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Center section countersink question

Jwputnam

Well Known Member
I am making final dimples and counter sinks on all center section parts. I carefully read the posts here and in the instructions especially in regard to countersinking the F-904A which calls for double counter sinks for five double flush rivets on either edge of the bottom panel and the other four C/S rivets specified. I elected to C/S that entire line as suggested by the common questions section. This 904A forward half of the 904 wing spar then does not require dimpling.

My question is then what to do for the bottom skin attachment to the 904B rear half of the wing spar. I am assuming that the skin is dimpled and this section of the wing spar is countersunk all the way across. The "skin" is actually two overlapping layers of aluminum just as with the 904A. This would require dimpling both skin surfaces, followed by the C/S of the bulkhead flange.

This is probably obvious to you guys, but I have made enough stupid mistakes that I thought it prudent to check first! I saw nothing mentioned in the instructions or here in the forum.

HOLD THE PHONE! Maybe the better way is to dimple both the skins AND the 904B here. The flange is not that heavy in this area. Maybe easier and even stronger? What do you think?

OR.......I could do the same as with the 904A attachment and C/S the skins completely and not touch the spar flanges.

Thanks for any help.
 
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John, I will take a stab at answering your question. I believe you are asking if you should machine c/s the -904 spar flange or just dimple it.

I'm going from memory here and I'm building a -7, not a -9A. You are correct in that both spar flanges need to be machine c/s or dimpled to receive the bottom skins that are dimpled. I dimpled the flange as itis thin enough to dimple (I forget the actual gage of the flange). But caution here as there are a couple of places on the forward flange that do require a machine c/s. These holes will later accept the floor stiffeners.

Hope this helps.
 
We are on the same page.

In regard to the 10 C/S on the back side of the 904A to accommodate double flush riveting so as to not interfere with the gear, I believe.....I did those per plan, but in doing so, the process reminded me how fragile a C/S in this .032 flange is. I don't like doing it, so my preference is to perhaps dimple all three layers, the outer skin (972 fwd bottom), the middle skin (976 center bottom) and the 904B flange.

It is my opinion though, that it would be easier and perhaps even stronger to dimple the top skin, C/S the middle skin and leave the .032 904B flange alone. The dimple fits into the C/S, the parts are riveted together to a flat flange.

I am actually looking to see if this idea is acceptable since I accidentally created this identical scenario for the 904A flange by C/S the 976 instead of the 972......long story, but I believe that the above solution works very well.

I did write to Vans about this. I just thought that I would get some opinion while waiting.....and it is ALWAYS appreciated.

Thanks
 
I just went back out in the garage and looked at the description for riveting these sections together and the process that I am suggesting may actually be what the plans show.

The plans show a countersink through the 976 skin as well as the 904-A flange. This is in fact what I did, but I am still unsure if this was to include the 972 which overlaps both. It is unclear to me. The drawing points only to the 976 skin and the 904A (which would not receive a C/S through the 976 in my opinion....too deep...and why would you want to C/S the 904 A anyway?).

I have probably confused you to death........but....suffice it to say that my plan remains to dimple the top skin (972) into the C/S in the middle skin (976) which will rivet FLAT to the 904A flange. I guess I just need to know if this is structurally sound. (Maybe it is Van's intention in the first place and I am just misinterpreting the diagram and plans......and I am very capable of that)
 
The way I understand this issue is that all 3 parts involved - the 2 skins and the F-904A/B flange are too thin to countersink. You wouldn't be able to open up the hole enough in a single skin to accept a rivet head/dimple. This leaves you 2 options for these holes, 1) dimple all 3 layers, and 2) countersink the F-976 through to the 904 flange (basically removing material from both mated pieces to accommodate the dimple from F-972). Option 2 is the only option for the 4 places that will attach the skin stiffeners because the dimple will interfere with the stiffener.

I think you are describing option 2 as your approach. The one thing to note is that you need to counter sink the 976 skin in place on the 904 flange, it will need to cut into both pieces and the resulting hole in 976 will be noticeably bigger than the -3 rivet shank.

I don't know if you saw it or not, but Vans has an FAQ on this subject that describes it quite well. You can find it on their web page, under Support, Construction FAQ and it's titles "A common RV7/7A and RV-9/9A fuselage construction question".
 
Thank you Claud.

Your response confirms my final thinking....and yes, I did read that article even before I proceeded.

What confused me was the lack of instruction regarding the top (972) skin. I now realize that it is intended to be dimpled. Days after reading that article and carrying out what I now know was the correct process, I looked back and was panicked because I thought that I missed action on the 972. I was over-thinking again.

When I countersunk the 976 while clecoed to the 904A, I did not think that the process touched the 904A at all, but as I look at it with my magnifiers, I realize that the hole is enlarged and the 904A is very slightly C/S, so your analysis is correct.

I really don't like to C/S the 904 flanges, so I have decided to dimple all three surfaces for the 904B bulkhead attachment. I'll just feel better about it.

Your response has reassured me and put me at ease. Much appreciated. I can move forward in confidence now.
 
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