Elevated Bridge B/C

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Re: Elevated Bridge 2009

Post by gh »

tehkubix wrote:...I don't know where I read this but if you soak it in water and then bake it it decreases the mass by a lot....
Soaking wood in water with allow force the cells to expand, if they were crushed during handling. That will decrease density once it dries, but won't decrease mass. Baking the wood will make the moisture leave the wood, which does decrease the mass. However, moisture in the air will return to the wood as soon as it cools down. IIRC, the Boomilever Wiki recommends a few grains of rice in the box your device is carried in to absorb the moisture and decrease the mass.
tehkubix wrote:Also, if you wanted to do that you should take one strip of balsa and cut it in half length wise. That's about the only way to get two really really similar pieces density wise imo.
I told you that. :D
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Re: Elevated Bridge 2009

Post by Aia »

Pleiades wrote:what was the mass of your bridges?? have you tested them yet?
I can't mass my bridges because I had to check my scale back in to my coach. Likewise, I can't test my bridges because all the loading equipment is up my coach's classroom.

If you ask me those questions again in 2 weeks, I'll be able to answer them. : )
gh wrote:the Boomilever Wiki recommends a few grains of rice in the box your device is carried in to absorb the moisture and decrease the mass.
Does that rice trick actually work? Has anyone tried that?
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Re: Elevated Bridge 2009

Post by Pleiades »

i have a digital scale that i use to mass my geckos but it only has a 1 gram accuracy. both of my bridges currently weigh 17 grams but they are both made out of chopsticks so once i start using balsa the mass will drop siginificantly.. hopefully
Last edited by Pleiades on July 20th, 2008, 10:17 am, edited 1 time in total.
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Re: Elevated Bridge 2009

Post by andrewwski »

So what's the accuracy? 1 gram or 0.1 gram? Because I think you mean the accuracy is 1 gram, but the way you wrote it means that it's 0.1 gram.
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Re: Elevated Bridge 2009

Post by Pleiades »

1 gram as in no decimal points
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Re: Elevated Bridge 2009

Post by NamG »

You can find a digital pocket scale on ebay for cheap (less than $20). It might be worth looking into as having your own scale is extremely useful. I bought one off ebay that is accurate to 0.01g.

Also, you can make your own loading block out of plywood. Or Pitsco sells them. The rest of the loading equipment can be found/made fairly easily. The sand may be the only thing you have to buy, unless you want to harvest it from somewhere. Or you could improvise the weights. Use free weights, water, gravel, books, or a car jack.
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Re: Elevated Bridge 2009

Post by Aia »

Okay, I built yet another bridge yesterday. On this bridge, I tried to design it as to limit the ends from sliding away from each other upon loading. I don't think I quite achieved my goal.

Any thoughts on how to build a rigid structure without making it excessively heavy?
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Re: Elevated Bridge 2009

Post by Pleiades »

I just tested my first chopstick bridge and it ended up getting an efficiency of 106. The bridge weighed 19g and it held just over 2kg. It's not THAT BAD considering it's made out of chopsticks glued with elmer's school glue but i know that balsa bridges will be getting efficiencies WAY higher. A lot of the joints failed. Of course the major problem was the two sides pushing away from eachother. it's going to be tough to avoid that. The chopsticks were also really weak. As for ideas on how to make a rigid structure.. since it's my first year doing building events i'm not quite sure but i'll probably be able to answer that once i become more experienced with this event. I cant wait until late september when i can get my balsa and glue and actually build a legitimate bridge. Since i'm new to this i dont really know what size balsa to get. I'm assuming i'd need 1/8 and 1/16. Also is it ok to use duco cement for a glue?
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Re: Elevated Bridge 2009

Post by carneyf1d »

ive never done boomilever or bridge, but i do know a thing or two about balsa wood from planes. i've always seen CA(cyanoacrylate used to bind balsa). ive also heard of ambroid, but ive never used it. remember the strength of a joint is proportional to the area of balsa joined.
when shaping balsa, you can wet it. i think its the cohesion, or adhesion(forgot which is which) forces within the wood and water cause the wood not to break. could be wrong, but as long as the wood bends im happy. but be careful when drying it...if you put it in the oven, the stresses in the wood can cause it to warp when drying. i usually let my balsa air dry. and put it next to a hot lamp or something.
remember there are different types of grain. A grain is for bending, and C grain is stiff. and make sure to keep checking your densities so you can get exactly what you need.
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Re: Elevated Bridge 2009

Post by gh »

Joints made with ambroid/Duco glues are flexible. They are also weaker than CA joints. Thus, they are good for planes, where robustness of joints leads to a more crash-resilient plane, but not as good for structural balsa building, where strength matters more. In addition, thin CA will soak into the wood further than ambroid/Duco, leading to more overall strength. However, CA dries fairly dense, so be very conservative when using it in planes or balsa structures. Listen to bah's advice about gluing.
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