Sep 11

Construction Start

Horizontal Stabilizer

Horizontal stabilizer drawing

I officially started construction of my RV-9A on July 17, 2012. The first major sub-assembly to be made is the horizontal stabilizer. Construction began with fabrication of the two HS-908 Attach Angles. The instructions indicate that these and the two HS-909 shims are the only items that need to be made from scratch so I decided to those pieces first. The kit comes with a piece of angle aluminum 2″ x 2.5″ x 5″ long that is technically enough for three pieces but the thickness of any cutting blade will make it difficult to make an extra angle. I tried using both a hacksaw with a new fine tooth blade and a Dremel with a metal cutting wheel and found the hacksaw to work better for me. The Dremel would shoot dust and debris all over the place and a new hacksaw blade gets through the aluminum fairly quickly.

HS-908 attach angle rough cut

A single HS-908 attach angle rough cut.

After the rough shapes of the attach angles were cut, I shaped them to the final specifications using a vixen file and a bench grinder with a medium grade ScotchBrite wheel. It’s important to smooth down any tool marks because those can become sources of stress fractures due to airframe vibration over time. The sharp corners were rounded down as well. I then drilled the nine 1/8-inch (#30) holes in each attach angle using the drill press.

The kit includes two pieces of aluminum that are the correct width for the HS-909 shims. I just had to cut them down to the correct length with snips. I deburred and rounded down the corners using the bench grinder but a fine grade file would have been sufficient. It’s just that the grinder makes quick work of it.

Now that the messy fabrication work was out of the way, I could begin preparing all of the parts of the horizontal stabilizer for assembly.

I clecoed together the left and right HS-902 front spars to the front spar doubler plate (HS-907) and match drilled all of the 1/8-inch holes including those of each attach angle. The doubler and spars already had a hole to line up with the top middle hole pre-punched. I then did the same with the left and right HS-903 rear spars, the rear spar doubler plate (HS-906) and the eight outboard hinge brackets (HS-912).

Front Spar Attach Angle

Front spar clecoed with HS-908-L attach angle.

Front Spar Clecoed

The front spar assembly.

Rear Spar Hinge Brackets

Rear spar hinge brackets

After the front and rear spars were ready, I clecoed the nose ribs (HS-905) and the main ribs (HS-904) to the front and rear spars and match drilled the holes that would attach them together. These holes are also 1/8 inch. I then took everything apart and deburred all the holes and edges. The Dremel with a grinding wheel works well on the lightening holes in the ribs and spars.

Horizontal Stabilizer Interior Structure

Horizontal stabilizer interior structure

With the interior structure of the horizontal stabilizer ready, I could move on to the skins. The kit includes two flat pieces of wood that have the cross sectional shape of the horizontal stabilizer drawn on it. I cut those sections out with a jig-saw and mounted each to a piece of 2×4 lumber. These formed a jig in which the skin can be assembled.

I clamped the jig to a work bench and lined each support with duct tape to prevent scratching the skin. I put the left-hand skin (HS-901) into the jig first and clecoed the interior structure into the skin starting at the leading edge: the nose ribs, left-hand front spar with doubler plate, main ribs with HS-909 shim, and finally the left-hand rear spar with doubler plate. Using a 3/32 (#40) inch drill bit this time, I match drilled all of the holes in the skin to the interior structure.

Left-hand Skin In Jig

Horizontal stabilizer left-hand skin in jig

Horizontal Stabilizer Interior Structure Clecoed

Interior structure clecoed into left-hand skin

After the holes were drilled on the left-hand horizontal stabilizer, I disassembled everything, deburred the holes and dimpled the skins using the DRDT-2 dimpler. I had previously built a “cradle” that fit between two workbenches on which I placed the dimpler. Then by putting a piece of carpet on each workbench I could slide the skin around without worry of scratching it while dimpling each hole. I used the Tatco hand squeezer to dimple the ribs. I used Cleveland Tools tank dimple dies which provide slightly more room to allow the dimples in the skin to nest deeply into the dimple on the rib. I used the counter-sink cage on the hand air drill to counter-sink the holes in the spars and doubler plates. Before using the counter-sink cage, it is best to test it on some scrap to get the depth setting just right. I deburred the edges of the skins as well with a file. After all this work on the left-hand side was complete, I put the left hand parts aside and repeated the entire process on the right-hand side.

Now that all of the drilling and deburring was complete, the next step was to wash and clean all the parts that need to be primed. I had decided to prime the spars and ribs even though they had an Alclad layer because these parts were handled the most and would be more likely to have fine scratches on the surface. The attach angles are the only parts that have no corrosion protection so those definitely needed to be primed. I washed the parts in a solution of Dawn dish detergent and dried them with a clean shop towel to get the excess water off. I let them air dry at least overnight to get completely dry. There were some spots of adhesive residue from where stock labels were attached at the factory but these were easily cleaned up with some acetone. I also used acetone to clean up the insides of the skins but I only intended to prime the rivet lines.

I used Duplicolor self-etching primer to prime all the ribs, spars, doubler plates, attach angles and skins along the rivet lines. It only takes a light coat. As a test, I weighed the ribs before and after priming and found the primer only increased the weight by 9 grams (less than 1/3 of an ounce) over all of the ribs which amounts to 0.7% increase in weight. While the parts felt dry to the touch after 15 minutes or so, I let them dry for a few days to fully harden before moving on to the riveting.

Primed spars

Primed horizontal stabilizer spars

I began by riveting together the front spars to the front spar doubler plate and attach angles. I used the rivet gun set to about 35 psi and AN470D4-6 rivets. While some of the shop heads were slanted because the bucking bar wasn’t held perfectly perpendicular to the rivet, after checking them against a rivet gauge and measuring with calipers, I decided the finished heads were well within acceptable tolerances.

Following the front spar assembly, I proceeded to the rear spar assembly. Here, I switched to using the squeezer. It definitely required more physical effort and fatigue would be a problem if many rivets needed to be set in one session. I switched back to the rivet gun to attach the hinge brackets (HS-912).

Front and rear spar riveted

Riveted front and rear spars