RV-8F

N772RW

Building, testing and operating an experimental aircraft

Drilling the Horizontal Stab

01 Jan 2010

Nothing like a little airplane work to ring in the new year. Dad and I made quite a bit of progress on the horizontal stab, completing layout and drilling of the left side and most of the right. The dreaded HS-405/702/404 junction turned out great, thanks to the heads-up from several other builders’ logs.

Drilling the HS-702 Drilling the HS-702

Match-Drilling the Understructure

Following Van’s instructions, we prepped and fluted the ribs, then assembled the understructure and match drilled all the rib-to-spar joints. We put the HS-801-PP skin on the skeleton and clecoed it into place – a two-person job unless you enjoy frustration. The nose ribs are a very tight fit, and we found it easiest to cleco from the leading edge aft, and from the inner ribs out toward the edges. We found it nearly impossible to move misaligned nose ribs once the spars were clecoed in place.

Horizontal Stab Understructure Horizontal Stab Understructure

Next we test fit, marked, fluted and clamped the HS-404 and HS-405 ribs into place, carefully ensuring that the skin’s pre-punched holes aligned with the centerline we had drawn on the rib flanges, while also attempting to align the flanges as closely as possible to the edge of the skin, with no under- or overlap.

Match-Drilling the HS-405 Match-Drilling the HS-405

Once the 405 was where we wanted it, we match-drilled it to the HS-603 spar, and then drilled it to the skin, two holes at a time, alternating top and bottom, working from the rear spar forward (or “up” in the photos). It took a little tweaking after drilling each hole to keep the un-drilled section of the flange aligned where we wanted it.

When it came time to drill the inboard section of the HS-702 forward spar, we disassembled the skeleton from the skin so we could draw a centerline on the 702. I don’t think this step is called out in the instructions (either that or we missed it), but it would have been nice to do beforehand. Either way, we learned our lesson and did it correctly on the right half of the stab.

HS-405 Edge Distance

We called it a night at this point, then reassembled the left stab the next morning. Using the centerline on the 702 made it easy to keep that aligned as we match-drilled it to the skin, working outboard to inboard.

HS-405 Flange HS-405 Flange

When we finally drilled the last hole in the 702, we encountered our first big “issue.” Although we had been as careful as possible to ensure things were aligned before drilling to maintain proper edge distances, we ended up with a hole too close to the forward edge of the flange on the HS-405 (red circle in the photo).

There was simply no way to avoid this problem, since the hole position is set by the pre-punched skin, and the 405 was pre-formed at the factory. I put an SOS post on the VAF forums, just to make sure it was okay to build on, and the consensus was that the 405 is simply “built too low” (to quote Foghorn Leghorn), and many other builders have encountered this problem. In fact, one builder even received this reply from Van’s regarding the same issue:

The HS-405 rib flange has been slightly short ever since the first kit RV-4. Engineering is OK with that rib and the less than perfect edge distance. Ignore the edge distance in this case and move on.

So that’s what we did. The next step was to drill from the aft side of the forward flange of the HS-405, through the 702 and 404. This is where other builders had encountered problems with over-trimming and edge distance.

Per the instructions, we had already marked and pilot-drilled the four holes in the 405 forward flange, so we just verified that the locations were going to give us proper edge distances. We had to move and re-flute the HS-404 a bit to get the aft flange to sit where we wanted it before drilling.

To get things lined up properly, we simply measured from the inboard end of the HS-702 spar (the end that touches the right stab’s 702) to the center of our pilot holes in the 405 flange (on the aft side of the spar), then measured the same distance on the forward side of the 702 to the aft flange on the HS-404 to make sure we weren’t going to drill the holes too close to the edge.

Drilling the 405/702/404 Drilling the 405/702/404

Then we clamped everything down and drilled with the 12” #30 bit, from the aft side forward through our pilot holes. To get the right angle on the drill bit I needed to remove the rear spar (HS-603), since it was getting in the way. An angle drill might have been nice for this task, but the 12” worked fine.

Thanks to everyone for pointing out the potential pitfalls in this critical area! We’d definitely have screwed the pooch had we not read these other blogs. You can see the results in the photos, and while the edge distance might not be perfect 2X diameter, we’re calling it good.

2X Edge Distance? 2X Edge Distance?

Finally, we match-drilled the skin to the rest of the understructure and then moved on to the right side of the stab. The last few photos illustrate the process we used to layout the pilot hole spacing on the 405 flange.

HS-405 Layout HS-405 Layout

HS-405 Layout Detail HS-405 Layout Detail

Punching the HS-405 Flange Punching the HS-405 Flange

Van’s measurement callouts are correct and accurate, but they assume you’ve trimmed the aft flange of the 404 perfectly, and no one does. This layout method seems to work a little better, and it really helped me to visualize exactly what was going to get drilled and what the problems could be.

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