Boomilever for 2013

[Faustina]

Wow, the discussion is getting interesting. I have a question, though. Will longer tubes hold less than shorter tubes of the same thickness and diameter? For example, iwonder said his tube was "3.75" long, weighed 1.2g and held 50kg before failure." So will a 16 inch tube (~40 cm) hold less than 50 kg or not?

[SLM]

Wow, the discussion is getting interesting. I have a question, though. Will longer tubes hold less than shorter tubes of the same thickness and diameter? For example, iwonder said his tube was "3.75" long, weighed 1.2g and held 50kg before failure." So will a 16 inch tube (~40 cm) hold less than 50 kg or not?

A compression member can fail in one of two ways. If the cross-sectional area of the member is too small and/or the wood has low density, then the member fails when the compression force reaches a certain value. This mode of failure however is not a function of the member's length. If the 3.75"-long tube failed in this mode, then a longer tube (with the same cross-sectional size and density) would also fail (theoretically speaking) at the same applied load.

The other mode of failure is buckling. Buckling strength of a compression member does depend on the member's length. If the 3.75"-long tube failed due to buckling at 50 kg load, a 16"-long member (with the same size and density) would fail when the applied load approaches 2.75 kg.

[iwonder]

Of course, identifying which mode of failure the tube was in when it failed is easier said then done. Even so, the longer the is much more likely to buckle, even if the shorter tube was in compressive failure.

[jander14indoor]

Unless the buckling and compressive loads are VERY close its generally pretty easy to tell which happened in a post mortem. The broken parts look very different.

Compressive failure involves little change of part shape and failure kind of looks like parts driven together with surface exploding outward in multiple directions.
Buckling the parts center will move distinctly one direction sideways and failure looks like a fold or bending failure.

Jeff Anderson
Livonia, MI

[iwonder]

Ahh... Thanks for the correction... In that case, the short tube underwent compressive failure, I have a loner tube and I'm going to build a boom soon to see how it all holds up... Hopefully the base/tension members do fine

[retired1]

The longer a tube is that is unsupported, the more prone it is to a buckle. Tubes are extremely strong for their weight. Unfortunately, I think that a tube will be subject to the same criteria as apiece of wood, IE: 1/4 x1/4"
You can not say that it is a built up member, because if you build a tube from material that is 1/4 or less, it will fail or you are extremely good or lucky.

[iwonder]

Kinda confused about your terminology... What do you mean by a built up member? Also, what is a tube built out of 1/4 or less? And what would make it so failure prone? Finally, what criteria would a tube be subject to that's the same as a rectangular section? Unless you mean a .25" Square piece of balsa and a .25" diameter tube, which would exhibit the same properties(or similar properties).

[retired1]

Built up member- a piece that is made up of several parts as a box beam or an I beam or a laminated beam.
Rule 2b of the trial event is very specific in that your starting wood an not be larger than 1/4 x 1/4".
If you made a tube out of say 1/16 x 1/4 , it would be nearly impossible to have it round and you would have numerous glue joints to fail. It would add a lot of weight, negating some of the advantages of a tube.
I recant on the tube diameter-I was not thinking of the 1/4 starting wood when I said that. There is a possibility that they would allow a 1/4 dia tube, but that would have to be built with 1/32 or thinner balsa and I question the strength of it. Remember that a long tube will need to be supported and a thin wall tube is not going to accept most bracing (based on airplane tubes)
I hope that the 2013 rules clarify that. If not, it will be an early question on the rules.

[iwonder]

It's been brought up before, and thoroughly discussed, as to why the .25" square cross section will, in all likelihood, not apply. Also, a few pages back jander14indoor had a very good link to a method that produces very nice round tubes of ~.25" diameter without much glue. The bracing issue has also been discussed, but because the cross-section limit will most likely not apply, it hasn't been a large problem.

[retired1]

Well, the rules will be out in 6 weeks and then we should know or at least have more discussions on interpretations.