We use a standard carbon wing joiner for most of our projects. It’s slightly conical and has a 6 degree V-shape/Dihedral. See the picture below for the sizes of the joiner resulting from the mould. The mould for the wing joiner is close to 20 years old. It’s a pretty simple setup, made of wood and aluminium. It has been used countless times and is still going strong. For the Elfe S3 we reduced the dihedral of the mould to 1 degree (the mould allows for this).
Here’s a description of how we build wing joiners out of this mould. All references to materials are to the R&G Webshop.
Setup:
The setup consists of 1) a roll of CF-Roving Tenax-E HTS40 F13 24K 1600tex (Art. 205.0024), mounted on an easy-rolling dispenser; 2) a 70ml syringe with a short piece of tube (I use motorcycle fuel lines) mounted in a self-made holder (to drench the roving with resin)(the syringe is held by a standard broom holder); 3) a small turntable to help cut the carbon roving to the right size; and 4) the mould. See the pictures below for more detail. The syringe and turntable are screwed to my workbench to make sure they stay in place.
Waxing the mould:
We apply three coats of liquid wax. Allow each coat to dry and then lightly polish with a piece of soft cloth. Apply some bicycle grease to the screws to close the mould (to avoid them getting stuck due to a bit of wayward resin).
Prepare the core of the joiner:
Previously we used a rohacell core for our wing joiners. For more recent builds we’ve found that it’s much easier (and cheaper) to use a balsa wood core. We use either 4mm or 6mm balsa. This is put into a 3K carbon sleeve (35mmØ 3K, Art. 200.4008).
Epoxy Resin:
I use two small joghurt cups to prepare the resin. Amounts needed:
- 4mm Balsa Core: Approx 120gr Epoxy (90gr Resin + 31.5gr Harder)
- 6mm Balsa Core: Approx 110gr Epoxy (80gr Resin + 28gr Harder)
I use black colourant to get nice and black wing joiners.
Set aside and thicken some of the resin in the 2nd cup (Aerosil) to apply to the mould and cover of the mould.
Building the joiner:
Apply epoxy resin to the balsa core (in the sleeve) using a small brush.
Apply thickened epoxy resin to the mould (this is to ensure that the surface of the wing joiner is nice and even, without air bubbles)
Then roll carbon rovings onto the turntable. I roll them in sets of 8. Use scissors to cut them into separate rovings at the right length.
Evenly put the rovings into the mould, starting at the edges.
Regularly spread out the rovings in the mould using an old credit card or a piece of wood .
The amount of rovings to use is as follows:
Wing Joiner with 4mm Balsa Core:
50 Rovings above and + 50 rovings below the core. In addition, add on each side of the core 4x13mm und 3x 17mm rovings in the center
Wing Joiner with 6mm Balsa Core:
40 Rovings above and + 40 rovings below the core. In addition, add on each side of the core 4x13mm und 3x 17mm rovings in the center
Note that you may need up to three rovings more on each side, depending on how much resin the rovings absorb (this can be adjusted by squeezing the tube coming out of the syringe – see picture)
Once all rovings and the core are in the mould, close the mould and let the resin cure for a few days.
