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1. 10:00 PM - Re: Wood size (taildrags)
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So I'm always the guy who wants to analyze things because I'm an engineer. Not
taking a position one way or the other on the longeron size issue, just analyzing
a little bit. Let's just say that the basis of design for the longerons
is that they are of spruce, that they are 1"x1" in cross-section, and that we're
interested in the strength of that member in bending. Spruce modulus of rupture
is 9400 psi so we assume that's the maximum bending stress that the longeron
will sustain. We can calculate the maximum bending moment that the longeron
can take, M, to be equal to the modulus of rupture times the width of the
longeron times the square of the height of the longeron divided by 6, with that
result divided by 12 to get it in lb-ft of bending moment instead of lb-in.
That calculates out to be a little over 130 lb-ft of bending moment.
If we make the parts to still be 1" high but only 7/8" wide, that drops to about
114 lb-ft of moment... a 12% reduction in what they can sustain in bending.
Some builders have gone to Douglas fir, which has a modulus of about 10,900 psi.
If fir is used for those 7/8" x 1" longerons, then they will resist a bending
moment of about 132 lb-ft and you've gotten back what you lost by shaving
down the 1" to 7/8". It comes with a price though... Doug fir weighs more than
spruce and you have to determine for yourself if the tradeoff is worth it.
As others have posted, unless you're willing to calculate stresses here and there
and everywhere in your airplane structure, it's best to stick to the plans
and only venture away if you want to do the homework. In the case of the longerons,
they are not in pure bending in any one spot at any one load condition,
so even if you check just bending in one axis, there may be other loadings that
are worse than the one you evaluate.
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Oscar Zuniga
Medford, OR
Air Camper NX41CC "Scout"
A75 power, 72x36 Culver prop
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=469913#469913
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