9.3.2 Baffles and Bulkheads

The factory supplies us with one, large, 4 x 8 foot sheet of Divinycell for making the bulkheads and baffles. The first step in the process is to cover both sides of the Divinycell with a layer of BID. Wow - I never realized how much fiberglass it took to cover a 4 x 8 foot sheet of anything! My last roll of BID went from really full to really empty very quickly! I also never realized how much epoxy it would take to first lay down a layer of Micro (EZ-Poxy mixed with Microballoons to create a slurry that protects the Divinycell!) followed by the epoxy required to stick the fiberglass to the Divinycell. Because the fiberglass isn’t wide enough, I had to cut it into three sections. You can kind of see each section in the photo below.

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Now that the templates are finished I need to lay them out of the pre-prepped Divinycel foam board so that I can cut out the actual pieces.

The next step is to mark each template on the sheet. I’m only doing one fuel tank right now so I need to make sure that I leave enough room for the other tank. I marked a line at the halfway point since I’ll need half for one tank and the other half for the other tank. Most of the pieces easily fit within the 4 foot by 4 foot section, but template “3 & 4” extends over, so I needed to lay out the other pieces leaving room for the “3 & 4” template on the other side. You can see my jigsaw puzzle layout below.

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Now that the templates are finished I need to lay them out of the pre-prepped Divinycell foam board so that I can cut out the actual pieces.

A Sharpie to the rescue again! The templates are marked and ready to be cut out!

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Marked up Divinycell templates ready for cutting.

Once the pieces are cut out and trimmed properly, it’s time to start installing them. See the photo below. I should point out that trimming the pieces, especially around the leading edge curves, took a lot of fiddling around! They need to fit as tightly as possible because, afterall, this is going to be a fuel tank. We don’t want leaks. The fiberglass layups, along with the initial application of Micro, help to fill in the gaps.

The pieces also need to be installed as level as possible. The bulkheads get sealed to the top of the strake. The baffles don’t get sealed so if they don’t perfectly fit, it’s not the end of the world. I did a lot of test fitting for each bulkhead piece with the strake top to make sure that everything looked correct. I used 2x4’s along with some clamps and duct tape to help hold that first bulkhead in place and level. Once the initial layups cure, it stays in place by itself.

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Installing the first bulkhead on the right side.

There are four bulkhead pieces that frame the tank along with the leading edge and a small section of the fuselage. The section to the right on the long bulkhead piece shown below frames out one of the small baggage areas accessible from inside of the fuselage. Clamping this long piece into place was a bit trickier. I had to clamp a 2x4 inside the fuselage then use that 2x4 to help clamp the long bulkhead piece in place. The shorter angled bulkhead piece more or less supported itself up against the long piece.

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Using clamps to hold the bulkheads in place while the layups cure.

This entire process took the better part of a couple of weeks to complete. There are a lot of layups here; along the bottom of each piece on each side, the sides of each piece, etc. In addition, I had to mark and cut out mouse holes in the baffles.

The baffles are the five internal pieces with the quarter-round holes cut in the corners. The baffle's primary purpose is to keep the fuel from sloshing around a lot inside of the tank. Fuel needs to flow between sections and that’s what the bottom mouse holes accomplish. The top mouse holes are needed to relieve air pressure from building both as the tanks fill up and as they are drained down. Eventually one vent connection, one fuel outflow connection and the fuel tank fuel level sensor get installed on the wall of the fuselage. Finally, the entire internal sections of the tank need to be sealed with a special epoxy called “Jeffco”. Jeffco will not react to aviation fuels and any additives added to the fuel, unlike some other epoxies. I’ll have more details on all of that later.

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Bulkheads and baffles are all in place on the right tank.

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Closeup photo showing the mouse holes in the baffles that let the fuel flow from section to section.

I ran an initial bead of Micro along the tops of each baffle as well as inside the curved section of each mouse hole. These baffles are made out of a foam core material and if there are any leaks, that foam would quickly soak up fuel. Not a good situation! I also have to make sure that there aren’t any gaps between the junctions of each baffle. If there are gaps, they need to be filled. I’m using a small inspection mirror to check the lower mouse holes for gaps. I did find some so they’ll need to be plugged!

In Section 9.3.0 I showed the cardboard templates that I made for sizing each bulkhead and baffle piece. That took quite a long time to get right. The good news is that, for the left tank, I already had all of the pieces made! What took the better part of a week for the right side, took only a day for the left. I had to trim each piece a bit to get them to fit, but in the end, it all went together fairly quickly. Here’s what the left side looks like with the templates in place.

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Cardboard template layout for the left fuel tank.

So, the left side templates are finished! Time to cut out the actual pieces and do all of the fitting and trimming before epoxying them into place.

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All of the bulkheads and baffles for the left fuel tank are now installed.

And just like that (after many hours of trimming, fitting and attaching) the bulkhead and baffle templates are converted into the real thing. Manual Section 9.3.3 discusses the installation of the baffles, but I merged all of that together here because basically it’s all one step. 

One additional step I’m adding is the installation of cap strips to the bulkheads. Some builders do this, others don’t. The original construction videos discussed a process for doing this even though the current construction manual no longer calls for this process. I decided to do it because, 1) it wasn’t that much more work, and 2) everyone that did it said it made sealing the tanks so much more reliable.

The first step in this process is to just make some ~1.5” wide strips of fiberglass. In this case, I used my i-beams to do the layup. I first covered the tops of the i-beams with duct tape so that I could easily release the strips. I made each strip the approximate length for whatever bulkhead top I was working on.

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In order to help seal the tanks, many builders use cap strips on the bulkheads to provide a wider area for sealing. The original factory videos showed this, but the manual no longer calls out the procedure.

Pretty simple, eh? Once the epoxy cured, I peeled these off, cleaned them up a bit on my belt sander to get rid of the sharp edges, then used a mixture of EZ-Poxy and Micro to glue them down to the tops of the bulkheads.

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The corners wanted to bend up slightly so I'm using some clamps to set the proper contour.

Because the layups are only 2 BID, they are very flexible and followed the corners of the bulkheads perfectly. The particular corner shown above just didn’t want to sit flat so I used some clamp persuasion to help it a bit!

Once the epoxy under the strips cured, I did 2 BID layups underneath each strip. This firmly attaches the strip to the side of the bulkhead. Before doing those layups I made up another mixture of EZ-Poxy and Micro applied to the corner joints underneath to help smooth the transition between the strip and the bulklhead wall.

Notice in the photo below that the cap strips are only applied to the bulkheads - the outside sections that define the tank. The leading edge also defines the tank. The internal baffles do not need the cap strips. They will get epoxied to the strake top eventually, but if the joint isn’t perfect it isn’t as critical since there are already mouse holes in the top sections of the baffles for tank venting purposes. In other words, the baffles already “leak” on purpose! 

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Here you can see the cap strips installed on top of the bulkheads. Once the strenghtening layups under the cap strips cures I'll trim these for proper fit.

Once the epoxy cures, I need to trim the edges to clean everything up, along with a bit of sanding, of course. (There’s no lack of sanding with this project!) I also need to re-test fit the strake tops to make sure they still fit properly. If the strake top doesn’t fit properly, then I need to do some additional trimming of the cap strips.

The final step, before closing up the fuel tanks is to seal everything with Jeffco, a special epoxy that is extremely resistant to aviation fuel and additives. The cure time with Jeffco is much faster than EZ-poxy (as I found out when trying to install the upper strake halves!) so I only made small batches at a time and sealed up just one or two compartments at a time. Cabosil is added to the Jeffco to both thicken up the mixture and to give it that shiny finish you see in the photo below.

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Thie fuel tank builkheads and baffles are now sealed with the special Jeffco epoxy (plus Cabosil). Let's hope they don't leak!

Before installing the strake tops, all of the fuel fittings and digital fuel level sensors need to be installed. I’ll detail those procedures in Section 9.3.4.

ui© John Trautschold 2018