[NSRCA-discussion] An education in balsa usage for pattern

[Gray E Fowler]

People who have said that you cannot build a pattern plane light enough 
with balsa are just wrong. The problem lies with the fact that most in 
this hobby are fairly ignorant of the properties of fiberglass, carbon 
fiber and kevlar. I have pasted a simple chart to help with this, that 
references everything back to balsa as the standard, that is make it "100" 
and everything else compares to this.



As you can see, fiberglass is not all that light, but it is much stronger, 
but not all that stiff.  Balsa is really stiff but actually very weak. 
T-300 is cheap carbon fiber.
Your plane, when stressed will ALWAYS fail in some sort of compression 
mode, another fact that eludes most hobby ist. Compression strength is 
also a property dominated by the matrix resin, but fiber interaction has a 
serious impact. Compression failures rarely break the fiber, instead they 
crush the resin holding the fiber.
This is why, in short, kevlar blows. Resin does not adhere well to kevlar 
and why it may not fail in tensile (the numbers so high and why everyone 
thinks it is so strong) it will fail in compression....just like balsa. 
Blasphemy right??? No Kevlar in Aeroslave kits, only core structure 
fiberglass (for light weight and stiffness) and two kinds of carbon fiber, 
T-300 and M-46J (has the modulus of steel or 3X that of T-300).

In summary, composites are better to fab a complexed curved fuselage with 
strength and weight advantages. If you use balsa and keep the shape simple 
since basla is all flat, great results can be achieved.

I am also convinced that the Communist Chinese and the French cannot 
pronounce "Kevlar". Buy American (or Australian!).


Gray Fowler
Principal Chemical Engineer
Composites Engineering
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