Chapter 14 Center-Section Spar

Chapter 14: Center Section Spar


The spar goes together quickly even though most of the layup sessions last 6-7 hours. There's nothing too technical about building the spar other than being careful to achieve the correct dimensions at the outboard ends.  Why is this critical?  The height of the finished spar determines the thickness of the strakes (since the strake skins are bonded onto the spar).  You want the strakes to match seamlessly to the wings.  If the spar is too thick, then you'll use alot of micro on the wings to get them to match to the spar. The archives are full of "too thick" testimonials!

There are two critical steps to consider.  One is when tapering the top of the spar from BL37 to the outboard ends.  The other is when cutting the spar cap troughs to the specified depth.  The plans give you the various templates and patterns to follow.  On paper, I found them to be a tight fit when matched to the wing templates, so I sanded the taper a bit deeper to ensure that my spar wouldn't be too thick.  I'm glad I did because my spar was still almost too thick when I matched it to the wings (built later in Chapter 19).

My spar weighed 44 pounds.

Lessons learned:

  1. I bought 14-inch particle board shelves and got the Home Depot service technician to use his Tim Allen Tool Time saw to rip the boards to the 12.5 inches required. This was easier and more accurate than buying a 4x8-foot sheet and ripping the long cuts myself. 

  1. I held all foam in place with drywall screws going through the jig and into the foam. The foam is so well supported by the screws that I didn't need the pine sticks. I didn't have the time to do the entire inside layup at one time, so I did the port interior half on one night, the starboard half the next night. We're only talking one layer of BID here, so I don't see any issue. I did overlap about 4 inches (instead of 1 inch) at the center splice.  To maintain squareness, I installed the forward face foam (the foam that faces up in the jig, the pieces Nat calls the "top") to hold everything square while the inside layups cured.

  2. Although it's not mentioned in the plans, it is allowable to cut 1-inch holes in the bulkheads for running wires and such through the spar. Just make sure the one-inch hole in the spar end bulkheads match up to the one-inch hole depicted on the wing hotwire templates.

  3. Before floxing the interior hardpoints, I took a fine tipped sharpie pen and marked lines at the exact locations where the LWA1's would go. Besides the benefit of helping to locate the hardpoints during the interior layups, the marks are useful later for assuring you've properly located the LWA4 and 5 external hardpoints. If done properly, you'll be able to see the marks through the glass once you've routed out the foam. Very reassuring to see those marks!

  4. The plans call for you to taper the top surface of the spar from BL37 out to the outboard ends.  You must get this taper correct or else the spar will be too thick.    To get an idea of what this taper should look like, I traced the spar end patterns (Section C-C, Page 14-9) onto the spar end.  I sanded with my 4-foot sanding plank from BL37 outward until I reached the traced lines.  I used the BL 62.5 template to double-check the taper.  It is worth noting that the section C-C spar end pattern should match exactly to the wing templates.  When I realized that it looked like an exact fit on paper,  I decided to sand the taper just a tad bit deeper to ensure that my spar wouldn't be too thick.  I'm glad I did because my spar was still almost too think when I matched it to the wings (built later in Chapter 19).

  5. Throughout the building of this plane, the builder is left to his/her own accord to develop the best method for doing things. Cutting the spar cap troughs are no exception. I settled on one that worked well for me. I roughed out the spar cap troughs with a dremel tool and hacksaw blade, then used a sanding jig to very gently sand the troughs to the final depth.  The jig is an 8x4-inch block of wood with sandpaper glued to the sanding surface.  A small strip of wood along the board's length serves as a fence to limit the width to 3 inches.

  1. Do yourself a favor.  Step 4 has you cut the 1"x1/2" angles off the front corners of the spar. Postpone doing this until after the spar caps are done. The foam tends to get dinged up a lot from handling the spar. Leaving the corners in place allows the corners to take all the abuse (and dings!) until it time to glass the shear web.

  2. I have heard horror stories of builders running out of spar cap tape while in the middle of a spar cap layup. So in preparing for the spar cap layups, I decided to lay out and cut all the strips first to see if I had enough tape to do the job. This also has the added advantage of ensuring I used the correct number of tapes.

  3. For the spar cap layups, I attached the fences to the shear web with small drywall screws. I also stuffed some dry flox into the crevice between the fences and the radiused edges of the shear web. Nat says you can use micro, but since it's such a small amount, I used flox. The flox stops epoxy from seeping between the fences and dribbling down the shear web. Before starting the spar caps, be sure to fill up your epoxy pumps. The spar cap tapes soak up A LOT of epoxy. It helps too to get the epoxy and shop area nice and toasty. If Mother Nature takes a break like she did for me (passing cold front, hangar temperature at 50 degrees…eck!), you'll need a few hair dryers to keep things warm enough.


  1. The access holes that are cut into the spar forward face look like they are perfectly made for a set of 1,000 Watt speakers! Too bad the plans don't call for 'em.