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Firewall and Forward Fuselage plans confusion - F-1049 F-1001A

1001001

Well Known Member
This has been driving me nuts for a day or so. There are some odd instructions regarding rivets connecting the Firewall Bulkhead F100A and the Forward Floor Ribs, F-1049x. I can understand sometimes the plans don't specify dimpling when flush rivets are used, leaving it up to the builder to know that of course flush rivets need dimpled or countersunk holes.

But the specificity of some of the steps here is confusing.

On Page 27-2, Step 6, the plans say "Dimple (flush head on forward side) all rivet holes in the web of the F-1001A Firewall Bulkhead..."

OK, that's fine, but there are some rivet holes in the web of the firewall that have not yet been match drilled to their joining parts, which are the F-1049A, B, C, and D Floor Ribs. Most of the time we want to match drill before dimpling/countersinking holes, but this instruction says do them "All." But, this doesn't seem right to me. Step 6 also says to "Machine countersink all parts that lay against the web of the firewall bulhead for the dimples..." One could take this as an instruction to read ahead and see that the F-1049 ribs join the firewall, and should be dimpled. But they are not otherwise mentioned on this page, so maybe it would be a good idea to find them in the instructions! So, reading a bit further:

Page 28-5 is where we start to work with the Floor Ribs. On Page 28-7, we start to interface these ribs to the Firewall Bulkhead. Here, in Step 1, we are told to "Final drill #30 the holes common to the ... Firewall Bulkhead and the forward tabs of the F-1049A... Floor Ribs. See Page 28-5, Figure 2. Sure enough, Page 28-5, Figure 2 calls these out as AN426 flush rivets.

At this point, we've been told to dimple the firewall bulkhead rivet holes and then to final drill these holes to their corresponding ribs. This is not usual practice, in my experience with any of the prior instructions.

On Page 28-10, Step 11 says "Dimple all #30 holes in the fwd fuselage floor ribs EXCEPT the holes in the forward and aft tabs and the most forward hole in the upper flange. See Page 28-5, Figure 2."

OK, that's pretty specific. These are not supposed to be dimpled here, and probably not supposed to be already countersunk.

Then, on Page 28-11, it says in Step 6,"Dimple the forward tabs of the fwd fuselage floor ribs." This page says nothing else about these ribs other than to rivet them to the F-1072 fuselage bottom skin. Not sure why it's important to do that now, rather than earlier, other than that dimpling them earlier might cause fit problems for other parts?

Then on Page 28-13 toward the end of Step2, we are finally instructed to rivet the forward tabs of the F-1049 Floor Ribs to the Firewall Bulkhead. I can't find any language in between 28-11 and 28-13 that says to dimple the corresponding holes in the Firewall Bulkhead.

So, I'm confused. I can understand waiting to dimple the floor ribs' forward flanges if doing so too early can cause fitment problems with other parts, but why suggest that the corresponding holes in the Firewall Bulkhead should be dimpled much further back in the process? Is it impossible or extremely difficult to dimple these holes in the Firewall Bulkhead this late in the game?

Why would the plans have us final drilling a dimpled hole and an undimpled hole together?

Please help, I'm going a bit nuts over this, and I'm concerned that if I don't follow the rather Byzantine instructions regarding this that I'll have a serious problem fitting parts together, but it also seems really odd to match holes for flush rivets after one of them has already been dimpled (which opens up the hole quite a bit and can't achieve a very good match if done in this sequence).
 
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Confusing instructions

I am just a little bit ahead of you, and experiencing a lot of the same frustration. I find myself spending an inordinate amount of time looking ahead in the plans to figure out what goes where later on. It seems that it wouldn't have taken that much time to continue to give clear instructions, as was done in the empennage instructions. I'm very grateful for the guidance I've gotten from others on the forums on these confusing steps, but it makes me feel even more incompetent than usual.
From what I've heard, this confusion gets worse later on, and there are lots of places where we are essentially expected to "reinvent the wheel" to figure out how to make certain attachments (tight rivet spots, for example). I'm still trying to figure out how to rivet the firewall recess into place. A lot of these pieces are very expensive to replace if a mistake is made.
To answer one of your questions, I dimpled the holes for attaching the fwd fuselage floor ribs as instructed. When you cleco them later, it becomes obvious that it might be very difficult to dimple it well because you have the tunnel attached. I did all of the dimpling (with a few exceptions) of the firewall on the DRDT-2. Here is a pic of the floor and ribs. You could probable do it with a long yoke on a pneumatic squeezer, but it seems like it will be fine to dimple it before final drilling.
https://flic.kr/p/2k1i8bA
 
To answer one of your questions, I dimpled the holes for attaching the fwd fuselage floor ribs as instructed. When you cleco them later, it becomes obvious that it might be very difficult to dimple it well because you have the tunnel attached. I did all of the dimpling (with a few exceptions) of the firewall on the DRDT-2. Here is a pic of the floor and ribs. You could probable do it with a long yoke on a pneumatic squeezer, but it seems like it will be fine to dimple it before final drilling.
https://flic.kr/p/2k1i8bA

Thanks! I appreciate the reply!

I'm perfectly OK with them leaving out obvious things like dimpling holes for flush rivets, or leaving the assembly sequence entirely up to the builder. It's quite another thing, in my opinion, when (like in this case) they give seemingly very specific instructions for half of an assembly, and vague instructions for the mating parts.
 
When I ran into an instruction that I didn't quite get, I simply left the part of the step undone with a note to make sure it was done at a later date. In most cases I found the step made sense as I moved along. Once in a while I would simply have to complete the step because I could see that I was going to get boxed in soon.

Leave clecos in place (a sure sign attention was needed at some point), a yellow sticky, and typically I would highlight and flag the text in the instructions so I didn't loose track. If you don't flag the step in some obvious way, I guarantee you will forget something until the point it becomes a major PITA to go back and fix it.

As for the rivets in the firewall recess corners, they are a PITA no matter when you do them. Word of advice, do them before attaching the box to the firewall, or at least before the other bracing is installed. Stuff on the firewall will get in the way and make the job even harder. I would also recommend not drilling the control cable holes until you are routing the cables if you are going with a quadrant. 2 of the three were in the wrong place for me and had to be patched.
 
When I ran into an instruction that I didn't quite get, I simply left the part of the step undone with a note to make sure it was done at a later date. In most cases I found the step made sense as I moved along. Once in a while I would simply have to complete the step because I could see that I was going to get boxed in soon.

Leave clecos in place (a sure sign attention was needed at some point), a yellow sticky, and typically I would highlight and flag the text in the instructions so I didn't loose track. If you don't flag the step in some obvious way, I guarantee you will forget something until the point it becomes a major PITA to go back and fix it.

As for the rivets in the firewall recess corners, they are a PITA no matter when you do them. Word of advice, do them before attaching the box to the firewall, or at least before the other bracing is installed. Stuff on the firewall will get in the way and make the job even harder. I would also recommend not drilling the control cable holes until you are routing the cables if you are going with a quadrant. 2 of the three were in the wrong place for me and had to be patched.


I keep a list of deferred steps on a whiteboard in my shop, and note on the page the date of deferral as well. I try not to defer too many items because of my concerns, like yours, of being boxed in later.

I did already drill out the control cable holes, so too bad there!
 
Sections 28-29.

I match drilled the ribs to the firewall (as a component) when I read the firewall holes had to be dimpled. Worked out fine. I’m not too far ahead of you....riveting front side skins now on fuselage. If you think this section is exciting, wait until section 29. Many gotchas in this section you should read up on. Call if you want to walk through these. May save you time and frustration. 419-344-1519.
 
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