Primitive Archer
Main Discussion Area => Bows => Flight Bows => Topic started by: Badger on March 13, 2018, 04:53:31 pm
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I am getting started on my giant bow project. 80 ft across. I am going to be forced to use a soft wood for the limbs because of the length. Looks like I will be limited to spruce, western red cedar or Doug fir. I realize I will have to make some serious design modifications but will need a little help from someone who can read the wood property mechanical sheets. What would be the best wood from those choices? I am also wondering how much less bend can I expect for a same thickness as a typical whitewood bow? I do plan to build several scale models and work up but am kind of looking for a starting point.
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Just curious, but what kind of draw weight are you looking for? What will you use for the string? How are you going to draw it? I hope you keep us updated with pictures as you go.
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You know you won't get the performance that you would out of osage......:)
DBar
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Steve,
You win!!!!! Hahaha!
What is the history behind this project? Does it have to be 80 feet long? What about 20 feet? I think it will be really challenging to do with any kind of wood. I think the procedure required just to string this thing would be enough to kill me. A deflexed design might help in case you find it has to remain strung for long periods of time. Could you splice hardwood laminates together into some kind of mega-bow blank?
I bet the incredible scale of this project will give the neighbors something to talk about.
Alan
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Steve
this came up earlier in another thread, I cannot recall what was said, but I ran some calcs in wood bears spreasheet on a doug fir gluelam. you can buy any length up to 60' long at the local lumberyard.
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strains for the few larch and DF bows I have tillered were about .6 vs .7 for birch, for the same amount of set
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You may consider white oak? I know its used as planking for plank on frame ships. They like to get the planks long, so perhaps source boat lumber.
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strains for the few larch and DF bows I have tillered were about .6 vs .7 for birch, for the same amount of set
You were able to get the bows to 0.6% strain and live? That’s pretty impressive. I keep my hardwood bows around 0.5%, depending on species & condition.
Alan
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White oak would be great choice!
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If possible I want to make the limbs 1 solid piece. I plan to use 2 timbers about 6" or 7" thick, 2 feet wide and 32 feet long attached to a stationary center piece about 20 ft long. I am open to a stack leaf design if I can get it to work right. I am hoping for a draw weight of about 8 to 10,000 pounds and about a 24 ft draw length. I plan to draw it to destruction going up incrementally until it fails. Hoping for a little over 100,000# stored energy.
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White oak would be great choice!
White oak is my first choice but I doubt I can get that in 1 piece. If I absolutely have to I will cover the backing with FG cloth but I really want to avoid that.
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You may need to tightly bind it with rope every few feet to keep it from throwing a 2x4 sized splinter!
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Ok Steve why a 80' foot bow? What's the deal. Worlds largest bow? Plywood laminated together. Arvin
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You will need a very tall archer to avoid the lower limb hitting the ground ;)
Del
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Ok Steve why a 80' foot bow? What's the deal. Worlds largest bow? Plywood laminated together. Arvin
Da Vinci had drawn up plans for an 80 ft war machine but never built it.
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You will need a small tree for the arrow, and really big feathers for the fletching! And a large crew to operate the drawing mechanism. I'd like to see that work - from a ways off! Ol' daVinci had some good ideas, a lot of them worked! I sometimes wonder if he had alien influence, or some funny smoking stuff, or both!
Hawkdancer
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Make a take down Steve! Use white oak, the shipyards use stave quality wood, to steam bend ribs and planks. What you need is out there. Call a few up, im certain they can offer help on sourcing it for you.
Gannon and Benjamin come to mind. A well known outfit up in Massachusetts.
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daVinci had some good ideas, a lot of them worked! I sometimes wonder if he had alien influence, or some funny smoking stuff, or both!
Steve, your teasing us. We are having to guess too much. Is there a trip to burning man in your future?
2 feet wide and 32 feet long
a giant doug fir stave :o
I am open to a stack leaf design if I can get it to work right.
like a car leaf spring? some side by side stacking wold allow you get away from the 2 foot wide requirement
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I think it would need to be wider than 2' for a 80' length. Not enough back for 24' draw. Arvin
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Arvin, building on a 12X scale. wider would be better but even getting a 2 ft piece is kind of tough.
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Yes I understand Steve about the availability. I have so many questions about this project. Seams very interesting though. I will be watching for sure! Arvin
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Oh this wil be fun to watch! Remember the other Davinci show you did, saw that way before I ever touched a bow and realized years later it was you when I saw the show again ;D
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if you made laminated limbs out of sheets of plywood it could easily be 48" wide... :BB
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I was trying to build each limb self bow but I may have to go laminated.
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Do you plan to simply scale up an 80” bow made from the same material in order to get the dimensions? I wonder how well this would scale? For various reasons, the bending strength properties of large samples are not as good as small clear test samples, so you will probably have to go much more conservative.
Alan
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I was intuitively thinking the same thing Allen, theoretically the surface of the wood can't be under any more strain than a small bow but it seems there would have to be some extra strain from something so large I am not accounting for. I actually want to establish the feasibility of the project before I go to far. Working with a large piece of lumber would really simplify it but I am having my doubts as to whether or not they could handle sufficient strain.
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Sadly, the old growth trees that would yield some 30' limbs are getting scarce. I wonder just how much one would have to scale back their expectations for the strength of materiel with a lot of well done lamination?
it seems there would have to be some extra strain from something so large I am not accounting for.
I think the limb strains will scale with a static analysis, although the dynamics of the tip return speed might be something to look into.
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I wonder if limbs will sag under their own weight?
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Sleek, depending on the wood the limbs will weigh somewhere between 1,000 and 2,000#, I don't believe they will sag, hopefully not anyway.
Willie, just from some small tests I have played with it doesn't seem like it is going to scale up very well. The 20 footer should tell me a lot.
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Sleek, depending on the wood the limbs will weigh somewhere between 1,000 and 2,000#, I don't believe they will sag, hopefully not anyway.
Willie, just from some small tests I have played with it doesn't seem like it is going to scale up very well. The 20 footer should tell me a lot.
The testing will be based on a percentage of stored energy rather than peek draw force. Example. a 66" stores roughly 50# energy and we shoot an arrow that is about 1/1000 of 50#. So arrow weight of 1/1000 of stored energy is what the testing will be using.
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So arrow weight of 1/1000 of stored energy is what the testing will be using.
I am curious what draw weight you will tiller the 20 footer to, in order to make the efficiency comparison?
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I will do it by physical dimension but I believe it goes up 4 times while the stored energy goes up 8 times. I want to see how well this holds up when actually built as opposed to theory. I built a 25 footer for the Da Vinci show and it came our pretty much exactly how I planned weight wise.
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Aluminum flag pole for arrow. Lol or a tele pole. Arvin
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interesting scaling ratios, Steve. I plugged a 64" by 1.25"wide bendy handled bow into a spreadsheet and scaled up times 5 to 26'8" x 6.25" wide x 3" thick
for the same strain,
the poundage went from 40 to 1000 or 40 x 5 x 5
the stored energy went from 36 to 4500 or 36 x 5 x 5 x 5
and the arrow speed at 10 gpp from 169 to 357 :P
the scaling of stored energy seems rather impressive, and an arrow of 1/1000 SE works out to 4.5 lbs or 31.5 gpp
so if I plug a 4.5 lb arrow back into the calculator at that gpp, the fps drops back to 223
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Willie, I am glad you did that, I used my primitive math skills to try and estimate speeds and was somewhat disappointed and about ready to throw in the towel. At 357 fps I am much more encouraged. Can you blow that up 1 more time and see what you get, say 52 ft
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I can, but wouldn't you rather have the numbers for your current design length, with the 20 stiff handle? what wood are you currently considering. etc ?? can furnish a few more specs?
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Yes, I would prefer that. I used a virtual mass method for making a speed calculation and the dry fire speeds were much lower than I had hoped for.
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IF 10gpp is the way to standardize all bows against each other, then arrow speed should be average.
( Im saying it wont)
But, with such a long draw this is gonna show what I have been saying for years, and hopwfully get rid of the 10 gpp rule. The draw length must be calculated into the formula when figuring out arrow weight.
Steve, this is not news to you, but I really hope this project reinforces this point of mine.
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I base grains per pound on stored energy and not peek draw force. A bow drawing 10,000# but storing 100,000# energy because of the long draw is radically different than a 50# bow storing 48# energy. Grains per pound really doesn't work well on giant bows or miniature bows either for that matter. It does work well for normal size normal draw length bows.
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Yes, I would prefer that
Ok, I will try to "draw" the bow from scratch in woodbears spreadsheet, what are your revised specs and dimensions?
Grains per pound really doesn't work well on giant bows or miniature bows either for that matter.
If I understand you correctly, you are saying efficiencies do not scale well? Besides the problem of finding quality staves for giant limbs, like Alan mentioned, what other reasons come to mind that might be the cause of lesser efficiency?
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If I understand you correctly, you are saying efficiencies do not scale well? Besides the problem of finding quality staves for giant limbs, like Alan mentioned, what other reasons come to mind that might be the cause of lesser efficiency?
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Finding a decent string will be a challenge. Standard rope materials will probably be much more elastic or heavy unless you want to spring for a very expensive spectra rope.
Alan
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Yes Alan that's a lot of sinew ! Steel cable would be saver with out stretch. Don't know about ship rope. Still watching. Arvin
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Spectra rope for sure, expensive but available. Once I get a solid plan down with some drawings I can see about how much sponsorship I can get.
White oak in 12 ft long straight grained pieces can be found. Maybe scale back to 60 ft, use a 15 ft rigid center. Da Vinci drawing was an 80 ft long crossbow. I think It will require engineering all the way. A good idea for a show would be to put on a mock siege. All the pieces carried it in on wagons and then assembled by the soldiers on sight. Would have to come up with a good use for a giant bow in a siege. Fire rate would be extremely slow.
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Scaled back to 60 foot is still impressive! A nice graphic like similar to this will go a long way for getting folks onboard.
http://hexus.net/media/uploaded/2016/12/c8cf1d80-6528-4afa-a44c-ebb9927ae7c1.jpg
A series of test bow possibilities to consider would to be to build a 3', 6' and 12' bow of the same design. you could probably get all three from the same board. I think if carefully tillered to the scaled draw weights, a lot could be learned about how well you can scale the efficiency.
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Scaled back to 60 foot is still impressive! A nice graphic like similar to this will go a long way for getting folks onboard.
http://hexus.net/media/uploaded/2016/12/c8cf1d80-6528-4afa-a44c-ebb9927ae7c1.jpg
A series of test bow possibilities to consider would to be to build a 3', 6' and 12' bow of the same design. you could probably get all three from the same board. I think if carefully tillered to the scaled draw weights, a lot could be learned about how well you can scale the efficiency.
60 ft might be more realistic, unless we can come up with a different construction method, I wonder if a limb could employ airplane wing technology.
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I wonder if a limb could employ airplane wing technology.
To some degree there might be ways that maximizing stiffness/weight ratio might be applicable to bow builds, but in esssence, the primary goal in wing design is to build a light limb limb that does not bend too much while our goals are more about bending and energy storage.
Of course, a sponsor with deep pockets and an unemployed aircraft engineer looking for new challenges would be an interesting team to have onboard
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I was just thinking in terms out adding light weight ribs to a hardwood back and belly, might be worth a simple test. Maybe something like 2/3 of the center empty 1/3 of the space occupied by ribs. Might be an easy way to preset tiller at the same time
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Always worth an experiment. :) I think your challenge is going to be shear forces inside the limbs
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Del might have have an observation or two, as I think he was working on a hollow crossbow limb earlier this winter.
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The giant bow should not be able to throw an arrow any faster than a regular size bow. Somewhere north of 200 fps with a reasonably sized arrow would be a major success. But the trajectory of the huge arrow from the huge bow should travel much farther than the same speed arrow at normal scale. Air resistance will be much less a factor on the big arrow trajectory.
Alan
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The giant bow should not be able to throw an arrow any faster than a regular size bow. Somewhere north of 200 fps with a reasonably sized arrow would be a major success. But the trajectory of the huge arrow from the huge bow should travel much farther than the same speed arrow at normal scale. Air resistance will be much less a factor on the big arrow trajectory.
Alan
Allen, that's what I was coming up with. Even the dry fire speed calculations were disappointing. The only way to make the giant bow short that I can think of is to have an extra long draw length. I was hoping for 500 fps plus and it doesn't look like I will even come close to that.
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The draw length will be what makes the difference. The longer the better.
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The giant bow should not be able to throw an arrow any faster than a regular size bow. Somewhere north of 200 fps with a reasonably sized arrow would be a major success. But the trajectory of the huge arrow from the huge bow should travel much farther than the same speed arrow at normal scale. Air resistance will be much less a factor on the big arrow trajectory.
Alan
Allen, that's what I was coming up with. Even the dry fire speed calculations were disappointing. The only way to make the giant bow short that I can think of is to have an extra long draw length. I was hoping for 500 fps plus and it doesn't look like I will even come close to that.
I took a few minutes to refine the previous calculation a bit. Accounting for a spectra string that has a breaking strength at 5 times draw weight, the arrow speed drops down 4%
The initial calc also assumed a 10% energy loss for hysteresis. I should mention that the sheet is estimating a max and min spread for kinetic energy and arrow speeds , perhaps giving a broad definition to what constitutes hysteresis? Nevertheless, I have to use a value of 60% energy loss to hysteresis, to bring a 10GPP arrow speed down to 226 FPS at the lowest estimate of it's speed, and an 80% loss for average speed. 5 GPP speeds are of course higher.
Perhaps a reasonable vibration loss for a "normal" bow is lumped in with hysteresis to create the hi/low spread, at any rate with the exponentially high energy storage, something else is not scaling very well. I would prefer to suspect that hysteresis is what doesn't scale, otherwise there might be some potentially limb shattering vibrations in a bow 10 time normal size.
Steve, can you refer me to your source of the formulas for the dryfire calcs? It would be interesting to see which method scales better.
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Willie, my estimates on speed were based on 100,000# stored energy at 60% efficient at 1/800 arrow weight of stored energy. From that I established a virtual mass not accounting for any additional hysteresis which I know I would have. I did all my figures on a scratch pad which I trashed but can easily do them again
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I did find some examples of virtual mass calcs, let me see if I can duplicate your results, just for the sake of a learning exercise. what did you get for the fps?
125 lb arrow? if I understand you correctly?
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I don't remember now, I just remember I was disappointed
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Here’s my quick & dirty estimate:
The estimate for stored energy of the giant bow is pretty straight forward. Simply take the energy stored by a 80”X2” bow, and scale it by volume.
Drawn to a conservative 28 foot draw length would be the equivalent of a 28” draw on the 80” bow. Assuming the 80” bow draws 50# at 28”, the stored energy would be conservatively around 45 ft-lb.
So for the giant bow, stored energy, SE = 45 ft-lb * 12 * 12 * 12 = 77,760 ft-lb
The same scaling applies to the arrow. A 300 grain flight arrow for the 80” bow becomes: 80 x 12 x 12 x 12 = 518,400 grains giant arrow = 74 pounds.
Virtual mass of the bow also scales by volume. If the 80” bow has 200 grains of virtual mass, the. The giant bow has VM = 200 x 12 x 12 x 12 = 345,600 grains.
Velocity for the giant bow 74 pound flight arrow: V = sqrt((2 * 77,600 ft-lb) / ((518,400 grains + 345,600 grains) / (7000 * 32.2)) = 206.2 fps
Dry fire speed = 339 fps.
But, the distance the giant bow flings the giant Flight arrow could easily be 100 yards farther than the equivalent 80# with its flight arrow, but this would be just over 400 yards at the best.
Alan
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We are pretty similar Allen, I had a higher virtual mass I believe and came out with lower numbers for distance. But I do believe they are within a reasonable range of reference.
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Has any one seen actual usage of virtual mass formulas that scale bow size? My research only uncovers the method being used to proportion energy between arrow and bow. Klopsteg said that a bows virtual mass remained constant, but isn't a scaled up bow a different bow?
just thinking out loud here,
if ......
a typical bow is said to have a a limb deflection of 1/3 the draw length? let's say for instance, 10".
a typical powerstroke is said to take 1/40 th of a sec, or 25ms
then.....
the tip speed when shooting our arrow is 10 x 40 = 400 inches/sec, or 33 FPS
If the bow is 12 times as long, and the powerstroke has to be of the same duration to produce comparable velocities, then the limb tip speed now has to be 10 x 12 x 40 or 400 FPS
is wood strong enough to survive these accelerations?
can wood actually accelerate itself that fast? What is the hysteresis of wood trying to unbend at these velocities?
or perhaps the powerstroke does not have to be 1/40 of a sec to provide comparable velocities?
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Willie, I've found that tip movement is only about 1/3 of draw length. Confused me foe a while but I think the brace height uses a third that we don't get to use. I reserve the right to be wrong. :D
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Good point Don,
correction made,
thanks
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Another way to look at it is that the scaled up giant bow will have the same ratio of arrow mass to stored energy as the normal size bow.
Playing around with the numbers
Normal bow: 300 grain flight arrow / 45 ft-lb = 6.67
Giant bow: 518,400 grain flight arrow / 77,600 ft-lb = 6.67
How about draw weight? The proportion of the draw weight X draw length to stored energy for the normal size bow will be the same as the giant bow.
Giant bow draw weight = 77,600 ft-lb * (50lb * 28”/12” per foot) / 28 ft = 7,185 pounds!
In order for the giant bow to complete its launch in the same amount of time as the normal size bow, the acceleration of the arrow would have to be much higher in order to accelerate the giant flight arrow over a much longer distance, but this isn’t the case. The acceleration of the arrow at the moment of release for the flight arrow of each bow is as follows:
Regular bow arrow acceleration:
Mass to be accelerated = 300 grain flight arrow + 200 grains virtual mass = 500 grains = .00222 slugs
Acceleration = force/mass = 50lb / .00222 slugs = 22,500 ft/s^2
Giant bow:
Mass to be accelerated = 518,400 grain fligh this arrow + 345,600 grains virtual mass = 3.83 slugs
Acceleration = 7,185 lb / 3.83 slugs = 1,874 ft/s^2
The ratio of the arrow acceleration for the regular bow to the giant bow is the same as the scale ratio of the large bow to the small bow. In other words, it will take the giant bow 12 times longer to complete the shot compared to the regular size bow.
Length scale ratio: 80 ft/80 in = 12
Acceleration scale ratio: = 22,500 ft/s^2 / 1,874 ft/s^2 = 12
;)
Alan
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Regarding virtual mass, the ratio of virtual mass to stored energy should be similar for the giant 80 ft bow as it is for the 80” bow.
Virtual mass is not a perfect predictor of how fast a bow will shoot different mass arrows however. Virtual mass approximates the bow using a single degree of freedom model, but the bow has infinite degrees of freedom. The value for virtual mass will fluctuate some amount depending on whether a natural frequency is triggered in the bow limb, string, arrow, or any combination of all three. It usually works pretty well over a limited range of arrow weights, and works better for some bow designs over others. For example, with most of my modern footbow limbs, the virtual mass often decreases significantly going from 10ggp, to 1/2ggp. I even had an extreme example where the efficiency of one of my bows increased with a lighter arrow before sharply falling off again with an even lighter arrow!
Alan
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I agree virtual mass can't be regarded with any real accuracy but it does indicate an rough area of performance you might expect. I was surprised you bow actually gained performance wit a light arrow. I wonder what mechanism allowed that to happen? might be worth looking into.
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How about draw weight? The proportion of the draw weight X draw length to stored energy for the normal size bow will be the same as the giant bow.
Giant bow draw weight = 77,600 ft-lb * (50lb * 28”/12” per foot) / 28 ft = 7,185 pounds!
Alan
Your 7185 lbs draw weight compares favorably with 7200 lbs for a similar bow modeled in woodbears spreadsheet. Your original arrow was 6GPP (300gr @50#)? Scaling up using this ratio giving an arrow weight of 6.2 lbs. Our often used arrow weight per lbs draw (GPP) doesn't scale at all like the virtual mass method. It might be interesting to see how your mass/stored energy ratio correlates with distance observations.
Thanks for you insights on the degree of freedom limitations with the virtual mass method. Like Steve, I agree it wold be interesting to further examine vibration and harmonics of limbs and strings. In your extreme example where efficiency increased before dropping, were your arrow velocities obtained from a chrono, or were bow and arrow considered together, using velocities assumed from actual arrow distances?
I hope my questions are not too pointed, this study of badgers project has brought some good concepts to light.
PS a 12/40 th of a sec powerstroke does seem more reasonable
substantial editing to original post
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Willie, because of the lower acceleration rate I thing the 6# arrow would be the better choice which would be based on peek draw force as opposed to stored energy which I originally allowed for. I also think the bow would need to be scaled up from a flight bow type bow, not so much in reflex but in the limb shape. All the dynamics of it would respond much better using draw force instead of stored energy for the arrows.
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In your extreme example where efficiency increased before dropping, were your arrow velocities obtained from a chrono, or were bow and arrow considered together, using velocities assumed from actual arrow distances?
I was bench test shooting a real bow using a shooting machine fixture and two chronographs. The shooting machine fixture precisely releases the arrow at a pre-determined draw length. I also have a paper holder for the arrow to pass through so I can determine the quality of the arrow Flight based on the shape of the hole it makes in the paper.
I don’t think it is necessarily a good thing to see efficiency jumps like this. I’d prefer that it behaves much more predictably over a very wide range of arrow weights. I think what I am seeing has to do with a couple of natural frequencies cancelling each other out at certain arrow weights and shot speeds.
Alan
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Willie, because of the lower acceleration rate I thing the 6# arrow would be the better choice which would be based on peek draw force as opposed to stored energy which I originally allowed for. I also think the bow would need to be scaled up from a flight bow type bow, not so much in reflex but in the limb shape. All the dynamics of it would respond much better using draw force instead of stored energy for the arrows.
Shooting a 6-lb arrow out of the giant bow would be the equivalent of shooting about a 1/2 ggp arrow out of a 50# bow! This will be a dry fire situation!
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Guys you know I am not a math guy! I think the problem is in the material not the mass. The wood is not 10 times as dense. It's just bigger and heavier. I also reserve the right to be wrong cause I don't know. I have said before that mass is mass but maybe not with giant bows of the same material. Arvin
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Guys you know I am not a math guy! I think the problem is in the material not the mass. The wood is not 10 times as dense. It's just bigger and heavier. I also reserve the right to be wrong cause I don't know. I have said before that mass is mass but maybe not with giant bows of the same material. Arvin
Arvin, scaling up the composition of the material is tricky. My thinking is that it should be much less dense to be ideal. Not even sure why i think that. Maybe in my head I am thinking if I scaled up the cell structure in the wood it would be lighter.
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Sometimes scaling things up or down just flat doesn't work. That model schooner that I posted a while back is scaled down exactly from the original and it won't float upright by itself. It needs 5 pounds of ballast hung 18" below the boat or the slightest breeze knocks it over. Sails have (basically)2 dimensions(square0 and hulls have 3(cube) and they have to be of the original design or it won't work.