Empennage

Tonight, I’ll start this post a bit differently from usual, with a story of sorts. A couple years ago, I decided to get myself a new tool: a cordless impact driver. My reasoning for this was to facilitate swapping out wheels on my car at track events. Not too long after that driver came in, I had occasion to remove a couple wheels on the car. Right after finishing that job, I think I actually said out loud, “why in the world did I wait so long to get one of these?”

Yeah, I had another one of those moments tonight. Recall that the last time I worked on this SB, last weekend, I spent 3.5 hours painstakingly drilling out 24 rivets, and then worried heavily that those were relatively easy compared to the 16 inside the stab. Well, tonight, over the course of a little over half an hour, I drilled out all 16 of those rivets, using the new tool I got from Cleaveland. I think it’s safe to say, at this point, that this thing was well worth the money.

After popping the heads off all those rivets, I managed to get one of the reinforcement angles out without too much drama, just by hitting the rivet shafts with a center punch. The other angle didn’t want to come out so easily, so I left that for tonight. Probably I just need to work in the rivet shafts there with a punch and hammer, but this was about 10:00 at night, and my neighbor has a newborn…I figure it’s good to try and keep the noise down.

But hey, this is great news – with the exception of getting that other angle knocked loose, I’m now finished with disassembly tasks. After this there’s some trimming of the spar flanges, fitting of the reinforcement doublers, and some modification to the ribs…then it all gets riveted together again. I think there may be a good chance I’m able to get this SB work wrapped up this weekend.

Whew…it’s a hot weekend in Houston. The rest of the summer has been relatively balmy, with his peaking in the 90s, but this weekend (and for the next several days), we’re sitting up over 100°. I got an early start this morning, but by mid-morning I was already in a state where I’d work for about half an hour, then retire inside to cool off for a while. Still, I made good progress.First up was getting that last reinforcing angle out. That just took some strategic application of force, and eventually it popped right out. An inspection of the rivet holes in the spar web looks good, none of them appear to have gotten oversize or had any damage done around them. Success! Then I just had to drive all those rivet remains out of both angles. No big deal there, just a bit tedious.

OK, so I didn’t get the SB done this weekend like I thought I might. Temperatures here have finally been crossing into the 100s this weekend, which definitely is keeping the work sessions short. And today, I spent some time helping a neighbor unload an aircraft he bought, and also went to a flying club safety meeting that ended up running about three hours. So not a lot got done today, but hey, we’ll talk about it anyway.

First up was demurring the doublers. I ended up reinstalling the vise on the workbench for this, it made it a whole lot easier. It’s also necessary to radius one side of the doubler forks, to ensure it can nest properly inside the spar channel.

Next up are adding some markings to help with fitting and drilling the doublers. They already have some prepunched holes, but these are in addition to the holes that already exist in the spars. Those existing holes will be used as guides to add the additional holes in the doublers, and it’s necessary to ensure they have proper edge distance. We start by drawing a horizontal line using the prepunched holes as a guide.

Next, I added my own touch. I watched a Youtube video demonstrating the SB, and the guys doing that handled the edge distance issues by clamping the doublers, marking the holes in the spars, removing the doublers, evaluating edge distance, and then retrying a couple times. It seemed a lot easier to me to simply draw lines around the perimeter of the doubler, which would represent the closest that any hole could be to the edge. This way, when fitting the doublers, I can just ensure that none of the edge distance lines are visible through the rivet holes.

One of the marked doublers:

Next, it was time to fit the doublers. However, there’s another fitment concern here – it’s possible for the doublers to interfere with the vertical stab attachment, which goes right in the middle of that forward spar joint. I was starting to consider how I could mark the area to prevent this, and I realized that there was a much easier approach.

The vertical stab attaches at its forward spar using a spice plate, which is drilled and riveted to the vertical stab spar, and then bolts to the horizontal stab forward spar. The bolt holes already exist in both parts, though they’re currently undersize. But since they’re prepunched, all I had to do was dig up the splice plate and cleco it in place. This also has the benefit of tying the two spar halves together, making the h-stab a bit more stable in its partially-disassembled state:

That was about as far as I got today. I did start fitting the doublers, getting them clamped in place and seeing how much fun it would be to position them properly, but I didn’t get any photos of that. I’ll pick back up on that task probably tomorrow night. For what are probably fairly obvious reasons, it’s really important to get the installation of these doublers right. Badly-placed or subpar holes here definitely have the potential to compromise what is an extremely important structure. So there’s going to be a lot of “measure twice, drill one” going on here.

Whew…more fun. Tonight I got the doublers fitted and all the drilling done…and there was quite a bit of it.

As of last night I’d clamped the doublers in place, but not really done anything in terms of positioning them or anything. That was what started tonight – lots of tweaking the position while peering inside the stab body with a flashlight and examining the lines I’d drawn. Once I had them aligned the way I wanted, I decided that in the spirit of “measure twice, cut once,” I’d mark each hole location with a Sharpie and remove the doublers just to make sure the locations were good.

Here’s the result of that – good edge distance all around:

Then it was just a matter of clamping them back in place, carefully aligned, and getting started on the drilling…and as mentioned before, there was a lot of it – 30 holes per doubler, many requiring use of the angle drill at awkward angles. Unfortunately, this didn’t go completely well – it turned out that the first #30 bit I was using had gotten dull, and I didn’t notice immediately. The result was that, for a couple holes that were drilled using the spar web as a guide, the bit ended up wandering a bit and widening the hole in the spar web. These spots will need to be opened up to a larger size and I’ll use a 5/32 diameter rivet in them. The real fun here is that all of these holes are common between the ribs, so I’ll have to open the holes with all the parts in assembly, and limited access. The good news is that there’s plenty of extra material around these holes, so edge distance shouldn’t become an issue when enlarging the hole.

Anyway, here’s the scene after I finally got all the holes drilled – just a solid forest of clecos:

That’s enough for tonight. Next, I get to remove these doublers, see if all the holes ended up looking decent, and do a bunch of deburring. Then it’s on to fabbing up the new angle flanges for the main ribs. Before long it’ll be time to so some really fun tight-quarters riveting!

Well, this isn’t the most pleasant post to write, but it’s the way things are. Upon reviewing the work I’ve done so far on this service bulletin, the long and short of things is that I’ve ruined some parts and I’ll be substantially rebuilding the horizontal stab.

I mentioned in the last post that I’d had a few holes that got enlarged in the spar web, due to using a dull bit in the angle drill. At that time, I figured I could just replace those with 5/32” rivets and be fine. But I took a closer look at those holes tonight, and while two of them that are oversize can be safely opened up, the other two are far too large. I thought briefly of asking Van’s support about the possibility of doing another doubler repair here, but some reading on VAF reveals other builders who have gotten some very direct guidance on this topic, and that guidance is that any hole that can’t be fixed by opening up to #20 calls for replacement of the spar. Since I damaged both spars, I’ll be replacing both of them.

There are also some edge distance and hole spacing issues with the doublers I drilled, but that’s kind of beside the point right now, as the doublers will need to be re-drilled with the new spars anyway. While I’m at it, I’m also going to replace both reinforcing angles and all four of the inboard ribs. This is basically all fo the inboard/center structure in the stab. It’s possible that the ribs could be salvaged, but at this point I figure I might as well go all-in on using post-SB parts.

At the moment, my plan is to hold off on ordering anything until after I’ve done all the disassembly required for this replacement work. This is to allow for the possibility of damaging other parts beyond repair; better to make one parts order instead of two. I thought about going ahead with that disassembly work tonight, but I think it’s better that I take the rest of the night off and let this sink in before jumping into more work.

The good news is that the financial damage isn’t horrible. Replacing the parts listed above will cost about $140. Things only start getting significantly more expensive if I have to replace parts of the rear spar or the skins. I don’t think saving the rear spar will be a problem, but the skins might be a different matter – there are a lot of rivets to drill out carefully. Then again, my fancy new rivet removal tool will hopefully make this go better.

At this point, I think my goal will be to complete disassembly by the end of this weekend, and order the parts early next week. A stretch goal might be to get disassembly done tomorrow night and order the parts Friday, that’d save me a few days waiting, but again, rushing disassembly is likely to just make things worse.

Anyway, that’s it for tonight.

Finally got started on tearing this thing apart tonight. Over the past few days, I spent a fair amount of time thinking through this process. The main outcome of this was that I decided to go ahead and order my replacement parts after all, rather than waiting until after disassembly like I said last time. I figured it’d be better to get the parts in hand sooner, to minimize downtime. I can find other things to do while waiting on stab parts, but I’ll be limited in what I can do some I’ll have a workbench full of stab parts.

I also decided not to go ahead and replace the inboard ribs after all. After reviewing the stab assembly procedure, I was reminded that those ribs don’t come predrilled, and have to be drilled in assembly with the skin. That seems like an opportunity to cause problems, as well as being more work, so I’m going to use the ones I have – after all, there’s nothing wrong with them

Anyway, all that stuff shipped today and should be here middle of next week. In the meantime, back to disassembly.

My basic plan here is to reverse the assembly procedure – first, drill out all the rivets to the rear spar and extract it, then drill out the rivets between the skin and ribs and remove those, and finally remove the front spar. Tonight I figured I’d just see how far I could get on the rear spar. The answer was “pretty far.” I started by drilling out the solid rivets securing the structure together. Then there were the four Cherry blind rivets that secure the rear spar to the center ribs. Getting the heads off those was surprisingly easy, I was kind of expecting some drama, but it didn’t happen.

Next were the skin rivets.I got all these drilled out and removed fairly efficiently – instead of my usual painstaking three-step process, this time I just center-punched and drilled, being careful to try and keep the drill centered. It didn’t go perfectly, but I got all the rivets out without any major damage. There are a couple holes where I may need to go to a NAS oversize rivet, but mostly things went great.

At this point, I was ready to extract the rear spar, declare victory for the night, and retire inside. Except I couldn’t seem to get the spar to come out, it seemed hung up in the middle of each side. Right…oh, right where I took the heads off those Cherry blind rivets. The shafts of those rivets were still in place, and once I thought about it…of course they’re probably still holding things together, the whole idea with them is that they grip the sides of the hold really well. “Shaft” is really not a great word, what’s actually left is just a thin metal tube. One of them had its edge bent a bit, and I was able to get a punch on that one and drive it through, at which point that part of the spar got looser. That confirmed my theory – I just need to knock out the other three shafts. Well, first I need to figure out how.

I started trying to use a punch to catch the edge of the sleeve in one hole, then thought about the possibility of damaging the hole. I also thought about how I might be bending the rib flange inside the stab by doing this – I should really be backing that up with a wood block or something. The bottom line was that rather than trying to force this issue, it was better to just call it a night, let the problem marinate in my head, and revisit it tomorrow.

So that’s what I did. I’m happy with the progress I made today, even if it was sort-of backwards-moving progress. Tomorrow we’ll keep it going.

Update: I got the spar off after all. It suddenly occurred to me that all I needed was something to use as a punch that would fit snugly in the rivet hole. I didn’t have an actual #21 punch, but I did have a #21 drill bit. And it did a bang-up job of driving the rivets right out. Easy peasy.

As usual, lots of other stuff going on today, including the monthly Houston RV lunch, which went pretty long since we had some out-of-town guests in attendance. But I made good progress on disassembly.

My initial plan for the spar replacement was to only partially disassemble the stab. I was figuring on leaving the forward main rib in place, and maybe the tip rib as well. It seemed easy to only remove what I absolutely had to to get that forward spar out. I started the morning by drilling out rivets on the aft main rib; in my initial plan, the next step would have been to drill out the blind rivets attaching that rib to the spar and the forward main rib. However, after looking inside the skin at this blind rivets, I started to reconsider:

Getting to those was going to be a bit of a pain, and it seemed like a good opportunity to do more damage while trying to get at those rivets. At this point, I’d gotten pretty comfortable with drilling out skin rivets efficiently, so I decided it was only a tiny bit more effort to just go ahead, drill out everything, and completely disassemble the stab. So that’s what I did. After an hour or so of work, I was back just a bare skin for one half of the stab:

Drilling out those blind rivets was far easier with this piece of skeleton liberated from the skin. And so now, the left half of the stab is completely broken down. Tomorrow, I’ll repeat this process for the right half of the stab. I think I might be able to get it done in about an hour.

It’s been kind of surprising how painless this disassembly has been. I’d expected to invest a lot more time in this. Looks like I’m going to have to figure out something else to work on once this is done and I’m waiting on the new parts. I have a couple items in mind, though…

Nothing exciting to report today, no awesome photos or anything. Just drilled out a bunch more rivets and got the right half of the stab torn down. Overall I’ve been happy with how this went, drilling out the rivets was surprisingly painless and fast, and while there are a few holes that will need some attention when I reassemble, I didn’t mangle any more parts.

So that’s about it – I’ve got a pile of parts now, and I’ve just got to wait for the new stuff to come in later this week. Then I get to remember how to build this thing…

Not logging this as any actual time tonight since it was mostly 15-20 minutes of tinkering around…but today my nice care package from Van’s arrived. New spar channels, reinforcement angles, doublers, and some assorted hardware. The one thing I forgot to do was to print out the updated h-stab assembly instructions at work today. I have toe up-to-date manual on my laptop, but I want a hard copy to refer to while I work. I think I have a decent handle on the procedure, but I’d rather not wing it since I’m already rebuilding.

So all I did tonight was cleco some random stuff together and take a look at test-fitting the various parts. Interestingly, the doublers that come with the new kits appear to be different from the doublers that I got with my SB kit. The overall shape is the same, but they have a lot more prepunched holes. I can only assume this is a slight difference between new-construction and retrofit procedures.

Anyway, I’ll see about picking up with some actual work tomorrow evening.

So with new parts in hand, I got right to work this evening. It was a good night for it, too – some rain during the day today seems to have dropped the temperature nicely.

The funny thing is, I waited until today to work on this so I could print out the updated construction manual pages, and then I proceeded to mostly ignore them today. They seem to be kind of inaccurate, actually – for example, they speak of drawing rivet lines on the doublers to help lay out the holes, but the doublers I got are already almost totally prepunched, so there’s no leeway to play with spacing here.

With that in mind, I decided to start by drilling the reinforcing angles to the spar channels – at least the center holes, inboard of where the channels will get bent to match the sweep of the spar. Doing this meant I had the holes in place that I needed to cleco the doublers and the v-stab splice plate in place to see how everything fit together. As you’d expect from prepunched parts, there was no interference between anything.

Next, I removed the reinforcement angles and marked the required tapers on their ends, then made the tapers. I rough-cut on the band saw, cleaned up the long cut lines with a vixen file, and then used the scotchbrite wheel to round the corners and clean everything up.

With that done, I got to thinking about how I could verify that I’d have good edge distance on the new doublers. Seeing as how they’re prepunched, I figured they ought to be fine, but since I’m rebuilding after ruining the old spar, I’m feeling a bit conservative with this stuff. The only holes that aren’t prepunched in the doublers are those in assembly with the inboard ribs, and those that will match the outboard holes the angles (outboard of the bend line). Those outboard holes aren’t prepunched in the spar; they get drilled in assembly with the angles. That meant I couldn’t easily mark the prospective hole locations on the doublers either…hmm.

Then it hit me…I could just cleco the angles to the splice plate and doublers, without the spar involved. The alignment wouldn’t be precisely the same as when everything is together, but it’d be plenty good enough for verifying the edge distance. And it was super easy to do:

So I marked the holes, disassembled everything, and then marked an edge distance line around the perimeter of the doublers. The results are not really what I’d hoped for, though. The holes on the bottom are OK, but the top is a different matter. The inboard holes are slightly out of spec, but the outboard ones are way worse. The edge distance lines drawn here shouldn’t overlap the edges of the holes:

So now I get to decide how to handle this. I can only think of two real options: first, accept the edge distance as it is and build on. Second, try and modify the upper angle a bit to tweak the holes downwards. That second option actually isn’t as wacky as it sounds at first. The angles already have to be bent to match the swept portion of the spar – I could maybe apply the mallet to bend the “ears” down a bit to migrate those holes. It wouldn’t take much at all, really…but it is still finagling with a kind-of important structural piece.

And of course I had to hit this on a Friday night, when there will be no Van’s support available until Monday. I guess I’ll start by asking around on VAF and see what kind of stuff I hear. I really don’t want to stop work for the weekend…

UPDATE: Actually, this isn’t as bad as I thought. The common rule of thumb for rivet edge distance is 2x the rivet diameter to the hole center, or 1.5x the diameter to the edge of the hole. However, a closer look at the mil-specs provides a more precise number…specifically, for 1/8” rivets, the minimum distance to the hole center is 0.219”, or about 7/32”. Using that number, and measuring with my calipers (much more accurate the two sharpie lines), I’m find on edge distance everywhere except the inboard most hole on that top angle, and in that case it’s barely out of spec. With that in mind, I’m going to build on.