First off, thanks to everyone posting their tips and processes on this forum. I’m learning a lot.
Here’s one way to build foam cored wings that works for me. I’ve only built about 5 of these (2 backs and 3 fronts), so definitely no expert. This is what works for me with my limited skills. What I tried to achieve is a relatively quick process with minimal waste to produce strong wings with a fairly accurate profile.
It’s essentially a high density foam core (Corecell or crossed-linked PVC) that’s shaped by hand or CNC, one-shot vacuum bagged with epoxy carbon skins and then finished with either a balloon mix or 4oz glass gloss coat.
The one disadvantage of this method, that I believe I’ve managed to overcome, is the flush-out (is this the correct term?) on the leading edge where the top and bottom skins join. It messes up your LE profile and you end up having to reshape it with lots of filler and sanding afterwards. Can’t sand the joint down too much without weakening the LE either.
Advantages are as mentioned:
Relatively quick and only one trip to the vacuum bag – less costly consumables.
No moulds required – at the moment, moulds are a waste of time and material for my one-off DIY wings. I don’t need the fancy anhedral curves or mirror carbon finish right now. Maybe later.
Light strong wings. Except or the finishing/sanding layer, the majority of the layup is bagged so less resin and straighter fibre matrix.
Process:
Core shaped by hand or CNC. I’ve done two by hand, a plywood and Corecell core, and even though I achieve a surprisingly accurate profile, I was concerned about the effect of flat spots in hand shaping. I managed to find a guy that charges around $50 to CNC a core for me – usually from off-cut foams he has lying around. It doesn’t save me much time, because now I have to make the effort in Fusion360. The profile is imported (using RG8 - http://airfoiltools.com/airfoil/details?airfoil=rg8-il) and offset for the thickness of the skin.
I prefer the higher density Corecell or cross-linked PVC foams. Had a bad experience with one wing when using lower density Corecell that crushed under the pressure of the fuselage connection and caused the wing to break. Don’t know how you guys build Styrofoam wings that don’t break…
I prep the centre section of the core where it will connect to the fuselage as follows. Drill/cut 30mm holes where the bolts will go. Close the bottom of the holes with masking tape, then fill with a few layers of carbon. This makes a strong layer to countersink for the bolt heads at the end (6). Fill the rest with balloon & microfiber resin mix. I let this lot dry, sand the top flat and then drill the hole for one (centre line) bolt. I put a little self-tapper screw in that hole - this makes it easy to find the hole after the whole wing’s been laminated. (I use this one whole to position and drill the other holes at the end.)
The top and bottom laminate on the front wings are both 2 x 300gsm UD plus 2 x 300gsm BiAx plus 4oz e-glass as the outer layer. For back wings 1 layer of each is probably enough?
The trick with the LE flush is as follows: sand a 3-4mm flat strip into the top part of the core’s leading edge. When bagging the laminates onto the core and while the vacuum is increasing, pull and pinch the bag to ensure that the top and bottom laminates join at the lower (sharper) edge of this strip. Then once I have full vacuum, bend the flush-out upwards. This creates a channel as shown in the pic (3). I later fill this channel with 1 or 2 strands from the UD carbon to make it even stronger (5). This is optional if you’re using balloon mix as a finish layer as it will fill the channel anyway, but preferred when doing a gloss coat finish.
After bagging I use a metal saw and 40grit to trim back the flush-out. The leading edge I leave about 5mm (3) and the TE about 10mm (4). I also rough sand the top and bottom to get a better key to the finishing layer.
For the finishing I’ve tried two different methods. Both work quite well. One is the normal thick balloon mix that is then sanded back, the other is a wet layup of 4 oz with a gloss coat. The balloon mix is a bit softer and probably best for the top of the wing that should not get scratched too much. It is lighter and better for smoothing out the surface of the laminate, but requires more sanding than the glass & hotcoat layer. Both look good though.
For the gloss-coat option, I mask the other side first (7) and try to wrap the glass around the leading edge as far a possible (8). For even stronger LE, wrapping glass on both sides is probably best.