The Long EZ Build: Strake bottom skins

With the strake ribs bonded to the airplane, it was now time to make and install the strake's bottom skins.

I started out by bonding a few sections of 0.35" thick H45 foam...

I then set up a few stands under the strakes... making sure I would be able to reach into each compartment with ease...

Since the main gear is in the way, I couldn't fit another stand... so I slid a board across the two stands..

I then placed the untrimmed bottom skin under and used a bunch of scrap foam blocks to shim the skin up onto the ribs' bottom...

I then used a ruler butted up against the fuselage side wall.. and used the 4" mark to make a small dot on the foam.. I repeated this for 20+ fuselage station locations... I then removed the skin and splined the dots.. this gave me the perfect contour to cut to match my fuselage side. I then reinstalled the skin and test fitted it... no gaps between the skin and fuselage... perfect!

Next step was to trace out all the ribs and bulkheads onto the skin so I could trim it to final shape... I used as many foam blocks as I could to get the skin up onto the ribs...

I then traced all ribs and bulkheads...

Along the aft edge too.. notice how I traced the fwd face of the spar (not into the bevel)..

I then removed the skin and placed it on my bench...

I added 1" to the trailing edge to account for the bevel...

As for the outboard diagonal, I measured 32.4" from the BL23 (where the kink is). Page 21-2 in the plans shows the 4.9" and 32.0" dimensions of the right triangle... so applying Pythagorean theorem (oh come on!! You remember that, don't you?!)...

And voila! We have the outboard diagonal drawn....

Now I needed to bevel the aft edge... I lined it up flush with the edge of my work table.. and used a belt sander....

Fine finish by hand...

That's a nice looking bevel!

Before bonding the bottom skin in permanently, I wanted to make sure all surfaces were prepped for glass-to-glass bonding.. since it would be much easier to get this out of the way now...

Also, I knew the inboard forward compartments would have a hard time bending to the shape of the fuselage cutout. So I made a few holes every 5" or so...

This allowed me to pass long toothpicks through and hold the skin exactly where it needed to be...

I then shimmed the skin up onto the ribs again.. making sure everything was within tolerance...

I also used shims to hold the aft edge into the spar's bevel...

After checking and double checking that the skin was in perfect position, I mixed up micro and applied a bead along every edge of every rib and bulkhead.

I let everything cure and removed the stands from underneath. Before I could move on to doing the inside bottom layups, a little prep work was required. First, I had to patch up the inboard wing attach cutout. I made a plug out of scrap H45 foam...

And used 5-min to bond it on.. after cure, I sanded it flush (this pic was taken BEFORE sanding it flush... obviously...)

I then focused on the fuel drains... the plans don't give precise measurements as to where to make the 1.25" dia hole.. so I guesstimated the center to be 1.5" from the fuselage and 1.5" from the fwd face of the spar...

Using a 1.25" dia hole saw... I drilled away!

I then sanded the bevel rim around the hole... this was harder than it sounds... since it's a tight space and didn't want to break the foam. If I were doing it again, I would probably do this step when the skin is on the worktable prior to installing it...

Next up were the water drains... plans call for 1/8-27 NPT... (per CP#28 LPC#60)... if you are unfamiliar with NPT, it stands for "national pipe thread" and is a specific thread designation where the thread tapers to obtain a good seal.

I practiced a few times to make sure the bottom of the valve would be flush with surface...

Perfect!

I alodined them...

I used a dremel to mill out 1/8" so that the puck would be flush with the skin...

To prevent epoxy from getting inside the thread, I filled it with hand clay... also, notice the black dot on the upper left of the puck.. this was to keep track of the puck's orientation. Since it is an NTP thread, it is very important to know which side is up!

With everything prepped, it was finally time to apply the 1-ply BID to all compartments! I should note I did not do ALL compartments in one day.. instead, I broke it up into a 3 day job...

I started out by painting plain epoxy on any mating surfaces...

Slurry on the foam and a drier micro on all edges to get a nice fillet..

Showing the water drain puck embedded in the foam...

Since I built my center section spar with Aeropoxy (not rated for avgas)... I had to apply 1-ply BID to any surface of the spar that will come in contact with fuel. For ALL fuel tank surfaces, I am using EZ-poxy 87.

I peel plied the 2 UNI plies that went on the outboard diagonal...

For the inboard compartments, I applied a flox corner to all edges that mate with the fuselage wall. AFTER doing this layup, I realized that page 21-6 section C-C shows that this layup should have overlapped 90 degrees onto the fuselage's inside wall.