Thoughts on MDF & Torsion Box Design

[Khalid]

i didn't see anywhere planes made out of MDF..Usually balsa wood is used that is very light weight and expensive..

[High Seas]

Ger 21 - Et all
sorry for the late reply...

the_extreme
Right> Thanks.
I'm talking of shear failure in the WEBs -- not the edges. A common failure in foam cored applications - not unlike MDF in this case - is the actual failure of the material between the surfaces. The flat upper and lower surface are supported by the WEBS - but if the WEBS fail - due to sheer stress - the torsion box fails.

Now, for most light weight applications - may not require FEA to get to a solution - consider this:

MDF is a "foam-like" product when used as "core" be it a door - or a torsion box. (Mind, you doors are an interesting structure in themselves)

The reason planes are not made of MDF - very high weight and very poor strength properties.

Gir:
The comparison to aircraft comes form my experience as an enginner and aviator --- and now "boy-machinist." Weights, loads, stress, and strength required in a torsion bosx is similar to an aircraft (win) - but -- yes the router DOESN'T need to FLY! -- BUt weight usually equals CO$T. Damping is what is required -- yes - and there are better materials for that than MDF!

Kahlid :-
balsa is not as expensive as it sometimes seems - given its properties--but not always available worldwide -- Supply and Demand!.


So, I'd offer the following:
Stability - in humid conditions or changing humidity
Rigidity - stable and won't bend or sag (mind you small - but expect you want good/constant repeat-ability at different sites)
Durability - no glue failure due to moisture intrusion
Mechanical - ease of attaching fittings, screws, etc...

Now - MDF could be fine knowing given the considerations offered above -- but -- I'd opt for good quality ply.! {BTW I dumped a 24x24x8 (in) set up in MDF for all the above -- now using it to support my dremmel scroll saw in the garage!}


Jim

[randyf1965]

Does the analysis of the torsion box also include the inner ribs and skins on all 4 sides? In the pictures it appears to have an open front.

My first test of a torsion box had 10 ribs vertically and 4 ribs horizontally and was very flimsy until it was skinned on top and bottom.

[the__extreme]

Originally Posted by randyf1965 Does the analysis of the torsion box also include the inner ribs and skins on all 4 sides? In the pictures it appears to have an open front. My first test of a torsion box had 10 ribs vertically and 4 ribs horizontally and was very flimsy until it was skinned on top and bottom.

Hi Randy,

The model is basically a lengthwise cut-out of one section of the torsion box, supported at both ends the way most of us are building/have built our routers. The top and bottom are skinned, but the open sections you see are actually the sides, not the front. The sides were left open so that the internal structure could be seen, which should not affect the analysis because the load was applied vertically, at the center, above the lengthwise rib so that no twisting would occur. Although the loading would not be identical, you could imagine connecting 4 of these sections side-by-side to give you the 4 horizontal ribs that you used in your torsion box. The center-to-center spacing of the ribs in both directions is 6" and the ribs are 4" high, with 0.5" thick skins.

I agree that this structure would be very flimsy without the skins, and if it was to be twisted, even with the top and bottom skinned, would perform poorly because the sides are open.

Shawn


As a little bit of a side note, for anyone wanting to scale up or down a torsion box, in order to get the same deflection under the same load, increase the box thickness (ie. rib height) proportionally with the supported box length. The deflection varies with the supported length cubed, and the resistance to bending with the thickness cubed. Increasing both by the same percentage cancels out.

[dimitar]

You can download a stl file for blade:
http://www.partenovcfd.com/software.html

[rowbare]

Shawn,

Can your software calculate what would happen if you would fill the cavities in the torsion box with expanding foam?

[Jack000]

are there any longitudinal beams? The horizontal ribs aren't really doing anything in the pictures, it's the long beams that are supporting the weight.