Joining the Fuselage


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 getting a heater was not just an option if I was to continue working.  But you can read about that in my Getting Ready for Winter section.

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) 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 spliced 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.   I think a "slide show" type presentation would be best here.  As usual, pick the thumbnail photo to download a larger photo.

OK, let us begin at the end.  This is the finished project... well sort of.  This is a shot of the lower fuselage joint after I had applied (but not yet cured) the final ply (you can see the peel ply here).  I thought it would make the following photos more understandable if you knew what the goal was.  Besides, my photos of the upper join are better.
First step in joining the upper fuselage was to invert it.  I'd be crazy to attempt this task working over my head, as would any SQ2000 builder.  The fuselage is not very heavy, but it was a bit too big for me to handle alone, so my buddy Mike came over to help turn it over.  You can also see the foam pads on the floor quite clearly in the full-size photo, as well as the main spar hanging on the wall.
The bottom of the fuselage is relatively flat, so it sat nicely on the foam pads.  The roof, however, is highly curved and it wasn't very stable once inverted--especially with a 190 lb. oaf such as myself climbing around inside.  My solution was to notch out a section of the mohawk and build a small cradle to fit under the nose.  I then placed a sandbag inside the fuselage--just forward of the cradle--to stabilize it.
The first task was to fill any imperfections in the base laminate (gray areas), then install 1 ply of glass across the joint.  Nothing too complicated here.  You can also see (in the full size photo) my electric heater and a thermocouple.  Rather than keep the full volume of my garage heated during the over-night cure, I plugged the open (aft) end of the fuselage with a 1" thick piece of styrofoam and left the electric heater running inside.  It had an adjustable thermostat so that I could maintain a nice epoxy-friendly 80 during the entire cure.  It worked out great.
Once the entire length of the joint had been glassed, it was time to build those damn foam blocks.   This is what took the longest.  The blocks had to fit tightly in the "valley:"  Too much material and they wouldn't sit flush, too little material and they would be loose and result in a thick bond line.  The tricky--nay, frustrating--part was that the "valley" was ever-changing.  The bevel angles, width, and in some areas even the depth would change from inch to inch.  It took a couple of hours to cut, bevel, file, sand, grind, or do whatever it took to make each piece fit perfectly.  Looking back, I may have been a bit too precise with this job considering that the foam isn't really the load bearing component here, the fiberglass is.  But it is a critical joint, and I didn't want to take any chances.  It would be a real bummer to be cruising along at 10,000 feet someday and have the fuselage separate.

By the way, the material I used here was unlike the .25" high-density bulkhead foam core I had been using.  This stuff was lighter weight, and .75" thick.  Despite it's lower density, I actually found this stuff more difficult to work with as it was tougher and less responsive to my files and sanding blocks.  I also wasn't sure how long to make each piece.  The manual only showed a cross-section and never specified a length... I arbitrarily chose 16".

Can you find the image of my cat in this photo?

Once the I had achieved the proper contour, the faying surface of the foam core was coated with a mixture of epoxy and microballoons (known as "micro").
Micro was also applied to the joint valley.
100 pounds of dead weight was placed on top of the core and the micro squeeze-out was removed.  I let it cure over night like this, once again with the electric heater inside the sealed fuselage.

I found that "Play Sand" provided the best gravitational attraction.  Note that I wrapped the sand bags in plastic to prevent contamination.

Once all the foam core pieces had been bonded in place and any gaps or irregularities were filled with micro and sanded flush, the whole joint was covered with a 12" wide ply of glass (peel ply shown here).  The glass stops just past the windshield threshold, as this piece will eventually be cut out.
The next step was to repeat the process for the hatch, which is just forward of the windshield.  Same procedure here, except I used .25" thick foam core instead of .75".  I also laid 2 plies of fiberglass sans core up to the nose of the aircraft.
The last step in the joining process was to cut out 2 foam ribs and glass them in place to stiffen the hatch.
 

Just a little side bar here.  After working inside the fuselage for many hours, I discovered there was something missing from the minimum required tool list: KNEE PADS!  If you plan on building a SQ2000, be sure to get yourself a set of knee pads.  You'll thank me later.  Also, if you have a bad back, don't attempt to build this aircraft, as you'll spend many hours hunched inside the fuselage.

 

Now that the joint was complete inside, it was time to work on the outside. I removed the flange by cutting it with my body saw to within .25" (approx.) of the fuselage, then used a grinder for the rest.  The last little bit I sanded by hand to avoid any skin damage or major "oops".  The blue stuff is low-tack masking tape I put down to protect the skin while cutting and grinding.   $8.00 a roll, by the way, at Home Depot.
Once there was no more evidence of the flange, I applied some lightweight filler to the joint.
Then I had my Dad sand it flush.  Well, one day anyway, when he came to visit for the weekend.
Final feather sanding made the joint invisible... well, it will be invisible once it's painted.  The fuselage contours came out nearly perfect up by the nose and didn't require much filler.  But as I worked aft, things weren't looking too rosy.  It took many many iterations of fill-and-sand   to get the correct fuselage contour on the roof line.  The bottom surface also required a lot of filler to achieve a nice, flat look.  I guess that's something KLS Composites could improve: making sure the fuselage halves come off the mold with 100% correct contour.

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