If the graphite rope were smaller than the glass rope, like using a smaller
percentage in the layup, then the failure of the graphite would occur much
earlier in the loading,  (like under normal flight conditions.)

This is a simple analogy and nothing in the aircraft is so simple.

Mark,

 

Think on this.  With a composite glass/carbon construction, the more flexible material (glass) does provide some structural support along with the stiffer one(carbon).  In both compression and tension the glass has no problem because it is not stressed to the same amount as the carbon.  The bond (shear) between the two is less stressed than if both materials were of the same stiffness because the distance (arc) over which the compression or tension has a different radius.  It's no wonder those aluminum spar caps have so many rivets.  

 

Once again, consider our 320/360 main spar.  It was designed to have a certain strength and stiffness at the various BL stations - more demand inboard than outboard.  Built entirely of glass it may have been too heavy and not stiff enough.  Built entirely of carbon, it may have been too stiff. The distance of separation (web) of the caps also contributes to the stiffness.  So, we have generally a "[" spar where the caps (top and bottom) are encapsulated by molding it within the web glass that is continuous from the aft edge of the bottom cap to the aft edge of the top cap.  Where the wing spars are bolted to the main spar, both spars have an aluminum plate embedded in the web glass, a bushing bonded thru each spar to tie everything together and a few countersunk bolts taking some of the load between the plate and the web glass in addition to the giant bolts that keep the wing attached.  Futhermore, certain parts of the wing and main spar are double webbed  i.e. "[]", and this leads to further stiffness, strength and resistance to twist.

 

Consider the Legacy.  Monocoque construction contains a skin solely of carbon - sort of.  Just like spars, the "web" between the outer and inner skin is either paper (honeycomb) or foam. Orthogonally organized paper (or foam) is certainly no match for the carbon, but this is what holds the Legacy together.  The total thickness of such skins is much thinner than that if it were made of glass because the greater stiffness of the carbon allows for a smaller web.  Of even greater interest is that the Legacey skins are all held in place by glass ribs and bulkheads.

 

Also, our wee glass Lancairs use foam in the top stub wing because the glass/nomex honeycomb combination is not very resistant to puncture wounds, those easily caused by a high-heeled wench or a dropped wrench.  I don't remember the core composition of the Legacy stub wing skin.