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Boy, lot of stuff today on combining carbon with e-glass with wood longerons
etc.  My punchline:  build the plane (structurally) exactly as the manual
describes.  If you don't trust the factory's research into these issues before
they offered it up as a kit and find this disconcerting, consider selling the
project.  Here's some supporting thinking:

Ed Armstrong is concerned about the mechanics of load carrying in the longeron
system.  Does use of carbon fiber make it worse than e-glass.  Answer:  we don't
know.  

Jeff Chipetine does a great job laying out what could go wrong and finishes with
what I think is the only way to settle the issue which is testing.  Scott
Dahlgren goes into a bit more detail with numbers leading us to the conclusion
that picking the right numbers for bulk modulus in a given direction is tricky
based on biases and builder technique.  He also lays out load transfer
principles that seem consistant with what I learned 15 years ago and reminds us
that foam core has less strength than spruce (so why not worry about the foam
core?).  Guy Buchanon lays out some data and states that in a pure tug, spruce
will fail first followed by e-glass, then Carbon.

Everybody is essentially right but whether or not there is a problem depends on
how much load is being applied, how much of each material is available to take
the load, and what the effective properties (modulus, strengths) of those
materials are in the loading directions.  Nothing will break if no load is
applied.  Anything will break if too much load appears.  Where are we between
these two?  This structure is further complicated by the fuselage section below
the longeron.  Using carbon instead of e-glass in the structure is probably fine
because someone's numbers suggested the wood isn't carrying much load in either
situation.  No hand calculations are going to be accurate enough to base builder
modification decisions on.  Good enough results might come from physical tests
or with a finite element analysis.  Talk to the factory to see if they might
share the thinking that went into the design.  They may  have a "Stress Book"
that looks at critical areas or it may be as informal as years of fixing what
broke (or else they've been lucky).

The arguments against just beefing it up with carbon are already out there.
I've come to feel that we should use only two materials, one to carry load
(carbon for some of us, e-glass for others) and the second to build section
(foam, honeycomb, doesn't much matter).   As was already pointed out, combining
materials can cause unforeseen troubles.  Of course, poor structural design with
simple materials can also cause unforeseen trouble too.  

Unless we are competent to re-engineer the plane, we are at the mercy of the
factory's structural design.  For the rest of us, altering load bearing sections
will at worst hasten failure and at best add weight.   Fortunately for us today,
this plane has a relatively long history of success.  While I don't know how the
engineers designed the plane, it has been proven.  So I'm comfortable with my
ignorance and my risk tolerance is probably average.  If any of us are not
comfortable, then try to do what it takes to get comfortable without creating
new problems (beefing up incorrectly).  And if that doesn't work, sell the
project.

So why isn't the carbon fiber MKII tail on the e-glass airplane a problem as Tom
Giddings asks about?  Because it is a completely different structural system
from we've all talked about above.  The interaction with the vertical stab. is
pretty localized and not likely sensitive to material differences with the
e-glass partly because the whole bond is so stiff.  There are other potential
issues that have been debated here such as flutter but they have nothing to do
with the material differences at the bond area.  If someone doesn't understand
why the carbon tail issues are completely different from reinforcing e-glass
with carbon, then don't stray from the directions.  

Ed de Chazal
Rochester Michigan