Section 8

  • Horizontal stabilizer
    Empennage Section 8
    Jun 6, 2017

    While in principle the horizontal stabilizer build should be pretty straight forward, I definitely made several aspects of it way more difficult than it needed to be. First was in countersinking the spar flanges. Supposedly the stops on most countersink cages are .001", and convention is to drill until the rivet sits flush, then stop in .007" further to accept a dimple. Supposedly a 112-degree countersink bit is ideal, but everyone seems to only have the standard 100-degree bit. Nevertheless, it felt like I had to countersink far deeper than that to get an acceptable flush mating between the skin and the spar and I still don't have a great explanation why.

    After dimpling the skins, riveting in the rib tips, and attaching the front spar I notice this...

    Yep, I riveted in the spar with 36 missing rivets on the spar caps that are now inaccessible from the back side. Bah! I decided I wasn't willing to drill out all the flange rivets, and contacted Van's support to get some options. They said putting Cherry Max rivets would be just as strong, and an easy fix right? I purchase CR3213-4-5 rivets to replace the AN470AD-5 that should have been in there and installed them with a little bit of flexing of the inspar ribs and some chewed up knuckles from scraping against the dimples while operating the rivet puller. Afterwards, while riveting the remaining flange rivets I notice the Cherry Max rivets were starting to back out. I went to the Cherry specifications, and it turns out the grip size on the 4-5 rivets are way too large and they weren't remotely fastened from the backside. Aaargh again. Now the challenge of drilling them out. My first attempt was to drill it out like any other rivet, so I got a 12" long #30 bit and tried to center it as much as possible. It turns out this is extremely difficult, as the Cherry rivets have a steel center section where the mandril goes through, and it's really quite hard. I dulled my bit rather quickly, and just made a mess of the rivet. My next attempt was to use a small high speed cutter bit on a Dremel to grind off the head. This was working okay, but due to the really tight working space it was difficult to navigate in there and get any precision. Needless to say the tool slipped, and I made a nice nick in the spar:

    I contacted Van's support again, and this is the first time they didn't have a clear answer for me. It's "probably okay" they said, but it's in a grey area of not being able to guarantee the impact on the spar's strength. I tried my best to measure the depth of the nick, and I think it's under 10% the thickness.. so I decided to scotchbrite the sharp corners off and continue. Furthermore, it's about midway from the center near where they cut out the lightening holes in the spar. Fingers crossed it doesn't become an issue.

    Now back to the issue of drilling out the Cherry rivets. As usual, and found some inspiration from the VAF forums, which suggested to punch out the mandril first. I went out to Harbor Freight and purchased some cheap punches I didn't care about destroying, and a tiny hammer, and attempted to hammer out the mandrils from the center. Needless to say, they're pretty stuck in there. I'll repeat what many have already said.. but these Cherry rivets are definitely robust. What ended up working pretty well is attaching the punch to a flush rivet set using some heavy tape, and punching out the mandril (plus the steel sheath) using the rivet gun. This actually worked fairly brilliantly. Afterwards, there's just a bare aluminum rivet with a nice hole in the middle that can be drilled out with a #40 bit no problem.

    I purchased the correct Cherry Max rivets (always check the grip size before ordering them!) and got them installed. Phew!

    Next hiccup was when riveting the skins to the flanges, I noticed I somehow missed dimpling about a dozen of the holes. There are so many rivets already fastened that I can't imagine taking it apart, so now I'm trying to improvise how to get it dimpled with the rivet gun. I've heard of people doing it.. so I improvised an attachment to the gun that would accept the male die, and placed the female end into a piece of wood backed by a bucking bar.

    This indeed made a dimple, but it's a little bit too shallow - and there's a noticeable indent in the skin that's distinct from the nice flush appearance from the DRDT2 dimples. Once riveted, the rivet protrudes out ever so slightly due to the shallow dimple. It's just cosmetic.. but it totally bothers me. 

    Everything else went fairly according to plan. Riveting the interior rivets is definitely a challenge and any slip-up will earn you a nice smiley shaped dent in the skin that will infuriate any detail-oriented person. I purchased one of those rubber cupped swivel rivet sets, but noticed that they can easily leave the manufactured head raised up from the skin due to the stiff rubber cup. Instead, I decided on using the standard rigid mushroom set that makes much more solid contact but seems to have almost no tolerance for error. I think I ended up having the gun bounce on two rivets leaving a small smiley on each one, which resulted in much cursing and swearing.

    One of them was rather bad, so I made some attempts at hammering it out. The smiley is largely gone, but the area is noticeably rougher now. Hopefully it will be less noticeable when painted.

    Last step was to attach the rear spar, and all the edge rivets could be squeezed which is super easy compared to riveting with the gun. I'll elect to use the squeezer any time it will fit. Now on to the elevator!