Chapter 21: Making the Strake Ribs, Bulkheads, and Skins

Strake Ribs

  1. Making the strake ribs is easy but time-consuming because of the large number of pieces to glass and peel-ply.  There's a running debate as to whether it would be simpler to glass and peel-ply several sheets of foam, then cut out the ribs with a jigsaw.  I cut all the foam out first, then glassed.  I didn't want to take the time to figure out the sheet orientation to ensure the proper weave orientation.

  2. There's also a running debate on whether to use two-inch peel-ply on the edges of the ribs, or go ahead and peel-ply the entire part.  I used the 2-inch peel-ply on the first go round, sanded the parts, put down heavy, second coats of epoxy and peel-plied all parts.  If I was to do this over again, I'd peel-ply the entire part for the first and second coats.  I hate prep-sanding.

  3. There's more running debates about how to treat the exposed foam on the baffle holes (semi-circles).  There's really no technical reason to treat the exposed foam.  The blue PVC foam is impervious to avgas, and since it is "closed cell", the avgas will not migrate through the foam from exposed areas.  Still, I sealed them off anyway with a wet flox mixture to minimize the amount of blue foam flecks that may go free and find their way into my fuel  system.

  4. The archives say that the location of the B33 viewing hole is supposed to be 6 inches ahead of the spar instead of the 3 inches stated in the plans.  I waited until the ribs and bulkheads were assembled, then simply put the hole in line between the sump hole and where the fuel caps will go.  Please note that I'm going to install my fuel caps more outboard than specified in the plans.  So your B33 viewing hole location will probably differ from mine.

Jig Tables

I built the jig tables from medium density fiberboard and 4-inch fir boards.  I made the tables 60x48 inches.  I am using lag screws as footpads on each leg so that I can level out the table.  I built two tables as I am going to fabricate both strakes concurrently.

Strake Skins

Here's another one of those times when the plans are a wolf in sheep's clothing.  The strake skins look so easy to construct, when in fact the work is tedious and lengthy due to cutting and gluing all those foam remnants together, not to mention shaping and fitting the foam to match fuselage and spar.   If you can find 8x4 sheets of this foam, DO IT!  Each strake skin can be cut out whole from one 8x4 sheet.  (Well, according to Chapter 21, Page 4, Figure 3, you'll be shy one inch at the outboard tip....49-inch width for the strake skin versus the 48-inch width for the sheet.)  

Sure, the foam layout sketches in Section 2 show you how to minimize your foam purchase, but what a pain in the patooty to cut and glue all those foam remnants together.  Since Wicks cuts whole sheets in half for ease of shipping, an alternate idea might be to buy 8 sheets of foam, make up four 8x4 sheets, and cut out the strake skins whole.  I did it the HARD way :-(

Of course, I used The Hinge Method to glue the pieces together.  You really do want to oversize the skins by an inch or two, trial-fit to the spar and fuselage, then trim to size.

Be aware that the archives and FAQs say that once you've built one bottom strake skin, you should be able to use it as a template to cut out the other three skins.  The top skins are supposed to be the same as the bottom skins except that they're 0.2 inches longer.  But...DON'T DO THIS!  I found out later that the fuselage curvatures on the top skins were NOT the same as the bottom skins.  Due to the geometry of the scored cuts in the leading edges, the top inner strake skins pull away from the fuselage side, leaving a substantial gap that must be filled.   (Reference:  The Big Whoops!)  My suggestion for the top skins is to do what the plans say -- fit the inboard and outboard skins separately, then glue them together.


Process (for bottom skins):

(Step #5 does NOT work for top skins!)

  1. I drew a line on my jig table to show where the R33 rib and R57 ribs go.

  2. I taped down one yardstick on the R33 line with the end of the yardstick at the center section spar forward face.  I taped down another yardstick parallel to the first, but against the fuselage.

  3. With my tape measure, I measured the distance from the R33 line to the fuselage side at three-inch intervals.  I recorded the measurements all the way along from the main spar to the FS60 location.

  4. Next, I transcribed the marks onto the inner skin foam.  I measured each mark plus one inch to give myself room for error and tweaking.  (If you followed my suggestion and used 8x4 sheets, then do cut all dimensions about 2 inches oversize!)

  5. I cut out the inner skin foam and placed it against the fuselage side.  I held the wide tip of a marksalot on the foam, but with the side of the marksalot's tip against the fuselage side.  I then marked along the length of the foam.  What you end up with is a mark on the foam.  Some areas had a wider black mark (indicating where the side contour matched the fuselage perfectly) and some areas had thinner marks (indicating a gap between the foam and fuselage).  I removed the foam from the fuselage side and gently sanded the foam until all marksalot lines JUST disappeared.  Doing this left me with skins that matched the fuselage perfectly.  You can achieve the same results with a set of dividers.

  6. I  marked the R33 line on the foam and trimmed off the excess.  I also transferred the R57 rib line to the bottom foam, measured the distance of the bottom airfoil of each rib and located these points on the rib lines.  I connected the two points with a straight edge and made the leading edge and end cuts.