## Towers B/C

hogger
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### Re: Towers B/C

Just a quick and dirty approximate budget calculation.

To get to a score of 3333 for Division C and no bonus point, you need a 4.5 grams tower.
- Baking it before weighing, you can lose about 10% of the weight, so you need to build 5 grams tower.
- For the 4 legs, let's use 1 gram 36" stick of 1/8 x 1/8. For Div C each leg will weigh about 0.67 grams (1 * (61/2.54) / 36). ( 1 gram * (length of the leg in inches) / length of the stick).
- So for 4 legs, the weight is 0.67 * 4 = 2.68 grams.
- The budgeted weight for the braces then become 5 - 2.68 = 2.32 grams.
- Each side of the brace gets 2.32 / 4 = 0.58 grams. You can do this with 1/64" thick pieces of balsa with appropriate width to cover the length of the pattern you need to do to complete the side braces for maybe 4-5 intervals.

Then you carefully build the tower, and just cross your fingers hoping it fails close to the buckling force you budgeting it for.

PS: Oops, I missed the glue, probably close to 10%, maybe you can do with 5% of the total weight or 0.25 grams. So, you need to take away some from existing budget for the legs and the braces. It is getting pretty crazy tight at this point, every glue joint must be done carefully with no excess.

Balsa Man
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### Re: Towers B/C

hogger wrote:Just a quick and dirty approximate budget calculation.

To get to a score of 3333 for Division C and no bonus point, you need a 4.5 grams tower.
- Baking it before weighing, you can lose about 10% of the weight, so you need to build 5 grams tower.
- For the 4 legs, let's use 1 gram 36" stick of 1/8 x 1/8. For Div C each leg will weigh about 0.67 grams (1 * (61/2.54) / 36). ( 1 gram * (length of the leg in inches) / length of the stick).
- So for 4 legs, the weight is 0.67 * 4 = 2.68 grams.
- The budgeted weight for the braces then become 5 - 2.68 = 2.32 grams.
- Each side of the brace gets 2.32 / 4 = 0.58 grams. You can do this with 1/64" thick pieces of balsa with appropriate width to cover the length of the pattern you need to do to complete the side braces for maybe 4-5 intervals.

Then you carefully build the tower, and just cross your fingers hoping it fails close to the buckling force you budgeting it for.

PS: Oops, I missed the glue, probably close to 10%, maybe you can do with 5% of the total weight or 0.25 grams. So, you need to take away some from existing budget for the legs and the braces. It is getting pretty crazy tight at this point, every glue joint must be done carefully with no excess.
A good exercise; thought provoking..... how might the seemingly impossible be done?
.
Absolutely agree pushing down into this range requires a set of very special sticks- out at the statistical extremes for density vs buckling strength. Finding them means either serious luck, or sorting through a lot of wood. To get a 3333score, as you note, for a tower not qualifying for the 29cm circle, you need a tower weight of 4.5gr. For a tower qualifying for the bonus, the weight goes up to 5.1gr. I do believe, from calculations, that the bonus ‘pays off’; but I could be wrong…

So, I’ll throw out, just for pondering, some numbers for C, (qualifying for the 29cm circle bonus), 1/8” legs, using 1/5 bracing interval, with a ladders (at 1/8”) and Xs (at 1/16 x 1/64) bracing configuration, based on the very best sticks I’ve found. These are single stick results; haven’t come up with a full set at these numbers. I’ve had the luxury of being able to go down to Specialized Balsa, and spend…..hours going thru what they had in stock, weighing, and doing single finger push-down buckling strength testing.

At 1/5 bracing interval, the legs need a 36” buckling strength of 68.33gr. Using the 2.3 effective length multiplier that I’ve discussed/described in earlier posts, that means 36” push-down BS at 29.7gr (29.7gr x 2.3=68.33). The lightest stick I’ve seen that’s close to this is one at 1.09gr w/ 36” push-down BS at 29gr. So, I think its entirely possible there are 1.0gr/36” sticks w/ BS at/> 29.7gr. So, a leg set at 2.68gr.

Looking at ladders, the lightest that has a BS above 1kg for the lowest (longest) ladders that I’ve seen is 0.69gr/36, and for ladders above it, 0.66gr/36. That gives a ladders weight of 1.66gr (up to 4.34gr).
For Xs (providing a tensile strength of =/>1kg), 1/16” wide strips from 6.5gr, 3” x 36” sheet- an X-strip total weight of 1.21gr, so we’re at 5.55gr.

Glue…. The best glue weight on the builds the kids I’ve been working with is a hair over 1gr, so let’s say with real care….0.9gr….that ends up at 6.4gr; 17000/6.4 means a score of 2656.

If you assume you can find sticks 10% lighter (w/ the needed strength- conceivable, but we’re talking a VERY small percentage of sticks) than weights noted above, (or could actually get, under competition conditions, a 10% weight reduction from baking/dryingtower- my experience is its more like 5%, and low density wood rehydrates quickly-within maybe 30 seconds its falling off, and gone within 3-4 minutes) weight at 5.76gr; 2951 score; 0.66gr heavier than what would score 3333.

So, conclusions……? Where might that 0.66gr lighter come from? Possibilities I can see are:
Using 3/32 for ladders above the lowest one….

The 2.3x effective length multiplier for converting 36” single finger push-down BS to in-place BS (when the leg segments are braced) is incorrect/too conservative. It wouldn’t have to be much higher to explain the difference.

Recognizing the variability (in density vs strength) in balsa, measuring BS at 36”, vs measuring at….line 62cm (just over actual leg length) could be missing portions of sticks with higher BS that what you read at 36”….

Using a bracing approach other than ladders and Xs (Xs only, using 1/16 sticks) (but the little info/data I have says that the bracing interval needs to be tighter w/ this approach), or….configuring the legs so that they’re….induced to bow/buckle outward, so the bracing only needs to act in tension to prevent that…..

Sure makes me wonder what that 4.37gr tower looked like- basic configuration…….
Len Joeris
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Complexity02
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### Re: Towers B/C

With regionals approaching in less than two weeks, I have finalized my design, but desperately looking for ways to lose some mass on my towers. And in these forums, suddenly this "baking" of towers popped up, and has me intrigued. I am interested in doing this, but I want to proceed with caution. A few questions:
Should the baking be done to finished towers?
Is there a risk of glue melting off the tower as a result?

I looked at some old forums, and they say to bake the tower for about 20 min at 200 degrees, but that's about all the information I can dig up.

P.S: I had not posted on these forums for a while, and my old account got deactivated for some reason. I was unable to reactivate it, so created a new account with almost same username

Balsa Man
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### Re: Towers B/C

Complexity02 wrote:With regionals approaching in less than two weeks, I have finalized my design, but desperately looking for ways to lose some mass on my towers. And in these forums, suddenly this "baking" of towers popped up, and has me intrigued. I am interested in doing this, but I want to proceed with caution. A few questions:
Should the baking be done to finished towers?
Is there a risk of glue melting off the tower as a result?

I looked at some old forums, and they say to bake the tower for about 20 min at 200 degrees, but that's about all the information I can dig up.

P.S: I had not posted on these forums for a while, and my old account got deactivated for some reason. I was unable to reactivate it, so created a new account with almost same username
Let me start by saying that 200 degrees is too hot. I've used 140 as an upper limit, and it works fine.
You want to make sure the oven is off- as in heat it up, turn it off, let temp drop to ~140.
10 minutes will get you all the dehydration you're gonna get.

To maintain the dehydration, you're going to need an airtight box, and you may want to have a desssicant pack (silaca gell) in the box, too. If it's not airtight, it will....re-equilibrate with atmospheric moisture within a few hours.

Is all this trouble worth it? When you open the box, within less than 5 minutes, it will re-equilibrate with atmospheric moisture. Half of what you loose 'baking' will be back within, oh, a minute (happens quicker with really low density wood). So, if you can officially get it on the scale before it re-equilibrates, yeah, you'll pick up a little. If its a rainy, humid day, you'll gain a bit more. My experience is that 5% is the most you're going to see, and, as I said, it goes away quickly. So, on a 10gr tower, as a practical matter, if you pick up two to three tenths of a gram, you'll be doing well.
Len Joeris
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### Re: Towers B/C

Balsa Man wrote:
Complexity02 wrote:With regionals approaching in less than two weeks, I have finalized my design, but desperately looking for ways to lose some mass on my towers. And in these forums, suddenly this "baking" of towers popped up, and has me intrigued. I am interested in doing this, but I want to proceed with caution. A few questions:
Should the baking be done to finished towers?
Is there a risk of glue melting off the tower as a result?

I looked at some old forums, and they say to bake the tower for about 20 min at 200 degrees, but that's about all the information I can dig up.

P.S: I had not posted on these forums for a while, and my old account got deactivated for some reason. I was unable to reactivate it, so created a new account with almost same username
Let me start by saying that 200 degrees is too hot. I've used 140 as an upper limit, and it works fine.
You want to make sure the oven is off- as in heat it up, turn it off, let temp drop to ~140.
10 minutes will get you all the dehydration you're gonna get.

To maintain the dehydration, you're going to need an airtight box, and you may want to have a desssicant pack (silaca gell) in the box, too. If it's not airtight, it will....re-equilibrate with atmospheric moisture within a few hours.

Is all this trouble worth it? When you open the box, within less than 5 minutes, it will re-equilibrate with atmospheric moisture. Half of what you loose 'baking' will be back within, oh, a minute (happens quicker with really low density wood). So, if you can officially get it on the scale before it re-equilibrates, yeah, you'll pick up a little. If its a rainy, humid day, you'll gain a bit more. My experience is that 5% is the most you're going to see, and, as I said, it goes away quickly. So, on a 10gr tower, as a practical matter, if you pick up two to three tenths of a gram, you'll be doing well.
One of my students baked a tower, and set it on fire. I don't think baking is good, because it damages the wood quite a bit, and loosens glue points.
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Complexity02
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### Re: Towers B/C

Balsa Man wrote:
Complexity02 wrote:With regionals approaching in less than two weeks, I have finalized my design, but desperately looking for ways to lose some mass on my towers. And in these forums, suddenly this "baking" of towers popped up, and has me intrigued. I am interested in doing this, but I want to proceed with caution. A few questions:
Should the baking be done to finished towers?
Is there a risk of glue melting off the tower as a result?

I looked at some old forums, and they say to bake the tower for about 20 min at 200 degrees, but that's about all the information I can dig up.

P.S: I had not posted on these forums for a while, and my old account got deactivated for some reason. I was unable to reactivate it, so created a new account with almost same username
Let me start by saying that 200 degrees is too hot. I've used 140 as an upper limit, and it works fine.
You want to make sure the oven is off- as in heat it up, turn it off, let temp drop to ~140.
10 minutes will get you all the dehydration you're gonna get.

To maintain the dehydration, you're going to need an airtight box, and you may want to have a desssicant pack (silaca gell) in the box, too. If it's not airtight, it will....re-equilibrate with atmospheric moisture within a few hours.

Is all this trouble worth it? When you open the box, within less than 5 minutes, it will re-equilibrate with atmospheric moisture. Half of what you loose 'baking' will be back within, oh, a minute (happens quicker with really low density wood). So, if you can officially get it on the scale before it re-equilibrates, yeah, you'll pick up a little. If its a rainy, humid day, you'll gain a bit more. My experience is that 5% is the most you're going to see, and, as I said, it goes away quickly. So, on a 10gr tower, as a practical matter, if you pick up two to three tenths of a gram, you'll be doing well.
Well, I guess I won't be baking it then. It does seem to be more trouble than help for inexperienced hands like mine. My next tower will be my regional tower, the worst tower to experiment baking on. I wouldn't be comfortable personally to see it go into an oven for 10 min Aiming to keep it under 8 grams at bonus.

aldragon
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### Re: Towers B/C

How do you get a higher density wood but lighter? It seems that every time I get wood, the lighter density the lighter the weight. My balsa sticks for 1/16 by 1/32 ranges from .2-.4 and 1/16 by 1/16 ranges from .1 to .22. Which wood am I better of using? I saw the past few post about bucking strength but I'm not sure which wood is better off using.

To add on, My legs are 3/16 by 3/16. Each stick is about 2 grams so it should cut down to 1.7. I'm trying to aim for a max of 8 grams. Also, are 1/16 by 1/16 braces enough to hold all 15 kg?
Region/PUSO/SOUP/States 2017

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Towers: -/-/15/3

PUSO/SOUP/States 2018

Heli: 9/6
Towers: x/51
Mousetrap: -/-

Balsa Man
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### Re: Towers B/C

aldragon wrote:How do you get a higher density wood but lighter? It seems that every time I get wood, the lighter density the lighter the weight. My balsa sticks for 1/16 by 1/32 ranges from .2-.4 and 1/16 by 1/16 ranges from .1 to .22. Which wood am I better of using? I saw the past few post about bucking strength but I'm not sure which wood is better off using.

To add on, My legs are 3/16 by 3/16. Each stick is about 2 grams so it should cut down to 1.7. I'm trying to aim for a max of 8 grams. Also, are 1/16 by 1/16 braces enough to hold all 15 kg?
OK, some major disconnects going on here. Let’s go through element by element.

How do you get a higher density wood but lighter?
You don’t, you can’t, and I don’t understand where your disconnect on this is coming from. Density is mass per unit volume (pounds per cubic foot, grams per cc, etc). When talking about balsa sticks (of the same cross sectional size), it can be expressed gr/36” stick. A 1.2gr/36 1/8” stick has twice the density of a 0.6gr/36” 1/8” stick.

It seems that every time I get wood, the lighter density the lighter the weight. My balsa sticks for 1/16 by 1/32 ranges from .2-.4 and 1/16 by 1/16 ranges from .1 to .22.
Yes, as noted above, the lighter the density, the lighter the weight. I’m not sure where you’re getting your stick weights (which I assume are for 36” sticks). I have never seen a 1/16” x 36”stick much under 0.2gr (maybe 0.17, 0.18).

Which wood am I better of using? I saw the past few post about bucking strength but I'm not sure which wood is better off using.

To add on, My legs are 3/16 by 3/16. Each stick is about 2 grams so it should cut down to 1.7.
Assuming a 36” stick length, and a C tower meeting the 29cm circle bonus, leg length will be about 61.3cm. So, a 61.3cm piece out of a 2gr 36” (91.6cm) piece should weigh 1.34gr, not 1.7gr. Don’t know how you got that number. To figure out what bracing interval you have to use to brace those legs so they’ll carry design load, you need to know their buckling strength. Then you can run inverse square calculations to get the bracing interval required. Ideally, you’ll want a set of four with closely matched buckling strength. The weakest will fail first, so the bracing interval needs to be enough to support the weakest leg.

I'm trying to aim for a max of 8 grams. Also, are 1/16 by 1/16 braces enough to hold all 15 kg?
At some density, 1/16 bracing could successfully brace the legs so they could hold 15kg. What that density needs to be, you’ll need to calculate (again, you need to have measured the buckling strength to do this).

The question is, is this the lightest way to go. Getting under 8gr on a C tower (that will carry full, or very close to full load) is...... a pretty serious goal; it will not be easy. You're not going to be able to do it unless a) You build with very good precision/symmetry (see discussion on jigs), b) you know/track both the buckling strength and density (stick weights) of your wood supply, c) you know the strengths you need for your design, and get/use pieces that provide needed strength at the lightest weight.

Hope this helps; the info is here to get close to that goal; hope you can pull it off. Good luck. If you come up with new questions that haven't already been answered in previous posts, do feel free to ask.
Len Joeris
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### Re: Towers B/C

Does anyone here have any recommendations for gluing techniques? After examining my joints, it seems like they are definitely weaker than the balsa itself. I'm currently using the widely accepted method of picking up small drops between two pins and placing them at the edge of the joint, but it seems like this just isn't giving me the strength I need.
Last edited by HandsFreeCookieDunk on March 2nd, 2017, 10:04 am, edited 1 time in total.

aldragon
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### Re: Towers B/C

Thanks Balsaman. I probably misread and misinterpreted the posts (I did read the past post you told me to read but I was referring to some post on the previous page) and I'm also on a time constraint with no time to build for my next competition (which is in a few days).
Region/PUSO/SOUP/States 2017

Heli: 5/20/6/4
Towers: -/-/15/3

PUSO/SOUP/States 2018

Heli: 9/6
Towers: x/51
Mousetrap: -/-

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