*Originally posted by Kevin Courter *
**I've also read that hi may in fact make the blade stiffer, like the waves in corrugated cardboard.**

Stiffer *per unit weight*. If you look at the bending moment equations in a mechanics textbook, you'll find that if you take any cross section of material in any shape and remove material from it without changing any of the exterior dimensions, it will be less rigid in all axis.

If you keep the same amount of material and change the cross section, all bets are off.

As an example take this overall shape

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This will have a certain stiffness in each axis of bending.

Now take a chunk of material away from it

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*** ************

There is no way for this cross section to be more rigid than the old one. It's simply got less material to resist deformation.

There are 100 asterisks in the first box; there are 90 in the second shape. So I took away 10% of the material. But I bet I didn't take away 10% of the rigidity. So per unit mass, the second design is superior, but on an absolute basis, it is not.

If you can avoid buckling issues, the largest contributors to overall blade stiffness are material properties (elastic modulus) and overall cross section dimensions, basically the width and height of the cross section in the longest dimension. This is why I-beams are so prevalent in contruction--they're an easy to manufacture shape that is very, very stiff per unit mass.

dhc

_________________

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Made weak by time and fate, but strong in will

To strive, to seek, to find, and not to yield.

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