Monday, December 28, 2009

The Aft Bulkhead

Or, if you will, I could also title this post "A New Third Eye." But that comes late in the story.

The aft bulkhead will eventually be installed (wait for it....) at the very aft (or 'back end', you landlubber) part of the tail cone. It will not only seal the back of the tail cone which is, after all, nothing but a big tube (shaped somewhat like a cone, surprisingly enough, given its nomenclature) but will also provide attachment points for all kinds of important things like the vertical and horizontal stabs, the electric elevator trim motor, and all of the sorta complex stuff that makes the Flettner tab work.

With all of that going on with a part that's probably only one square foot in area, it's not all that surprising that there's a lot going on with regards to riveting and the attachment of various pieces-parts. And, with that much going on, it's also not surprising that this particular page in the plans/manual is up to its third revision. And considering that the downloading and reading of revision pages from Van's web site was one of the research methods I employed during the run-up to starting this project, it's equally unsurprising that I have read this page many, many times prior to actually reaching this step.

With all of the different pieces to attach, there are obviously going to be different size rivets used, each depending on how many layers it has to penetrate. The manual has a chart to help you determine which rivet to use for each hole:

There are some interesting things going on with this chart.

First, note the size difference between the third and fourth rivets sizes in the list. AN470AD4-4 is the third down in the list. The '470' indicates that this is a round-top rivet (as opposed to flush), it is a #4 width (which means a tough squeeze), and it is a -4 in length. The fourth in the list is a AN470AD4-6, which is a -6 in length, 50% longer than the -4. The integer gives the length in 1/32s, so a -4 is 4/32" long.

So, why and where do we use the longer rivet? Look towards the bottom of the bulkhead and you will see a couple of brackets are being riveted onto the bulkhead. It would make perfect sense for the longer rivets to be used in the holes that have to pass through the flanges, right? But if you check the sizes used, you will see that it is the shorter rivets that go through the flanges, at least on the left bracket. On the right bracket, the rivets that go through the flanges are different lengths depending on which side of the bracket they go through. This makes no sense until you realize that the longer rivets are also being used to rivet the tail skid in place. This becomes obvious a little bit later in the process.

But what's going on at the top of those brackets? If you look at the top flanges and determine the type of rivet being called out, you will find that it is the LP4-4, which is a blind rivet. Just on the other side of the big hole in the middle of the bulkhead, you will see a collection of eight CCR-264SS-3-2 rivets. Those are #3 size flush blind rivets. So what's will all of the blind rivets? We never see the use of blind rivets in areas where a lot of beefiness is required. My theory here is that those rivets are out of the reach of the 3" yoke of the rivet squeezer, and rather than have the builder spend $100+ for a larger, seldom-used yoke, blind rivets were employed instead. As we will see, though, I'm still going to have ti use an alternate method (which for me will be a rivet gun) to drive some of the more awkwardly placed rivets.

Thus familiarized with that which is to come later in the page, we jump back to the first step on the page which is to fabricate those brackets we see at the lower part of the bulkhead. These require separating from a longer piece in the way we've become accustomed to, and then a quick deburring as has become our wont. But wait! Because we read ahead (it's hard to miss - it's a big, bold-font warning just before the official Step 1) we know not to deburr the larger hole in the middle of each bracket piece. Instead, we rivet the two bracket pieces that have rivet holes in them to create the right side bracket. We then "deburr only enough to locate the bushing."

Ok, that seems odd. I have been locating small parts like bushings by digging through the paper bag inventory list. In all of the deburring that I have done, I don't recollect ever locating a part by performing the act. Clearly we are learning a new Van's term here. Because there is a restriction on where to deburr, my first step was to nominate one side of the two-flanged bracket for deburring and mark it. As long as I was doing that, I marked the appropriate side of the one-flanged bracket to avoid a mistake:

Before locating the bushings with deburring, I located them using the inventory sheet. Test fitting them to the bracket holes helped discern what was meant by the statement "oversize before press fit" in Step 1. In this case (and in most, I wager), the word 'oversize' is not a verb. No, we do not attempt to make the bushing oversize; it already is. So, 'oversize' is an adjective, and an appropriate one to boot. Those bushings were having nothing to do with those holes. In fact, we are informed that a C-clamp and a small socket may be employed to coerce the bushings into the holes.

To make that happen, though, the bushings must be located by deburring. With a clearer picture of what was desired, I realized that what I needed to do was deburr the holes just deeply enough for the newly slanted edge around the hole to guide the bushing into place:

From there, it's (or should have been) an easy process to put a C-clamp on one end of the bushing and use a socket on the other end of the C-clamp to push against:

I say "should have been" because getting all of that in position was a three hand (or two hands and a chimpanzee foot; in either case I am anatomically ill-equipped to the task) job, and even then the bushing insisted on going in crooked. I removed the (Harbor Freight, he said in ominous foreshadowing) clamp to see what the problem could be. Oh, this might be it:

After a short search of the clamp shelf, I found a straight(er) clamp and tried again. Success:

Step 2 took me awhile to figure out. Basically it states that we are to "slightly flute at shallow notches, 4 places" only enough to "reduce the slight pucker in the flange." The drawing showed the notches in question, but it took me awhile to figure out where the slight pucker was, mostly because when they say "slight," they mean "almost non-existent":

Once you find the slight pucker, it's just a matter of squeezing it down with the fluting pliers. That was a little harder than I expected it to be because the fluting pliers that I have are so big that the tiny little pucker was hard to get a grip on. You can see where I had to nibble at it:

With all of the prep work done, we start to cleco things together. Not, as it turns out, in order to start riveting. Rather, there is some drilling to be done. Remember that fancy tail skid bracket that had so much cutting and grinding to do? Well, it's not done yet. The two rivet holes already drilled into it are just starter holes to get it clecoed into place for final drilling:

The plans also have you clamp the part down prior to drilling, but I found my collection of clamps to be lacking in their ability to make any serious contribution at the higher end of the skid. Having seen what can happen when match drilling parts only being held by clecos, I knew to keep a close eye on the fit of the part being drilled into after each new hole. Sure enough, it didn't take long for a chip-filled gap to appear:

Remove. Deburr. Cleco. Drill. Repeat.

It got better as I worked my way down into the area that the clamps were able to reach, culminating in a nicely drilled part:

That is an interesting picture in that it finally shows why we went through all of the rigmarole of cutting corners and slants on the tail skid bracket. It wasn't make-work after all. Now that it's in place, it's obvious that the cuts in the lower flange were to allow the threaded hole to protrude out of the bottom of the plane without interference from the flanges of the bulkhead. But what about that big (ugly!!) slant that I butchered into it? Well, that big hole above the tail skid is where the counterweight bar will pass through into the tail cone. As the bar moves up and down with the horizontal stab, it needs room to move in the vertical plane. If we hadn't cut that slant into the tail skid bracket, it would have interfered with the counterweight bar, which would have in turn interfered with the pilot's ability to use up elevator. And that, my non-pilot friends, is a definite problem.

Once drilled, the tail skid needed to be removed for deburring one more time. As along as I was doing that, I took the whole assembly apart and sprayed some primer on it. As you know, I haven't been priming much of the airplane because it really doesn't add any corrosion protection. That said, it does act as a bit of a talisman (or "Dumbo feather," as I like to say) in situations where the criticality of the area in question is such that a little peace of mind is nice to have. And, just like taking timeouts into the football locker room at half time, primer left in the can has no use whatsoever. Once the primer had dried, I clecoed it all back together again:

NOTE: I have returned from the future with a warning! See those two holes in the lower left corner? There's a big one (3/8", to be exact) and a little one. See them? Ok, good. Go to Step 14 where it tells you to "Insert the bushing into the F-1211 Assembly called out in Figure 3" and retrieve the bushing from the appropriate parts bag. Make sure it will fit into the hole. Don't put it in all the way just yet - just put it in far enough to ensure that the hole is the correct size. Mine needed to be drilled out to 7/16" and it was a lot harder to do at the point I did it than it will be to do it RIGHT NOW!

I started with the blind rivets because they're so much easier than the squeezed #4 solid rivets, then worked on around the periphery. This too was a three-hand operation, but I tried to substitute the edge of the work bench for the third hand. That worked out fine, right up until the point where it didn't. The thing about squeezing a #4 rivet with the robust Cleaveland Main Squeeze is that once it gets past the force-enhancing cam, it has a tremendous amount of "spring-back" potential energy stored. If one were to somehow lose his grip on the part being riveted in just the wrong way, that part (in this case an aft bulkhead with all kinds of sharp-edged protuberances on it) could potentially fly back and hit the riveter (the guy squeezing the rivet, not the actual tool) right in the kisser. How do I know this, you ask? Well, a picture = 1000 words:

Ouch! And, truth be told, not much of a mood enhancer, unless the mood you're trying to enhance is "crappy." A "third eye" like this is a great conversation starter, but it's unfortunately a conversation you'd rather not have. Good thing I'm not going to work this week!

I pressed on with the riveting, though, trooper that I am. But without the good mood that I started with, it start to feel a little like work, a feeling that I have sworn to use as an indication that it's time to quit for the day. I hate being halfway through a job, though, so I pressed on. Through a strange twist of fate, everything went fine until I reached the inner parts of the bulkhead where I found that the yoke on the rivet squeezer couldn't reach all of the rivets. I just left those clecoed (or taped over) until I can go to the hangar to drive them:

Funny thing about that picture: note the two groups of three clecos. You would expect (if you're the type of person that likes symmetry) there to be four clecos in each group. Well, what happened with the fourth holes is something I didn't even thing possible: I got "smileys" on squeezed rivets! That's not uncommon when driving rivets; it happens if you let the rivet gun slip off of the rivet. I was shocked to see it happen on a squeezed rivet. So shocked, actually, that I tried again on other side. Which achieved the same (worse, actually) result:

Between that and my throbbing forehead, it was definitely time to stop. I'll have to carry the bulkhead out to the hangar and drill out the smiley rivets out before driving in the eight (now ten) remaining rivets. Then on to the eight nutplates!

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