FireFlys' Cozy Mark IV, #1500

Dedicated to those that lost their lives on 9/11


Chapter 14 

Centersection Spar


Chapter 14 Research Summary (PDF Document) - Please contact me if you have tips/FAQ's that should be added


Step 1 - Centersection Jig

Step 2 - Assembly of Foam Parts

Step 3 - Interior Layups

Step 4 - Install the Foward Face

Step 5 - Outside Hard Points

Step 6 - Spar Cap Layup

Step 7 - Shoulder Harness Blocks and Outside Layups

Step 8 - Hardpoint Reinforcement

Step 9 - Access for Wing Attach and End Layup

Step 10 - Installation in the Fuselage


  • If I had it to do again, I would not use the Urethane foam - It is easy to shape, but because it is so fragile, it's a pain in the @$$ to work with.
  • Step 1 - Cut and extra CS6 to be used at the centerline in Step 3.
  • Step 3 - Used "reminder" labels on interior bulk heads to keep me aware of the orientation of the entire part. 
  • Step 4 - I used the Fein saw to cut the micro joints to the depth I wanted before sanding.  Also, if you used the soft foam, you can just "wiggle" the spar cap templates into the foam to assure the right shape at each position.  Then sand the remaining surface to those points.
  • Step 4 - I waited to cut the angles on the top and bottom of the forward edge until just befor I glassed them in Step 7
  • Step 4 - Made a simple depth guage by marking a piece of paper, then used it frequently to verify progress.
  • Step 6 - As with the canard, I had my UNI tapes cut and labelled prior to starting the layup.
  • Step 7 - Be careful not to cut through interior lay-ups.



Step 1 - Centersection Jig  

14.1.1 Make the spar jig - I had a friend who is interested in building a Cozy make this for me.  He is an excellent carpenter and the work was terrific.  Moved the jig to my table and verified all the measurements. I also covered all the surfaces with clear tape for an easier release of the completed part.         Step Done 






Step 2 - Assembly of Foam Parts

14.2.1 Cut all the foam parts - Per plans, just go slow and easy with the shaping. Note - You need to cut a third bulkhead like CS6 & CS7 for the centerline of the spar used in Step 3.     Step Done

14.2.2 Assemble all the foam parts - Per plans. Since this part is built on its side in the jig, the bottom is really the aft, the top is really the front, the left is the right, etc, etc..., just go slow and easy.  Bulkheads are measured from the center line, but are perpendicular to the spar. Used a square to mark where centerline intersect the aft edge, then used the square to mark the perpendicular position.       Step Done






Step 3 - Interior Layups

14.3.1 Fabricate and prep aluminum pieces - Cutting all aluminum hardpoints per plans (also cut pieces for Chapter 19 at same time). Tried using hacksaw first, then decided to go with a jig saw instead.     Step Done  

14.3.2 Interior lay-ups - The plans have you do this entire layup in one session.  Not too difficult, but it is tedious.  First 1 ply BID at 45 degrees. (beforehand, I had the glass cut then folded, rolled and placed in order that they were to be placed). Then, CS6 and CS7 are set with micro (using nails through outside foam to hold in place) and BID on each side. Reinforcement layups, with hardpoints (interior and outboard) are next.  Finally, another bulkhead is placed at the center and reinforced with BID.        Step Done  








Step 4 - Install the Foward Face

14.4.1  Glass the inside of CS4 with 1 ply BID  - This is the side that will eventually be facing inside the spar.  After cure, prepped the pieces so they could be bonded to the spar.     Step Done

14.4.2  Cut conduit holes in the bulkheads  - Cut holes, cleaned foam from the interior edges, then filled with micro.  After cure, sanded openings to make sure they were smooth and couldn't snag anything.     Step Done

14.4.3 Attach CS4 to the spar - Plans call for a micro joint to close box.  Decided I wanted to flox the joint instead; this makes for a stronger corner. Made channel for the flox, filled, and placed CS4.  Weighted pieces until they cured.    Step Done  

14.4.4 Make spar cap templates - Copied the 8 templates from M-20 onto paper, glued them to masonite, and cut/sanded to shape.    Step Done  

14.4.5 Carve top and bottom spar cap troughs - Removed the spar from the jig, then cut edges per plans.  Used the templates to verify correct depth.  Like others, I made a sanding block to make the job easier.  Three tricks I used here were: precut the micro seams before sanding, "wiggled" the templates into the soft foam (see picture to right for better idea of concept) to mark the correct depth at each location, and made a simple depth guage (marks on piece of paper) to check my progress.    Step Done  

14.4.6 Cut the angles on the top and bottom edges along the forward face - Since the soft foam is so fragile I waited to complete this step until just before I glassed them in Step 7.   Step Done  


Step 5 - Outside Hard Points

14.5.1 Install the 6 outside hard points - Per plans.  Measure carefully, clear foam, prep area, and flox into position.   Step Done

14.5.2 Prep for shear web layup - Per plans.  Rounded corners and protected areas not to be glassed with tape and paper.   Step Done

14.5.3 Install the 4 ply UNI shear web at 45 degree orientation - Per plans. Cut the UNI ahead of time then rolled them out, making sure to butt ends together.  Peel plied the lay-up when I finished.   Step Done





Step 6 - Spar Cap Layup

14.6.1 Position the spar with top side up and build dam - Per plans.  Make sure you take time to cover any contact points with clear tape so you don't have any inadvertant additions to your spar caps!    Step Done  

14.6.2 Cut the 40 strips of UNI tape - The plans don't specifically tell you to precut the pieces (they recommend dispensing as you go), but I wanted to make sure I had enough tape on hand.  In addition, I found it much easier to lay and track the rolls if they were arrange in the order I used them.  Just make sure to cut each piece slightly longer than the schedule, and trim to shape.     Step Done  

14.6.3 Lay the top spar cap - Per plans.  Nothing new here (if you've built the canard).  I used exactly 23 layers for the top cap    Step Done  

14.6.4 Position the spar with bottom side up and build dam - Per plans.  Same as 14.6.1    Step Done  

14.6.5 Lay the bottom spar cap - Per plans.  Same as 14.6.3, except I used 17 layers of the UNI tape.    Step Done  









Step 7 - Shoulder Harness Blocks and Outside Layups

14.7.1 Cut 4 harness blocks from spruce - Completed per plans.    Step Done  

14.7.2 Remove urethane from top surface to fit blocks and micro into place - Completed per plans.  Blocks placed at 5.25" and 12.5" from center of spar on both left and right sides.  This step should have been easier than I made it.  When removing the foam, I marked what the cutout should have been, then plunge cut with the Fein saw.  Carelessly, I was too aggressive and cut through the interior layup. Not the end of world, but will require extra work to repair.  I went ahead and more carefully cleared the other areas. FROM STEP 4 Cut the angles on the top and bottom edges along the forward face - Since the soft foam is so fragile I waited to complete this step until just before I glassed them in Step 7.  Nothing special here.  Just made reference lines, and used the Fein saw to get me in the ball park.  Then I sanded to the finished shape.  Once the edges were complete I floxed (vs. micro) the blocks into position (making sure no floxed oozed to surface).  Before I can complete this step, I will repair the interior when I open the access holes during Step 9     Step NOT Done 

14.7.3 4 ply UNI layup on top, aft, and bottom (at 45 degree orientation) - It's funny how a single sentence can mean such a long time in the build process.  I knew this would be a long layup when I started, but I made it longer.  I finally bit the bullet to complete this step and had everything ready to go.  All seemed to go well, and several hours later the spar was all peel plied and everything was right in the world...until I pulled the peel ply off.  Spotty dry patches (mostly on the vertical top and bottom sections).  End result, the layup marginally met the acceptance criteria.  However, I wasn't satisfied with the part and decided to correct it.  Fortunately the bulk majority of the dry spots appeared to be in the upper two layers.  New step (using the repair criteria): two layers UNI off, 2 new UNI layers with a BID cover.       Step Done  



Step 8 - Hard Point Reinforcement

14.8.1 Prep LWA-2 and LWA-3 (outside hard points) and flox into position.  - Shaped these pieces at the same time that I did the interior hard points.  While I could still see the interior hardpoints, I made  pencil marks to transfer the center of each to the surface.  I then carried these "centerlines" to the Top and Bottom of the spars.  Then, with the hardpoints in place, I drew "boundary" lines and transferred these lines to sides as well.  Then sanded the aft surface for a good bond, and using the lines on the top and bottom, remarked the position lines on the aft surface.  Finished prepping the top and bottom surfaces, and floxed the parts in place.      Step Done 

14.8.2 Add UNI and BID layups over hard points.  - Made flox corners around LWA-2 and LWA-3.  Then cut and laid out: 3 pieces UNI (9" wide by 17", 16", and 15" long; 3 pieces UNI (9" wide by 10.5", 9.5", 8.5" long); and 4 pieces BID (2 @ 5"x7" and 2 @ 5"x5" (all 4 with 45 degree orientation).  Wet the UNI pieces out between wax paper, then cut each in half lengthwise to make the 4 UNI strips.  Used the 17" strips, centered, on the outboard hard points, and used the 10.5" pieces on the inboard points.  Then covered the tapes with the BID over the aft surface only.  Peel plied the layup and allowed to cure.     Step Done 






Step 9 - Access for Wing Attach and End Layup

14.9.1 Cut and shape 5" x 13" access holes - Pretty straightforward.  Measured from center per and outlined the opening.  Used a small bottle as a template for the rounded edges.  Then I drew lines 6" x 14" outline to mark where I wanted the bevel to begin.  The plans call out for a glass to glass edge at the interior of this cutout.  However, I wanted something a little more stout, and most likely less sharp.  So I deviated from the plans and set-up for a floxed edge by removing some foam with the rotary tool.    Step Done  

14.9.2 Lay 2 plies of UNI.  -  45 degree orientation, alternating direction between plies, and lapping onto the top and bottom beveled edges.  Plans prefer Safe-T-Poxy because it is slightly more fuel resistant.  However, I stuck with the MGS at this point.    Step Done  

14.9.3 Cut 2.25" access holes in bottom of spar - Plans specify a single hole at B.L. 65.5 (2" from end)- assume it is implied for two (unless you plan only to attach 1 wing).     Not Done  

14.9.4 1 Ply BID layup on outboard bulkheads - 45 degree orientation with flox corners on all edges.  I prepped for this lay-up by creating a channel for the flox with my Dremel tool.  The plans say to make a flox corner on all edges, however, I chose not to create a flox channel below each spar cap (it seems excessive).   Not Done  




Step 10 - Installation in the Fuselage - Complete this Step after Chapter 16, 19, and 24 (Fairings) are complete

14.10.1 Make two EM-2 (engine mount reinforcements) - See Plans,    Step NOT Done  

14.10.2 Flox EM-2 onto the firewall and cover with 1 ply BID - See Plans,    Step NOT Done 

14.10.3 Level fuselage, then position spar so it is centered at fuselage center - See Plans, Mark all shims used so you can duplicate positioning.    Step NOT Done 

14.10.4 Prep and flox spar into position and let cure - See Plans,    Step NOT Done 

14.10.5 Verify position, secure spar with 5 ply BID tapes  - See Plans,    Step NOT Done 

14.10.5 Secure spar to firewall (lower) with 3 plies BID  - See Plans,    Step NOT Done 

14.10.5 Secure spar to upper firewall (when installed) with 3 plies BID  - See Plans,    Step NOT Done 


Chapter 13    Chapter Index    Chapter 15

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