Boomilever B/C

[iwonder]

Wow, that's a pretty nice jig for building the smaller box-beam members... and it's pretty neat that your calculations arrived at a member 3/8" high and my current members are 3/8" high

[Balsa Man]

Wow, that's a pretty nice jig for building the smaller box-beam members... and it's pretty neat that your calculations arrived at a member 3/8" high and my current members are 3/8" high

Thanks. As I've said before, proper, effective jigging is a real key to booms/towers/bridges.
Yup, your sense of the science is pretty darn good. Well done.

The dropping of the old, 'no piece can have a cross-sectional dimension greater than 1/4" rule opens possibilities. Back in '08, we hadn't figured out how to do "I" calculations, but got to a 3/8" square ...modified box beam- using 1/4" width sidewalls, with 1/16th... corner stringers- walls and stringers at pretty high density - giving outside dimensions at 3/8 x 3/8. It got the compression/buckling strength needed. Heavier than the just walls, no corner stringers discussed above.
That's how I knew this year that a ~3/8th x-sect for balsa would....get to the needed stiffness range for an un-braced 40cm boom.

Turned out to be quite a complicated build- like a long-handled slingshot, with a little "Y" at the distal end- 2 tension members coming in near the ends of the Y- through slots drilled through the member- seriously complicated, compound angle drilling.... This year, the design mantra's been keep it simple! A compression member, a tension member- simple (in overall configuration) as it gets- still some complications in how to actually build that, but we think we've worked them through.

[iwonder]

Next question... The idea of hardwood dowel pins through the base has been brought up, and in principle it sounds like a really nice idea, but if you use end grain balsa, what's the pin going to 'push against'? When the tension force is loaded on it, wouldn't it simply slice through the grain(since the grain of the tension members and, to an extent, the base is parallel to the tension force)?

[Balsa Man]

Next question... The idea of hardwood dowel pins through the base has been brought up, and in principle it sounds like a really nice idea, but if you use end grain balsa, what's the pin going to 'push against'? When the tension force is loaded on it, wouldn't it simply slice through the grain(since the grain of the tension members and, to an extent, the base is parallel to the tension force)?

Good question, as usual.
I think you must be referring to my comments 3-4 pages back - on what I referred to as “shear pins” as an….ultimate option for improving the shear strength in the glue joint where the tension member is held in the base/attachment plate. I’d indicated low density balsa, in end-grain orientation, would work for base/plate. I’d thrown out a diameter of 1/32nd for such pins- I guess technically “dowels”, ‘cause they have a circular cross-section.
Yes, with end-grain orientation in the base/attachment plate, grain orientation in it is…..close to the grain orientation of the tension member, and the tension force on the t-member. The strength with which the balsa would resist the pin being pulled through it would be it’s crush/compression strength, parallel to the grain. When you…..squish a piece of low density balsa from the sides, it’s pretty soft/weak. The side/perpendicular crush/compression strength is a lot less that the …..end-grain/parallel to the grain strength. If you look back to my post on this, the suggestion/step/stage before going to a pin, was higher density (than the end-grain block/attachment plate) balsa ‘facing’- similar to Aia’s ‘wedges’ – to improve shear strength of the glue face. So, in this approach, the pin outside the t-member would be into/engaging higher density balsa.

It’s been seen/acknowledged that a pair of 3/32nd bass t-members will work (i.e., have sufficient (and maybe more than sufficient) tensile strength for a C-boom, hence more than enough for a B-boom); Aia’s experience is that with the suggested wedges, and a ½” thick block/plate, joint shear strength is sufficient, so I don't think there is a need of pins in this set-up. I’d really included this idea in the range of possibilities as a possible way for getting a single tension member to work. Experience and the physics involved in that suggests that with this doubling of forces, hardwood to hardwood is the solution. If a pin is needed with that, it’s engaged in hardwood on both sides, so there's no tear/slicing issues…..

[iwonder]

That sounds good, I was looking at that kinda of thing because there wasn't enough strength in the glue/balsa for nice, small, hardwood tension members to work in a boom

Still not sure how the distal end is done with a single tension member...

[Balsa Man]

That sounds good, I was looking at that kinda of thing because there wasn't enough strength in the glue/balsa for nice, small, hardwood tension members to work in a boom

Still not sure how the distal end is done with a single tension member...

In both cases, this falls into the....."glad to discuss in more detail later in the season" category. Suffice it to say, it can be done, at both ends

We, as a team, barring a miracle, aren't gonna be at Nationals, so any "competitive secrets/advantages" really only relate to our State.
I'm more than happy every year to provide (sometimes quite detailed) insights/discussion at a general issues sort of level, starting early in the season - help get the level of technical understanding, and solution options up, and provide some "how to" ideas. I enjoy doing that, and I think it....helps up everybody's game.

Each year, I push our kids to brainstorm new/modified ideas- moving ahead from the....existing body of knowledge - how do the physics and the rules, and what's been done before, come together in possibly better ways. Out of respect for those efforts, if an idea or two that provides a competitive advantage comes up, the details stay with us until we hit Regionals. I encourage the kids to talk w/ the competition at Regionals - let them see what we have. If there's a key "how", that get's discussed at State. I....sort of track that opening of information here.

[iwonder]

Completely understand... I tend to keep certain things guarded myself when it concerns certain details with devices. Still, it's a very interesting idea and I'll just have to figure it out for myself

[liutony66]

Can someone please define the terms:

attachment base
left bolt/right bolt
bolts in general
contact points

preferably with a picture. Thanks!

[Balsa Man]

Can someone please define the terms:

attachment base
left bolt/right bolt
bolts in general
contact points

preferably with a picture. Thanks!

I'm guessing, from these questions, you haven't read either the rules, or all the information that's already been posted here.
Do yourself a BIG favor, and do that. You will be amazed at how much you learn/understand.

Then feel free to ask more questions.

very quickly-
the attachment base is what you use to attach the boom to the wall (which is done with bolts)
left or right bolt depends on which way you're looking at things- there are 3 boltholes in the test wall- a middle one, and one on either side (left/right)
bolts in general are.... bolts, rules call for 1/4 inch (diameter) ones, and,
contact point is the point at which something contacts something- as in arm of boom contacts wall

[liutony66]

Friends getting on my account and posting during meetings... Sorry about that. Thanks anyways.