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

In summary, wood aircrafts are generally more subject to aeroelastic effects compared to aluminium ones. This is because wood is anisotropic and does not obey the "linear elastic" laws you learn in materials science.
  • #1
Murmur79
10
0
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?
 

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  • #2
Notice the thickness of the wood compared to the aluminum?
 
  • #3
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.
 
  • #4
pantaz said:
Notice the thickness of the wood compared to the aluminum?

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

Aero51 said:
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/ [Broken]
 
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  • #5
Thank you all for your answers.
AlephZero said:
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)?
 
  • #6
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.
 
  • #7
Murmur79 said:
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.
 
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  • #8
AlephZero said:
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.
 
  • #9
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.
 
  • #10
Aero51 said:
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.
 
  • #11
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.
 

1. How does the stiffness of wood compare to that of aluminium?

In general, aluminium is stiffer than wood. This is due to the atomic structure and bonding of aluminium, which allows it to resist deformation under stress more effectively than wood.

2. Why is aluminium often used in construction instead of wood if it is stiffer?

While aluminium may be stiffer than wood, it also has other properties that make it a more suitable material for construction. These include its strength, durability, and ability to withstand extreme temperatures and weather conditions.

3. Can wood be stiffened to match the stiffness of aluminium?

It is possible to stiffen wood through certain treatments such as laminating, but it is difficult to achieve the same level of stiffness as aluminium. Wood also has inherent variations in stiffness due to its natural growth patterns and composition, making it challenging to consistently match the stiffness of aluminium.

4. How does weight factor into the stiffness of wood vs aluminium?

In general, aluminium is heavier than wood, which can affect its stiffness. However, the weight of a material does not directly determine its stiffness. Other factors such as the material's cross-sectional area and the distribution of its volume also play a role in determining stiffness.

5. Are there any advantages to using wood over aluminium in terms of stiffness?

While aluminium may have a higher stiffness compared to wood, wood has some advantages in certain applications. Its lower weight and flexibility make it useful in structures that require some degree of give, such as suspension bridges. Wood also has a more natural appearance and can be easier to work with in some cases.

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