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Scott,
I'm not sure of your point.
In the cases you describe, the graphite is designed to cary the whole load
and the glass is only the supporting structure. That's good engineering.
The L2 spar and legacy skin are perfect examples. The carbon sections are
thick and strong enough to carry the whole load. (Webs of beams,
ribs and bulkheads only support the load bearing parts).
If you were to make a spar cap by substituting glass for half the graphite,
you would have precisely the two rope scenario and 25% less ultimate
strength.
Mark, Speed Demon Ravinski
Sent: Wednesday, March 31, 2004 12:59
PM
Subject: [LML] Re: mixing carbon and
e-glass
In a message dated 3/31/2004 7:22:53 AM Central Standard Time, mjrav@comcast.net writes: 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.
Scott
Krueger AKA Grayhawk
Sky2high@aol.com
II-P N92EX IO320 Aurora, IL
(KARR)
LML, where ideas collide and you decide!
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