Towers B/C

[sluelway1973]

Okay all, so we are ordering wood from Colorado and we need suggestions on getting these towers from a dry environment to a humid environment. The issues are evident and we are becoming frustrated with the end results.

Thanks

[sluelway1973]

Are the Jigs laser cut?

[Crtomir]

Okay all, so we are ordering wood from Colorado and we need suggestions on getting these towers from a dry environment to a humid environment. The issues are evident and we are becoming frustrated with the end results.

Thanks

Our girls duct taped quite a few silica gel desiccant packs to the inside of their tower transport box. This helped to keep the humidity low.

We thought about putting the towers in an oven and baking out a lot of the moisture and then sealing the dry tower in a plastic bag. The idea would be that they could take the dry tower out of the bag just before check-in and weighing. However, we could not find an oven big enough to try it. I think you could lose as much as 0.5g from your tower weight if you could, though.

[Balsa Man]

Okay all, so we are ordering wood from Colorado and we need suggestions on getting these towers from a dry environment to a humid environment. The issues are evident and we are becoming frustrated with the end results.

Thanks

Our girls duct taped quite a few silica gel desiccant packs to the inside of their tower transport box. This helped to keep the humidity low.

We thought about putting the towers in an oven and baking out a lot of the moisture and then sealing the dry tower in a plastic bag. The idea would be that they could take the dry tower out of the bag just before check-in and weighing. However, we could not find an oven big enough to try it. I think you could lose as much as 0.5g from your tower weight if you could, though.

Over the years, have a) done a number of tests to look at how much weight reduction you can get by….drying and how long it lasts, b) had ….atmospheric conditions at State or Regionals such that relative humidity was significantly higher than normal (rain/high humidity moving in), so we knew that drying could help on weigh-in weight, and c) had Nationals in a place with much higher humidity than we normally have here (in Colorado).

I’ll say up front that there’s simply no issue/concern ordering wood from Colorado, or the Atacama desert of Peru, or, oh, the Amazon rain forest, or Houston TX, where the humidity is almost always….really high. Exposed to the air, the wood does gain and loose weight from ambient humidity. The only real question/issue is how you manage exposure in a relatively short period before weighing for competition.

Second, I’ll say, it is important to stay within ‘safe’ limits reducing moisture content. We’ve used 140 F as an upper limit. Beyond that, or something close to that, you DO risk affecting glue/joint strength.
So, what have we seen/measured, and done….?

The magnitude of effect/variation is …smaller than some suggest. In a range from ~20% relative humidity to 90-ish %, we’ve never seen an….immediate effect over about 7-8%. On a 10gr structure, that’s about 0.7-0.8gr; on a, say 6gr tower, that would mean 0.4 to 0.5gr. This is from heating a large oven to 170F (the minimum setting), turning it off and with thermometer in the oven waiting till it’s down to just above 140F, and popping the structure in. The data are very similar when you put the structure in a large plastic storage box, and use a hair dryer, heating/drying the air in the box and the structure- keeping a hand near the structure, and making sure the structure is not seeing/getting air blown on it at a temperature that is uncomfortable to hold your hand in.

The duration of the effect is pretty short-lived. In a number of tests, baking/drying, and putting the dried structure immediately on a scale, you can literally see the weight increase happening (if the ambient humidity is….elevated. Looking back at data, within a minute/minute and a half, you’ve lost around half of the effect, within 4-5 minutes, it’s gone. The ….adjustment rate to ambient humidity is a little slower with higher density/larger cross section, and faster with lower density/smaller cross section, but within the range noted.

With a really air-tight box (we’ve used an Underwater Kinetics box ‘good for’ 10 meter submersion- good for smaller structures- but not big enough towers this year), drying and quickly placing in the box, along with ‘properly activated silica gel’ (baked at 325-350F for 30-45 minutes), the drying sustained itself for …multiple days. With anything short of that (a storage box with lid taped, or bag if you have one that'll fit the structure), the sustained dryness is of….shorter duration. So, my take is for, say, a trip to a competition in a high humidity location, the best way to go is doing the hair dryer treatment at the competition, immediately before going to compete.

The issue, and the bottom line in terms of benefitting from managing structure moisture content, is how you can manage the time between first exposing the structure to ambient humidity, and officially getting structure weight measured. Since the rules don’t address this… issue/technique, I’ve always a) made sure the kids understand the….dynamics of the effect- time is working against you once the box is opened, and b) advised that you want to do what you can, given the way the folk running the event are operating/running things, but only what you can- if you have the option/opportunity to get the structure on the scale before its measurements are checked, take it. If not, get the measurements done as quickly as you can. However check-in goes, by the time you’re actually testing, the structure’s moisture content will be at/very close to ambient conditions.

[Random Human]

Quick Question:

Are the buckling strengths of a say 1 gram 1/16^2 and a 1 gram 1/8^2 members the same? And this can be applied to any situation.

[Balsa Man]

Quick Question:

Are the buckling strengths of a say 1 gram 1/16^2 and a 1 gram 1/8^2 members the same? And this can be applied to any situation.

Short answer, no, they won’t be the same; the 1/8” will have a buckling strength a little over 2 ½ times that of the 1/16”. Where does that number come from?

OK, review time. In the massive book of useful information we’ve compiled this year, the information to understand and get the answer is there. As we’ve discussed before, in calculating buckling strength, there are two relationships at work, and we need to look at both together.

Buckling strength (Euler’s equation) is E x I divided by (effective) length squared. I is the second moment (sometimes referred to as the cross-sectional moment) of inertia. For a square cross section of dimension d, I = d^4/12. So, looking at 1/32 increments from 1/16 to 1/8:
5/32” (d =0.15625); I = 0.000004967
1/8” (d =0.125); I = 0.0000020345
3/32” (d =0.09375”); I = 0.00000064373
1/16” (d =0.0625”); I = 0.00000012716

What does this tell us, first, assuming same density? That a small increase in cross section gets you a big increase in I. If we say a 1/16 stick has I = 1, I for a 3/32 stick = 5.0625, I for a 1/8 stick = 16, (and I for a 5/32 stick = 39.0625).
OK, let’s now look at density, and how it affects buckling strength. Let’s set the weight of a 1/16” stick at 1. A 1/8” stick of that density will have a weight of 4. So, a 1gr 1/8 stick will have a density that is ¼ of the density of a 1gr 1/16 stick.
E (in Euler’s equation) is the modulus of elasticity (sometimes referred to Young’s modulus); which is a measure of the inherent stiffness of a piece of wood (or anything else). E varies by density- the higher the density, the higher the stiffness (E). As we’ve discussed a lot this yearThe best data source out there (and the only one I know of, other than the data I posted this year from testing of a lot of 1/8” balsa sticks) for the relationship of density to E is an old study done by the U.S Department of Agriculture- US Forest Service: http://ir.library.oregonstate.edu/xmlui ... /1957/1286

I posted a link to this a few years ago, Crtomir posted the link a few weeks ago.

And remember, as we've discussed many times, there is always some variability that's just inherent in wood, in terms of density vs strength.

Using average values from Fig 7 in this study, we see that at ¼ the density, the value of E drops to 0.177
I also posted data from BS testing on 1/8” sticks at a range of stick weights. Looking at the relationship of density to buckling strength in these data, we see that at ¼ the density, the measured buckling strength drops to 0.1555
So, to bring things together, let’s use 0.166. The lower density drops buckling strength by a factor of 0.166; the bigger cross section increases buckling strength by a factor of 16. 16 x 0.166 = 2.656; the lower density 1/8” has a buckling strength 2.656 times that of the higher density 1/16”. And remember, of course, there is always some variability that's just inherent in wood, in terms of density vs strength.