One of the properties of balsa that a builder looks for and greedily covets
is high stiffness ratio (modulus of elasticity) balsa for spars on light,
high duration models; wood that maximizes strength and reduces weight in
these models.
Several people have studied and published methods to determine higher
stiffness balsa by testing. The most popular test was developed by Bernard
Hunt and John Taylor, called the "Stiffness Coefficient" (SC). Modulus of
Elasticity (a stress-stiffness ratio - "E") was massaged to provide a ratio
of stiffness over a range of densities - 4# to 6# for the SC test. The SC
test was easy to conduct compared to other methods and relied on the Euler
buckling formula (slender columns) for determining E of beams. For the model
builder, this would include wing spars and pushing a sheet of balsa into the
table of an electronic weighing scale, which was an easy task. Joseph
Maxwell developed a slightly more complicated test and formula based on
deviation from average E throughout the density range of indoor balsa; thus,
much more accurate in assessing stiffness than the Hunt-Taylor SC test.
True-weight Indoor Balsa (TWIB) adopted the Hunt-Taylor SC test to grade its
balsa. Based on the SC, algorithms put increasing price premiums on the
volume of a sheet the higher it tests. The algorithms priced volume, also.
As most who use this testing for finding good spar wood, I followed the easy
route and established a relationship with Hunt on the several websites that
offered calculators for the stiffness coefficient test.
<indoorduration.com>, the parent site of TWIB, offered a downloadable
version of the calculator for desktop use. Quite convenient. Then I
discovered that the downloaded calculator was modified from the original
published formulas used by Hunt and later posted by Don Slusarczyk, on his
web site. This was a couple years ago, or more. I questioned Tim Goldstein
about it at the time and got some response. And, eventually forgot the
matter; until recently when I used the same calculator, with outrageous test
results. "Outrageous" means checked wood and pretzels (mush, set, junk) were
yielding impressive results.
I was pissed, for several reasons.
With this introduction, the following may make some sense. In responding to
the post by Tim, I decided to perform some pseudo-scientific testing. I had
already determined that the Hunt-Taylor test was not so good. The testing
would force a more disciplined procedure to confirm gut-feel about the test
in general; and, demonstrate validity of the modified calculator.
Fast Back:
_____
From: Indoor_Construction_at_yahoogroups.com
[mailto:Indoor_Construction_at_yahoogroups.com] On Behalf Of Tim Goldstein
Sent: Friday, March 10, 2006 5:50 PM
To: Indoor_Construction_at_yahoogroups.com
Subject: RE: RE: [Indoor_Construction] Units of measurement?
> -----Original Message-----
> In response to the diversion, test and compare the
> calculators and discover what has been "helped".
---snip----
> So, take potshots at honesty
>Bruce,
>Take your medication and rejoin the real world.
>The calculator was flat out non-linear when comparing it's output to the
>real world of testing....>
Let's check and see what is "non-linear", whatever that means, and "real
world". But we need to bring in Tim's explanation for the adjustment to the
Hunt SC calculator, found in the prior post, to Don. It follows:
>What I have found is that for the range of balsa I deal in primarily, 100
is
about an average value. The adjustment I made was specific to thick sheets.
My experience over many thousands of sheets of wood is that unlike density,
stiffness is pretty consistent for a particular block of wood. I base this
on taking a block and cutting it entirely to the same thickness sheets and
then testing them. While the density will vary (sometimes considerably) the
stiffness coefficient for the wood from that block is quite stable. But what
I noted was that if I cut some .030" sheets from a block and then some .100"
sheets there was a great disparity in the stiffness even if the density was
consistent. More interesting was that the by handling and feel was a very
stiff and "snappy" piece regardless of thickness. My desire was to get the
scale more linear in regard to varying thickness of wood so that what by all
real world tests was a great sheet of stock would also have numbers that
reflected this in regard to using 100 as "normal".<
What I understand from this explanation is that the Hunt SC (we'll call this
"THS" for "The Hunt System") values don't work correctly with thicknesses
over .100". The adjustment was specific to thick sheets. And, presumably
stuff under .100" would remain the same, since Tim found those averages
satisfactory. This is because the averages were close to 100 (average) on
the THS calculator for thinner sheets. *****How a thickness difference would
affect the calculations is beyond me. Hunt used .125" X .25" test pieces, if
I recall, to build the SC formula.
I would tend to agree that the stiffness values are pretty consistent - that
is, good vs. poor - within a block. Note, "stiffness" is related to modulus
of elasticity; "stiffness coefficient" is related to the Hunt method. Sheets
from a block are usually representative of the block, as are strips from a
sheet. Next. I can roll a 36" long strip of 7lb wood into a coil nearly
double and it will whip back very close to its original shape. A nice piece
of wood that was well below 90 (poor) on THS scale. 10 and higher densities
from good blocks do the same thing. However, I believe THS calculator was
designed for 4# to 6# lumber. Everything over that density looks worse on
THS the denser (harder) it gets.
Tim made an adjustment to the calculator to better handle thicker sheets.
I'll call this the Goldstein Adjustment (GA). We know the problem can't be
in the Euler Formula or the real World would be in a world of hurt. Hunt's
calculations are suspect.
To evaluate this adjustment I ran a series of tests of my stuff for a
comparison of the calculators' output. To paraphrase, I broke some sticks.
Material ranged from 0.6# (pounds per cubic foot) to 16.2#. I didn't concern
myself too much with thickness, but everything happened to be about .090"
and thicker in the first series. The balsa samples were up to .311" wide and
18" long. These sizes are used to isolate density and grain variations for
more accurate tests. Tim usually tests material 1.2" up to 2" wide, which in
my TWIB stock usually includes density bands and grain shifts which will
distort results.
The following results are organized as: Density / THS + GA = % (percent
difference). THS and GA are the stiffness coefficients for the two
calculators. Densities are weighted toward the design range of the original
calculator. All balsa samples exceed the thickness:length ratio of 100.
Sample thickness for common 18" lengths must remain under .180" to use the
Euler buckling method for beams.
0.6# / 12.05 + 20.8 = 172% (rigid packing foam, just to avoid more
diversion);
3.6# / 109.7 + 135 = 123%;
4# / 117 + 152 = 130%;
4.9# / 93.8 + 117.2 = 125%
6.5# / 73.4 + 87.3 = 120%
10.8# / 63.8 + 76.1 = 119%
16.2# / 50.46 + 60.1 119%
A second series of sticks were broken [not really] which included samples
under .100" thick to check for among other things a closer deviation from
THS. According to the premise, thinner samples should match or have a
tighter margin of difference in the comparison of the calculations. Samples
were about .065" thick.
0.88# / 6.72 + 7.63 = 114%; (foam, and E= 386 psi);
4.1# / 92.23 + 105.1 = 114%; E= 115482
4.24# / 109.74 + 126.4 = 115%; E= 146014
5.22# / 108.3 + 123.3 = 114%; E= 218314
12.3# / 57.45 + 64.7 = 113%; E= 644414
I noted and added the modulus of elasticity for this second series. The foam
density variation of .6# and .88# represents the hazards of the weighing
device getting too close to its lower limit of .01g accuracy. I was going to
include some TWIB sheets, but I couldn't find enough clean sheets and the
results were just too depressing. I'll have a white elephant sale soon. [I
already had it; and frankly, feel like an accomplice to a crime even after
suffering a heavy investment loss.]
What do all these numbers mean? The second series results indicate a closer
margin of difference between the Hunt System and Goldstein Adjustment. The
adjustment margin is tighter in the thinner samples. And, even though it
didn't matter in my TWIB stock (some of the depression), marginal values
were boosted into the costlier levels of the price algorithms used for TWIB
wood. It was only in the last of my purchases that SC numbers began to match
the grade marks; and they were based on the GA calculator. Since any reason
for buying the higher graded wood is for stiffer spars, or vanity, the
natural inclination is to go for the highest rated, which is what I found
myself doing. Then I realized I could buy 20+ board feet of what I wanted
for the same price of a 15-sheet TWIB order, about 2 square inches.
The quick list:
1. The GA (Goldstein Adjustment) has nothing to do with thickness
variations. Euler doesn't discriminate. This is BS (Blowing Smoke). The
adjustment has everything to do with pricing, and moving less desirable
wood. I pointed out this complication before TWIB went public.
THS + GA = BS; BS = $.
2. Tim is open about the adjustment. This is BS. There is no indication
of the adjustment, until he is confronted about it. This public response
would make Hyde green with envy. In other words, don't be critical,
especially in private.
3. The Hunt test has problems. This is NOT BS. My mental image of the
SC scale is one of a missile intercepting another moving on a parabolic
curve. The two cross paths at about 5# density. Averages under 4.5# move
higher on the SC scale as density drops, and visa versa.
4. I have abandoned the Hunt SC test for wood stiffness, not because a
marketer has corrupted what it represents, but because it has restricted,
limited value for determining stiffer wood through the density ranges. Using
and comparing modulus of elasticity, together with the Maxwell method is far
more representative of basic structural engineering.
5. Don't ever trust someone else to determine the stiffness
(elasticity) of your wood. This is NOT BS. My mistake was very costly in
money and time.
There is no need to read the rest.
-------------------------------------------------
>..... more balsa than you have probably ever seen.<
I'm not SNIPPING, so this fragment remains. It's nothing more than a
derisive insult to validate Tim's expertise in grading balsa and
understanding the material, generally. Granted, from photos, his garage has
more wood than mine. I just have a few hundred sheets and several feet of
boards from various suppliers going back maybe thirty years. But, not much
material is older than 8 years from the stump. Also, I've culled a far more
representative variety of the material than is in both of our garages.
Just a clarification, the SC test is geometrically more accurate the smaller
the samples are, so a hundred board feet is meaningless.
I won't go into how many hands Tim has for testing, grading sheets as he
calls it.
>But as Don points out it is immaterial as long as you use the same
calculator to
test both pieces. And if you use 2 different calculators then the results
are not comparable.<
Duchy, for the comparison. I've pointed out that the "immaterial" is
actually very "material". See Quick list, item 1; and the tests.
>I have no clue what you think is dishonest. Yes I sell balsawood that is
marked for stiffness. With the exception of you none of my other customers
(who are far more successful fliers I might say) seem to have any
disagreements with the stiffness grading. I have very openly told anyone
that is interested that I modified the utility a couple of years ago to more
closely model real world results. You seem to be the only one that feels the
initial calculations are some sort of holy grail. If you have a problem with
my updating the calculator so be it. Enjoy breaking stick and trying to make
balsa match an incorrect model.
>If the hobby you enjoy is breaking balsa sticks and blaming your
performance
on a tool that I freely gave to the hobby then I say more power to you. In
the mean time I will continue with my plans to increase my supply of balsa
tested the way I feel is should be. If you disagree with my methods I don't
care as many other have very favorable comments about my grading and the
material I supply.
>Tim
[Denver, CO]
Indoor balsa wood & supplies
Now carrying Carbon Plate, rod, tube, and strip.
www.F1D.biz
Indoor flying info
www.IndoorDuration.com <
Some more BS.
Camera magazines always have complementary reviews of cameras. That's
because camera manufacturers advertise in camera magazines. Sometimes,
manufacturers create their own magazines. I build and fly models. I wonder
if Tim does? Are you selling low tack, despicable adhesive, Tim? Have you
used it yourself?
Stay tuned for an update to the state of indoor balsa; a review. I don't
have advertisers for my reviews.
Bruce McCrory in Seattle
(Will this get through the Gestapo?)
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Received on Fri Mar 31 2006 - 23:15:11 CEST