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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