Boomilever B/C

[UQOnyx]

The problem with a triangular box (by box I assume you mean tower) design is that where would the loading block go? The third comp. cord which wold be in the center would interfere with the loading block chain or hook.

[Can_Of_Tuna]

I suppose I could make the bottom beam shorter, and have two supports extending from that point to the ends of the two top beams... This should leave enough room for the bolt to go through.

[UQOnyx]

Maybe someone else could answer if the forces on each member would be equally spread out since I do not know enough to say.

[nxtscholar]

We're talking about a compression beam with a triangle cross section, right?

If so, I actually initiated the topic a while back in the forum. You can read the comments starting from here: http://scioly.org/phpBB3/viewtopic.php? ... &start=210

The general idea (and thus in general terms) was that while you'd effectively knock off about 1/4 of the weight (3 sides instead of 4), it wouldn't be worth the loss in compressive strength as per the area moment of inertia formulas.

[Can_Of_Tuna]

Huh. Haven't considered that. I might make one for comparison with a regular box-beam anyway. My other idea is a double compression tube. I realize that the two sides would be hard to make exact, but what are your thoughts on this idea?

[chinesesushi]

Huh. Haven't considered that. I might make one for comparison with a regular box-beam anyway. My other idea is a double compression tube. I realize that the two sides would be hard to make exact, but what are your thoughts on this idea?

for the double tube design, i imagine it would be difficult to cut down on the weight of the double tube as a single tube alone would be 5-10 grams optimistically, which means a double tube would likely be around 10-20 grams.

[iwonder]

For the triangle, yes, you save a little bit of weight, but try it, and you'll find that members with triangular cross sections like that tend to twist under compressive loads. It's rather strange, but I had it happen to me in towers a few years back (I'm pretty sure asking that question is why I joined the forums actually :O ) and the answer was that non-symmetric 4 beam members and three beam members have a tendency to twist. You'll have to increase your bracing strength to make up for it. It's hard to tell if it's worth it or not without trying it.

[UQOnyx]

Yeah, like a helix. It's really hard to explain, but something like this happened to me. If the angles are not PERFECTLY all 60 degrees it will twist into a weird shape. But for those interested, try it out yourself and you will see what I mean.

[GeorgeInNePa]

1) See the hole in the tube at the distal end? The loading block eye-bolt goes through there.

2) The tube is hollow. It's 1/16" balsa, soaked in water and "wrapped" around a piece of 1/2" copper water pipe.

3) The tension member goes through the tube and is glued into a wooden "plug", coincidentally, the loading block also bears on the same block.

4) We've had good luck with 1/16" balsa. The finished booms usually weigh 10.5g to 11.5g, depending on the sheet density. I want to test some 1/32" this year.

We will be testing a tube at the Rustin Invitational tomorrow. It's heavy, over 13g...

This is kinda old but information pertaining to the tube booms, isn't copper water pipe banned...? And how would you "wrap" around the wood..

You remove the copper thing to make a hollow tube. And you soak it so you can wrap it around. GeorgeInPA or anyone else, how long do you soak in the water for?

Exactly, the copper pipe is just a form. It's not part of the finished Boom.

We never really timed how long the soak is. It's usually, "Hey, that balsa is still in the sink...".

I would guess 5 minutes or so would be fine.

[GeorgeInNePa]

Huh. Haven't considered that. I might make one for comparison with a regular box-beam anyway. My other idea is a double compression tube. I realize that the two sides would be hard to make exact, but what are your thoughts on this idea?

for the double tube design, i imagine it would be difficult to cut down on the weight of the double tube as a single tube alone would be 5-10 grams optimistically, which means a double tube would likely be around 10-20 grams.

I assume they mean twin, smaller diameter tubes. We had considered trying that, but getting the balsa to bend to a small diameter isn't easy...