Joining carbon fibre tubes - dihedral !?
 

I'm building a lightwieght slowflyer - rudder / elevator, with a fair amount of dihedral and a very fragile undercambered wing.

To reinforce the wing without adding too much weight I got hold of some incredibly strong yet lightweight C/F tubes....3mm outside dia / 2mm inside diameter.

My question is - how to join the spars at the dihedral break (centre of the wing) I've thought about:

- Gluing a dowel inside and then gluing the dowel ends (no strength !?)
- Ditto with music wire bent to the angle and wrapped with something to fill the tubes
- Using bent music wire top and bottom and then binding with kevlar thread

The main problem of course is that I don't know what I am doing and have no experience with this material !

Would appreciate some help.

Thanks,

Nigel

RE: Joining carbon fibre tubes - dihedral !?
 

If it is a lightweight slowflyer, then it doesn't need to be incredibly strong. I would cut and bend some 2mm music wire to fit inside the tubes, and bend the dihedral angle into the wire. The wire should go into each tube about 2cm, so the wire should be ~4cm long. You can add some light fiberglass around the outside once you have made the joint, but it should not be required.

Good luck,
-David

RE: Joining carbon fibre tubes - dihedral !?
 

Think about using 1.5mm birch ply dihedral keepers - like the old style ff models :D:D:D use two for each join, one each side of the spars, and epoxy in place. Run the keepers 2 ribs either side of the join.

As David has said - it does not need to be incredibly strong. Just strong enough.

RE: Joining carbon fibre tubes - dihedral !?
 

If the carbon tubes have the fibers aligned straight (rather than spiral wound), then music wire inside the tube will tend to crack the end of the carbon tube. Once that starts, the carbon tubes split easily. The carbon is strong stuff, but like wood, it can be weak along the "grain". A short bit of bent aluminum tube that the carbon tubes slip into would be better, and probably weigh less than the music wire. I am not sure how strong the bent aluminum tube would be, but it would probably be "strong enough".

RE: Joining carbon fibre tubes - dihedral !?
 

Gentlemen,

Thank you for your help, much appreciated !

Nigel