Wed Mar 24 2010

Drilling horizontal stabilizer to fuselage

First step today was to rig up a way to check that the horizontal stabilizer is perpendicular to the centerline of the fuselage. I mounted this aluminum angle on a couple of hardware store bolts through the upper motor mount holes and tightened the nuts finger tight until the angle was tight up against the forward edge of the side skins.

Here's a closeup of the mounting bolt on the right hand side along with a flat head screw. The hole for the flat head screw is exactly on the centerline of the top flange and centered on a line that I drew parallel to, but 3/4 inch away from the top firewall gusset.

The end of the tape measure hooks on the flat head screw, then measure back to a common point on both the left and right sides of the horizontal stabilizer. I measured to the corner where the skin is notched out for the elevator counterbalances. After tweaking the horizontal stabilizer several times, the measurement was 162 15/32 inches on both sides.

This clamp held it in position for drilling. Since the clamp would interfere with the drilling jig, I drilled the left side first. Note that I drilled right through the 1/8 spacers.

Here's a shot through the drilling jig pilot hole. By biasing the jig against the longeron, the jig controls the position of the hole in the left/right direction...the forward/aft direction is located by sighting down through the pilot hole. I had previously drawn a centerline 7/16 inch off the edge of the can see it through the pilot hole.

Once I had the hole started (the position was set), I removed the jig and taped over the big hole in the aft deck to prevent a bunch of shavings from going down in the fuselage.

Here's the #30 hole where it comes through the aft deck (gray area in the upper left corner of the photo), longeron (going diagonally through up and right, and F-701 angle (extending to the left of the photo). Edge distance looks great!

I didn't have a long #12 drill bit to drill through the pilot hole, but I ran to the the local Ace Hardware and they had a 6" long 3/16 inch diameter bit. Close enough! (#12 = 0.189 vs. 3/16 = 0.188).

Drilling the inboard holes was quite a pain. The angle on the top edge of the horizontal stabilizer forward spar prevents drilling straight down, so you've got to use an angle drill. But the dang angle drill is too long to fit into the space provided. What I ended up doing is shoving the air hose in one of the lightening holes in the F-709 bulkhead, looping it around and out the other lightening hole. That allowed me to hook the angle drill up and shove the ass end of the drill into the appropriate lightening hole to drill the bolt hole.

The inboard holes didn't come out quite as well as the outboard holes. The one on the left got slightly oblong when I was drilling #11 (didn't have a #12 threaded drill) and the drill bit grabbed and twisted the drill in my hands. Grrr. It's not bad enough to cause a problem, but isn't as perfect as I would like.

Before drilling the mounting holes through the rear spar, I verified the forward end of the fuselage was level, then setup the machinists level on some aluminum angle to verify the horizontal stabilizer was level. It wasn't! After some shimming under the front spar... was good to go.

What I found is that on the left side of the aft deck, I needed an 0.016 thick shim. I'm really stumped why I have this problem since I was very careful to ensure no twist in the fuselage. Somehow it appears I ended up with a bit of twist in spite of the effort. I'll make a proper shim tomorrow.

The instructions say to verify zero degrees angle of incidence before drilling the rear spar and to measure the front and aft tooling holes to check it, but there's no good way to take a measurement of those tooling holes. Add to this the newly found twist and I decided to take a more careful approach to setting zero degrees. After a run to Aviation Depot to pick up a piece of 3/16 steel rod, I cut two pieces to fit into the tooling holes. Here's one piece of rod spanning between the left and right inboard ribs at the aft end of the horizontal stabilizer.

...same thing at the front end of the horizontal stabilizer.

After several rounds of careful measurements both front and back, I found that I needed 0.055 inch shims in addition to the 3/16 spacer the plans describe.

1 55/64 from the center of the aft deck to the top of the cross rod near the back of the horizontal stabilizer...

...and the same measurement at the front of the horizontal stabilizer.

All that was left was to drill through the supports and the rear spar. Took all day, but it was nice to get this thing mounted. I'll take it all back off, deburr everything, and make that shim tomorrow.