My parts arrived from Van’s last week. A couple of ribs and a HS-710 angle that I had ordered because I was tired of that nagging feeling of something not right in the structure of the horizontal stabilizer. Yesterday, I finally got to drilling the offending part off, with the intention of making a new one that satisfied all the edge distance requirements. This is the old HS-710:
You can see how beat up this thing is, and that’s from me sucking at riveting a long time ago. Not only that, see the holes along the flat part, near where the angle starts? Those holes are too close to the edge of the part. The general rule is that the center of a hole should be 1.5x the hole’s diameter from the edge of the part or another hole. This isn’t even close.
This part is one of four structural elements holding the tail onto the airplane, so it’s kind of important and it has to be perfect. No ovaled holes, plenty of edge distance, good rivets, deburred holes and edges. This part connects the inboard ribs and both spars as well, so it’s all got to fit together in such a way that all those requirements are satisfied. The plans for this section suck, and the design could be better too. This is one of the first things you have to do as a builder, and a fit this important could do with a little more explaining of the whys, and some clarification on the steps. It’s almost as if it’s deliberately designed to get your nose off the paper and understand the structural priorities for yourself, and the lessons are taught in mangled metal and UPS shipments from Aurora, Oregon. The main lesson here is that you have to trim off just enough of the nose rib flange to let it fit between the HS-710 and HS-714 angles, and that it’s better to be closer to the edge on the aft rib than on the angle.
So, armed with 4 years of aircraft construction savvy between then and now, I set about replacing the HS-710 angle and the inboard ribs.
This is the shiny new HS-710, ready to install. The tapered ends didn’t get as much of a taper this time, leaving more metal on the edges. All good, right? Wrong.
Another important lesson: Get rid of all annoyances and irritants before you do anything critical. Find your neutral space. My jeans were falling down (belt not tight enough, getting less fat, good news really), the drill charger fan was running, the psytrance station started playing something screechy, tweaky, and annoying, and the shop vac hose and air lines were completely underfoot. I drilled the new HS-710 in exactly the same place as the old HS-710, through the spar in the old hole. I needed a whole new class of profanity to accurately describe the situation at that point. I didn’t have one. So I threw stuff. Nothing heavy or expensive, but even watching my hearing protectors explode in a bright flash of red plastic and skip across the concrete didn’t satisfy the frustration and disappointment. Even the time I drilled a hole in my finger wasn’t this maddening.
At this point, I wasn’t much into taking photographs. I spent the next few minutes contemplating selling the whole mess and buying a Cessna. Slower, but for the money, hauls more cases of wine back from Napa. But then the ‘F— You’ took over. F— You happens at the darkest hour, when you just say F— You, this is not stopping me. F— You, you are just soft metal. F— You, I will get this done. Today. So I made a new HS-710 from some angle stock. That’s right. Made one. And I can make more. I’ve got plenty of angle stock. We can do this all weekend, so settle in, princess. This time, there was way more measuring than there was drilling, and I now have a fully functional, structurally sound, properly installed HS-710. Here are all 3 HS-710’s, old, new, and new new.
Getting that little bastard on was only the first of the fun things to do. This setup is designed to be assembled before the skins are on, and getting it all together requires some basket weaving and the use of the double-offset rivet set, which is a deep, dire pain in the ass. The procedure is: Stick the aft ribs in, drill them through the spar, then use those holes to match drill the forward rib making sure the edge distance is good. Before you drill that, make sure the skin-to-rib flange lineup is good. Get this part locked down, then do the skin holes. But riveting the flanges to the spar is a bitch. The double-offset set likes to walk around, and you really need to secure the assembly and keep the set seated or you’ll mangle the rivets and that will mean drilling out and possibly enlarging the holes. Trust me you do NOT want to go up to an AD5 rivet in there. The rivet gun just barely makes it in there to get a good lock on the rivet heads. Like I said, it was designed to be done without the skins in place.
If you get it right, you end up with something like this:
See how there’s plenty of room between the center of the rivet and the edges of the metal?
Squeezing the skin rivets is easy. I even remembered not to rivet the holes for the empennage fairing this time.
Assembled and resting. The next horizontal stabilizer I build better be on the next airplane.