H-stab fitting: Deep Thought edition

Well look, here we are after another break in getting stuff done. My excuse this time revolves around the two weeks we were out of town in early October, which of course 100% explains the six weeks it’s been since I did anything. I suppose being a little hesitant to actually commit to drilling holes figured in, as well. But today was the day to get moving again.

First order of business was to reverify that the fuselage was level and that the stab was clamped on straight and true. After all, it’s been six weeks since I did all this work. I was pleased to see that everything was still 100% kosher. So after triple-checking everything, I brought over the drill and drilled the four attach pilot holes between the attach bars and the rear spar. Whew – they ended up centered nicely on the spar reinforcement bars!

That was the easy part, though. The forward spar attaches with four bolts, which go vertically through a bunch of pieces. From top to bottom:

  • The horizontal stab’s lower reinforcement/attach angle
  • A thick spacer between the stab and the aft deck
  • The aft deck itself
  • The upper longerons (for the outboard holes) or a piece of spacer material (for the inboard holes)
  • A reinforcement angle riveted to the bulkhead

The good news is that all this stuff is pretty well fixed in place. Everything below the aft deck is riveted together, and the position of the stab itself is dictated by aligning it properly and squaring it to the fuselage. The spacers I’m a little concerned about, even though they’ll be tightly clamped when I drill, I worry they might move around.

The bigger concern here, though, is positioning the bolt holes – especially the outboard ones – such that they don’t compromise edge distance and thus weaken the structure. The plans give you measurements for laying out the holes, but being concerned about positioning, I decided to take some time to wrap my head around the layout and reason things out a bit.

To start with, I removed the stab and proceeded to draw the outlines of the relevant pieces on the aft deck:

I also took those photos and made some annotated layouts with Photoshop:

Rv stab all

Rv tab detail

This is where it gets fun. By the book, for a 3/16” bolt hole, I’d want the center of that hole to be 3/8” from the edge of any part. But there’s only 5/8” of overlap between the longerons and the bulkhead angle, so the absolute best edge distance I can hope for here is 5/16”. The good news, based on my research, is that this is not a show-stopper, and in fact it sounds like almost everyone runs into the impossibility of getting “by the book” edge distance here.

Overall, I think I have a solid strategy here to make this as good as it can get, but this seemed like something where I should really let everything stew in my brain overnight before actually doing any drilling. I also posted to VAF just to get a sanity check from the peanut gallery there. It may even be possible that I’m way overthinking this.

So we’ll see how I feel about all this tomorrow, and what sort of feedback I get. I still have to decide how I want to drill the actual holes, even if I can lay them out nicely on the aft deck, I need to transfer those hole positions to the stab angle in order to actually do the drilling…

Posted in Empennage, Fuselage | Hours Logged: 1.5

H-stab fitting: the beginning

OK, so this is really a post for the entire weekend, sort of. I didn’t really have anything like a dedicated work session, but I did still get some stuff done.

In short, I moved on to the beginning of fitting the tail feathers, starting with the horizontal stab. The first step to doing this is to get the fuselage dead level, both front-to-back and side-to-side. This meant taking it down off the rotisserie setup and getting the forward portion sitting on the sawhorse I built for this eons ago. Then I ended up making stacks of shims out of thin plywood until I got everything nice and level.

With that done, it’s time to set the h-stab in place on the aft deck. But it doesn’t sit directly on the deck; under the forward spar splice angle there are a couple spacers (at this point, bolts will go through the bottom of the angle and through the deck). The aft spar, meanwhile, sits flush against the stab attach bars, which are in turn riveted to a bulkhead and project vertically through the aft deck. At this mount point, a 3/16” spacer is placed under the spar to properly position the stab.

To start with, the stab is centered on the fuselage, using the joint between the rear spar channels as a reference and centering that joint between the attach bars. Then the stab is loosely clamped into place at the rear spar. Next the stab needs to be leveled side-to-side. To do this, you make a couple of spacer blocks, which are placed approximately mid-span on the stab, and a long level laid across them. I used this level as a base for my small smart level, to get a more accurate reading. I ended up with the left side slightly low, so I made a thin shim from scrap stock to place on top of the left-hand spacer under the front spar, and that got my lateral level set.

At this point, the clamps on the rear spar are tightened to set that joint in place. Next, the stab needs to be squared to the fuselage centerline. This is done by measuring diagonal lines from each tip to a reference location on the fuselage side. The stab then gets tweaked until those measurements are equal (or, by the book, within 1/4”…I got closer than that). At this point, the front angle is clamped down tightly.

This ends up being the point at which I stopped work. The next step in the procedure is to use prepunched holes in the stab attach bars and drill through the rear spar, creating pilot holes for the attach bolts. I did not feel at all ready to commit to that without maybe doing some reading on the procedure. Somehow it all felt a little too easy to me. I suppose there is one more thing to be done before I commit to the holes, which is to check that the incidence is properly set. To do this, I’ll need to use a line between the tip rib tooling holes as a reference, and make sure that reference line is level just like the fuselage.

I also need to think about how I’m going to go about planning the attach holes a the forward spar. This is a spot that builders frequently mess up – getting proper edge distance on the bolt hole through all the various pieces can apparently be challenging, and it’s extremely important to get things right here. Overall, the entire empennage assembly is attached to the fuselage by just eight bolts. I definitely don’t want anything compromised back here.

So we’ll see about picking back up this week, and maybe I’ll work up the gumption to actually take a drill to this stuff.

Posted in Empennage, Fuselage | Hours Logged: 1.5

H-stab v2.0 – finished!

Well, so ends this little detour on the road to a complete aircraft. Tonight I wrapped up the stab, so I can move forward with more fun stuff again. Basically all I had left to do was to rivet the rear spar to the skins, plus a few other rib rivets. Oh, and towards the end of the night I remembered that a few holes between the inboard ribs and skins are suppose to be left open for nutplate attach points for the empennage fairing down the road. And yup, I already riveted all those. So I got to drill out a few more rivets.

But yeah, it’s done, and once it was done, I decided todo this, just because I could:

The next big thing will be fitting this to the fuselage!

Posted in Empennage | Hours Logged: 1

H-stab v2.0 – yet more riveting!

And so the theme continues, chipping away at this a bit at a time. Previously, I was happy to be finished with the “annoying” riveting. In my mind, I just had to go around the perimeter squeezing skin rivets, plus a handful of structure rivets, and be done. Of course, it wasn’t quite that easy.

I started with the easy part – setting the blind rivets that attach the rear spar to the center main ribs. Next came the large rivets that attach the rear spar to the root and tip ribs. The root ribs weren’t too bad, but the tip ribs required some extra work. A couple of these holes got damaged during disassembly, so I drilled those out to #21 and used -5 rivets in place of the -4s. That required finding where in the world I put the stock of -5 rivets I bought a while back, along with the proper tools.

Next I started in on the tip ribs. These should have been easy, but I got a bit complacent at first. It’s a bit harder to mess up rivets when squeezing rather than bucking…but not impossible. I wasn’t careful in holding the skin side of the squeezer flush and so I ended up with a few proud rivets, which I’ll need to drill out and replace. I decided to just keep squeezing and mark those to be dealt with later. Then I got most of the root rib rivets squeezed as well, before deciding it was time to call it a night.

That just leaves a few more hard-to-get-to rivets on the ribs, and all the rear spar rivets. I really should be able to get that done in one more work session. Let’s see if I live to regret typing that last sentence…

Posted in Empennage | Hours Logged: 1

H-stab v2.0 – even more riveting!

So this hasn’t been a productive airplane week – in addition to the usual household chores, I’ve been working on getting the car ready for my next track event next weekend. That took up more time than I expected since I had an issue crop up with the power steering. I knew I was in danger of getting back in the habit of not working on the plane, so today I made sure to force myself out of bed when the alarm went off so I could get to work.

Today was just lots of riveting – as of last time I’d shot one row of spar and center main rib rivets. Today I knocked out the other three spar/rib lines, which wraps up all of the riveting that a) requires the rivet gun and b) involves bucking rivets in the tight quarters inside the stab. After I’d finished this riveting, I took some time to wipe down the inside of the skin with acetone – I deposited plenty of sweat inside there today. Best not to leave that in there.

With that done, I finally grabbed the rear spar assembly and clecoed it in place. Now I just have to go around the perimeter and squeeze all of those rivets, which should go more quickly than the interior rivets. Then there are a coupe more structural rivets to tie the spar into the ribs, and I will once again have a horizontal stab.

Here’s the obligatory photo of the stab all clecoed together and ready for final riveting:

I think the next order of business is going to be finally finishing up the riveting and assembly of the forward fuselage. I’d kind of like to get that done before moving on to fitting the horizontal stab, though I suppose it’s not necessary – the stab won’t be staying on long-term. That matters because my rotisserie setup isn’t going to work with any of the tail feathers in place. Which is why the stab won’t stay on – I’m a long way from being finished working inside the fuselage canoe…

Posted in Empennage | Hours Logged: 2.5

H-stab v2.0 – more riveting!

Well, this wasn’t the productive day I’d hoped for. I got up early and got started on some riveting before heading over to the neighbor’s hangar for the usual Sunday morning coffee. Came back over and shot a line of rivets on one side other stab, then retired inside for lunch. Decided to take a nap, and…next thing I knew, it was 6 PM. And I still needed to get the grass mowed.

So yeah, I did get those rivets shot, better than nothing. I also had a moment of frustration when I noticed that I’d somehow failed to rivet one of the nose rib holes. And that area was now sealed off since I’d blind riveted the spar in place. So I just put an MK319 blind rivet in that hole. Of course, it had to be on the top of the skin, too, not hidden on the bottom, but oh well. Not worth drilling those blind rivets out again and risking damaging other parts. I’ll probably fill the center of that blind rivet at some point so it’s less obvious after paint.

Posted in Empennage | Hours Logged: 1

H-stab v2.0 – riveting!

Well, this little detour appears to be drawing to a close. With the prep work I did Thursday evening, the only thing left to do today was to start riveting things together.

First up was assembling the entire front spar – the two channels, the two reinforcement/splice angles, and the doublers.Lots of fun with long rivets here, but I was able to squeeze them all. Then I added the inboard ribs. Look, a skeleton is coming together!

Next it was time to bring the skins over to the workbench. The assembly procedure starts here with riveting the center nose ribs to the skin. This is a bit of a run procedure, for two reasons: first, bucking the rivets has to be done in pretty tight quarters, since the stab isn’t exactly roomy. Even better, the skin has to be pulled around the ribs to conform, so there’s a fair amount of force trying to pull the skins away from the ribs. Even with clecos in every other hole, the skin wants to not sit flush against the rib. However, this was relatively easy to mitigate by riveting with the stab sitting flat on the workbench and pushing down hard with the rivet gun. The downside was that doing this and holding the bucking was a little awkward.

The tungsten bucking bars really came in handy here. I honestly am not sure how I riveted this stuff the first time around when I hadn’t even gotten the tungsten bars yet. Those things are absolutely worth every penny.

Anyway, with the nose ribs riveted, it was then time to introduce the spar and get everything clecoed together. Firs the spar is assembled to the skins, then the remaining ribs are added. Riveting begins with putting the blind rivets that go through the main center ribs, the spar, and into the center nose ribs that were riveted previously. Getting these blind rivets done was way more annoying than I expected; again, it’s tight quarters in there, which makes it tough to get the rivet into the hole, then the puller on the rivet stem, and then figuring out how to exert force in that small area.

But hey, it’s starting to look like I have a stabilizer again!

That ended up being the last work I did before switching to dinner-making mode. Assembly is coming along nicely, and tomorrow I can pick up with beginning to shoot a whole lot of skin rivets. It’ll still be tight-quarters work, but should be better than working up in the leading edge. I think there’s a good chance I can close this thing up tomorrow.

Posted in Empennage | Hours Logged: 3

H-stab v2.0 – prep work

No photos tonight, nothing to really see. I tore the stab apart, got the reinforcement and splice angles primed, then finished final-drilling the spar channels and dimpling the necessary holes. Everything is now prepped and ready for riveting!

Posted in Empennage | Hours Logged: 1

H-stab v2.0 – main rib fitting

So tonight I finished up the last bit of major fabrication for this SB/repair job. I initially had some car-related stuff to do after I got home, but still found an hour to spend out in the shop.

Tonight’s job was fabricating the “replacement” forward flanges for the inboard main ribs. The addition of the doublers on the aft side of the spar necessitates this, since the original flanges used to mate with the spar channel. The new flanges are made of angle material, which are riveted to the rib web. Normally the four attach holes would be drilled in assembly with the spar, using the existing rib attach holes, but since I’m starting over, I don’t have all of those. I do have the two “inner” holes, which are where the the forward inboard ribs attach, and which I drilled last time. The “outer” holes, though, I get to lay out myself.

Anyway, to start with, the angle pieces are clamped to the ribs and the ribs clecoed in place in the stab assembly. Then the angles need to be carefully slid forward to mate tightly with the spar. This means loosening those clamps while still maintaining the vertical positioning of the angle. Next, the angles need to be clamped tightly to the spar prior to using the two existing holes to drill.

The end result is kind of a mess of clamps, and sort of difficult to get a photo of. To the left is the rib, to the right is the spar, and sort of barely visible in the middle is the angle:

With those two inner holes drilled in each angle, they’re now positively located to the spar, and the ribs can be removed to better show the assembly:

Now comes the more fun part: figuring out where the two outer holes need to be located. These holes will go through four pieces: the rib angles, the doubler, the spar channel web, and the reinforcement angles – and I need to make sure I maintain good edge distance on all of those. Really, the only items of serious concern here are the angles. To make it easier to do this layout, I decided it was best to disassemble the angles and doublers, then cleco those items (minus the spar channels) on the bench to be able to better visualize things. With that done, it was easy to use a straightedge to trace the inner edges of the reinforcement angles onto the rib angles:

Then it was easy to lay out the desired rivet locations, center punch them, and drill a #40 pilot hole in each location. Then I was able to cleco all those pieces back together with the spar, and use those pilot holes to final-drill #30 through the entire assembly. With the rib angles fully fitted to the spar, I could now cleco the ribs back in place and get ready to drill the rivet holes between them and the angles.

Once the ribs were in place, I clamped each rib to it angle with two small C-clamps. The C-clamps were the right tool for the job, as I need these things to be clamped super tightly. Once the clamping was done, the rib and angle was unclecoed and removed as a unit, so I could drill the pilot holes (already in the rib) to final size in assembly with the angles.

The pieces clamped and ready for drilling:

And the pieces all drilled and ready to go:

And that’s it for the fabrication stuff. There’s still some disassembly, demurring, dimpling, and priming to be done, but overall I’m pretty close to being ready to start riveting all this stuff back together!

Posted in Empennage | Hours Logged: 1

H-stab v2.0 – skin and rib fitting

Good solid day of work, sprinkled in between the usual weekend stuff. I started off this morning by finishing all the drilling on the spar assembly, using the prepunched holes in the doublers and angles to drill through everything. Took everything apart to deburr and then double-checked the edge distance on the doublers – all good!

Next up was figuring out how to fit the inboard ribs. Here I was going to be inventing my own procedure, since I already had pre-fitted ribs, and I’d need to use those to lay out the rivet holes through the spars. I figured the way to go here was to cleco the skins to the underlying skeleton, and essentially treat this entire procedure as if I was back to performing the service bulletin on a fully-assembled stabilizer. So I broke out the bucket of clecos and got to work. Now we’re back to something resembling an airplane part:

At this point, I had the inboard nose ribs in place, but not the main ribs, and here I ran into my first bit of challenge. The new spar channels are only partially prepunched where they met with the skins – only outboard of the center ribs (this is why the span wise line of clecos above stops halfway in). The problem was, that meant the inboard portion of the spars wasn’t clecoed to anything, and the result was that it didn’t sit flush against those inboard nose ribs:

This is an issue, since I’m going to use those holes in the nose rib flange to locate and drill the rivet holes through the spar assembly. I need all this to fit tightly together. So I had to get creative and figure some way to pull the spar forward to get it tight against that rib flange. It wasn’t too hard, nothing that couldn’t be solved by a long clamp applied in a super precarious manner:

Drilling was fun with this setup too, since that clamp kind of got in the way of accessing that rear flange. I’d originally intended to use the angle drill to make these holes, but it was impossible to get it perpendicular to the surface because the rib web was so close. So I ended up doing things in two stages: I started the holes using a 12” #30 bit, but didn’t go all the way through. I was thinking of how I enlarged those holes in the old spar doing something similar to this, and I was more concerned about that since it can be hard not to apply a side-load to the long bit. I drilled just deep enough with the long bit to get a good start on the hole, then removed each rib and finished the holes using the angle drill. It was a bit painstaking, but it got the job done.

Here we are with both of those rear flange holes complete:

That wasn’t the end of the challenge, though. Next I’ll need to fit the aft inboard ribs, including the piece of aluminum angle that will be spliced in to act as the replacement front flange. To fit this properly, I can’t have those clecos sticking through the holes in the spars, but I still need the spar to fit tightly against the nose ribs for the fitting to work right. So I decided this was the time to final-drill those spar holes in common with the skin. With those done, I was able to remove the nose ribs and still have the spar stay in place, since those holes in common with the skin now positively locate the spar.

My last act of the night was to take the piece of angle material provided for the splice flanges and cut off two 2 15/16” pieces. I cleaned the cut edges up on those and then set them aside – I’ll pick up with that bit of work tomorrow night (or maybe Tuesday). Properly locating these splice angles will require some care, especially since I’ll be inventing my own method for locating the holes. But once that’s done, I’ll be finished with major fitting, and I can move on to some final prep work – final-drilling a few more holes, dimpling the spar, and priming the pieces that need it – and then it’s on to riveting this thing back together.

The end (of this little side journey) is in sight!

Posted in Empennage | Hours Logged: 3