Re: Units of measurement?
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 Fri Mar 10 2006 - 18:21:06 CET
This archive was generated by Yannick on Sat Dec 14 2019 - 19:13:44 CET