flyingwatermelon wrote:Does anyone have any recommendations or plans for building a testing rig? I'm planning to install a testing rig on the side of my garage but that involves nailing the rig to the beam of the house. I am sure there is a better solution to this, does anyone have ideas?
Attaching with screws would make it much easier to take down when you're done...
Well I was thinking of a transportable testing rig that didn't have to be installed to a wall.
Then you're talking a fairly serious construction project- a wall that's high enough to clear a bucket under the boom, framework/stiffening so the wall doesn't flex under load. Simplest way I can think of right off the top of the head - use pressboard shelf piece, like 4 ft by 12 or 15" for the wall, glue and screw 2x4s along the back side edges to stiffen (the 2" side of the 2x4s against the wall piece. Use a 2 ft sq plywood piece for the base, attach bottom edge of wall to back edge of the base piece. Run like 2x2 brace struts from top corners of the wall to the outside corners of the base piece; that ought to give you bucket room.
Have fun.
jma wrote:If the distance from the face of the testing wall to the center of the loading block is at least 40 cm, does that mean that i have to build the boomilever at least 42.5 cm long?
Well, you have to support the load block, so 40cm + 1/2 of 1/4" (the eyebolt thru the load block), and then enough to support the load block beyond the eyebolt hole. Does that have to be the full 2 1/2cm beyond the center of the load block? No, it's really a judgement call how much you need.
Okay, so I am used to doing glider and plane events, but I ended up doing this event this year because no one else had the time. Does anyone have any ideas for where I could start? Unfortunately my partner has no real idea what to do either, but he is pretty sure that we can build it all out of 1/8th inch balsa and/or bass. Is this even possible? We don't really have much of a budget at all.
314chocolate wrote:Okay, so I am used to doing glider and plane events, but I ended up doing this event this year because no one else had the time. Does anyone have any ideas for where I could start? Unfortunately my partner has no real idea what to do either, but he is pretty sure that we can build it all out of 1/8th inch balsa and/or bass. Is this even possible? We don't really have much of a budget at all.
You can easily do it with 1/8" balsa and bass. You could also use 3/32" balsa and bass, which would save a bit of weight. Balsa is ridiculously inexpensive, as is bass; the expensive bit is stuff like the saw and glue. If I remember correctly, wood costs $.32 per piece, a saw is ~$8, and glue is ~$7. You can get enough wood to build a boom with $5 (8*balsa stick + 6*bass stick+ etc. = ~$5), so your initial expenses should be ~$20. After that, you should only be paying $5 per boom. If you go to 3 tournaments, and build 2 test models before the first, and then one test model before the next two, the event will cost you $50 dollars, in $5 payments, by the end of the season.
Balsa is a bit more expensive in my area, but for the main compression members, I would certainly try to not use 8" long balsa since you will have joints in heavy compression.
In my computer model, we used 2 sticks of 24 pound 1/8 balsa. 22 pound was ok and 20 pound was marginal for our design.
For the 2 tension members, 3/32 basswood will give you about a 10% safety margin.
I paid dearly for the exactly 24 pound balsa. It turns out that I had some over 18 pound 1/8 balsa out of a package of 1/8 sticks that was too heavy for last year's tower event. It was poor quality, so The high priced was OK. Shipping eats up the dollars quickly if you do not have several "hobby shops" in a reasonable driving distance.
The amazing part was how small some of the balsa could be when the bracing was optimal. Some 1/32 sq, some 1/32 x 1/16 and some 1/16 sq. Only a couple of pieces of 3/32 under the load.
Building started for "B" last week. The computer says it will hold the full 15kg and weigh under 8 grams.
OK, worth a minute to go over, again, some real basics.
It can be hard to tell from a short post (either a question, or statement/suggestion) what level of understanding someone is working at. My sense is that both 314chocolate and orangeJuice could use some very basic help/understanding. If I’m wrong, I apologize –and this should be helpful to anyone who is struggling to get their heads around how this event works, and how to do reasonably well.
This has all been said before.
1) Take the time to carefully read through both Boomilever threads- this one and the Ongoing Contest one. Follow the links, and read/digest the info there. The information ranges from very simple/basic to very detailed and specific on some pretty high-end concepts. If you don’t understand something- after having read everything, ask specific questions.
2) Understand the basic forces at work in a boomilever – this is all discussed in detail – you have tension members running from the bolts to the boom arm- in a C-Boom, with two tension members, they’re each carrying about 22kg when you have a 15kg load on the boom. In a B-Boom that tension force is about 32 ½ kg. The boom arm is under compression loading- in a C-Boom, about 40kg at a 15kg load, in a B-Boom, about 30kg. Assuming you’re trying to get a boom that carries full load, the tension members have to have enough tensile strength to carry those forces, and the arm/compression member has to have enough strength to not buckle under the compression load.
3) Understand that BOTH the size and density of wood matters a lot. Just saying ‘will 1/8th work?’ is really meaningless. Both the tensile strength, and buckling strength vary a LOT, depending on both size and density. Heavier/denser wood is stronger than lighter/lower density wood. The density and strength of balsa varies A LOT; for bass the variation is less. For the same density, bass is generally a bit stronger; the density range of bass is toward the high end of the density range of balsa. Because wood is grown and not made, there is some variability in strength, even at the same density; assuming a 20% variability is reasonable.
4) To have any control on the strength of the structure, you HAVE to know the weight- the density – of your wood. Any “paired” members (for instance two tension members) need to be pretty closely matched. If you build a structure and it fails well below max load, you need to increase either the size, or the density, or both of the piece that failed to get a stronger structure. If you just build with “a bundle” of, say 1/8th sticks, and it fails at an 8kg load, and you build another one without knowing stick weights, it might fail at 4kg, it might go to 10kg- you have no way of knowing/predicting; no way to refine/develop your design.
Retired1 speaks of 20 pound, 22 pound, and 24 pound balsa; what he is referring to is density in pounds per cubic foot. Balsa varies from around 5 lbs/cu ft to over 20, approaching 25. For 1/8 sq, 36” long sticks, a 20 pound (per cu ft) stick weighs 2.95 grams; a 22 lb (per cu ft) stick weighs 3.25 gr, and a 24 pound (per cu ft) stick weighs 3.55 grams. A ‘middle of the road’ stick- with a density of 15 pounds per cu ft, weighs 2.2 gr. A light one- say 7 lbs/cu ft – about the lightest you’re likely to run into in a hobby store wood bin will weigh 1.05 gr. Looking at just tensile strength, that 15 lb/cu ft stick will carry about 36 kg tension, the 20 lb/cu ft will carry about 49 kg of tension, the 22 lb/cu ft will carry about 55 kg of tension, and the 24 lb/cu ft one will carry about 60kg, and the light 7 lb/cu ft one will only carry about 14 kg. Retired1 also notes that two 3/32nd sq bass tension members will carry the required tension load. In the link to Aia’s discussion of boom building you will find back near the beginning of the boom thread, you will find the same information. Our testing indicates that for mid-density bass – a 3/32nds stick at 24” weighing ~1.5gr, this is true. Buckling strength also varies by density.
5) You have to build with a pretty good degree of “repeatable precision’ to have a chance of developing/refining a design. If you can build to exactly the same shape/alignment each time, then all you need to do is adjust up the strength of the part that fails- if the shape/alignment is the same, the load on that piece will be the same, and if you adjust the strength enough, it will hold. If the shape is different, the load the piece will see is different. For instance, say for one boom, you get the two tension members accurately, symmetrically aligned; at a 15kg load, they’re each carrying 22kg. get them off/mis-aligned/asymmetrically aligned, one could easily be seeing 30 kg, and the other 14kg- the one seeing 30kg will fail prematurely- at about 2/3 of full load….
6) Last thing to consider (and why the suggestion for two 3/32nd bass tension members is a good starting place)- one of the challenges to getting a boom to work up to a 15kg max load is the glue joints at the ends of the tension member. You will see when you go to competition – and probably in your own testing, a lot of failure at these glue joints-there’s a lot of force, and pretty small glue area. While the tension members have to have enough tensile strength, there is another property that comes into play, shear strength parallel to the grain. The glue is a lot stronger than the shear strength of the wood- even if you were using pine, or even oak. If you put enough pull on the joint, you’ll get a shear failure- a thin layer of the wood on one side of the joint will shear off. Shear strength is also greater at higher density, so while that 1/8th balsa at 15 lb/cu ft appears to have enough tensile strength, it’s shear strength is probably not enough to work….
Please correct me if I am wrong:
This year 2013, According to rule 3.i. there is no limit on cross section length and sizes of wood pieces ....
While in year 2008 - there was limit of 1/4" x 1/4"
I have 2008 boomilever scoring sheet / checklist, can i strike out this condition from it. Does somebody has sample 2013 boomilever scoring sheet?
vikasdewan wrote:Please correct me if I am wrong:
This year 2013, According to rule 3.i. there is no limit on cross section length and sizes of wood pieces ....
While in year 2008 - there was limit of 1/4" x 1/4"
I have 2008 boomilever scoring sheet / checklist, can i strike out this condition from it. Does somebody has sample 2013 boomilever scoring sheet?
You're correct.
Haven't seen a score sheet for 2013.
Differences would be a) the no 1/4 x 1/4 limit, b) dimension limits for both B- and C- (was only for C- in '08 - same 40cm wall to load block bolt/15 cm centerline of bolts to lower limit of wall contact; for B- 40 cm x 20 cm height), c) I think the tie breaker of how far above the lower limit of wall contact boom touches the wall is new.
