A few of my friends and I are building a classic trebuchet for physics class and we are going to launch something and take several various measurements and write a lab about it. Anywho, the other day in the shop we started arguing over what would be a better way to position the counterweight. Should the weight be hung of the lever arm so that it falls in a straighter path downward, or should it be directly attached to the arm so that it swings in a circular path. My physics teacher couldn’t decide if either way would have a benefit over the other, but on Wikipedia the article on trebuchets said that it is more efficient if the weight falls more nearly straight down. I can’t prove, or figure out for that matter, which would be better, but it seems like a hanging weight would be better. This picture shows the simple difference between the way the two weights would swing.
Smithsonian Magazine covered a launchoff between the two types of trebuchets. I don’t think have the article around here, but I seem to recall that any difference in performance was negligible.
Remember though, a fixed-weight design has to have wheels to allow the trebuchet to move back and then forward as the weight moves through its arc. Otherwise, the forces will try to tear the machine apart, and there may (will?) be a loss of power. A trebuchet with the weight in a cradle doesn’t need wheels (and probably shouldn’t have them, unless guessing at where a war machine will roll is your idea of a fun time. :eek: )
…And of course the usual disclaimer–I am not a medieval miltary engineer. I’m just some schmuck who has thought about building one of these things as a father-son project.
(Wow, somebody posting a mirror in case the first site won’t load–I’m impressed!)
Neither design is the most efficient, as both waste energy when the counterweight moves horizontally. This wasted energy gets translated into extraneous back-and-forth movement of the carriage, necessitating wheels so the whole frame can move instead of tearing itself apart.
Ideally, the weight should drop straight down, which is what it does in a floating arm trebuchet. The pivot shaft moves back and forth on a track, allowing the weight to drop straight down with no horizontal movement at all, thus translating all the energy into the arm and thus to the payload. This results in a much longer throw than a either of the other designs using the same weight.
There was another article on the trebuchet in Scientific American around 1995 or so. IIRC, they said that the primary advantage of a hanging-weight trebuchet is that the recoil on the frame is less when the trebuchet is fired, and thus your John Q. Medieval doesn’t have to re-aim it as much between shots. In other words, it’s more efficient in the sense that it delivers a larger number of heavy objects to your enemies in a given amount of time.
Of course, the less energy goes into the recoil of the frame, the more will go into throwing said heavy objects. So maybe it’s better in both respects.
If you have the time, go to trebuchet.com and buy one of their “convertible” kits and try both modes. When you’re done, you’ll have a cool desktop toy. Or, buy the “ATreb” simulator program if you want to stay purely theoretical and want no fun toys when you’re done.
And yes, as cornflakes said, the machine as a whole needs to be mobile in a fixed-weight design or the thing will either twist itself to death or flip over.
(I’m unrelated to trebuchet.com, except as a customer.)
I logged back on to suggest basically the same thing. Why not build a convertible and use that as one of the input factors. For control of the experiment, I’d place the center of mass for the counterweight in fixed mode at the same place as where the pivot is when the weight is suspended. Also, the base would need to be locked in place when using a suspended weight and you will need some sort of a stop to maintain the same range of motion in both modes.
I agree with spingears that this board doesn’t exist to give homework answers, but you’re asking about set-up, not asking for answers. IMHO, there is a difference. I don’t know how the boards’ moderators feel about this; we may know in time.
For what it’s worth, I seem to remember that small changes in the angle of the hook that holds the releasing end of the sling can have a dramatic effect on release angle and in turn on distance. You may want to include it as an input to your experiment.
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Just out of curiousity, has anyone here built one of these things? If so, what kinds of wood are acceptable to use? I’ve thought about building one with my kids as a shop/science project. I’ve got plenty of #2 yellow pine laying around, but I’d be afraid that yellow pine would split after just a few launches.
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I’d imagine quite a few people here have made one. I once made one out of a cardboard box, a chopstick, and a pill bottle full of pennies. Obviously not very large, but it threw balls of aluminum foil ~30 feet (estimated, as my apartment isn’t that big). I made it on a whim one evening, and was restricted to materials that I had handy.
As to whether pine will be strong enough, it all depends on the scale you choose. If you’re building a smallish device to throw tennis balls or the like, it should be fine. If you’re looking to build something capable of launching pumpkins, not so much, unless you used pretty substantial stock. After all, I’d imagine that the full-sized models throwing 300lb projectiles used pine throwing arms, as hardwoods seldom grow really straight and long.
Worst case something snaps while firing, which could result in the projectile flying off in an unexpected direction.
Thanks, Gorsnak. I’m thinking of building one about 8-10 feet high for throwing pumpkins or cantelopes.
I’m guessing that the ones made way back when were probably made of old growth pine. New growth pine studs can split pretty easily. I’ll probably laminate the arms out of 1X6 pine stock if I build it.
I take it there are no maples in Toontown? There’s a reason their genus name means “straight”. Maples can grow every bit as straight as pines, and are much harder than most pines, to boot (redwoods might be harder, but not much else).
And while this is sort of a homework question, my hunch is that the OP is still going to do the lion’s share of the work, in actually constructing and loosing the thing. So it sounds OK to me, though IANAMAM. And besides, it’s a trebuchet! I mean, you can’t say no to another trebuchet thread!
This is Saskatchewan, man. There’s a tree down near Swift Current, but that’s it.
Actually we have numerous types of trees, but no maples to speak of, unless you include the Box Elder, which 'round here is known as the Manitoba Maple. Anyways, I’ve never seen any deciduous tree compare to a conifer for straightness, but I’m perfectly willing to be corrected on that point.
Hardly. Read the OP again, and notice the part where he mentions the discussion he and his partner have been having with their teacher, none of whom could figure it out. Obviously, a grade isn’t riding on this discussion, but rather a true fighting of ignorance in finding the most efficient use of energy for a trebuchet. Settle.
The kit I got from trebuchet.com is oak. Not sure if that’s a reflection on oak’s strength, or its wide commercial availability in ready-milled 1/2 x 3/4" s4s stock (smooth on all 4 sides)
If you’re looking to make a “full” scale treb, have a look at glu-lam lumber - it’s essentially large timber made of many smaller layers bonded together - much stronger than normal hunk of tree.
One of the best places to see trebuchet designs pitted against one another in a relatively controlled test is Delaware’s World Championship Punkin Chunkin Competition. The trebuchet category allows any size machine or counterweight, but standardizes the ammunition: one pumpkin, eight to ten pounds in weight. This year the Yankee Siege trebuchet team lofted their pumpkin a world record 1,394 feet (link shows a video of this throw). Notice how their counterweight shakes the huge trebuchet back and forth on its 8-foot diameter wheels. Here’s another clip of the same trebuchet in action. I’m a little surprised that they don’t use some sort of braking system to slow down the oscillations after the toss, especially on a machine that big.
Photos of other teams’ trebs are available, and you can check the records to see what kind of distances they got. A little mensurration of the photos should lead you to the proper proportion of arm length, counterweight mass, etc. and also give you an idea of which counterweight design is more efficient – at least as far as pumpkins are concerned.
By the way, you should bug your physics teacher to let your school enter a team for next year’s competition. My high school physics class went there in 1995 (back when nobody was breaking the 1,500-foot mark, even with air cannons) and it convinced me that I wanted to become a mechanical engineer. Road trip!
Useful information, but considering the last line and your previous post, I can’t help but think you’re being snarky at the expense of the OP. Uncalled for, IMHO. The answers to nearly all of the questions posted in GQ could be found “on the internet with a few simple searches,” but then I can also teach myself the fundamentals of the TIME CUBE with a few simple searches.
The nice thing about GQ is that an expert, or at least someone with some experience, can interpret and explain. This particular question is not really all that easy, as I suspect the answer probably depends more on practical construction considerations than theoretical calculations. And just because a question is school-related doesn’t make it homework.
I thought it was interesting that the chassis did not move until after the pumpkin was launched. This makes me think that any energy lost to fore and aft forces during launch are minimal when a suspended weight is used…
To echo what zut said… What’s your point? The OP basically asked, “I don’t understand the difference between these things” and is looking for an interpretation. Coming to the SDMB is going to provide someone with much more patience and understanding than any 58-page .pdf document ever will - aside from your snarky comments.
Interesting executable. The calculator seem to generate the farthest ranges when the length of the counterweight-side of the beam and the length of the cord holding the counterweight are as large as possible without the counterweight hitting the ground, but the exact ratio of those two lengths for the farthest throw seems highly dependant on the other attributes of the trebuchet. The calculator can’t handle setting the length of the cord to 0, but setting the cord length to .0001 worked well enough.
