page 4-27 of AC 43-13 and the info provided from an EAA SportAir work shop manual for the sheet metal class that I attended some time ago. The SportAir manual says to add the thicknesses of both sheets being joined and then multiply that by 3 to determine the rivet size to use. The example given in AC 43-13 on page 4-27 clearly shows that they only use the thickness from a single sheet times 3 to determine the rivet diameter size to use. So this is wonderful, now I am unclear as to which to use, and the problem is that if I use one formula it will require me to use 5/32 rivets, whereas the other formula will require me to use 1/8 rivets. So that's just great. The sportair manual refers to all this as a "rule of thumb", so my decision is to follow the example provided in the higher power source document, which is AC 43-13. This method allows me to use 1/8 inch rivets which is what I was expecting to use.

The rivet size is supposed to be about 3 times the thickness of the sheet, rounded up to the next largest rivet size. So 3 x .032 (single sheet per example from AC 43-13 = .096. A 3/32 rivet is .094, which is under the required .096 diameter. SO the next higher standard size rivet is 1/8 inch, which is .125. the other consideration is that if the parts to be riveted are considered to be structural, then for most standard aircraft aluminum sheet thicknesses that are used, the 1/8 inch AN470AD4 rivets should be used. So I will be using 1.8 inch rivets for this mod. See page 4-27 in AC 43-13.

The number of rivets to use is based on a formula. An example is provided on page 4-27, and the table it references - table 4-10, is on page 4-38. For my situation, 4.9 rivets are required for each 1 inch of width for aluminum with thickness of .032 inches. In my case I may be using 2 inches of "width" or less for my parts. There is a note on this table, note C, the explains that 75% of the number of rivets shown can be  utilized for single lap sheet joints,which is basically what I am doing.

.75 x 4.9 = 3.675 rivets per inch of width.
3.675 x 2 inches of width = 7.35 or 8 rivets total.

If I choose to only use 1.5 inches of "width" then the formula becomes 3.675 x 1.5 = 5.51 or 6 rivets. Using only a 1 inch overlap would require 4 rivets.

Lastly, after the number of required rivets is determined, you need to determine the rivet layout. AC 43-13 provides a couple of examples in figure 4-5 on page 4-20. The examples show the difference in strength when a double or triple row of rivets is used. This is where you have to account for the size of the part you will have to make so that it fits the area you are working with, and also the number of rivets required to attach it. The primary rule is that no matter what, the area has to be large enough to provide edge distance and enough area to use the required number of rivets as determined above. Notes that I had written on my rib from last year as I began calculating all this indicated that I determined that I needed 5 rivets on one side, and 6 rivets on the other. I think I am going to use a double row pattern. So this requires 2D edge distance around all edges of the part, or .25 inches (2 x 1/8 inch rivet diameter), and 4D distance between rivets, both next to each other and in between rows, or .5 inches. So if I decide to use only a 1 inch overlap, requiring a minimum of four 1/8 inch rivets, the part will need to be at least 1 inch x 1 inch where the rivets will attach the new part to the rib web. for each side of the part, I will need .25 edge distance, plus .5 distance to the next rivet, plus another .25 inches for the edge distance on the other side. so a 1 inch x 1 inch part should be all that is required for the joining surfaces, assuming I mark and drill my holes correctly.

Disclaimer - this information is provided for my benefit,and is based solely on my interpretation of information in a number of different reference materials as I try to determine how to fabricate custom parts for my planned modifications. Anyone else choosing to follow or utilize this information does so at their own risk. Builders must assess their own situations and conduct their own research to address their specific needs,and this info may not provide the correct information for a different application.

There now, so after going through all that, are you still sure you want to modify any part of your aircraft? It takes a lot of brain power and thinking things through before you actually do something different from the plans. As I have said before in other posts, I do NOT take any of this lightly, and I realize that any of the actions I am taking for this modification may put me at an increased risk. So I strive to stringently follow all established procedures and practices to mitigate that risk as much as possible.

Tomorrow I will make final decisions about the dimensions of the part and cut the basic shape, and hopefully bend the flanges, now that I know what bend radius to apply. As a footnote, the bend radius of the W-709 rib is only .0625, or 1/16th of an inch, compared to the thicker 408 rib, which is .09375, or half of a 3/16 inch diameter. They used a smaller bend radius on the W709 rib because it is not as thick as the 408 rib. I found that to be interesting, but I am a bit anal about such things sometimes.
Remember, the devil is always in the details. More tomorrow.