Re: Elevated Bridge B/C
Posted: May 1st, 2010, 10:08 am
I haven't posted in quite a while now, and certainly haven't taken the time to read through the entire 70+ pages. However, I wanted to say just a couple things.
First, Balsa Man, you say that to get beyond 2000 in B division takes some magic mix of design, finding some special pieces of wood, and very precise building, and predict that the top one or two at nationals this year will score around 2500. Well, I agree with the design and 2500 part, but not the special pieces of wood and precise building part. Last season, we had 10 balsa and 88 bass pieces in the national entry, which weighed 5.96 grams, did not fail during loading, and took first place. This year, the design is very similar to last year (see the picture in the rules manual), but has 14 balsa pieces and 80 bass pieces. The bass was purchased at northeasternscalelumber.com, and the balsa at my local hobby store. Nothing special about either, except picking a density for the balsa and seeing whether or not it was high enough. By precise building if you mean perfect angles and great workmanship, then I disagree. If you mean great glue joints and just average workmanship, then I agree. A great glue joint, to me, has neither not enough nor too much glue (odorless CA, gap filling viscosity for us) and has no gap between the wood pieces where they contact each other. On our builds, the joints with higher stresses coming together at the node points are designed to have more glue surface area and use more glue, and vice versa. Considering that many of the 94 or 98 pieces in one of our builds has two ends, that's a lot of glue joints. I tell the students over and over that you don't want one of the higher stress joints to fail prematurely, causing the bridge to fail before it should. Rather, you want structural failure to occur somewhere away from a glue joint.
Now for my second point. Assuming that you can get structural failure to occur repeatedly, then how do you go about maxing out efficiency and pushing the 2500 efficiency mark? Probably the most notable thing is to figure out just how the bridges are failing. A week ago, I tested two prototypes that I built. The efficiency of the lighter one was about 2000 and the heavier one (by a whopping 0.11 grams) was just short of 2300. Both had the same failure mechanism, which was that the trusses pinched together at the center of the member between nodes 2 and 3 on our 9 node design. These members are the ones next to the members that the loading block rests on (see picture in rules manual). So, as each bridge was being loaded, you could watch how the cross piece and Xs on either side of the center cross piece for the members started bowing upward. At the point just before structural failure, the bowing increased dramatically. This happens very quickly and is therefore tough to catch for light builds. So, what's the remedy for this pinching/buckling? Use larger wood for the trusses? Find that magical piece of wood that doesn't break under the stress? These certainly are options, but not good ones in my book. I very much prefer to keep the trusses exactly the same (wood type and dimension) and use slightly larger cross pieces to connect the trusses, while using the same size Xs to connect the trusses. This worked for us last season leading up to nationals, and we will be doing the same this season. As far as the cross pieces, the website above sells three sizes that you can't get in most hobby stores (1/32" square, 0.040" square, and 3/64" square). Midwest Products also sells 0.0406" square pieces that are 11 inches long, with 15 per pack. Some hobby stores carry it and others don't. Of course, almost everyone carries 1/16" square. And by the way, I'm talking bass, NOT balsa. Getting back to the tests last week, they both used the same nominal size cross pieces and Xs. However, I noted as I was building them that the cross sectional area of one was a bit less than the other, as a new purchase was used for one and an old purchase was used for the other. The one with slightly smaller cross sectional area failed first. So, our national build, which the students are working on, will use the next size or two larger cross pieces between the two trusses in the center of those particular members, and the same size X pieces as my prototypes. So, I hope everyone is getting a clear picture of how to max out on your designs.
Good luck to all at nationals!
First, Balsa Man, you say that to get beyond 2000 in B division takes some magic mix of design, finding some special pieces of wood, and very precise building, and predict that the top one or two at nationals this year will score around 2500. Well, I agree with the design and 2500 part, but not the special pieces of wood and precise building part. Last season, we had 10 balsa and 88 bass pieces in the national entry, which weighed 5.96 grams, did not fail during loading, and took first place. This year, the design is very similar to last year (see the picture in the rules manual), but has 14 balsa pieces and 80 bass pieces. The bass was purchased at northeasternscalelumber.com, and the balsa at my local hobby store. Nothing special about either, except picking a density for the balsa and seeing whether or not it was high enough. By precise building if you mean perfect angles and great workmanship, then I disagree. If you mean great glue joints and just average workmanship, then I agree. A great glue joint, to me, has neither not enough nor too much glue (odorless CA, gap filling viscosity for us) and has no gap between the wood pieces where they contact each other. On our builds, the joints with higher stresses coming together at the node points are designed to have more glue surface area and use more glue, and vice versa. Considering that many of the 94 or 98 pieces in one of our builds has two ends, that's a lot of glue joints. I tell the students over and over that you don't want one of the higher stress joints to fail prematurely, causing the bridge to fail before it should. Rather, you want structural failure to occur somewhere away from a glue joint.
Now for my second point. Assuming that you can get structural failure to occur repeatedly, then how do you go about maxing out efficiency and pushing the 2500 efficiency mark? Probably the most notable thing is to figure out just how the bridges are failing. A week ago, I tested two prototypes that I built. The efficiency of the lighter one was about 2000 and the heavier one (by a whopping 0.11 grams) was just short of 2300. Both had the same failure mechanism, which was that the trusses pinched together at the center of the member between nodes 2 and 3 on our 9 node design. These members are the ones next to the members that the loading block rests on (see picture in rules manual). So, as each bridge was being loaded, you could watch how the cross piece and Xs on either side of the center cross piece for the members started bowing upward. At the point just before structural failure, the bowing increased dramatically. This happens very quickly and is therefore tough to catch for light builds. So, what's the remedy for this pinching/buckling? Use larger wood for the trusses? Find that magical piece of wood that doesn't break under the stress? These certainly are options, but not good ones in my book. I very much prefer to keep the trusses exactly the same (wood type and dimension) and use slightly larger cross pieces to connect the trusses, while using the same size Xs to connect the trusses. This worked for us last season leading up to nationals, and we will be doing the same this season. As far as the cross pieces, the website above sells three sizes that you can't get in most hobby stores (1/32" square, 0.040" square, and 3/64" square). Midwest Products also sells 0.0406" square pieces that are 11 inches long, with 15 per pack. Some hobby stores carry it and others don't. Of course, almost everyone carries 1/16" square. And by the way, I'm talking bass, NOT balsa. Getting back to the tests last week, they both used the same nominal size cross pieces and Xs. However, I noted as I was building them that the cross sectional area of one was a bit less than the other, as a new purchase was used for one and an old purchase was used for the other. The one with slightly smaller cross sectional area failed first. So, our national build, which the students are working on, will use the next size or two larger cross pieces between the two trusses in the center of those particular members, and the same size X pieces as my prototypes. So, I hope everyone is getting a clear picture of how to max out on your designs.
Good luck to all at nationals!
