The Firewall construction was not unlike the other bulkheads I had recently completed: the core was cut to match a template then fiberglassed on each side.  The main difference here was that I used a .25″ thick birch plywood sheet as the core instead of high-density foam sheet.  This made the part substantially heavier than the other bulkheads, but since this is what the engine ultimately mounts to, weight = strength = good.

None of my tools were suited to accurately cut a highly curved piece of plywood, so I borrowed a jigsaw from a friend which worked quite nicely.  Plywood doesn’t sand as easily as foam (duh), so I cut the piece as close to the template line as possible to minimize the amount of edge sanding required.  Glassing was as before, except these were the largest pieces of fiberglass I had used yet – roughly 3’x4′. The other significant difference between this part and previous bulkheads was the inclusion of integral hardware.  (Hey, I finally got to use my cool Ingersoll-Rand pneumatic drill.)  If you look closely at the photo, you’ll notice 4 silver dollar sized circles in the “corners” which are the engine mount attach points.  They are actually .25″ thick aluminum plugs which were bonded into the plywood before the fiberglass was applied. Firewall screws Additionally, after the first side was glassed I bonded in 6 (.190″) countersunk screws (3 on each side) which serve as attach points for the rudder cable pulleys.  (Hey, I finally got to use that 100 degree countersink I bought at Boeing Surplus.)

The hardware was bonded to the plywood using a slurry of epoxy and cotton flox (which is finely ground cotton–almost like a powder–for those of you not “in the know”).  I had some doubts about the structural integrity of the screws bonded in with this mixture, and their ability to withstand a decent torque.  But once it cured, I was a believer: cured flox is some tough stuff.  The heads were flush on the other side, and glassed over, so unavailable for a screwdriver.

That was about it.  The completed part was set aside for installation sometime in the future when it will get fire-proofing materials applied and a metal face sheet.

FS-37 Bulkhead Part 2

As mentioned in an earlier post, the FS-37 Bulkhead – one of the more complicated layups – was my first casualty.  After the initial trim and final cure, I was sanding down the edges to meet the final contour.  The top of the part is straight, so I figured I’d sand that with the belt sander that my Dad gave me (surplus from his shop).  That damn tool was way too agressive for this composite part.   It sanded right through the edge in no time, and made a neat concave shape.   By the time I finally had a straight line, I had trimmed away much more material than planned.  Now what to do?  I had several hours (not to mention materials) invested in the part – I hated to throw it away and start all over again.  It may still be okay.  I could wait until installation and see how it fits, right?   Well, probably.  But I had just enough material to make another part, so I convinced myself to scrap this one and start all over again.  After all, this part takes 90% of the canard lifting loads and transfers them into the fuselage structure, so it’s not a trivial part.

It was a bit of a setback, but the second part came out even nicer than the first.

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Hindsight note: After installing the canard (much later), the original FS-37 bulkhead with its slightly concave top surface would have been just fine.  A little filler between the canard and the top of the bulkhead would have been fine, provided the edge distance of the crush plate inserts was still adequate.

Forward Fuselage Bulkheads

Finally, armed with a fresh shipment of epoxy resin, I could actually start building parts.

This first part I built was the FS-16 Bulkhead.  (FS-16 represents Fuselage Station 16, meaning it will be installed 16 inches aft of aircraft datum.)   The process for building all the bulkheads is essentially the same, but the FS-16 bulkhead one is the most straight forward, which is why it’s a good starting point.  First, I obtained a piece of high-density, .25″ thick foam and trimed it roughly to the shape of the FS-16 template that I built a few weeks earlier.  The foam was then sanded/filed until it exactly matched the contour of the template.  Hey look! I made a bulkhead core.

Next, I carefully weighed out 3 parts of epoxy resin and 1 part of hardner, and mixed in some microballoons to form a light-weight slurry.  This slurry was then spread into the surface of the foam to fill any open cells, which in turn cuts down on the total amount of epoxy required for the layup, and ultimately leads to a lighter finished part.  For subsequent parts I used the Clearstream (Hudco) 8040 filler in lieu of the microballoon slurry.  Had I not taken that Aircraft Composites Course a few month earlier, I would not have learned that trick.  The SQ2000 Builder’s Manual doesn’t go into detail like that.  It specifies which materials to use, ply orientations, etc., and covers some techniques, but it isn’t an all encompassing guide to aircraft construction.  But I digress…

While the slurry was still wet (tacky), the appropriate type of fiberglass cloth was cut slightly larger than the core and layed in place.  A new batch of epoxy was mixed up (no microballoons this time) and spread over the dry fiberglass with a brush until nicely wetted out.  A small texture roller can be of assistance here to force the resin into all areas of the fiberglass cloth.  Then, while the epoxy is still wet, a layer of peel ply was layed over the fiberglass and wetted out.  The peel ply is a thin, nylon cloth which epoxy has a hard time sticking to.  After the epoxy has cured, the peel ply is removed (“peeled” away–hence the name), leaving behind a nice, even texture which is perfect for subsequence epoxy bonds.  Even if no further bonding is necessary, the use of peel ply is encouraged because it leaves such a nice finish.

I did the layup on a 1″ thick piece of particle board with Formica facesheets.  This is a nice, smooth, flat working surface.  During the over-night cure, I sandwiched the bulkhead between 2 of these boards, and placed some additional weight on top to make sure it would cure flat.  Warped bulkheads can only lead to trouble down the line.  Since it was the middle of summer, I didn’t need to worry about heat during the cure: the garage never got below 70 degrees.

The only tricky part about this process is the trim.  The manual recommended that I let the epoxy cure for a few hours until it’s tacky, then knife-trim the excess fiberglass to the shape of the foam core.  This didn’t strike me as a very good construction technique, and it limited me to glassing one side only.  If I glassed both sides, then allowed a full cure, I could just cut/sand/file the excess glass.  So that’s what I did.  Turns out that cutting the cured glass to the exact shape of the foam, without damaging the foam was harder than I thought.

For the next bulkhead, I followed the manual’s advice and knife-trimmed the excess.  Timing becomes an important factor here: if you wait too long after the epoxy has been applied, the composite becomes too hard to cut with a knife.  If you don’t wait long enough, it’s too sticky and gums up the blade.  And the ambient temperature will affect this window of opportunity, so if you’re heading off to your girlfriend’s house for dinner on some warm summer night after a layup, be sure to warn her that you have to leave in a couple hours to “trim your bulkhead.”  I’m sure she’ll understand.

And so it goes for the other bulkheads.  The shapes and layups become more complex, but the process is the same.  The FS-37 Bulkhead has numerous plies and 2 cores, and took quite some time to build, so it was the most challenging for a new builder such as myself.  As luck would have it, this part was also my first casualty, but you can read about that here.