Wood vs aluminium stiffness. What doesn't add up?

[Murmur79]

AFAIK, the wood used for aircraft structures should have a specific stiffness, that is specific Young's modulus and bending strength, somewhat higher than aluminium (see attached image).

If that is the case, why wood aircrafts are generally more subject to aeroelastic effects compared to aluminium ones?

 

[pantaz]

Notice the thickness of the wood compared to the aluminum?

 

[Aero51]

Also, wood is anisotropic and does not obey the "linear elastic" laws you learn in materials science. How it behaves specifically, I don't know because I am more of a fluids guy.

 

[AlephZero]

Notice the thickness of the wood compared to the aluminum?

That's exactly the point of what "speciifc stiffness" means.

Also, wood is anisotropic and does not obey the "linear elastic" laws you learn in materials science.

"Anisotropic" doesn't mean "nonlinear". Wood behaves just as linearly as many other structural materials.

To answer the OP's question, planes are not designed to carry the structural loads through flat sheets of material that bend, because that is a very inefficient way to use material. The more relevant comparison is with the honeycomb. Your picture doesn't say what material it is made from (from the color, the core could be nomex) but all-metal honeycomb structures are easy to make.

Actually, all-wood honeycomb structures could be even more efficient than all-metal. Some speciies of wasps already build their nests that way (they chew up the wood to make something simiilar to paper), but it would be hard work training wasps to build aircraft.
http://www.crosspestcontrol.co.uk/blog/get-inside-a-wasp-nest-2/

 

[Murmur79]

Thank you all for your answers.

To answer the OP's question, planes are not designed to carry the structural loads through flat sheets of material that bend, because that is a very inefficient way to use material. The more relevant comparison is with the honeycomb. Your picture doesn't say what material it is made from (from the color, the core could be nomex) but all-metal honeycomb structures are easy to make.

Actually, all-wood honeycomb structures could be even more efficient than all-metal.

Of course the rigidity depends on the type of structures used. But, structure being the same, let's take a classic semi-monocoque design, with frames, stringers and stressed skin: since specific stiffness of wood is even better than aluminium, a wooden aircraft could be in theory made as rigid as an aluminium one (total weight being the same)?

 

[Aero51]

Anisotropic doesn't mean "nonlinear". Wood behaves just as linearly as many other structural materials.

Yes, I know that. As far I remember the only materials (with few exceptions) which have linear elastic behavior are metals.

 

[AlephZero]

But, structure being the same, let's take a classic semi-monocoque design, with frames, stringers and stressed skin: since specific stiffness of wood is even better than aluminium, a wooden aircraft could be in theory made as rigid as an aluminium one (total weight being the same)?

If the structure is loaded mostly in tension, the relevant parameter for specific stiffness = $E/\rho$.

For a beam in bending, it is $E/\rho^2$ or $E/\rho^3$, depending how you choose to scale the size of the beam.

For tension, metals beat wood by a small margin. For beam bending, wood beats metals by a big margin.
http://en.wikipedia.org/wiki/Specific_modulus#Approximate_specific_stiffness_for_various_materials

Another issue is that metals are homogeneous, but wood is not (for eaxmple it has a grain) - which is not the same issue as wood being anisotropic! Therefore the margin of safety for a thin metal structure can be less than for a thin wooden structure, and that overturns wood's small specific stiffness adbantage over metal.

 

[pantaz]

That's exactly the point of what "speciifc stiffness" means. ...

I wasn't familiar with "specific stiffness" as an engineering term. Now that I've looked into it, the photograph and OP makes much more sense.

Thanks.

 

[Aero51]

Another issue is that metals are homogeneous, but wood is not (for eaxmple it has a grain) - which is not the same issue as wood being anisotropic!

I do not think you are correct. The first paragraph on the 10th page of this paper
[Analysis of Elastic Anisotropy of Wood Material for Engineering Applications] reads:

Wood is probably the most commonly recognized anisotropic composite material on earth.
As wood possesses a complex fiber-composite structure, it varies in its most properties with
the directions, called anisotropy. It is the best described mechanically as an orthotropic
material and given the orthogonal symmetry of wood, the orthorhombic (a kind of elastic
anisotropy) elasticity concepts developed to describe crystal characteristics.

 

[AlephZero]

I do not think you are correct.

What don't you think is correct? I didn't say that wood was isotropic (of course it is not). And your quote says nothing about whether or not wood is inhomogeneous.

You didn't give a reference for your quote, but here it is anyway:
https://docs.google.com/viewer?a=v&...XPIJXa&sig=AHIEtbSAd1LkkIXiksfRDvjU5PXYk6xkWQ

In fact I can't see what the paper as a whole is trying to say - but it makes the elementary mistake of calliing balsa a softwood. "hardwood" simply means the plant is an angiosperm, and "softwood" that it is a gymnosperm. The terms have nothing to do with the hardness and softeess of the wood. And in any case, "hardness" is not the same as "stiffness" - yet another schoolboy error in terminology.

 

[Aero51]

Well email the authors explaining their errors if you disagree. I am not a structures expert however that paper says otherwise. If you have a references to counter their claims please share.