Combining Carbon / Foka 4

[martynk]

I am working on a design for a 1/4 scale Foka 4 (span 3.8m). Planning on a 3 piece wing with a predominately spruce built up structure. The inner panel - encompassing the upper fuselage will be about 500mm and have a ply boxed joiner tube assembly. I really wanted  to use 10mm carbon joiners but there just isnt quite enough space when you get out to about 300mm for a tube.

As I am going to have the ribs laser cut, it dawned on me that I could use 10mm carbon tube for the inner section, then tapering through a series of telescoping carbon tubes out to 7mm at the tip, i.e. 10 > 8 > 7 (and possibly 6mm). Wing will be fully sheeted with 1.5mm balsa then glassed.

My question is - does a 8mm Carbon rod sat within a 10mm Carbon tube (1mm wall thickness) provide the same rigidity/strength as a single 10mm carbon rod? Model will be used for slope soaring and aerotowing so I am not expecting huge loads but I do want to avoid anything that resembles an uncharacteristic droop.

Your thoughts please Maestros