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

1. Nov 28, 2012

### 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?

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2. Nov 29, 2012

### pantaz

Notice the thickness of the wood compared to the aluminum?

3. Nov 29, 2012

### 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.

4. Nov 29, 2012

### AlephZero

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

"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]

Last edited by a moderator: May 6, 2017
5. Nov 29, 2012

### Murmur79

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. Nov 29, 2012

### Aero51

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

7. Nov 29, 2012

### AlephZero

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.

Last edited: Nov 29, 2012
8. Nov 29, 2012

### pantaz

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. Nov 29, 2012

### Aero51

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:

10. Dec 1, 2012

### AlephZero

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: