Monday, June 21, 2010

[Insert Clever Title Here]

Usually as I'm working on the plane, there's a background process running in my head trying to come up with a clever title for the ensuing post. Nothing really came to me this time. I'm still working on the lower half of the firewall, but I'm very nearly done with it. Unfortunately, I need another little dollop of firewall sealant. It's a shame that I had to throw away about 200 times the amount I am now in need of. This time I'll order one of the little one ounce bottles.

At the end of the last session, I had just riveted on the bottom skin. The next step was to rivet in the cooler stiffener, which was the part that gave me so much consternation as to whether holes that seemed like they ought to be countersunk but hadn't been called out in the plans as having countersinking required truly ought to be countersunk. As a reminder, I threw caution to the wind and countersunk them. The fun with this part was not quite over; it had three different rivet types required to get it installed.

Once that was done, a half dozen nutplates were clecoed onto the inside surface of the bottom skin. That was one of those reach-around jobs where it's a chore just to get the things in place.

On the plus side, the reach-around jobs nearly always end up using blind rivets.

With the latest batch of nutplates finally settled into their final resting place, we moved on to installing Cowl Attach Plates. These are nothing more than flat pieces of aluminum that host a collection of (see if you can guess.....) nutplates. There's a little twist here, though, in that the plates can't be flush with the edge of the skin. They have to be recessed just a little bit up into what will be the engine compartment so that the skin of the cowl will be flush with the skin of the fuselage. This spacing is accomplished with the highly technical expedient of... spacers. 

The spacers are simply thin rectangles of thin aluminum. They are to be cut from "AS3-020," whatever that is. It took awhile to find because I thought that it would be a smallish piece of metal shipped in one of the brown paper bags. There was such a piece of metal, as it turns out, but there was only one rather then the two required. I finally found the strip of material over on the parts shelf.

The fabrication is quite simple: measure out and cut a couple of 1 1/2" pieces and round the corners.

The drawing showing how it all fits together is easy to follow.

The little spacer plates get placed inside the flange of the firewall:

The part that will hold the nutplates only has one hole already drilled in it. You use one cleco through that hole and a clamp on the other side to hold the assembly in place. The parts are flat but the flange has a little curvature, so there's a gap:

I took it all apart again and bent a little curvature into the spacer and the Cowl Attach Plate, then clamped and clecoed it back into place.

Once clamped in place, two more holes are match drilled up through the flange and through the spacer and the Cowl Attach Plate. When it's all still attached would be a good time to go ahead and machine countersink the rivet holes for the nutplates. That will keep you from doing what I did, which is to countersink the wrong side of the attach plates. I thought I was doomed, but it turns out that even though the plates were match drilled in different locations from each other, they were still interchangeable side to side. The holes matched up every bit as well as every other hole I've match drilled, which is to say they were approximately aligned.

That done, the last step on the page called for the use of a Uni-Bit to enlarge the hole the gascolator fuel line (I think) will pass through the firewall to 9/16". I got a kick out of this: "To keep the hole centered draw a cross hair out from the hole for reference." Why did I think that was funny? Because I knew that there was absolutely no way a cross hair was going to keep that hole centered. All it was going to do was graphically display just how off center my hole was!

I started with regular bits and worked my way up in size. I was able to keep the hole pretty centered.

When I got to where I needed the Uni-Bit, I marked the step at which I wanted to stop. Uni-Bits have a way of just keeping on going well after the spot where you needed to stop.

I then proceeded to use the Uni-Bit to get the hole off-center.

I'll burn that bridge when I get to it.

The next step was band saw work and I usually enjoy that. It's kind of like going someplace warm on a winter vacation and enhancing the enjoyment of the temperate weather by comparing it to the awful conditions back home. I like to use the band saw because I remember what a pain the hacksaw was. 

In this case, it was the simple separation of four Support Channels. These will sit on the shelf that attaches the lower firewall to the upper firewall. They will support the rudder pedals, unless I miss my guess. The plans call fro the aft two holes of each to be dimpled, but fail to give any written direction as to which end is the aft. Placing them on the shelf shows the aft holes.

That's not the side that gets dimpled, though. You have to turn it over.

Some matching holes get dimpled in the firewall shelf and, as it turns out, so do some non-matching holes. I got lost with the dimpler and put dimples in four holes that didn't need them. It's not hard to un-dimple, but it was an indication that my head was no longer fully engaged in the process. Rather than press on and risk a much more permanent mistake, I just clecoed everything in where I think it belongs. I'll look at it again when I'm fresher to make sure that I have all of the nutplates in the right places. 

Besides which, I need some fuel sealant to run between the shelf and the lower firewall before I can rivet it together.

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