OK, I mentioned several elements for rotor design, and the section you have contains them all:

The forward section is completely filled, and that is traditionally called the spar in rotor design. It carries bending and torsion loads and some of the tension. The forward aero shape is also in the spar;

The aft section is light and is all aero;

The metal strap is either for tension or for attaching to the hub.

Now to do a reasonably efficient (weightwise) rotor in fiberglass, you start with a rotor foil, chord and span selected (which I think you already have). Then you would determine the loads at several stations along the rotor span. Loads are tension, lift, pitching moments and from that the total tension, torsion, shear and bending loads on the rotor blades at each station. The methods for doing this should be in text books that cover rotor design.

Next is education in composites, which can be weird. I am partial to Tsai and Hahn, but I have another at home that is good too. How is your Matrix Algebra? Everybody works in matrix algebra for structural computations in composites.

You would do a rough sizing for the skin and spar. I would probably make a stout I beam at the 1/4 chord point and forward built up over hotwired blue foam with a skin over the whole thing, just like the Rutan method. Remember, it carries tension from centrifugals, shear and bending from lift, any pitching moments, and internal loads. Then you compute the ABBD matrix for each station, input the loading (include your factor of safety), compute the section strain, then lamina strain, then check the failure criteria. Anywhere that failure is indicated, you have to beef up something and iterate your design until it all passes.

Basic engineering here.

The spar should be designed to carry the tension, bending, shear, and any torsion loads. It will be beefy. The caps will be UNI cloth, UNI tape or some other material where almost all of the fibers run the long way out the spar. The caps will carry bending loads and tension. The shear web will be fibers at plus or minus 45 degrees to carry shear.

The skin will carry aero loads to the spar and what torsion loads you have. These will be pretty thin. Two plies of BID is probably overkill. If your failure criteria say the skins are overloaded, you probably need to beef up the shear web or the caps.

The lift changes as you approach the tips and hub, and the active loads change a bunch as you travel in or out along the rotor, so you might need more spar and maybe more skin thickness in different places. As you approach the hub end, you will need to get the centrifugal and pitching laods form the spar into the hub, and that transition is one to sweat over. Go to the rotor design books for help on this and use BIG factors of safety for this joint.

If all of this sounds big, well it is. Monkey-see monkey-do might actually benefit you here. Are there any fiberglass rotors the size of what you are thiking about on the market? This is called benchmarking. You want to check out how the other guys build their skins, spars, and attach their hub fittings.

Good Luck, Billski