Ongoing Contest(Scores)

[iwonder]

Here's the other way of figuring this out... Pick the loading block/bucket up with your hand and try to unevenly load the bucket so it gets the desired effect... You'll find it's quite impossible

[dholdgreve]

Not yet, if you don't mind indulging me a little further. My temrinology was imprecise - I am aware that the center of mass must always be directly below the suspension point - in fact, that is the root of my confusion. When the mass is distributed unevenly, the center of mass moves back underneath the suspension point. If the handle-bucket pivot is sticky, however, this would have to push the handle itself (not the whole system, mind you - that was never what I was thinking) out of plumb. I guess my question is, is this situation not then repeated at every joint all the way up the chain? As the handle of the bucket tries to rotate in the s-hook, is there not some tiny lateral force opposing its motion?
Now that I put my unformed thoughts into words, though, I realize that at each step such a force would be in the opposite direction which, besides being very small, would help explain why it is of no practical significance.

Okay, I think I can stop worrying about it now. Thank you for your explanation.

I think you are getting way too hung up on the bucket (no pun intended)... Focus on the chain... the one, single point of connect between the bucket and the structure... If the chain is straight, vertical, plumb... whatever you want to call it, it makes absolutely no difference where the sand is within the bucket... if you could figure a way to keep the sand in while the bucket hung sideways, it just doesn't matter. IS THE CHAIN VERTICAL? It is impossible for a vertical chain to pull harder on one side than the other... well, wait a second... if you are bearing on two compression beams, and one is slightly lower than the other, the eyebolt is tight in the hole through the loading block, then yes, the higher of the two compression beams will receive more load than the lower... but assuming that both compression beams are level, there cannot be an uneven load applied through a vertical chain... no matter where the load is in the bucket...

Now don't get this confused with a load shift... If the bucket is hung, so that at half load, the bucket shifts because the chain had a kink, or twist, causing the bucket to swing, this is a dynamic load applied and totally different than the sand being off center. NOTE that the if the bucket is swinging, the chain is no longer straight, so the statement about it being impossible no longer applies.

In summary... FORGET the bucket! FOCUS on the chain! Make sure there are no links twisted, make sure it's centered on the handle of the bucket, make sure that if you are using a dispenser, that neither the chain or the bucket is touching anything that could force the chain off of vertical.

And another thing... Don't screw around getting the sand loaded... Do it as quickly but smoothly as possible... Slow loading only forces your structure to hold the load longer... If you are using a sand dispenser, run it wide open! as fast as you can! Even if your structure breaks, the E/C is more likely to take too much sand back out of a slow load than a fast load...

[dholdgreve]

Yeah, hopefully the tilted bucket "problem" is clear for this year.

On a tube for the compression member. It all started with a conversation with a MechE friend- I usually chat with him at the start of the season to gather food for thought for our builders - basic physics/enginering, how could you do/build that, how could you get the precision needed; things like that. He helped me understand the structural advantages of thin wall tubes in compression. What played into that was the incredibly complicated jigging/building my son ended up doing for boom in '08 - single square box beam C-member, with a "Y" at the end to hold the block; 2 tension members. Also, I wanted to get to something really simple, because it was apparent our boom builders didn't want to put a whole lot of testing/development time in. We're not going to be going to Nationals, so the target we set was something EZ to build, that could do well in-State.

The tube we came up with is not a rolled tube. It is a 5/8ths i.d., 1/32nd wall tube made of 2 pieces of balsa. It is quite precise in wall thickness and straightness. Starting w/ the 2 pieces, it takes about 15 min to ...have a finished tube. Adding the tension member, another hour. Tension member is a hardwood. Couple tricks in the T-member, and how its attached to the bolt. I am going to hold off on the hows for now.

The Regional one is significantly over 10 gr- intentionally conservative. Its also significantly under 15. Density of tube wood is a bit under 12#/cf. The design calc sheets, looking at E X I at those dimensions, says that at a bit under 8#/cf, it should hold full, and get to a structure weight under 10gr. Ya never know till you hang it, though....We'll see how far we can push it between Regionals and State. Not a Nats winner, for sure; conceivably medal....maybe. It's easy to build, beautifully simple, really cool looking, and they're ready to be quite competitive here in-State.

Very interested in hearing the results of both your regional boom and the state boom. Although I certainly have the utmost respect for those that continue to push the envelope by thinking out of the box, I still believe strongly in the KISS principles, especially as it applies to Division B... The more joints they make, the more chance of a bad one.

[Balsa Man]

[quote="dholdgreve still believe strongly in the KISS principles, especially as it applies to Division B... The more joints they make, the more chance of a bad one.[/quote]

Amen, brother!
At every joint, glue has to be right, surface contact has to be right, and alignment of the pieces coming into the joint has to be right.

[fanjiatian]

It seems it's all the rage to build trussed compression beams this year.

Sorry, just making sure I'm up to date in this thread: you're referring to the use of a box beam vs. a simple solid member right?

[dholdgreve]

It seems it's all the rage to build trussed compression beams this year.

Sorry, just making sure I'm up to date in this thread: you're referring to the use of a box beam vs. a simple solid member right?

Good point, definition of terms is critical... To me, a box beam has actual solid sides, top, and bottom, probably from 1/32" material, glued continuously to corner material located inside the box beam whereas the diaphragm of the sheet actually provides the bracing as opposed to any form of X or Z bracing. This is quite possible, now that the rule about 1/4" cross sectional material max has been removed.

A truss beam, on the other hand, may be created by using (4) 1/8" x 1/8" corner beams as compression members, projecting horizontally from the vertical testing wall that have been trussed both vertically and horizontally to each other. In this scenario, there must be close to 80 nodes, or glue joints on the truss beam alone. That is 80 chances for failure. This model has no material larger than 1/8" x 1/8" and nothing really substantial the resist the shear of the loading block. It must take at least 3 times as long to build, with no real advantage that I can fathom. I coached this event several years ago when it was in Division C, and don't remember ever seeing any designs resembling this. Then, all of the sudden this year, probably at least half of the booms I've seen have that type of design. I must therefore assume it was demonstrated at a coach's clinic, or something similar.

As with Len, I'm really not too interested in giving away all the secretes quite yet, other than to say if you are currently using this design, consider the alternatives, as this design will not get you where you want to be. Keep it simple, simple is good...

I believe that the National Boom winner in Division B will be well over 1500 points, and it will be a very simply design, focused on very good material selection, precise, clean cuts, glue used very sparingly, and a simple design that maximizes every single piece on material in it to over 90% of capacity. By the way, I betcha it does not carry the full load, breaking at between 14.5 and 14.9 KG. Any takers?

[Balsa Man]

I actually expect the C- winner to be pushing, probably a bit over, 2,000 pts. With forces in a B-boom ~3/4 of those in a C-, B- should theoretically be able to score a bit higher, if someone can put the combo of good design, good wood, and careful build together. And yup, very close to full load, but under. Don't want to leave money on the table at Nats

[dholdgreve]

I actually expect the C- winner to be pushing, probably a bit over, 2,000 pts. With forces in a B-boom ~3/4 of those in a C-, B- should theoretically be able to score a bit higher, if someone can put the combo of good design, good wood, and careful build together. And yup, very close to full load, but under. Don't want to leave money on the table at Nats

WOW!!! Now THAT is a bold prediction! Even if it carried 14.99 KG, to score over 2,000, it would need to weigh less than 7.5 grams... I just don't see that happening. Certainly not with the design that we have been working on... That is the really cool thing about this event... it's like a chess game... relatively easy to learn, difficult to master, nearly impossible to perfect! Every design has an efficiency limit... Where each piece is 100% utilized... very, very few of us ever see that in our designs... The proof would be when every piece fails at exactly the same time. I believe our efficiency limit would be in the 9.0 to 9.2 gram range, but we will never see that... I can't even fathom what a boom might look like with a limit of less than 7.5! Amazing.

Len, will you be attending nationals?

[Balsa Man]

Oh, I'd love to, but, no, that isn't going to happen.
I got to go once, in '05, the year my older son was a senior; 2 C- teams went that year; we were the second. It was a great experience.

Bold prediction? Maybe... I sure don't see many at/above 2000, but one...wouldn't surprise me at all. Depends on where the institutional knowledge from the last boom winner at Nats is, and, to some extent, when you're pushing that far out to the edge of engineering possibilities, getting/identifying/using pieces of wood at the extremes of the physical properties envelope- those 1 in a hundred at the edge of the bell curve. The measured variability I've seen over the years is significant; greater than the variability in E vs density in that 27 sample set in the old US Forest Service study I posted a link to a few months back.

[dholdgreve]

Oh, I'd love to, but, no, that isn't going to happen.
I got to go once, in '05, the year my older son was a senior; 2 C- teams went that year; we were the second. It was a great experience.

Bold prediction? Maybe... I sure don't see many at/above 2000, but one...wouldn't surprise me at all. Depends on where the institutional knowledge from the last boom winner at Nats is, and, to some extent, when you're pushing that far out to the edge of engineering possibilities, getting/identifying/using pieces of wood at the extremes of the physical properties envelope- those 1 in a hundred at the edge of the bell curve. The measured variability I've seen over the years is significant; greater than the variability in E vs density in that 27 sample set in the old US Forest Service study I posted a link to a few months back.

OK, ... Lucky for me, this year's nationals are practically in my back yard, at Wright State. Should be a terrific venue for this competition, and having run "practice" invitationals the last three years, should have a cast of seasoned veteran volunteers, that will pull off the best national competition yet! I'm planning on attending and pretty much camping out at Booms, anxiously waiting this 2000 point structure! I sincerely hope you are correct, but a few years ago I went to the national's held at THE OSU and frankly was disappointed... There were a handful (15 to 20 at the most) that I would call competitive... The rest sort of looked the the first one they ever built. We will see.