Lower Fuselage Joint – Part 1

With all the bulkheads now complete, I could move away from the workbench and onto the airplane as I began Section 2, Chapter 9 of the Builder’s Manual.  As the title suggests, this is the point where I permanently bonded the 2 fuselage halves together.

Let me preface the chapter by saying this was a major task, and took way, way, way longer than I imagined.  I partially attribute this to the weather turning colder, and my generally lower enthusiasm for working in a cold garage.  Regardless of my own desires for comfort, working with epoxy resin requires an environment of at least 70°, so a space heater was now a necessity if I was to continue working through the winter.  But you can read about that in my Building in the Winter entry.

Before actually bonding anything together, the normal first step would have been to align the 2 fuselage halves. Recall, however, that I took care of that a few months earlier when I was still awaiting the arrival of my epoxy (see Aligning the Fuselage Halves) so I could jump right into the joint (sounds like a blues song).

The basic concept of the fuselage joint was pretty straight forward;  a single ply of fiberglass is laid up across the joint valley, then some foam core is bonded in, and another ply of fiberglass covers the whole mess.  After that, I could cut off the flange (mohawk) and fill the remaining gap flush.  Sounded simple enough, but it sure took a long time – highly attributable to my meticulous manner I suppose.

Typical of composite construction, when two pieces are to be joined together, the core is tapered down to zero so that a solid laminate joint can be achieved.  I referred to this area as the “valley.”

I started by sanding the valley area by hand to remove any imperfections and high spots.  Next, some light weight filler (epoxy/microballoon mixture (“micro”)) was applied to any small voids or low spots so that I wouldn’t have any trapped voids in the joint.  After a full cure, the filler was sanded smooth and flush.

Then comes the messy part: a heavy ply of fiberglass, about 12″ wide, is laid up across the joint (is it “laid up” or “layed up?”) and allowed to cure with a top layer of peel ply.  After cure, the peel ply was removed.  I started at the nose and worked my way aft over the course of a few days.

Layup over the lower fuselage joint.

Nose Gear Plates

The Nose Gear Plates, or NG-2 Bulkheads, are pretty much like the other bulkheads with a few additions. Construction began the same way: cut out the high-density .25″ thick foam core and sand it to match the template I fabricated a few weeks earlier.  Actually, in this case I built 2 cores, because both left and right parts are required, and they need to be perfect mirrors of each other.   “Why?” you may be asking.  As the name implies, the Nose Gear Plates serve as the attach points for the nose landing gear and its retraction devices.  If the parts are not alike, the gear installation will be skewed which could lead to bindin

g during retraction (or worse–during extension).

Adding solid hardpoints.

The inboard surface of each part was glassed with 2 plies of EBX-1800 cloth, cured, trimmed, and sanded to match the template.  On each part, there are 4 areas (bolt/attach points) that need to be strengthened by boring out the core and building up the area with glass plies instead.  This operation is a bit tricky because the hole saw needs to be positively controlled: cut too shallow and you don’t get all the core out.  Go too deep, and the hole saw cuts into the inboard plies that I had just layed up.  Using a drill press, and setting the cut depth using some scrap pieces of core helped a lot.  Then came the tedious job of cutting a bunch of circular pieces of glass.  Thousands of them.  Maybe tens of thousands.  Well, maybe I’m exaggerating a bit.  But I’m sure it was at least 100.  The circular pieces were then layed up into the holes, and the outboard sides of the parts were glassed.

But wait, there’s more.  On the outboard side of each part, 4 aluminum crush plates are bonded, faired around the edges with an epoxy/flox slurry, and glassed with some Bi-Direction (BID) cloth.  On the first crush plate, I made the mistake of sloppily applying the flox and allowing it to cure before adding the overlay BID ply, thinking it would just take a few minutes to sand it down to a smooth radius.   Wrong.  As I mentioned before, cured flox is some tough stuff.  I’d say it took about an hour to grind/file/sand that stuff down to a nice radius.  Needless to say, I didn’t repeat that mistake when installing the other 7 crush plates.

Original NG8 plate.

But wait, there’s still more.  On the inboard side, yet another aluminum crush plate (NG-8 for those of you taking serious notes) gets installed at the nose gear pivot point.  Unlike the other crush plates, this one is mechanically fastened rather than bonded.  The NG-8 plates that were supplied with the kit were in

New NG8 plate installed.

pretty bad shape: scratched, holes out-of-round, lousy countersinks, and generally not the type of parts I wanted on my airplane.  I faxed a drawing of the parts to my Dad, and he built me 2 new ones at his workshop which looked – and worked – great.
Once all the parts were installed, the last step was to mate up the 2 bulkheads, and open full size holes through both parts simultaneously.  The drill press strikes again.  That’s about it.

Nose gear plates complete.