Repairing flaperons

[Dave S]

Hi Gizmos,

This is a typical repair that is explained in the FAA's Advisory Circular 43.131b Chapter 4, page 4-26.

If you are not familiar with this AC - it is a large document, laid out in chapters that you can find on the FAA's website and you can download as much of it as you want - the entire AC covers just about any kind of repair/modification you can imagine. An excellent document for builders in the first place.

If I did this right - this should be the correct link for your specific repair, but you can drill down from the main FAA website to the Advisory circulars then search for 43.13 1B and find chapter 4

http://www.airweb.faa.gov/Regulatory...E?OpenDocument

The cut to the chase is if you have a sharp end to the cut it should be stop drilled, then a patch panel can be riveted over the top of the area. Extra strength if you hysol the patch when it is riveted.

I'd stay away from fabric covering of the aluminum flaperon as a repair - the fabric won't be much of a structural repair for aluminum.

Sincerely,

Dave S
KF7 Trigear Flying
912ULS Warp Drive
St Paul, MN

[gizmos]

Thanks Dave for the fast response. I got this kit from another builder, this was the only damage, the photo shows the worse. The other two places are only dented. Any other tips on the flaperons installation I should know? Stripping the protective coating was easy with paint stripper and a sharp plastic putty knife, I used several because they get dull. I found it was best to start pulling the coating off as soon it starts to get loose. If not, the stripper will break down the plastic and you have hundreds of pieces . I got lucky, two sides came off in one sheet. This kit was started in 1994. Thanks again

[jiott]

I think its best to leave the bent parts for structural reasons, straighten as well as possible.
For just a patch I don't think aircraft grade aluminum is necessary. If the patch requires any sharp bends be aware that the stronger, harder grades may crack at the bend line.
Measure the thickness with a micrometer at one of the hinge slots; I think it is 0.012" but not sure.

[patrick.hvac]

Flaperons are 2024 T3 clad ams 4462 Kaiser aluminum .016

[Eric Page]

I'm in the same situation as @gizmos was in back in 2013 (first post, above), with several flaperon hinge slot ends damaged (some dented, some cut) by improper storage/transport. I'm also in the process of devising a solution, but the guidance in AC43.13-1b is, quite frankly, confusing gibberish.

Starting with Figure 4-16, on page 4-32, we see the method for patching stressed sheet metal skins:

AC43-13-1B Fig 4-16.jpg

Notice that the crack length is 2.5" and they specify a patch size "not less than 2 times length of crack," so we can assume the patch shown is five inches long, measured "crack-wise." It appears that it might be a bit wider across the crack, but let's say 5" wide also.

Now look at an excerpt from Table 4-10 and its notes, on page 4-38:

AC43-13-1B Table 4-10.jpg
AC43-13-1B Table 4-10 Notes.jpg

Using their example with a skin thickness of 0.028" and 1/8" rivets, the table requires 4.9 rivets per inch of width. Note c. allows 75% of that number for a single lap sheet joint (3.675 rivets). If the patch is 5" wide, then the minimum number of rivets would be 5 times that number, or 18.375, which I would round up to 19.

Now look at the patch in Figure 4-16 again. They used a total of 8 rivets across the width of the patch, and the note calls out "4 rivets reqd each side." Confusingly, the drawing also shows 4 rivets along each edge of the patch, so the meaning of "side" is somewhat unclear to me, but let's assume it means "on each side of the crack."

The patch shown in Figure 4-16 has 32 total rivets. Does this mean that the calculated minimum of 19 is for the whole patch? If that's the case, then what guidance dictates the minimum of 4 rivets per side, and how is that calculated? What if the patch is much longer than it is wide; then how many rivets?

It gets worse. Looking at paragraph 4-57.c., on page 4-19, and Figure 4-5, on page 4-20, we see that for single and triple/multiple row rivets, edge distance should be at least 2 times rivet diameter and spacing should be at least 3 times rivet diameter. For double row rivets, spacing is 4 times rivet diameter.

AC43.13-1b Para 4-57c.jpg AC43.13-1b Fig 4-5.jpg

Using the most space-efficient layout, with the rivets staggered to achieve the closest possible lateral spacing (a technique that is not shown in Figure 4-5, but is used in Figure 4-16), there doesn't appear to be any way to get the required number of rivets across any patch. For example:

Spacing Example.jpg

A patch of 1.03" width would require 4 rivets (4.9 x 0.75 x 1.03 = 3.78), but only three will fit, and only if they're staggered. The math just doesn't work, no matter what size patch or how many rows of rivets you use. Even a single row doesn't work; it would require 1/2" total width to achieve 1/4" edge spacing, but a 1/2" patch requires 2 rivets.

If someone understands this and can offer a cogent explanation, I would both be very grateful.