More cowl fitting prep work

So, as implied by the title, today was all about getting set up for the joy of cowl fitting. I started this off last night by pulling out the line material that will be used to attach the cowl halves. This was in line with the first step of cowl fitting, per the instructions, being to cut and install the the hinge halves on the firewall. However, I ended up putting those back and delaying for later, mainly because I can’t really follow the instructions to the letter here. The main hangup is that I don’t (and won’t) have the forward upper skin riveted in place, which in turn precludes having those hinges riveted, since all these items are riveted in assembly.

Instead I moved on to a more fun problem, something I’ve been mulling over for the past few days. Briefly, the cowling halves are provided slightly long, and need to be cut to length in order to fit snugly against the firewall. The basic procedure for this is to lay out a line 2” aft of the firewall flange, lay the cowl on top so it overlaps the forward fuselage, and after doing all the fitting and positioning, measuring back 2” forward from that line lays out the initial cut line.

First problem: The firewall isn’t held in position unless that forward upper skin is installed. That’s not necessarily a show-stopper; I can cleco it in place, measure back from the firewall to mark the line, and then remove the checks so the cowl rests on the skin. But that leads to…

…the second problem, which is the right upper portion of the firewall. The left side is held securely in position by the skin, but the right side has a giant open spot where the forward baggage door will go. As provided, the skin didn’t have this piece cut out, but…I cut it out some time ago so I could match-drill that skin to the underlying structure. In retrospect, I wish I hadn’t done that. If that piece wasn’t cut out, then this skin would hold the right side of the firewall securely as well.

Anyway, it’s water under the bridge, and I’m left with this setup. You might be able to imagine how that portion of the firewall has a lot of freedom of movement here:

To solve this problem, I decided to temporarily add the baggage door to the party. Actually fitting this piece is a whole procedure that apparently gives lots of builders fits, but all I needed it to do was serve as a sort of filler piece to position the firewall. In practice, the top of the door attaches to the forward upper skin and underlying structure with a length of piano hinge; for this purpose, I cut a piece of .063” to sit in place of that hinge and attach the door. That piece of scrap was first drilled to the hinge attach structure, then I worked on putting the door in place on the fuselage before drilling the door to the scrap piece.

While doing this, I reviewed the baggage door instructions for inspiration. It turns out that that procedure calls for a nontrivial usage of duct tape, which I took inspiration for. In order to hold the door in the right position for drilling, I added strips of duct tape on the forward side, which also served to pull the firewall into proper position. Between that, and some more strips of tape to hold the bottom edge of the door in place, I finally got the brace drilled, which fixed the door in place pretty well:

Next, I worked between the clecos to place the 2” guide line off the firewall flange, then added another strip of duct tape to really ensure the firewall wasn’t going to move. Finally, after going all the way around the firewall and making marks for the guide line, I put down a strip of masking tape all the way around, which will be my actual reference when it’s time to mark the cowling:

So that takes care of allowing the back end of the cowl to be positioned and marked – the next task is to allow for positioning the front end. If we just lay the cowl atop the engine, the front edge sits on the crank pulley and is far too low. So I created a couple of adjustable plungers which allow for carefully raising the front. These were made using scraps from my first instrument panel cut; each piece attaches to a baffle attach point on a cylinder, and then has a 1/4-20 tapped hole for a long bolt. The bolts are topped with rubber vacuum line caps so as not to damage the inside of the cowl. I also added jam nuts on each bolt, to ensure I could prevent the bolts from moving, as well as taking some load off the threads in the admittedly thin brackets:


Finally, it was time for a trial fit of the cowl to see how all this worked out. Mostly I wanted to make sure that those bolts up front were long enough to raise the cowl to the level needed…or if I was going to need to buy some longer ones. So while this is in no way a careful fitting, I did take the time to roughly align the cowl with the prop spinner:

In this position, I still have a little bit of bolt length left, so it looks like I’m in good shape here. For the final fitting, I’ll need a way to precisely space the cowl off the spinner backplate, as well as a way to establish that the cowl is level from left to right. Really, I just generally need to review the fitting instructions, so I have an idea of the big picture of what I’m doing. It’s probably not surprising that I want to quadruple-check everything I’m doing before I reach the point of actually trimming the cowl…

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