Primitive Archer
Main Discussion Area => Around the Campfire => Topic started by: joachimM on March 15, 2016, 04:56:16 pm
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Hi folks,
I've been having an argument to and fro on PA about the efficiency of strong recurves versus less recurved specimens (http://www.primitivearcher.com/smf/index.php/topic,56255.0.html)
Theory at least suggests that strong recurves ("hooks") are less efficient because during much of the draw these bows are actually shorter bows but carrying the tip mass of a longer bow. Even though such bows have a fatter force-draw curve, their efficiency is supposed to be lower because of the dead mass at the tip of the bows during much of the draw.
So I decided to test performance of two bows that only differ in their shape (strong recurve versus contact recurve), with the same bow mass, same peak draw weight, same brace height, same length of recurve but different angle.
The below pics show the two bows, and their FD-curve, plus the 163 grain arrow used to shoot the bows.
Each bow was made from a 1 m section of electric wire PVC pipe (16 mm diameter)
AAAH, blasphemy!! PVC on PA!! - before you decide to lynch me: this is basically the same principle as the plastic food tray models Tim Baker advocated in TBB4. Plastic, yes.
The sharp recurve drew 16.8 pounds at 24", the contact recurve drew 16.7 pounds at 24". Each bow was shot and chronographed 12 times with the same arrow (c. 10 gpp), which was drawn to exactly 24" (one but terminal node of the arrow).
In order to get to the same peak draw weight, I had to deflex the sharp recurve a bit. At rest, the tips of both bows were level.
The draw-back of PVC bows is that you cannot tiller it by remove material, only by flattening it more or less. Since a contact recurve would be more stable at the tips than a sharp recurve, the tip mass could also be lowered. Here I couldn't do that. As a result, I expected both bows to perform equally, even though the sharp recurve stored more energy.
Here's the results
Average speed of contact recurve: 120.6 fps
Average speed of sharp recurve: 107.0 fps
Actually, the slowest shot with the contact recurve (114 fps) was higher than the fastest with the sharp recurve (111 fps).
So don't let force-draw curves fool ya. It's not about stored energy, what counts is the energy imparted on the arrow. Kooi & Bergman (1997) showed this extensively in their paper (google it to find the pdf).
Joachim
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This is just fantastic. Thank you.
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Could you at least paint them brown?
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Just out of curiosity Joachim, would you mind testing them both with a heavier gpp arrow and a lighter gpp arrow?
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For a control they should at least have the same amount of reflex these don't
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I enjoy reading your posts, fella.
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For a control they should at least have the same amount of reflex these don't
Bubby, these have the same amount of net reflex at rest. Both of these will break when drawn to 26" (I tested one till breakage). They are strained about equally right now.
Suppose you just removed the deflex in the highly recurved bow, it would have higher peak draw weight and then you'll be comparing two bows with different draw weight, because they are strained differently. If I remove the deflex, the highly recurved bow will break at c 23". I could redesign it non-deflexed and draw it to 21" (and get the same peak draw weight of c. 17#), and draw the other one to 24". But then I would be comparing bows with different power strokes, and the slightly recurved bow would store a lot more energy. Again, not what we want.
This here shows that if you have two bows with the same peak draw weight, the one with the fatter force-draw curve (which results from the recurved design) will not necessarily shoot better. Rather the contrary.
This confirms what mathematic models indicate, but what few people believe, dismissing the maths because it's just theory. I walked the walk.
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Are you assuming, or do you know, wood would react the same way? A lot of questions come to mind when I compare wood and plastic. Mass weight, flexibility, tip weight, reaction speed, stored energy and so on. All you really proved was two pieces of plastic shaped in two different ways reacts almost the same.
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Make the recurves the same length just bend one more to avoid lift off , apples for apples
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that one bow isn't what i would even consider a contact recurve it is basically a flipped tip. I'm with Pearl and his breakdown
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JochimM, if arrow weight isn't matched to tip speed well enough the amount of energy transferred will vary. Particularly in a stacking bow. This is why I was curious about arrows with higher and lower gpp. There could be a sweet spot here for that weight for one bow and not the other that reverses itself for a different weight arrow.
Not trying to argue the "which bow type is better point," but very interested in energy transfer dynamics. I do agree that F/D curves give only the static picture, not the dynamic one, and I had already read the papers you linked to before. I just think this is an opportunity to see how these bows react individually to arrow weight. Will they maintain the same distinction for different purposes?
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Precisely PlanB, we can see this looking at historical asiatic bows, the Ottomans for example used short ears contacting the string and they used lighter arrows, around 300-500g (I'm speaking in terms of war bows and arrows), whereas the Qing bows had exceptionally long ears and string bridges but they used arrows closer to English weights, 1000-1500g. I know ear length and recurve angle are not exactly the same but I suspect the principle is similar with respects to arrow weight.
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Posted by: mullet
« on: March 15, 2016, 04:30:26 pm » Insert Quote
Could you at least paint them brown
Lol! :D
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that one bow isn't what i would even consider a contact recurve it is basically a flipped tip. I'm with Pearl and his breakdown
Both recurves/flipped tips are exactly the same length, one is shallow the other isn't. Whether it's a flipped tip or a recurve is semantics. One has immediate lift-off, the other doesn't. The point was to prove that a FD-curve can be deceiving.
Pearl Drums: did they react the same way? Hell they didn't!! Look at the figures: one bow is >10% slower than the other.
Do wood and PVC react the same way in a bow: essentially, they do. in straight bows, you get a linear increase in poundage when drawn further and further, in both materials. But you're straying from the mechanics question: this is a question of the interaction between strain, leverage, and tip mass addressed in theory by Kooi & Bergman 1997 (http://www.bio.vu.nl/thb/users/kooi/kobe97.pdf).
Now, some folks reacted "well you shouldn't have deflexed the sharp recurve"
So I removed the deflex. It now stores a ton more energy (because it's strained much more) and has a higher peak draw weight than the other bow (at 24"), but still doesn't shoot noticeably faster than the contact recurve with a nearly straight FD-curve. It's about the same (average speed 117 fps, 4 shots 24" draw. During the fifth it buckled -a compression failure in PVC bows-, proving that the deflex made sense to compare the bows).
Even though I'm comparing two bows with different peak draw weight, the one everyone thinks would shoot faster doesn't.
To level the playing field again, I drew the contact recurve 1" further, drawing close to the same peak draw weight (actually a tiny bit more). Instead of shooting an average of 120.6 fps, it now shoots on average 129 fps. Again, a lot faster than the sharp recurve strained about the same amount. Which is logic, as the power stroke was longer. But at the same level of strain, the functionally longer bow wins.
10 gpp is a pretty standard method of measuring bow performance. It's not about the absolute amount of energy transferred, its about the amount relative to the stored energy: efficiency. Shooting heavier arrows will reduce the difference between both bows, but will not remove it entirely (unless shooting extremely heavy arrows). Shooting lighter arrows will likely exacerbate the difference. But I don't have lighter arrows than 163 gr.
But it seems many are missing the point: FD-curves fool the arm, not the arrow. I don't see anyone commenting on the core of the matter. Everyone is trying to find a way out of what seems to be an uncomfortable situation.
So I say: prove me wrong! I feel like I did my fair share of empirically testing the mathematic models. Come with your own hard data instead of opinions. . Shoot two bows which have the same peak draw weight at the same power stroke, and which are strained the same amount (the mass principle is a good starting point to know this). And prove me that a bow with sharp hooks/ bow with a fat FD-curve shoots faster at 10 gpp than one with "flipped tips" / bow with nearly straight FD-curve.
Mullet: here ya go :D
Joachim
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10 gpp is a pretty standard method of measuring bow performance. It's not about the absolute amount of energy transferred, its about the amount relative to the stored energy: efficiency. Shooting heavier arrows will reduce the difference between both bows, but will not remove it entirely (unless shooting extremely heavy arrows). Shooting lighter arrows will likely exacerbate the difference. But I don't have lighter arrows than 163 gr.
But it seems many are missing the point: FD-curves fool the arm, not the arrow. I don't see anyone commenting on the core of the matter. Everyone is trying to find a way out of what seems to be an uncomfortable situation.
Joachim, I wasn't trying to find a way out of an uncomfortable situation and I accept (and already agreed) that F/D curves don't necessarily represent what the arrow sees. I know 10 gpp is the usual standard comparison, and a dowel painted black can serve as a lighter "arrow" as far as a Chrony is concerned. I'm sure you have access to heavier arrows, and they don't have to be tremendously so. I was just curious if there would be a difference, and just how much difference there was. Of course you don't have to try them if you don't want. It was just a request.
As far as commenting on the core of the matter, here it is:
The string can only deliver to an arrow as much energy as that arrow RESISTS while moving. The resistance an arrow presents to the string takes two forms: inertia, and friction.
Ignoring friction, inertia is a result of mass -- the arrow's weight, in other words. A heavier arrow accelerates more slowly and absorbs more of the available energy.
A very light arrow has little inertia. That's a plus as far as speed goes. It can accelerate very quickly immediately after the string is released.
If a bow has very slow limbs / heavy tips, high stack, high internal resistance and a high draw weight, it may be possible for a light arrow to outrun the limbs (and string) soon after release. If so, it is gaining no energy at that point. No matter what the F/D curve says is the draw weight at that distance (and further along) before it leaves the bow. In fact, if the arrow nock is tight, it may actually be dragging the string with it.
A heavier arrow, accelerating slower, may absorb energy from release all the way to brace height, since it is not moving as fast as the limbs and string. It is also resisting the string more because of its inertia. It is technically a more efficient system, and its behavior will look a little more like the F/D curve, although it will never receive the forces that the F/D curve shows.
If those are extreme cases, it's also possible to imagine cases in between, where a light arrow receives a smaller fraction of the energy available than the heavier arrow from a somewhat faster bow, but where the differences are less extreme.
Since a light arrow mainly gets its energy from the furthest part of the draw, the difference in these classes of tip designs can favor (or not) that segment of the release cycle. So also can tip mass, and a whole lot of other variables. I personally think it would be very difficult to isolate all of the factors which contribute to initial limb tip speed.
In fact, testing arrows of different weights might be a very useful measure of whether a bow is slinging an arrrow for most of the draw length or only the initial part, and at what weight arror this transition occurs. And that is why I was just curious about the results using different arrows. Nothing personal, nor trying to prove anyone right or wrong.
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Plan B: point taken. I will do the tests with heavier arrows.
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joachimM, most people are not disputing your result. They are just curious about other aspects of similar experiments.
As with PlanB, I also would like see the same experiment using a heavier arrow, say 15 gpp. We do appreciate your effort a lot. Thanks in advance.
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Thanks Joachim.
Also just as a point of interest in conversation, I believe that hand shock is to some degree a result of the arrow absorbing little energy after the initial acceleration. That energy stays in the limbs, and has to go somewhere when the bow reaches brace height, so it overshoots -- a little like dry firing.
People say that it s caused by heavy limbs, and in a way that is true. But not directly The heavy limbs are slower and so an arrow can absorb less energy. A light arrow will only absorb mostly initial energy, leaving a lot in the limbs.
I think that it would be easy to prove it is an energy transfer problem, not just heavy limbs, by taking a bow with bad hand shock and trying a heavier arrow and seeing if the hand shock goes away or reduces. The heavier arrow would be able to absorb more of the bow's energy and there would be less left over at the end of the shooting cycle to transfer down the limb.
That's a guess...
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So I say: prove me wrong! I feel like I did my fair share of empirically testing the mathematic models. Come with your own hard data instead of opinions. . Shoot two bows which have the same peak draw weight at the same power stroke, and which are strained the same amount (the mass principle is a good starting point to know this). And prove me that a bow with sharp hooks/ bow with a fat FD-curve shoots faster at 10 gpp than one with "flipped tips" / bow with nearly straight FD-curve.
Seems like your work has stood the test, at least until a doubter presents something different, however I am not a doubter
I recently got a chrony and even found some pvc, and am following this thread to see if anyone thinks that there may be a potential use of pvc pipe models, scaled up in size, to improve designs for selfbows.
Of course we might have to incorporate some way to monitor the acceleration of the arrow, if efficiency is the goal.
In short, my question about pvc pipe is not to discount it's use for simple design evaluations as you have done here, but to see if its up to adding to our already nuanced understanding of bow/arrow dynamics.
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You know the original question was not non contact vs string that doesn't leave but when the string loses contac during the draw and what point of the draw lift off would be more efficient you seem to have morphed the test completely away from that. Why?
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Bubby-
Don't know about the original question part (have not gone back to reread everything), but that might be a worthwhile question to test with pcv, especially in light of Plans B;s inquiry of "at what relative arrow weight". Do you have an arrow weight in mind? or even better, bend up a bow with 1/2 pvc and post the specs, I can try to duplicate and verify with the chrony if you do not have one available.
willie
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mmh, I didn't think I moved away from that model, I just didn't explicitly link it up with when the lift-off of the recurves. Basically, theory indicates that lift-off should be as fast as possible so as to have as little as possible dead mass during any part of the draw. All the tip mass beyond the contact point with the string will be dead mass from that point onwards during the return of the limbs.
The real advantage, IMO, of recurves is that they increase string tension at brace, not the change in leverage (causing the fat FD-curve). There are other ways to increase tension at brace (see the angular low-stack designs with slightly reflexed/recurved limbs) without the cost of dead tip mass. Basically these are exaggerated variants of working contact recurves.
So if you think I didn't answer the right question, I'm up for any sensible challenge one might propose. Basically, make me a good proposition, set out the rules you think are good to test the difference in performance (plus a how you define performance) and I can try to make model bows to test hypotheses and shoot them according to the rules you set up. If we agree these are good rules to compare bows.
So shoot
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No chrony here, I've made a few plastic bows but use 3/4" or 1" but that's besides the point
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Plan B and PatM got me thinking about a few things that might explain why or when sharp recurves could actually beat other bow designs, namely for flight shooting.
A sharp recurve can be viewed as a shorter bow with higher tip mass, but with extra leverage at the end of the draw (and the start of the return).
Maximum strain on a sharp recurve is attained at a shorter draw length than with less recurved (or straight) bows (all else being equal). Say the recurve will have its max strain at 24" and the straight bow at 28". Total energy stored is smaller in the sharp recurve, but the slope of the change in energy transfer is steeper.
The straight bow will store more energy and be more efficient overall, but the short bow with extra levers during the first few inches of release could have a higher dry fire speed. Only the potential energy of the last few inches of the draw (when the recurves lift off) may matter for flight shooting if the arrow is so light that it doesn't decelerate the tips. A heavier arrow would decelerate the tips, and the speed of the arrow at exit would be influenced by the acceleration during the entire return of the limb, not just the start.
PatM wrote "there's no priZe for efficiency" (got the orthograph right). For flight shooting there isn't. For hunting with heavy arrows I guess there is.
But at least I can now get my mind around why some very fast flight bows are short static recurves with sharp hoooks.
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I said no prize, not no price. ;)
But yes there still has to be a reason for the 500 plus yard Osage statics having tips close to 90 degrees. They did make the tips as close to non-existent as they could.
No question that mass at the tips is the biggest enemy of the static but fwiw I have shot statics with still chunky tips way farther than the most minimal tipped straight bow.
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Joachim, you elucidated what I was thinking. The shape of a tip speed curve is important to what weight arrow it is attuned to.
It's unfortunate that we have a word for a rate decrease for speed (deceleration) but not one for a rate decrease for force (depression?!). A plotted out dynamic "depression" curve, along with the mass of an arrow would give you it's terminal velocity, and also the location in the release cycle where terminal velocity occurs. After that point the arrow is decelerating from friction. On a fast tipped bow with a reasonably massive arrow, that might be at or near brace height. On a very light arrow with a highly stacked bow with low brace tension, it might be well before that.
Since short limbs at high draw weight can have fast tip speed, I'm not surprised that fight shooting with light arrows works well with them. If a fast tipped bow also has low stack at the same draw weight, that would be ideal. A bow shape with short limbs might stack more than a similar bow with longer limbs, but the shorter limbs might have less inertia. Since two variables are present stack, and inertia, it's likely that two bow types will produce good results under different conditions (ie arrow mass). A longer version optimized for lower inertia where possible and extremely low stack, and a shorter version optimized for lower stack where possible and extremely low inertia. Then one or the other will be appropriate for a particular arrow type.
A very short bow with extremely low limb inertia, and some stack, shooting a very light arrow will depend on acceleration only in the early part of the release cycle. Because of this the results will be relatively inconsistent, and it will be highly dependent on the archer's release, bow condition, wind direction and resistance and many other factors.
A longer bow with extremely low stack, and some additional inertia, shooting a more moderate mass arrow will depend less on the early part of the release cycle. Because of this the results will be more consistent, the heavier arrow will gain more energy and moving inertia, be less affected by the above factors.
It may not shoot as far however as the best shot from the less stable alternative bow system. I don't know about actuals. In one case occasional records may be favored. In other cases consistent wins may be favored. It depends on whether you favor extremes or averages. Overall points, or one time maximums.
It seems to me that gentler recurves would likely reduce stack, and static and lever systems would increase initial force (accpression???!) early in the cycle.
It would be really useful to have a curve of limb tip speed. If that could be summed with the traditional F/D curve, a new curve could be generated which is much more characteristic of a bow. And if the arrow mass was known, the point at which terminal velocity could probably be determined, as well as total energy absorbed, etc.
These are just thoughts -- nothing proven. But just what seems reasonable to me.
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It would be really useful to have a curve of limb tip speed. If that could be summed with the traditional F/D curve, a new curve could be generated which is much more characteristic of a bow. And if the arrow mass was known, the point at which terminal velocity could probably be determined, as well as total energy absorbed, etc.
I think there is, it's just damn difficult to get it. it's the dynamic force-draw curve, as opposed to the static force-draw curve. The DFD shows which energy is effectively transferred to the arrow, but I've only seen it in mathematical models ???
Good thinking, but we need some of these other flight shooters to chip in, next to Pat.
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Just to throw my two cents worth in, I concur with the mathematical models, but I suspect they are missing a variable. The proof as they say is in the pudding and I too can't get past the fact that these sharp 90deg hooked bows were throwing these arrows farther than anything else out there, there has to be a reason for that, it can't be despite the hooks.
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I do not claim to be a flight shooter, but could some of the "excess" mass in the tips, be beneficial in the last few inches of the powerstroke?
Presuming an early liftoff design, the contact point is rapidly changing at the end of the powerstroke.
Could extra tip mass and their corresponding momentum, be delivering the extra oomph needed as they come around and shorten the string? Boosting the arrow acceleration, right at the last?
The excess mass delivering at a more effective moment, making up for the penalty they impose on the limb earlier in the powerstroke?
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Joachim, let's call it the Force/Release curve. I'm pretty sure a set of them if they could exist, would explain all.
Basically you need a high speed camera, and measuring nock positions at set intervals on a set weigh arrow. Plot that in a curve, and you've got it.
The nice thing is you could do it for the limb tips at the same time.
Casio makes some cheap high speed cameras, available used on ebay for the $100-$150 range that do a 1000 fps video -- but it's small format and the quality might not be good enough, dunno. Might have to wait a few years til cameras catch up.
To be honest, this stuff is interesting to discuss, but I get tired of it eventually and want to grab a piece of wood and just whittle while thinking of almost nothing at all.
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The mechanics of a limb with a static tip seem like they have less disruption of their power stroke.
You can find lots of discussion regarding the way a longer working limb has a "bulge" or ripple in the limb on the return.
Sometimes the wording that people choose to try to describe these actions may not be wholly accurate though.
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seem like they have less disruption of their power stroke
Pat- Could you explain better what "disruption of power stroke" is?
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If you look at slow motion videos you can see some interesting stuff going on as the limbs return. These movements of the limb have various types of impact on the energy delivery.
I'm not going to speculate exactly how.
It can all be explained mathematically though. ;)
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I have seen videos that show limbs flexing in mid- limb after the bow reaches brace height, but never one showing a limb with static tips, just before....
a link would be helpful
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I don't have a link, that's just how people have tried to explain things on other threads. Less working limb, less distortion.
With a straight bow you san even see the handle start moving towards the archer before the limbs return.