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Quote:
Balsa all by itself is a complex, composite material with a huge range of weights and densities. In addition, it has huge differences of compressive and tensile strengths in different directions, and huge differences in stiffness (Youngs) depending on the direction of grain versus applied force. Foam is a monolithic, non-direction material with the same properties from every direction; its advantage is its uniformity. It is also low weight, cheap, resilient at low loadings, and has the ability to be formed easily. |
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Balsa was dropped from assembly line production use from all but the most advanced, lightest, sailplanes.
Properly selected and graded balsa is far superior to foam in just about any/every application within an airframe other than cost and ease of use. Foam is fine for just adding bulk, but balsa becomes a load bearing portion of the composite structure as well as adding bulk. |
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OK, so out of all the foams out there, which would be the best for a load bearing core?
This isn't for a plane, btw. I just don't want to shape a balsa core. I can get a more accurate shape with a hot wire, than with planer and sandpaper. It can be a 'heavy' foam for my use (just need to keep it at or under 'heavy' balsa). |
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I think we need a bit more info on what you are creating.
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There are foams that have a compression strength that is over 200 psi. They can be toxic to wire cut though.
Dow makes a foam called High Load 100 that has a compression strength of 100 psi. It's extruded polystyrene. It can be hot wire cut. Owens Corning also makes Formular 1000 that is XPS and 100 psi. 3.0 pound density expanded polystyrene (can be wire cut) has a compression strength of 100 psi. 4 pound density has 175 psi. 4 pound density is pretty tough to come by though. The longitudinal compression strength of balsa is around 700 psi. I can find any firm figures on the axial compression strength of balsa. |
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Last edited by wyowindworks; Apr 08, 2011 at 02:50 PM.
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I see numbers for balsa of 3.7Mpa (530PSI) to 27MPa (3900PSI) for the compressive strength parallel to the grain.
Cross grain compressive strength of most woods is about 1/10th the parallel compressive strength. Balsa might be about 50 to 400PSI on that basis. http://www.scribd.com/doc/37966050/Balsa-Propertie-5 Kevin |
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http://f1.grp.yahoofs.com/v1/QGmfTdS...m_modulus2.txt
Elastic Modulus Of Common Foams last revision: 5 May 03 ================================================== ====================== foam rho E11 E22 E33 E33/rho data source type (lb/ft^3) (psi) (psi) (psi) (in/10^6) ---------- ------- ------ ------ ------ -------- ------------ Spyder foam 2.30 4000 6900 5.18 Mark Drela Spyder foam 2.36 1218 3358 7027 5.14 Phil Barnes Hi-Load 60 2.25 1100 3400 6000 4.61 Mark Drela Hi Load 60a 2.30 1229 2819 6212 4.67 Phil Barnes Hi Load 60b 2.30 1412 2888 6412 4.82 Phil Barnes Dow blue 1.75 2000 640 2500 2.47 Mark Drela Foamular 250a 1.63 2317 1899 2372 2.51 Phil Barnes Foamular 250b 1.63 2430 1972 2476 2.62 Phil Barnes Foamular 400a 1.98 2788 2946 2941 2.54 Phil Barnes Foamular 400b 1.98 2920 3088 3087 2.67 Phil Barnes Foamular 600a 3.04 4927 4927 5108 2.90 Phil Barnes Foamular 600b 3.04 4716 4807 5251 2.98 Phil Barnes Grey Foam a 1.54 2894 1054 640 0.72 Phil Barnes Grey Foam b 1.54 2845 1105 588 0.66 Phil Barnes Stylite a 1.93 1877 2906 3489 3.12 Phil Barnes Stylite b 1.93 1847 2854 3416 3.06 Phil Barnes Rohacell 31 1.90 5120 5120 5120 4.66 CST Rohacell 51 3.10 9950 9950 9950 5.55 CST 4.7lb balsa 4.70 160000 Mark Drela --------------------------------------------------- Definitions ----------- rho = density E11 = modulus along sheet (8' dimension) E22 = modulus across sheet (2' dimension) E33 = modulus across thickness E/rho = specific modulus For foam core applications, the relevant modulus is E33. For bare foam wings (slow fliers, etc), the relevant modulus is either E11 (span is along foam sheet), or E22 (span is across foam sheet). |
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As stiff as possible
Use Balsa ...Endgrain...Dont think you can get much stiffer than that
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