Area Moment of Inertia for Wood/Ply Spar

[Noor]

Hey Guys,

I am in the process of doing a stress analysis on a wood wing spar with a composite construction of plywood web with four spruce spar caps. I'm finding out that the analysing structures built from wood is not as "trivial" as those made from metal. Perhaps some of you composite materials guys can provide me with a sanity check on my problem.

First off, I obtained the material properties from ANC-18 for both the spruce and ply. When I want to calculate the moment of inertia of the spar section, I run into the problem of scaling the width of the plywood by the ratio of the moduli of elasticity of both materials. Not only does the plywood have a different modulus of elasticity parallel to, and perpendicular to face grain, the wing design calls for the face grain of the web to be at 45°.

What I did so far is just to assume that the plywood web is actually spruce (without modifying the width); however, my gut feeling tells me that it's a poor assumption and it's giving me a moment of inertia less than I should have.

If any of you have experience with wood structures I would appreciate any tips and enlightment you have to offer.

Thanks very much!

 

[robonave]

Hallo Noor,
if you have ANC-18 (which is a must for wood aircraft structures) I suggest that you read pages 36 and following for the stress analysis of wooden spar of composite structure.
I have a nomogram too from a different source (its italian) for the design of wooden box spar structure, let me know if you want it.
Last suggestion based on my personale experience,if the plywood thickness is not important let not consider its contribution to the bending resistence, and consider its strenght only for shear.
robonave

 

[Noor]

Thanks for the tips.

The critically loaded section of spar is actually contsructed with additional spruce filler blocks between the upper and lower spar caps as well as 2mm ply doublers (web is 4mm ply) on the fron and rear face of the spar. If I assume the spar is contsructed entirely of spruce (i.e. no correction to dimentions of the plywood components), then the ply components do not contribute significantly to the bending strength. However, I don't know how much they will actually contribute if I make a correction for the plywood (mainly because I don't know how to correct!).

This is an issue for me in this analysis because I have determined that the spruce components alone will not carry the require loads, and I want to be certain of this before I recommend a redesign!

I've talked to some fellow engineers and nobody seems to have experience with wood, so any little tips you toss me are very welcome.

Many thanks,

Noor

 

[robonave]

Ok Noor,

what I suggest You was to go through the design process using
figure 2-8 of ANC-18, this will give You a secure and acceptable method to validate the strenght of your spar.
Personally I suggest not to rely too much on the contribution of plywood ( I normally do not take it into account but it depends upon the rlative dimensions of caps and webs) anyhow if you want to You must know the number of plies ( on 2 mm birch it is normally for grade1 plywood 5) than calculate the mean modulus as indicated in chapter 2.52 pag. 42 of ANC-19 than use the normal method of reduction of plywood areas as you mentioned. I would even point your attention to the fact that the neutral axis is not calculated
as usual given the different values of modulus of elasticity and rupture for tension and compression, so utilize the figure 2-8 for bending strenght
after having defined the working areas as above, fig. 2-8 take into account the position of neutral axis

Let me know if You want any more help

robonave

 

[Noor]

Thanks robonave!

You've obviously done this before, and your guidance is very valuable to me.

I will take some time to digest all the information and see what I can do. If I have more questions for you, I will post again soon.

Thanks very much!

 

[aviat]

Here is a wood handbook that you may be able to use:

http://www.fpl.fs.fed.us/documnts/FPLGTR/fplgtr113/fplgtr113.htm

 

[joekm]

Based on your description of the spar, I'm wondering if you should not look at this as a diagonal semi-tension field design (i.e. a Wagner Beam). If you have access to Bruhn, "Analysis and Design of Flight Vehicle Structures", check out chapter C11.

Hope this helps...

Joe

 

[Noor]

I have access to a copy of Bruhn at work, so I will take a look at what you suggest... first I've heard of a Wagner Beam. Thanks for the lead Joe.