FireFlys' Cozy Mark IV, #1500

Dedicated to those that lost their lives on 9/11




Chapter 4 - Building the fuselage bulkheads

This is the chapter where you really begin building! The bulkheads will act as "stiffeners" for the fuselage.  

Chapter 4 Research Summary (PDF Document) - Please contact me if you have tips/FAQ's that should be added





  • I used the CAD drawings available on Marc's website - CLICK HERE .  Cut the blueprint copies along the lines, then used a fine point Sharpie to trace the outline of the cut copies.
  • I cut all the foams with a utility knife and metal straightedge.  Only needed a few passes on the Seatback and Landing Gear Bulkheads.  For the Last-A-Foam (F22, F22Doubler, F28, and IP) made multiple passes along the "outside" of the Sharpie line, then tapped a razor blade with a hammer along the cut line to finish the cut. Then a few light passes with a Perma-Grit block quickly shaped to the line.  Perma-Grit blocks/tools also worked great to fine tune Firewall cuts.
  • Don't get hung up on making the edges of the bulkheads "perfect"; you will get them right during the trial fit in Chapter 6.  I would mark the edges per the drawings, then leave a little extra (1/16" or 1/8") to work with where the part will meet the Longerons
  • Used the "Hinge Method" to 5 minute epoxy bulkhead pieces. See Chapter 3 Research Summary  for more details
  • Used Wayne Hick's method for sliding plies onto work, and recommend that new builders use it as well.  CLICK HERE to read more about it.
  • When working with UNI: use 1/2" masking tape to mark cut lines.  Then cut down center of tape to keep edges from becoming frayed.  Leave taped edges in place, cutting only where needed when laying plies down.  Also, measure carefully to maximize cloth use.  Also ended up doing this with the BID - It seemed to keep the cloth from deforming while handling.
  • If you're new to working with glass, identifying small air bubbles can be little difficult - especially when everything is "wetted-out".  To me, the small air bubbles below a ply of glass look like poison ivy blisters.  When I find them, I like to make multiple light passes using the squeegee, working the bubbles toward the edges with the "grain" of the glass.
  • Peel-ply - I seemed to achieve the best results, by laying glass (per plans orientation), then Dacron peel-ply, then a heavy (~4 mil) plastic "peel-ply" a la John Slade technique (see reference towards bottom of page).  This gave me the uniform finish that John mentioned, yet still allowed me to avoid extra sanding prior to future lay-ups.
  • Put some Dacron peel-ply down on F22 over the 3/8" space on each side of the doubler before glassing on top of the doubler.
  • Added .25" to bottom of IP
  • Consider waiting on getting an epoxy pump.  You may be able to save yourself some money and aggravation.  See Chapter 3 - Tools
  • Fein tool does a nice job of trimming glassed parts if you miss the knife trim window.
  • Used .25"x2'x4' MDF as temporary firewall; marked with W.L. 23, W.L. 0, and Centerline. 
  • For the firewall : Made 1.5" square inserts, beveled 45 degrees on two parallel sides, vs. 1" inserts called for in plan.  If you plan on doing the same thing, then adjust your order accordingly from the supplier. (I received a 2"x 3" section of 2024 Alum. from Wicks).  Also, for the blind screw installation, I ground down both sides of the heads and custom cut each channels to avoid the dreaded "spin" (see below).
  • Following tips from others, used highlighter in plans to mark completed steps.


Perma-Grit Blocks/Tools, Utility Knife, Jig Saw, 6' Metal Straight Edge, 12" Metal Straight Edge, Hammer, Scissors, Masking Tape, Permanent Marker, Highlighter, Decimal Tape Measure, Fein Tool, Roto-Zip, Sticky-Stuff Dispenser*, Detail roller, Hack Saw, Metal Files

* Still debating value

SEATBACK - Done...maybe 

Used a jigsaw (20 tpi blade) to cut 45 degree angle.  Varied from plans by not bringing fiberglass over sides and floxing to front sides.  I did, however bring the top and bottom edges together.  During fuselage assembly, I plan on channeling the sides out before floxing.  As a side note - I also had three "exotherms" (one pure epoxy, one slurry, one flox) - I'm assuming because I mixed too much epoxy at a time.  Weight - 5.7 lbs (Map pocket not cut).  I am considering rebuilding or modifying the seatback to add a "pass-through" panel on the passenger side. 

NOTES:  During trial fit in Chapter 6, this part required the most adjustment.  Leave yourself a little extra where this part, and the other bulkheads, will intersect with the longerons.  Also, keep in mind (for all parts) that the longerons will not have a 90 degree edge, but will be rounded slightly.  Therefore, the bulkheads should not have  "crisp" corners.  I would also just draw the center map pocket and heat duct cut-outs, and wait to cut them until Chapter 6. 


F22, F22 DOUBLER, & F28 - Done

The razor tap method worked fine.  However, I learned quickly that you only need to rough cut the high density foam.  The Perma-Grit block takes the edges down quickly, accurately and with no effort.  Varied from plans by glassing the forward side before doing the aft side.  Figured it would be easier to weight down with the thicker build-ups on each side vs. having the doubler on the top only.  Hopefully that won't come back to haunt me.  A little tip: Put some Dacron peel-ply down over the 3/8" space on each side of the doubler before glassing on top of the doubler.  This will prep the 3/8" area, and keep the "run-off" epoxy from building up there.  


Added .25" to bottom of IP.  Note at the time of the photo, only the center "connectors" had been sanded with the Perma-Grit block. Glassed on both sides, working on adding stiffeners.  Following the advice of others, I used sealed blocks wood as forms for the channels.  I had some uncertainty about adding the electric channels on the vertical stiffeners since it appeared that some others had not.  In the end, I went ahead and put them on - figuring I can always trim them off if the weren't needed.

Added picture: When I have excess epoxy at the end of a lay-up I try to have small items ready to be glassed/floxed.  One of those items are the leg holes in the IP.  I channeled out the foam and filled the void with flox.  After cure, I filed and sanded the edge flush.

NOTES:  I used the CAD drawings and added the .25".  When it came time to install in Ch.6, I had to trim most of the excess back off.  But, this was certainly easier than having to add more.  If I had to do it again, I would just glue the basic parts together, glass both sides, then trace the drawing onto the part and cut to shape with the Fein tool. 


I rough cut the foam around the areas adjacent to where the hardpoints will be be inserted.  That way I can adjust for any slight differences that may occur when I cut out the hardpoints. (Note the blueprint template to the side) -- One point of interest: the forward landing gear (designed to be installed at angle) was the one part that the CAD drawings appeared somewhat different than the M drawings.  I decided to stick with the CAD drawing shape since the dimensions appeared true and the M drawing contained an interruption.  Used 24 plies to make hardpoints, then used LG Bulkhead foam scraps as supports to control the thickness while the part was weighted for cure.  The finished part came out a perfect thickness.  Note that the picture at left makes it appear that there are more "dry" spots than there really are.  After cure, used a jig saw with metal cutting blades to rough cut hard-point shapes.  Then I held pieces together in a vise and used the Perma-Grit tools to finish shaping.  Taped the edges of the finished pieces (like the hinge method) when inserting in the bulkheads with 5-minute epoxy.  Then peeled quickly for a clean installation.

Side note - Tried masking off the sides of F28 and the LG Bulkheads to prevent the epoxy and micro run-off from seeping around to the underside of the part that I was glassing.  This didn't work, and certainly wasn't worth the time.

NOTES - If you are planning to use Wayne Hick's alternate Landing Gear Cover (as I am), then you may want to add an 1/8th inch along the bottoms, including the NACA cutout section, of each of the landing gear bulkheads now.  {Do not change the top half of the forward LG Bulkhead, as it does not come in contact with the fuselage bottom.}  In Chapter 7, you'll find that these bulkheads are designed to be below the fuselage bottom to create "joggles".  Wayne's method does not require joggles, so the 1/8th will need to be added then if you don't do it now.  Also, if you add the 1/8th inch now, then in Chapter 6 (Fig. 32) you will need to relocate your block to 3/8th inch below the scoop cut out to insure that the fuselage bottom mounts flush to the cutout.


Longeron markings on .25"x2'x4' for Temporary Firewall.  After rough cutting Permanent FW with 20tpi jig saw, finish shaped with Perma-Grit blocks (I LOVE THESE BLOCKS/TOOLS!!).  Used clamps to hold the two "connector" pieces together, and the PG Block onto the table.  This allowed me to get two identical parts.  I plan on using 1.75" round aluminum hardpoints (a la Bernard Siu).  After more thought on the matter, I decided to insert square hardpoints.  However, instead of the 1" x 1" size called for in the plans, I went with 1.5" x 1.5" squares.  This decision was based on some references that the 1" x 1" inserts had a minimal margin of error.   Additionally, I filed 45 degree bevels on two parallel sides of each insert.  Then I inserted the hardpoints with the bevels facing aft, resulting in an exposed 1.25" x 1.5" rectangle on the aft side (see picture at left).  Though the plans didn't specify, I used standard epoxy and flox vs. 5 minute epoxy to install the hardpoints.

After reading all the comments about the dreaded "spinning" of the blind screws, I decided to grind the tops of the screws to create a "T" shaped bolt.  I then carved each channel so the now rectangular shaped tops (see picture at left) had no room to spin.  Hopefully the increased resistance added by the channels, coupled with the flox, will prevent the "spins".

Note - The CAD drawings show the blind screws (on the two small pieces that connect the bottom and top firewall) sitting higher than the plans dimensions.  I put them in at this higher location and now I hope this doesn't come back to haunt me.  I looked at Marc Zeitlin's plane, and it appeared that his were at the higher location.  I didn't see any comments where this caused him problems. 

Chapter 3    Chapter Index    Chapter 5

N911HF - No 911 Heroes Forgotten