Chapter 21: Installing the Top Skins

Fitting the Top Skins

  1. Here are pictures of the starboard strake top.  Using a tip from the archives, I scored the foam at 3/4-th inch intervals instead of 1 inch intervals to help the foam contour better to the ribs.



  1. WHOOPS!  What in the hell?  What's that gap doing there between the skin and the fuselage?  *#!@!*??  Well, that's a rather disappointing development.  After being so careful to cut and carve the top skin to match the fuselage, somehow the geometry of bending the foam over the ribs causes the foam to pull away from the fuselage side.  I guess this is why the plans have you fit the top inboard foam first, fit the top outside foam, THEN glue the two together. (I can hear Nat all the way from Arizona..."I told you so!")  The fix is to glue in a foam filler piece, carve the top again to match the fuselage (this time BENDING the foam into place over the ribs), then glassing 1-BID over the foam splice overlapped to the existing UND layups.  I'll say what John Slade always says...."no one will ever know". :-)  

Making the Top Jig ("Stacking Lumber")

The plans have you "stack lumber" on the strake tops to hold them flat and to conform them to the ribs, but I'm sure the plans imply you need STRAIGHT lumber. I don't care what Lowes or Home Depot call "Choice Lumber", but it's NOT straight.  (And I don't have the wood-working tools to make the lumber straight...)  So I went jig-crazy again.  I basically took some left-over fiberboard from the strake tables and cut out a large triangular piece to fit over the flat, triangular section of the strake top.  I made two cradles that fit over the R33 and R57 ribs, then added stringers to hold this humongous jig perfectly flat.  I drilled a gazillion holes in the jig and used short drywall screws to hold the top skins securely to the jigs.  No, the screws did NOT go through the strake skin.  The foam is only 0.35 inches thick, so you can only realistically get about 4 or 5 threads of the screws into the foam.  But it still does the job if you're very careful and you don't have to mess with bondo, or with repairing the gouges in the foam.




Making the Cap Strips

At this point I decided to make cap strips on the tops of the ribs and bulkheads.  (See previous web page for this process.)


Floxing the Top Skins in Place

After the cap strips were completed, I  inspected the fuel bays one last time -- (a) ensured all peel-ply, duct, and box tape was REMOVED; and (b) searched for any pin holes.  I then prep-sanded the tops of the cap strips and the inside strake skin.  After vacuuming extensively to remove all debris, I applied a heavy coat of epoxy to the inside strake skin , a light coat onto the cap strips, and one last coat to the fuel bays.  I waited for the epoxy to reach the slightly tacky state.  If you don't wait for tackiness, the epoxy will flow to a low point.  I applied wet flox to the tops of the cap strips and to the spar and fuselage as per plans.  As an added measure for leak control, I dabbed on additional drops of epoxy onto the flox just prior to installing the upper skins in place.  I placed bricks onto the jigs to weigh down the skins and ensure a good flox squeeze.  I scraped away excess flox from accessible areas.  

I saw no fun in attempting 2-BID taping while hanging upside down in the plane, so I postponed taping the tops inside the baggage areas until I flipped the plane upside down.  But I did go inside several times to scrape away oozing flox and ensure it flowed into a fillet that was contiguous and free of pin-holes.