May 31 - June 2, 2010

Horizontal Stabilizer - Angle of Incidence, Drilling Rear Spar to uprights - 5.0 Hrs.

Over the last few days, I continued my work on the Horizontal Stabilizer. The issue facing me right now is to make sure the angle of incidence is exactly 0° or parallel to the line of flight, before drilling the rear spar of the HS to the uprights. I've spent a lot of time lately thinking about airflow over these surfaces and how important tiny imperfections can become. It seems to me that a lot of builders go the extra mile when mating the wings to get things exactly as they should be with respect to the sweep and angle of incidence of the wing. And that's as it should be. But what about the HS? I think it deserves all the care and scrutiny that the wings do. I don't see much being said about this.

Consider how small the trim tab is, in relation to the surface area of the horizontal stabilizer and the elevators. It's a very tiny surface area, yet small movements of the trim tab dramatically affect the flight of the whole airplane! With that in mind, just think how much a difference of even, say, 1/32" in the placement and angle of the entire HS would affect the flight of the plane! It seems to me that it's imperative and absolutely crucial to get these parts mounted in the most accurate manner possible. Inaccuracies could no doubt be trimmed out, but result in a lot more drag than necessary.

With all that in mind as I prepared to drill the rear spar of the HS to the uprights, I looked at ways to accurately measure the angle of incidence. The plans simply call for putting a 3/16" shim under the rear spar to set the HS at the right angle. Doesn't seem all that accurate to me. The plans also mention measuring the distance from the aft deck to the tooling holes in the ribs of the HS to make sure they're the same. This sounds better, at least in theory. So I fiddled and fussed around for a while trying to figure out the best way to accurately measure this, because the ribs are recessed inside the HS 3/4" or so and it's hard to just try to put a ruler in there and measure up to the tooling hole. Finally, I realized I don't need to know the distance, or the measurement at all. I don't care how many fractions of an inch it is, just so that it's the same for all 3 tooling holes. So I came up with the ultimate simple tool for this task.

The tooling holes in the ribs of the HS are roughly #12 in size, so I took my #12 bit and drilled a hole in a wood block in the drill press for a perfect 90° angle. I tried to drill it at the exact height of the tooling hole from the top of the aft deck. Then I put the bit down tight in the hole so it sticks straight out, as seen above. Some final sanding on the disk sander can shave it down to the perfect height.

You can see here that it just slides very nicely into this tooling hole on the forward end of the HS. Moving aft, I tried it on the middle hole and again on the rear hole. Well, guess what I found out? The 3/16" shim recommended in the plans gets you very close, but there was still a tiny bit of tilt to the HS. In my case, I needed to use a wedge-shaped wood shim on top of the 3/16" shim to raise it just a tiny bit. Only when the drill bit would go easily in all 3 holes on both sides, did I consider it done as accurately as I'm capable of.

Clamped in place, I drilled the aft spar of the HS to the uprights through the #30 holes put in long ago.

Then I enlarged the holes to #12 for the AN3 bolts that hold it in place. Bolted down, this really becomes solid.

Next, it's time to take it all apart for deburring, cleanup of aluminum shavings all over the place, and priming of the parts that need it.

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Contact me: swayze "at" europa.com (replace "at" with the @ sign... no spaces... you know the deal)