Chapter 19: Hotwiring and Assembling the Wing Cores


Getting the wing cores cut is another of those magical milestones.  Hot-wiring is easy and fun, fun, fun.  It's very satisfying when the cores look like they've been professionally done.

Hotwiring the Cores

  1. Norm Muzzy was kind enough to offer the use of John Epplin’s steel hotwire templates. The templates are VERY NICE!  So, merci beaucoup to the both of you. 

  2. I used 3M Temporary Spray adhesive and temporarily glued the foam blocks together. It’s easier and less risky than using nails (e.g., snagging the hot-wire).  If you use the 3M spray, you must buy the correct type (temporary bond) and you must use it sparingly. DO NOT leave the blocks glued together for too long.  "Temporary" becomes almost "permanent” after 7 days in a hot hangar. DO NOT use the permanent bond spray or it will melt the foam! If in doubt, always test first on a spare piece of foam.

  3. I used drywall screws to hold the templates to the cores. Be sure to angle the screws inward to avoid snagging the hotwire.  

  4. A helpful hint:  Remember when we hotwired the canard cores?  Remember that we used two different hotwire templates, right?  We used the first one to cut the complete airfoil, then slapped on a second template to cut out the spar caps, right?  Well guess what....the plans only give up one set of templates for the wings.  So a neat trick is to position mixing sticks over the top and bottom spar cap cutouts on the hotwire templates.  Hotwire the complete airfoil portion first, remove the mixing sticks, then go back and cut out the spar caps individually.  If you don't do this, the lag in the hotwire will cause the vertical sides of the spar cap troughs to be wavy.   Here’s a picture of George and me in action.

  1. For the FC1 cores, it took me a long time to convince myself that you simply cannot place the BL31 template at the same water line as the BL67.5 template without overhanging the template above the foam. So I just made sure that each template was level with respect to Mother Earth. I didn't have any trouble setting the other templates to the same water lines.

  2. It’s a crying shame that we can’t get thicker blocks for FC1 to keep from having to add that piece of foam to complete the wing root.  The hot-wire cuts this piece at such a shallow angle that it melts the foam edges too much and causes it to discombobulate (NASA technical word meaning to deform and otherwise look really ugly...).  I later pour-foamed that piece to fill in holes and voids, then I recontoured that area back into shape.

  3. If I had it to do over again, I would use the hotwire to take the end off FC1 (page 3, 2nd paragraph).  I micro’d first, then none of my friends that I beg, borrow, or steal from had a bandsaw tall enough to make that cut.  So I wacked it off with a cross-cut saw. 

Microing the Cores

  1. Prior to separating the core pieces for microing, I placed tick marks at each end and along the length so I could re-align the pieces perfectly.  I used drywall screws to hold the pieces together for cure.  The tick marks are washed out in this picture due to amateurish photo technique!

  1. Aim for using the minimum amount of micro as possible when microing the cores.  If you put on too much micro, then it will ooze past the surface of the cores and create an unwanted micro ridge.  My method was to thinly spread a viscous slurry to within a half inch of the edges.  Once the cores were mated together, the slurry would just almost squeeze up to the surface. For further insurance that it didn’t, I took a squeegee and ran its edge into the seam between the cores to remove about an eighth inch of the micro. 

  2. The smaller, individual core pieces fit together really well and required only the minimal amounts of micro to bond them together.  However, I was really surprised when it came time to micro the larger cores together.  My butt angles were not perfect and the large cores did not match together perfectly.  (I'm beginning to think this is a generic problem as many builders report the same problem...)  So I sanded the ends to get the best possible end-to-end fit before to minimize the amount of micro needed.  In one case the gap was wide enough that I added a thin wedge of blue foam to fill the gap.   I also lopped off an inch from each end of those aileron and torque tube cutouts that get left in the cores.  I did this so that they didn’t get micro’d to the wing cores.  I applied the micro very sparingly near these holes.

  3. Once the micro cured, I used pour foam to fill any remaining gaps between the cores.  I first lay down duct tape, then pour the goop into the cracks.  Once the pour foam cures, it’s easy to cut the excess off by carefully holding a hacksaw blade flat against the duct tape.  The tape also keeps you from sanding too deep, especially into the surrounding blue foam.  The tape also keeps the goop from adhering where you don't want it to adhere.

  1. Before moving on, now's the time to inspect the cores for micro ridges.  If you do end up with micro ridges, dremel them away with a cut-off wheel.  You must get rid of every one of them, else you’ll get a bump once you’ve glassed the surface and you’ll risk sanding through the glass during fill-n-sanding.  Either that or you’ll use a lot of micro to hide the bump.  

Jigging the Cores 

  1. Do yourself a very big favor…take the time to draw in the 17.4 water lines on the BL31 root core, BL67.5 leading edge core, and the BL169 tip core.  These lines come in very handy for jigging the wings completely level with the intended amount of twist prior to spar cap and skin layups.

  2. Do yourself another big favor….don’t break off the flashing on the leading edges of the cores just yet.  Use it as a reference for drawing in a line on the leading edge.  It really helps to have this line there for sighting the leading edge for straightness later in the jigging processes.

  3. Forget about “sliding” the cores into the jigs because the jigs fit very tightly to the cores.  It worked better to disassemble the jigs first, put the cores in place, then assemble the jigs around the cores.

  4. Once again, prior to glassing, inspect the wing foam for micro bumps and ridges.  Time spent here delicately sanding pays big dividends later.


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