Sat Mar 28 2009

Fitting extra tailcone stiffeners

Well, I went out for pizza and beer with a frined last night so I didn't get any work done on the plane. Today, however, was pretty productive. First, I started by tapering the aft end of the extra tailcone stiffeners. Since stress risers occur where there are rapid changes in the stiffness of a structure, tapering the end of the stiffener like this shoule spread the stress out and avoid skin cracks.

At the forward end, I wanted to see if I would be able to just leave the flange of the stiffener somewhat long and use it to form a tab which would be riveted to the F-706 bulkhead. As you can see here, because the stiffener is at an angle, when the flange would be bent up, it would miss the rivet holes that attach the baggage floor rib to the bulkhead. So I decided to cut the stiffener off aft of the bulkhead and make up some custom brackets.

First thing was to cut a scrap of aluminum and put the correct bend in it. This piece is too wide, but that's intentional. I'll trim up the width after drilling the holes.

I slipped a piece of .025 aluminum scrap under the bracket to create the correct clearance for the stiffener flange and held the pieces in place with a couple magnets and a spring clamp.

The holes are drilled. The ones through the bulkhead are #30 and the ones through the bottom skin are #40.

I offset the holes to define the edges of the bracket. The #30 holes got 5/16 inch edge distance and the #40 holes got 1/4 inch edge distance.

After some trimming and filing, here's what the bracket looks like. I think it came out quite well.

Making up the second one took about half the time.

After that, I started fitting cutting pass-throughs in the F-707 and F-708 bulkheads and fitting the doublers. This is the right doubler on the F-708 bulkhead as viewed from the centerline of the fuselage.

Here's the same one as viewed from the outside. Nice, even clearance all around the J-channel stiffener.

While I was working on fitting these stiffeners I noticed this note I had made on this edge of one of the F-708 flanges. I trimmed the edge off so that it would no longer interfere with another of the J-channel stiffeners. Both sides got this treatment.

Here's what the additional stiffeners look like as they pass through the F-708 bulkhead. Note how the doublers tie the stiffener into the bulkhead structure.

After some more work, I had the F-707 bulkhead and doublers fitted as well.