June 5, 2010

Horizontal Stabilizer - Drilling Elevator Horns, Prep work - 5.0 Hrs.

Today I took the HS off the fuselage. All the holes I drilled to get it mounted need to be deburred, all the aluminum shavings cleaned up, and priming done where necessary. All this was in preparation for mounting the vertical stabilizer and the rudder. While I was doing this, I noticed something very important that I almost overlooked. I still haven't drilled the elevator horns on the upper end for the bolt that holds them to the bearing in the horizontal stabilizer! So this became my first task today. But then I ran into another problem.

Remember the other day, I was talking about how I've been thinking of airflow over these surfaces. Well, today something else caught my eye when I was taking the elevators off the HS. The trailing edge of the HS is bent upwards, like a ski jump. I think the final riveting caused this. This can't be right! This is the kind of thing that causes turbulence, and hence, drag. Drag is a drag and I don't want any more than necessary! So I grabbed my straightedge to see just how bad it is. These pictures show it. This had to be fixed, if I could figure out a way. Below is a closeup.

See the gap under the straightedge? It may not look like much, but I flipped the HS over and found the same thing on the bottom surface. Thinking again about how dramatic differences in how the plane flies are brought about with little movements of the small trim tab, this is a large surface area! It's something I can't ignore. A nice smooth, flat surface is highly desirable here.

Using my seamers, I worked my way across the rear spar giving the trailing edge a good firm bend downwards. I was surprised how effective this was. In just a little while, I had all 4 surfaces corrected. Below is another shot of the straightedge on the skin. It's dead flat this time. Nice! I also eyeballed down the trailing edge to see if it was straight and not wavy. I made a lot of improvement here, too. This is now as good as I can get it.

Moving on, it was time to drill the elevator horns. Way back when I built the empennage, I bought a 1/4" drill bushing from McMaster-Carr with a 3/32" hole just for this purpose. So I dug through my stuff and found it. I know I spent at least $10 or $15 on this thing, with shipping and all. So I was very disappointed to find out it wouldn't fit in the bearing! I measured it with a micrometer and found it to be exactly 250-thousands in diameter. EXACTLY 1/4". But there was no way it would go in the bearing. My AN4 1/4" bolts go in the bearing just fine, though. So I thought it must be very close. So I ended up chucking it in the drill press and turning it, and using some emery paper, honed it down a little at a time until it would slide just snugly into the bearing.

Here it is, inserted in the bearing, with the right elevator clamped in place ready for drilling.

My 12" long #40 drill bit was the perfect one to use. Inserted in the bushing, it put a #40 hole in the elevator horn that's exactly perpendicular to the horn on both elevators, so there's no binding when the elevators are bolted together and move in unison. Then the right elevator was taken off, the left one put on, and the process repeated to drill the left one. The holes were then enlarged to 1/4" for the AN4 bolt that holds them to this bearing.

I also spent some time today spot priming some areas on the HS spars that had been scuffed or scratched over the last several years, to make sure the primer is all in good condition. This baby is now ready to be mounted on the fuselage again, and the fitting of the vertical stabilizer!

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