Re: Units of measurement?

From: tomv136 <tomv36_at_cox.net>
Date: Sun, 12 Mar 2006 12:13:29 -0000

--- In Indoor_Construction_at_yahoogroups.com, Don Slusarczyk <don5490@...> wrote:
Don,
Have you read Joe Maxwell's little book about balsa? If my memory is correct, he as done
nearly what you are suggesting. All of his data are empirical but his conclusion is that as
density increases so does stiffness. the increase in stiffness is not linear and increases at a
rate that is greater than the density increase.
The practical application of this phenomena is that a cross section of lower density can be
"traded" against one of smaller cross section and higher density (stiffness) resulting in a
structural member that meets the design deflection with lower weight. I recall that, for this
reason, Joe stated that 5.5# balsa is optimal for F1D spars.
Tom
> I have never heard of Bernard Hunt's SC formula as being the
> established indoor "standard" for grading balsa. It is simply a tool
> used in selecting the wood and should be treated as such. If Tim grades
> all his sheets using the same (modified) formula then I have no issue
> with that. I know that good 6# wood was about 100 SC and average was
> about 80 using Bernards scale. If you tested 4# wood and got 100SC then
> that was bad 4# wood, 130 was good 4# wood. The scale shifted based on
> density.
> If you are seriously wanting to make a better SC calculation then I
> have a few ideas which I never followed thru on. I was going to test
> about several hundred samples of wood in various ranges from 4# to 7#
> of similar size. I typically cut sheets into 18 x 1 sheets. Then based
> on the buckling load, back out the Elastic modulus. Then calculate the
> "beam stiffness" which is E x I . Then plot the elastic modulus vs
> density and the "beam stiffness" as a function of density. I then was
> going to hope that a nice relationship would fall out where one could
> predict estimated "normal" E values based on density. And actually I
> would prefer EI function based on density as the amount a beam (spar)
> deflects is proportional to EI. So I had hoped that a corrolation could
> be made (mathematically) so one could say a .030 x .060 spar of 130 SC
> 4# wood is equal in strength (meaning deflection in this case) of .030 x
> .060 SC100 5# wood. That way you can decide in real world numbers that
> if I use average 5# wood, then this piece of 4# wood when cut the same
> size will have the same strength but be 80% of the weight. Bernards
> calculation does not do that. I also know he tested mostly 1/8 x 1/4
> strips as I have a tube with about 20 of them in it he sent me. He was
> very fond of his Jim Jones balsa stripper and told me it was a
> convenient size to use.
> The method they used is an engineering method (Euler buckle load)
> but the actual SC formula they came up with was not based on an
> established engineering method of "Stiffness Coefficient" it was a term
> they created and derived themselves. That is why Tim modifying it is not
> an issue as the SC is an arbitrary unit and he used the same unit for
> all his sheets and defines his grades accordingly. Elastic modulus is an
> establish engineering term and is calculated based on a formula. If you
> fudge that formula then I can see a complaint. But right now a unit of
> 'SC' is no more defined than a unit of 'snuzzleflubbery'.
> I have always treated SC this way since the beginnig. It was a
> nice way to sort wood. I use to buy like 50 sheets of 3" x 36" x 1/16
> and 1/8 wood from SIG (contest grade) Cut it up into 1" x 18" pieces, so
> I woud have like 300 sheets of 1/8 and 300 of 1/16, then sit next to a
> scale and push. Say I wanted spars for a unbraced F1D (old rules is
> when I did this). I would sort the wood by density into groups. Then get
> a rough estimate of what force I needed. Say better than averag 5# was
> 300 grams of force, I would then load test all sheets in the 5# range.
> Then all sheets past 300grams were set aside, and graded further . I
> quickly found that maybe about 5-10% of the SIG wood was good stuff
> SC>115 for 5# density. Of those sheets I would then calculate in more
> detail each sheets SC and density more accurately. If I got a good sheet
> then I would split into 1/2" strip and retest. About half would test
> poor after splitting as a light sheet may have a few hard spots in it
> raising the stiffness. It was a very long and labor intensive process.
> I use to have boxes and boxes full of poor wood strips. Once Tim
> started grading I just bought his wood instead as it saved me hours of
> time and money having him do it. The reults of models built from his
> wood are very well seen. My models are lighter and stiffer than before.
> My father has noticed it as well. In fact my dad called me laughing one
> day as he tried to roll a motor tube from Tims wood for his Pennyplane
> and it was so stiff it cracked when tried to roll it. he had to use
> weaker wood to get it to roll. I remember before Tim started selling
> balsa that if you bought $1000 worth of wood you got about $300 of
> usable wood out of it. Now who was taking advantage of who? If you
> returned it, you ran the risk of not being able to buy ever again from
> them so you just ate the cost becasue there was no where else to buy it
> from. If you buy $300 from Tim you get $300 of damn good wood.
>
> Don
>
> > of mechanical difference. But there have been standards established
> > for structural materials. When the numbers are changed and advertised
> > as the genuine article, who suffers and who benefits?
> >
>
Received on Sun Mar 12 2006 - 04:13:33 CET