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The core segments are joined together using 3M77 spray adhesive (old formulation is better as the new "may" melt foam if you use too much too close - See SALGlider group for reams on 3M77).

The core TE's are trimmed using a template.

The cores are then sanded to remove all the shine (which is the burn kerf from the cutting process). The sanding is done in the foam beds produced when the cores were cut. This is to support the core and stop dings and dents appearing from stuff on the work surface.

The cores have slots sanded in to accept 0.050" carbon rods to strengthen the wing at the servo cut outs. The slots and pegs are on the upper surface, the cut outs are made to the lower face. The rods stop the top surface bucking into the servo hole. These slots are marked out using a template and created with a Dremel cut-off wheel in a block (you can just see it in the pictures below)

In a recent (Wed Mar 3, 2004) e-mail Phil updated his lay-up for servo cut-out reinforcement.

My vac bagging methods tend to evolve with time and so there are some things that I do differently now from the way I did them when I posted the XP3 lay-up pictures. I no longer use the carbon rod servo bay reinforcements or the little carbon servo bay doubler on the bottom of the wing. I now use fibreglass doublers on the top and bottom of the wing over the servo bays instead. I use 1.5oz (50g) glass 2.5" (6cm) wide that extend from the hinge line forward to the LE of the wing. This was done because they are easier to do and look better than the old way. I don't know if they are as strong but the XP3 wing has proven to be way strong anyway. I also now put a strip of glass at the centre of the lay-up. This glass is about 1" (2.5cm) wide and runs from TE to LE at the centre of each Mylar. This makes the cut line cleaner when the wing is cut in half on the table saw. All fibreglass doublers are placed directly on the Mylar as the first fabrics that are added.

Here we see Phil adding the LE to the XP3 core.

Phil's note on "Attaching fabrics to the leading edge of foam cores with 3M77"

If this is not done correctly you can have the following problem. The epoxy acts as a slight solvent for the 3M77 so the leading edge tape falls off or comes loose as soon as it gets wet with epoxy. The solution is to get the leading edge stuck better. 

My general philosophy about 3M77 is as follows: For a permanent bond you should spray both parts, wait for the 3M77 to get tacky but not wet, then stick the parts together. For a temporary bond (sticking sandpaper to a sanding block for instance) spray just one of the two parts. 

Leading edges are in a third category. The fabric is porous so the 3M77 can dry after the parts are joined by evaporating through the fabric. Normally for a permanent bond you would spray both parts and stick them together after they are no longer wet to the touch, but with leading edges it would be too messy to try and spray the leading edge of the foam core. What you should do is spray the fabric with a lot of 3M77 and immediately place it on the foam core. 

You must get it in place before the 3M77 gets too dry because you want some of it to transfer to the foam core and penetrate the pores as if it were sprayed directly on the foam core. If the 3M77 gets too dry before it is pressed in place on the foam core then you will have the temporary bond situation and it will fall off as soon as the epoxy hits it.

I hang the fabric leading edge material in the back of a spray booth (empty cardboard box about six feet long, standing on end) and spray it three or four passes with the 3M77. I then immediately apply it to the wing. It is not too wet unless it "swims around" on the leading edge rather than stick in place. Even in that case you just have to wait a few seconds for it to dry enough to stay stuck in place. I do an entire left or right wing panel in one piece. You may want to do it in two or three pieces until you get more practice. You must get the entire piece stuck down before it gets too dry.