Chuckin a 6 lb pumpkin 400lbs 17ft boom 8ft leads
Will longer leasd get more distance?
i am hesitant to add weight.
Getting a little sketchy
advise on getting more distance
Re: advise on getting more distance
Going to need more information to answer for sure. The answer is maybe, but maybe not.
67:1 Mass ratio (counterweight to projectile) is on the low side, so your sling line length might be close. Is 17' the long side or the total length? Some information on both beam lengths (long side and short side - known as the Long Arm and Short Arm respectively) would help. For a normal hinged counterweight trebuchet (is that what you have or something else?), sling lines of ~80% the length of the long arm are a usual good starting point, but that's for a 100:1 mass ratio. For a lower mass ratio like yours shorter is usually needed.
Have any video of shots, taken directly (or close at least) from the side? It's easy to help you tune it in with one of those.
67:1 Mass ratio (counterweight to projectile) is on the low side, so your sling line length might be close. Is 17' the long side or the total length? Some information on both beam lengths (long side and short side - known as the Long Arm and Short Arm respectively) would help. For a normal hinged counterweight trebuchet (is that what you have or something else?), sling lines of ~80% the length of the long arm are a usual good starting point, but that's for a 100:1 mass ratio. For a lower mass ratio like yours shorter is usually needed.
Have any video of shots, taken directly (or close at least) from the side? It's easy to help you tune it in with one of those.
Re: advise on getting more distance
For a simple hinged counterweight trebuchet, what are the parameters that can/should be tuned, what is a reasonable range of tuning values, and which parameters are most important? From what I've read online, it's something like this, from most important to least important:
1. Release Pin angle, 20 to 40 degrees
2. Counterweight to projectile weight ratio, 80:1 to 133:1
3. sling length to throwing/long arm length, 1:1 to 0.8:1
4. long arm to short arm length, 3.75:1 to 5:1
I'm designing a new trebuchet. It's a simple design, but large. My constraints are how much my tractor can lift, and the size of the barn doors it'll need to fit through.
The first three parameters on the list above are adjustable in my design, but adding an adjustable pivot to alter the arm ratio is making the design more complicated. I want to minimize rotational inertia (weight at the extremities). If I have an adjustable pivot, I need to do two sets of equations (highest ratio and lowest ratio) and build the arm to meet both sets of requirements. Not only is the math more difficult (and I want to get building, not do more math!), but it will require more overall mass in the throwing arm to provide sufficient strength for both configurations. The extra weight will decrease arm acceleration/speed.
But even if there is value in having an adjustable arm ratio, changing the arm ratio is significantly more difficult than modifying parameters 1-3 on the list above. So, in the field, I'm likely to just pick one setting and leave it there anyway. I might as well pick my one setting at the design stage.
So, is the list above accurate, and can number 4 be a design choice and not a tuning parameter?
1. Release Pin angle, 20 to 40 degrees
2. Counterweight to projectile weight ratio, 80:1 to 133:1
3. sling length to throwing/long arm length, 1:1 to 0.8:1
4. long arm to short arm length, 3.75:1 to 5:1
I'm designing a new trebuchet. It's a simple design, but large. My constraints are how much my tractor can lift, and the size of the barn doors it'll need to fit through.
The first three parameters on the list above are adjustable in my design, but adding an adjustable pivot to alter the arm ratio is making the design more complicated. I want to minimize rotational inertia (weight at the extremities). If I have an adjustable pivot, I need to do two sets of equations (highest ratio and lowest ratio) and build the arm to meet both sets of requirements. Not only is the math more difficult (and I want to get building, not do more math!), but it will require more overall mass in the throwing arm to provide sufficient strength for both configurations. The extra weight will decrease arm acceleration/speed.
But even if there is value in having an adjustable arm ratio, changing the arm ratio is significantly more difficult than modifying parameters 1-3 on the list above. So, in the field, I'm likely to just pick one setting and leave it there anyway. I might as well pick my one setting at the design stage.
So, is the list above accurate, and can number 4 be a design choice and not a tuning parameter?
Re: advise on getting more distance
ChrisBSR,
Yes, throwing arm ratio is typically a design choice and not something you tune with. Decide you want to change the ratio later? Build a new arm. 4:1 and 5:1 are the right range usually, unless you are running well under a 100:1 counterweight:projectile ratio. There's an old rule of thumb that says arm ratio should be 1/20th of the expected mass ratio, and that works OK if you're in the 75-110:1 ratio range. Above that, I'd stick with 5-5.5:1. Below, find more counterweight!
Pin angle, sling length, counterweight ratio are your usual tuning options for a standard HCW, yep.
Counterweight ratio: eventually, you want to run at max weight and not change it. Unless you are running some very extreme ratios (in which case you're competing in World Championship events and shooting 2000' plus), it's unlikely you'll get better performance with less weight. Projectile weight of course affects things, but most of us are shooting a narrow range of projectile weights (baseballs, 8-10lb pumpkins, 4lb ice balls, etc).
Sling length on a standard HCW is highly unlikely to be equal to the long arm length. 80% is a good starting point though.
Pin angle doesn't have a "typical" range as it can vary heavily. Pin angle is usually defined from the axis of the throwing arm, positive forward, negative backwards. It's not common, but I've seen negative pin angles. If your ratio and sling length aren't totally weird, 30 degrees forward is a solid starting point, adjust from there.
Normal tuning process I go through: for a given projectile and counterweight, start at the 80% sling and 30 degree forward pin angle. Video directly from the side, making sure the entire sling motion and first part of release is visible in frame. What you are looking for in the video is for the counterweight to bottom out roughly at the time the arm hits 12 o'clock. On an HCW, the counterweight frequently lags a little, and will be 10-15 degrees short of getting to bottom-dead-center when the arm is vertical - totally normal. You want the sling to be around 45-50 degrees above the horizontal when the arm hits that 12 o'clock position. Adjust the sling length so the sling gets to that position when the arm does - long sling delays arrival, short gets there faster (short slings swing swiftly). Once you have the sling arriving in position at the right time, release pin angle is used to tweak the final trajectory.
One non tuning-related item that I've seen many HCWs not do right is making the hanger arm for the counterweight as LONG as possible for your setup - the weight should barely clear the bottom of your frame/trough/whatever. Don't make it short and leave a foot of clearance, you're hamstringing yourself if you do that! Longer hanger arms maximize your potential energy, which is the source of power for a trebuchet! That said, the arm should cock somewhere around 45 degrees above the ground - you can go a little steeper, but much more than, say, 55-60 degrees and you are making the throwing arm itself too short to do much good! Somewhere in the middle is likely the point of diminishing returns, but I've never done the research to determine it as it would need a bunch of arms, hangers, and general messing around that I've never had time for! 45-50 degrees cocking angle works pretty well in most situations though.
Yes, throwing arm ratio is typically a design choice and not something you tune with. Decide you want to change the ratio later? Build a new arm. 4:1 and 5:1 are the right range usually, unless you are running well under a 100:1 counterweight:projectile ratio. There's an old rule of thumb that says arm ratio should be 1/20th of the expected mass ratio, and that works OK if you're in the 75-110:1 ratio range. Above that, I'd stick with 5-5.5:1. Below, find more counterweight!
Pin angle, sling length, counterweight ratio are your usual tuning options for a standard HCW, yep.
Counterweight ratio: eventually, you want to run at max weight and not change it. Unless you are running some very extreme ratios (in which case you're competing in World Championship events and shooting 2000' plus), it's unlikely you'll get better performance with less weight. Projectile weight of course affects things, but most of us are shooting a narrow range of projectile weights (baseballs, 8-10lb pumpkins, 4lb ice balls, etc).
Sling length on a standard HCW is highly unlikely to be equal to the long arm length. 80% is a good starting point though.
Pin angle doesn't have a "typical" range as it can vary heavily. Pin angle is usually defined from the axis of the throwing arm, positive forward, negative backwards. It's not common, but I've seen negative pin angles. If your ratio and sling length aren't totally weird, 30 degrees forward is a solid starting point, adjust from there.
Normal tuning process I go through: for a given projectile and counterweight, start at the 80% sling and 30 degree forward pin angle. Video directly from the side, making sure the entire sling motion and first part of release is visible in frame. What you are looking for in the video is for the counterweight to bottom out roughly at the time the arm hits 12 o'clock. On an HCW, the counterweight frequently lags a little, and will be 10-15 degrees short of getting to bottom-dead-center when the arm is vertical - totally normal. You want the sling to be around 45-50 degrees above the horizontal when the arm hits that 12 o'clock position. Adjust the sling length so the sling gets to that position when the arm does - long sling delays arrival, short gets there faster (short slings swing swiftly). Once you have the sling arriving in position at the right time, release pin angle is used to tweak the final trajectory.
One non tuning-related item that I've seen many HCWs not do right is making the hanger arm for the counterweight as LONG as possible for your setup - the weight should barely clear the bottom of your frame/trough/whatever. Don't make it short and leave a foot of clearance, you're hamstringing yourself if you do that! Longer hanger arms maximize your potential energy, which is the source of power for a trebuchet! That said, the arm should cock somewhere around 45 degrees above the ground - you can go a little steeper, but much more than, say, 55-60 degrees and you are making the throwing arm itself too short to do much good! Somewhere in the middle is likely the point of diminishing returns, but I've never done the research to determine it as it would need a bunch of arms, hangers, and general messing around that I've never had time for! 45-50 degrees cocking angle works pretty well in most situations though.
Re: advise on getting more distance
Thanks for the detailed advice, Matt! This is very helpful!
I hope to toss a variety of items: watermelons in the summer, pumpkins in the fall, and whatever odd items my kids or guests want to toss. So I am planning to make the counterweight adjustable. I'm also influenced by previous experience: my first trebuchet broke when I tossed a very-small pumpkin without reducing the counterweight. The light load didn't offer much resistance, so the counterweight fell quickly and didn't transfer much energy to the projectile. And all that excess energy found the weak points in my frame design.
I hope to toss a variety of items: watermelons in the summer, pumpkins in the fall, and whatever odd items my kids or guests want to toss. So I am planning to make the counterweight adjustable. I'm also influenced by previous experience: my first trebuchet broke when I tossed a very-small pumpkin without reducing the counterweight. The light load didn't offer much resistance, so the counterweight fell quickly and didn't transfer much energy to the projectile. And all that excess energy found the weak points in my frame design.
Re: advise on getting more distance
As, so you basically "dry-fired" and the machine couldn't take the load. I'd advise making things strong enough to sustain that event regardless, as dry fires do happen from time to time despite all of our best efforts!ChrisBSR wrote:my first trebuchet broke when I tossed a very-small pumpkin without reducing the counterweight. The light load didn't offer much resistance, so the counterweight fell quickly and didn't transfer much energy to the projectile. And all that excess energy found the weak points in my frame design.