Spar Reinforcement


While I was in my door hinge quandary, and basically not working on the plane much, I decided it was time to forget the doors for a while and work on something else.  Why not take that big ol' spar off the wall where it's been gathering dust for 2 years and work on it?  Conveniently, the Builder's Manual chapter that follows the door installation is Spar Reinforcement.  I had to get some help removing the spar from the wall, but luckily The Herminator was around to help me out with that.

Why reinforce the spar in the first place?  Good question.   The spar is a molded part, and is basically complete.  So why didn't the factory just build it to the required strength?  Would have been nicer for us builders that way.  All I can figure is that since all the reinforcement plies go on the outboard ends of the spar, the intent is to strengthen the area for landing gear attachment.  If I recall correctly, Glassic Composites originally intended to sell both a retractable and a fixed gear model, but developed the retract first.  I assume if they had actually produced a fixed gear kit, the spar would not need reinforcement as the gear would be mounted closer to the aircraft center line.  Just a guess.

Be that as it may, here's what I did.

 

The first order of business was to fabricate a fiberglass crush plate.  This was no big challenge, just seven plies of heavy E-glass layed up on a flat surface and cured in the poor man's autoclave.
The next step was to cut, grind, radius, sand, and otherwise shape the crush plate until it fit nicely on the forward face (inside) of the box spar.  The spar is oriented with the forward face down in this photograph.
Once I was satisfied with the fit, the crush plate was bonded in place and sanded flush to the end of the spar.
Now for the fun part... all of these plies are required to reinforce one end of the spar.  And they must all be co-cured, meaning this is one giant layup.  12 plies of glass were wetted out on the bench--one at a time--according to the layup schedule in the manual...
 ... then the whole mess was rolled up and inserted into the end of the spar (after coating the mating surface with resin, of course).  Working air bubbles and imperfections out of the layup was important, as with all layups, but was particularly difficult working arms-length inside a close box spar.
Finally 4 more plies were added to strengthen the area where an access hole will be cut out, then the whole layup was capped off with a single BID ply and cured.  Looks pretty good, eh?

Now, repeat the whole process for the other side.

The next step was to create 4 large slots in the lower side of the spar in order to gain access to the wing attach bolts.  This was done by cutting 2 intersecting holes with a 2" hole saw, then grinding out the remaining material to achieve a "slot."  
My hand-held drill motor didn't have enough torque to cut through the spar, so I ended up using a drill press.  This process took a long time, as the hole saw got extremely hot after just a few seconds of cutting and I didn't want the heat to cause a delamination of this critical component.  So I would cut for about 3 seconds then withdraw the hole saw and cool it with water and/or compressed air.
Then the edges were brush-coated with resin and sanded smooth. 

The 2 inboard slots also required edge potting, as there was exposed foam core in that area.

Spar complete and ready for installation.  Of course I'm not ready to install it yet, so back it goes to the garage wall.

 

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