Tunnel to belly pan transition

[Pbruce]

I am installing the belly pan and tunnel on my QB patrol. On the Patrol kit (uniquely, perhaps) the aft end of the tunnel is shrunk with fluting to turn it into a semicircular shape.
I understand that the forward end of the belly pan lays next to the belly former, and the tunnel goes on last. It will leave the “corrugated” or fluted edge exposed. I’m wondering if anyone has any suggestions for finishing this junction?

the accompanying photo is from below the fuselage, left side. It shows the belly former and the fluted aft end of the tunnel. The belly pan is absent.

[joehmason]

On ours, not all of the screws for the tunnel hit that former. The ones in the middle did, but as you moved towards either side the shape of the tunnel made the screws line up aft of the former. We just kept the screws centered on the rear flange of the tunnel and put nutplates in the belly pan.

My advise is to get the belly pan set exactly where you want it, make sure your tunnel fits nicely between the firewall and belly pan, then start in the middle drilling/clecoing, and work your way to either side. Let the tunnel determine where the screws will fall and put nutplates in either the former or the belly pan accordingly.

[Pbruce]

You do nice work. I know what you mean about the aft edge of the tunnel not lining up with the former once you get away from the center. I was going to trim the aft (fluted) edge even with the belly pan front edge and the former.

Now that I see what you have done I might keep it square and attach to the belly pan like you did. Cutting the fluting off might reduce the curveture, and since it’s approximately correct now, I don’t want it to change. Is that why elected to keep it square rather than trimming to match the former and belly pan front edge? Thanks for the input.

[Lee]

When I did my belly and tunnel, I laid out the screws for the tunnel on the former and then put 2 countersunk screws through the forward edge of the belly skin where they didn't interfere with the tunnel layout. The 2 screws were just to facilitate assembly to the two parts to the former. I also trimmed the flange on the tunnel to the aft edge of the former. I feel the tunnel lays on the belly skin much better after trimming.
Lee

[Lee]

I also put a piece of 3/4x3/4 angle on the front edge of the tunnel to hold the front edge straight where it transitions to the firewall. 3/4x3/4 was probably overkill but it is what I had in the hanger at the time.
Lee

[noema]

To make the belly work I had to shorten the tunnel in the front until the rear of the tunnel aligned nicely with the belly former. Also the corners of the tunnel needed some modification. I squeezes the ends flat in a vice to make all the belly panels flush.

Surprisingly the tunnel firewall transition was easier than I though. I was prepared to mount an angle extrusion. I found that was not necessary at all. I allowed it to be slightly convex in the front and folded the firewall slap over it. No issue at all.

Another challenge I had was how to make the boot cowl removable. I split the side panels slightly under the window. To get the panel out between the gear legs I added a small removable piece.

IMG_8620.jpg
IMG_8545.jpg IMG_8631.jpg

[Pbruce]

So how much of the aft edge of the fluted tunnel would you sweep to make it flat? 3/8” or so? More?
thanks

[Lee]

Like Stephan I found the aft corners of the tunnel a problem, although I also feel that the tunnel is a very difficult part to fabricate and all things considered done very well. My final decision was to place the aft fabrication seam of the tunnel right on the leading edge or the belly former and then trim the tunnel to the aft edge of the belly former. Prior to trimming I was ok with the way the tunnel laid on the belly skin. After I trimmed the tunnel I was surprised and pleased with just how much better the tunnel transitioned to the belly skin

I also cut my boot cowl just below the window line. I then remade the boot cowl top from a single piece of aluminum 4 feet wide and the seam came out just about where Stephan's is. No seams on top and the windscreen sets on a single piece of aluminum that once completed should never come off of the airplane.

Lee

[noema]

Lee, I like how you did the top piece. Would do it the same if I had enough sheet metal. Sourcing the thin Alclad is a bit of a hassle in Europe. Interestingly when working with the kit-supplied material I find it better to reverse the precut top piece. The reason is that it does not create another seam in front of the window tubes (on the outside skin). And on the inside dash board I want a service opening anyways.

[Pbruce]

So, I’ve got the belly pan and the tunnel pretty much fitting and in place.

Do the side panels get bent around the lower longerons and riveted to the belly all the way along the outer edge of the belly pan? Any suggestions to make that curl nicely? Thanks

[Lee]

The sides go from the firewall to the former. Put a straight edge on there and you'll start to get an idea how it runs. I actually put a couple of strips of wood in there just to hold the aluminum side panel flat while I was working with it. There's two separate biends at the bottom edge of the sides separated by the forward gear mounting point. On mine, neither Bend touched the fuselage tubing. I hand formed the bends around a two and a half inch piece of pipe and sometimes a one and a half inch piece of pipe when I needed a tighter radius. I used a piece of aluminum Spar material from a Kit Fox but a piece of ABS or PVC would work just as well. I'll post some pictures when I get home tonight.
Lee