Chapter 19: Shear Webs and Spar Caps


Lessons Learned

1.    I spent a lot of time ensuring that the wing was jigged right prior to each glassing session.   The jigs are not meant to be exact fits to the wing cores.  And with Virginia’s constantly changing temperatures this time of year (September - October), it’s amazing how much shifting goes on in the worktable, jigs, and in the cores themselves.  So it's up to you to use shims here and there to keep the cores straight.  For the shear web (with the jigs vertical), it was a simple matter of stringing the 17.4 water marks straight the length of the shear web.  For the spar caps (with the jigs horizontal), I used my smart level and held it up against the 17.4 water lines, then used shims until these were level. 

2.      Prior to glassing the shear web, I installed straight edges on each side of the cores to hold them bone straight.  (Where does the expression “bone straight” come from anyway?  As far as I know, there are no straight bones in the human body….)  My FC2 and FC3 cores were slightly bowed in the middles. These straight edges are shelf rails for holding those slotted metal shelf brackets.  Home Depot is a wonderful place!

   

3.    After the shear web is done, the plans have you micro on the leading edge cores.  In my opinion, this is a VERY SCARY step.  Why?  Because it is extremely important that the 17.4 water lines on the leading edge cores are straight in line with the 17.4 water lines on the aft cores.  I thought the jigs would take care of this for me.  But lo' and behold, after getting the cores micro'd onto the shear web and putting the upper halves of the jigs in place, I stood off to the side and could see very clearly that my 17.4 reference lines did NOT match up.  In fact, the leading edge core at BL67.5 was actually down at a negative incidence angle.  So I stuck in a few shims until the reference line was vertical.  Core alignment is very easy to check at the tip --- simply re-install the hotwire template or line up the reference lines that you hopefully drew onto the ends of the cores.  This is not so easy to check on the BL67.5 core... .you must either stand off to the side and eyeball it with the root core's reference line, or you can do what I did and extend a straight edge over the jigs at the 17.4 water line and drop a plumb line.  Once you have the tip and BL67.5 in the right places, it's easy to sight the leading edge to shim everything straight.  

4.    I  used the exact number of spar cap tapes as called for in the plans.  No more, no less…and the top and bottom spar caps filled up perfectly.  I had to squeegee really hard though to squeeze the tapes down into the troughs.  I also employed a new trick I learned when I helped Clark Canedy to glass his canard.  After each tape was glassed in, I dragged a small straight edge over tapes to ensure the tapes laid perfectly flat into the trough and that they did not swell above the troughs.


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