Something occurred to me while I was working on the rotor stops for my own "traditional" helicopter: what if you 3D printed a flat plate with the rotor incorporated into it, then glued the root edge of the rotor to the bottom of this piece? Another plastic piece would also be attached to the hub to protect IT from the impact of the rotor stop. Not only would you eliminate the rotor and hub splitting problem, if you incorporated predrilled holes in the plastic pieces you could continue to use your floss and duct tape hinging method.
I suppose an alternative to 3D printing would be to fabricate these pieces yourself using lightweight sheet plastic or fiberglass sheet. Thin ply would probably also work. Because of your rotor size (especially with your larger helirocs) the impact of them opening is quite significant so the idea is to protect the balsa from that impact by letting the plastic or fiberglass take the punishment.
