Boomilever for 2013

[T-B]

If we want to try making balsa tubes for the compression member, how thin should we go to get it to roll? I presume 1/64th and it rolls into a long pencil-shape with the grain going the long way, right?

Isn't there a huge risk of failure related to using balsa in that thin? Even 1/32nd is quite thin, but I say that as one of the worst wood evaluating teams in all history. Basically, how thin does it have to be to roll and do you have to soak it first like you do when making an arch?

[SLM]

There's been some talk about that... It's very likely that the rules will change in order to allow the larger cross section. And SLM, what about x's down the length of an I-Beam instead of plates, would that be lighter and still effective?

I could carve out the inside of the stiffeners but would keep the outer box intact, as shown below.



But, I am not sure if placing x-bracings down the length of the beam would help with local buckling of the flanges.

[iwonder]

The second picture(hollowed out) is what I meant, only expanded so the center of the x is the web(it arose from concern about the web breaking where the loading block sits on it), so that when you look down the length of the tube you see x's. Also, what software are you using for the images?

[SLM]

The second picture(hollowed out) is what I meant, only expanded so the center of the x is the web(it arose from concern about the web breaking where the loading block sits on it), so that when you look down the length of the tube you see x's. Also, what software are you using for the images?

Yes, that makes sense.

I use SketchBook Pro on iPad for creating the hand drawings.

[iwonder]

If I had an iPad...

By the way, what's the reason behind splitting the I-Beam into 4 flanges instead of 2? and even with all this talk I tend to think the tubes will come out lighter...

[SLM]

If we want to try making balsa tubes for the compression member, how thin should we go to get it to roll? I presume 1/64th and it rolls into a long pencil-shape with the grain going the long way, right?

Isn't there a huge risk of failure related to using balsa in that thin? Even 1/32nd is quite thin, but I say that as one of the worst wood evaluating teams in all history. Basically, how thin does it have to be to roll and do you have to soak it first like you do when making an arch?

A main reason for using a tubular section for the compression member is to eliminate bracings. This means the cross-section of the tube ought to be large enough for developing sufficient buckling strength. I am not sure if a single pencile-size (in diameter) tube would be sufficient for carrying the boom's compression force. This means you will need multiple tubes to handle the load, which means you will be forced to use bracings to hold the tubes together. Probably this defeat the purpose of using tubes altogether.

If you use a large enough single tube, 1/32"-thick balsa will work. However, I am not sure about 1/64" thickness, it may be too thin for the intended purpose.

[iwonder]

Even if you needed bracing for multiple tubes, would'nt it still be more efficient to build the member out of tubes becuase they offer almost the same buckling strength compared to a solid member, and weight a lot less?(they would offer the same strength, right?)

[SLM]

If I had an iPad...

By the way, what's the reason behind splitting the I-Beam into 4 flanges instead of 2? and even with all this talk I tend to think the tubes will come out lighter...

The top flange as a whole cannot buckle since it is attached to the web along its centerline. If the compression force in the member is large enough to cause the entire flange to buckle (say outward/upward), then the entire beam has to buckle (that is, the web and the lower flange need to buckle upward as well).

However, each half flange is supported by the web along one side only. Each half flange is free along the outer edge. That means each half flange can buckle independent of the web and the other half flanges.

[T-B]

So that means the primary value of using a tube is eliminated if you end up needing to use two tubes which would function similarly to two square or rectangular beams? I still need to think through the dilemma about how you would attach the tension chords to the tubes. Not much gluing surface. You would have to create it artificially which adds more weight. Oh, well. It is early in the year, so I can play with it and see how it works.

Something tells me that I should really be focused on learning the Structural Analysis Notebook. That would probably have better long term benefits. Does the SAN "know" the structural stats for Balsa and Bass, or do I have to generate those somehow?

Thanks,
T-B in CLT

[iwonder]

There are ways around the gluing issue, but that would be the trick right? I can't give away all my tricks

And the SAN uses whatever value you give it for modulus of elasticity, that's the only number it needs. The problem then, is that you can't build the structure with different densities of balsa, but that's not to big a deal. Speaking of the SAN, does anyone know why it's giving me displacements in members but I'm not seeing any stresses/forces?

Edit: Oh, and anyone looking to buy arrow shafts, if you don't have a local shop, the big name sporting goods stores sometime sell arrows for bow fishing(specifically, arrow blanks) that aren't carbon fiber and in the $50-$90 price range.