Designs

[iYOA]

oh and how about your testing jig?

[lucwilder42]

are you by any chance using a desiccant for your tower?
what technique are you using to apply glue?
are you sure your tower is perfectly straight?
i think a little bit more info might be needed

Accelerator, sometimes. I use it when I don't want to sit and hold the joint so for maybe 1/2 i did, 1/2 i didnt

Im applying the glue from the bottle onto the crosses then to the main structure pieces (4 main). Then holding tight onto the joint for 5 seconds and either accelerating or letting sit.

I'm absolutely positive its straight

There's a good chance your problem is the accelerator. While useful for helicopters and other flying events, it is not ideal for towers. CA takes about 24 hours to fully cure, and when you use accelerator, that curing process is cut way short and the CA loses a lot of strength. The glue should stick within a few seconds of pressing the the joint together anyways, so accelerator really isn't necessary. Patience when building is essential

[mrsteven]

are you by any chance using a desiccant for your tower?
what technique are you using to apply glue?
are you sure your tower is perfectly straight?
i think a little bit more info might be needed

Accelerator, sometimes. I use it when I don't want to sit and hold the joint so for maybe 1/2 i did, 1/2 i didnt

Im applying the glue from the bottle onto the crosses then to the main structure pieces (4 main). Then holding tight onto the joint for 5 seconds and either accelerating or letting sit.

I'm absolutely positive its straight

There's a good chance your problem is the accelerator. While useful for helicopters and other flying events, it is not ideal for towers. CA takes about 24 hours to fully cure, and when you use accelerator, that curing process is cut way short and the CA loses a lot of strength. The glue should stick within a few seconds of pressing the the joint together anyways, so accelerator really isn't necessary. Patience when building is essential

Ok thanks! So stop using accelerator lol
I know theres always an argument going on balsa VS basswood. Whats the take? I would think for the main structure using basswood and supports with balsa wood since it is good in direct compression. Any issues?

[lucwilder42]

There's only way to find out for sure :)

[mrsteven]

There's only way to find out for sure

haha ya, thats what im in the process of doing this week- although with a competition on saturday and huge homework ill have to pick one and go with it. Not enough time to make too many prototypes before then, just wanted to get the take of some most experienced towers builders- I'm more of a robotics/metal guy myself

[iYOA]

in my very personal opinion, basswood isnt worth spending time over. the argument about it being much more consistent than balsa isnt that significant because if you do stiffness tests of balsa wood sticks and make sure to measure the mass of each stick you use, building consistently is very feasible(also precision matters but that does not mean using basswood is a substitute for precision)

another thing is that if you have two sticks of equal length and equal mass, one being bass and the other being balsa, the balsa wood stick will be stronger because it will have a larger cross section despite being less dense.

[Balsa Man]

in my very personal opinion, basswood isnt worth spending time over. the argument about it being much more consistent than balsa isnt that significant because if you do stiffness tests of balsa wood sticks and make sure to measure the mass of each stick you use, building consistently is very feasible(also precision matters but that does not mean using basswood is a substitute for precision)

another thing is that if you have two sticks of equal length and equal mass, one being bass and the other being balsa, the balsa wood stick will be stronger because it will have a larger cross section despite being less dense.

Well, from the same context- personal opinion.
My opinion comes from 3 years of parent helping, then 7+ years of coaching (towers, bridges, and boomilevers).
I agree strongly, and have long preached the importance of precision. The two most important aspects being 1) control of the shape variable – as you develop/refine a design to get to the lowest weight that will work, you need to take out (as much as possible) the factor of each build being in slightly different shape, because those differences in shape will mean differences in forces (and where wood of some specification may have held in one build, it won’t hold in another build, and 2) getting things symmetrical (in 3 dimensions) – where the forces are equally distributed (for example, in the 4 legs of a tower).

On bass and balsa, I have to disagree (in spite of my handle) . For a number of years, I was “pure balsa.” Then listening to discussion of this long-debated subject, we decided to do some testing.

First, the statement suggesting that to be the same weight, bass and balsa sticks would have to be of different sizes (as in cross section) is simply incorrect.

The density ranges of bass and balsa overlap. You can have, over some range of density, bass and balsa sticks of the same length and cross section, that weigh the same.
A specific example- 3/32nds of an inch square cross section, 36” length. If you check Specialized Balsa’s website, you will see you can buy balsa from 0.4 to 2.2 grams (per 36” stick). While they don’t have bass listed by density/stick weight, I can tell you from having picked through/weighed.....hundreds of 3/32nd bass sticks, the weights at 36” range from about 1.8 to 2.6 grams. So, throughout the range from 1.8 to 2.2 grams, you can have bass or balsa sticks of the same size and weight.

The question is, of course, is which is “stronger”? The mention of larger cross section being stronger indicates iOYA’s talking about in compression loading – specifically resistance to column failure in axial loading (load applied along the length/axis of the stick); i.e., the legs in a tower. My experience, and some data says at the same density and cross section, bass IS stronger under axial compression loading - the load at which column failure occurs is higher.

The factors governing column failure are seen in Euler’s buckling equation (posted earlier for anyone that wants to look). One factor (which iOYA mentions) is cross section – technically the area moment of inertia (“I”), which is a function of cross section; a bigger cross section (all other properties being the same) is “stiffer”/resists buckling more than a smaller cross section. However, there is another term in the equation which isn't mentioned; “E”, the modulus of elasticity. This is a measure, essentially, of the inherent stiffness of a material.

The critical force (“F” – the force at which a column buckles/fails) is “E” times “I” (times pi squared), divided by the length squared. So, it is not just the cross section that determines resistance to column failure; its the cross section and the elasticity together.

Back when elevated bridges came in as the “balsa building event”, we did some compression/column strength testing, and the data said that at the same size (length & cross section) and density (i.e., the same weight), bass had higher column strength. In that testing, we also saw that the variability (at same size and weight) in balsa was higher than in basswood. In bass, the variability was on the order of 10%; in balsa, it was closer to 20%. Is that “significant”? It depends on how hard you’re pushing, how close to the edge you’re trying to get. At some level of competitiveness, yes, it is absolutely significant – for a tower, a half a gram, or a bit more.....

Now, there is not a lot of published data on the value of “E” for bass and balsa, but there is some. Here’s one source: http://www.conradfp.com/pdfs/ch4-Mechan ... f-Wood.pdf Look at table 4-1 on the second page; there are E values for both bass and balsa, and guess which one has higher values??

So, it is a combination of our own test data, and published data that forms the basis for my opinion that “if you have two sticks of equal length and equal mass, one being bass and the other being balsa, the BASS wood stick will be stronger” (i.e., will give you more column strength). If anyone has data that is different, I'd sure love to hear of it.

In the end, am I saying this means using bass for legs is "THE" way to go; that it is "the best" way; that a bass-legged tower will beat a balsa-legged one? Not at all - the efficiency of the overall design depends on more factors than the leg wood. But, on an equal weight basis, all the data I'm aware of indicates that bass legs do offer a strength advantage.

For this post, I guess I ought to sign off as Bass Man.....

[tcm1234]

4 legs! weight distribution

[thsom]

Did troy's tower hold all of the 15 kg?

[lucwilder42]

Did troy's tower hold all of the 15 kg?

Yup, whereas mine held 14.95 or something =/