Anybody wanna wrassle??

LOL

But I was shooting pretty well yesterday, including bare shafts at 30 or so.

One bow. Mix of 600 spine + 175, 500 + 175, 500 + 200. One of each in Centershots, a couple Black Eagle Vintage and a GT Trad.

And basically, they all group down the middle, maybe a bit weak. Definitely diving Right between 30 and 40.

I was noticing the same thing last weekend and - working on the theory that if it sounds too good to be true - asked someone who’s a helluvalot better shot and vastly more knowledgeable than I am about it. He told me that it’s not just the Blind Hog factor. Said if your release is pretty clean, you can get away with a surprising amount.

Good news (to my way of thinking) is that (as is often said) high FOC carbons are quite forgiving; they straighten up quickly even when your tune is off and for a number of reasons, they penetrate well even when they hit something fairly solid.

Bad news, methinks, is that they are maybe so forgiving that they can mask a substantial dynamic spine mismatch. Like the difference between 500 and 600. If those two (same length, same point weight) will “agree”, does that mean that they’re both wrong?

So I’m wondering if Flight shooters don’t use basically “0% FOC” at least in part because that’s as unforgiving as it gets, so when they know that they’re ON, they are really most sincerely ON. My BIL only knows enough about flying to have made CAG, but he talks about Nose Authority in such a way that makes me think that that is what FOC gives an arrow.

So here’s my question… (you’re welcome to argue any and all points I’ve floated here, but this is what has fired up my curiosity today)…

If you were to dial in your “perfect” dynamic spine rating (let’s assume a non-tapered/footed arrow for now) using “0%” FOC (let’s assume aluminum for precision), could you then dial in a HIGH FOC arrow of equal mass to the same rating and basically end up with an extremely forgiving arrow…. with very little to forgive?

How would you get a 0 % FOC with a metal point. Your nock & point would have to weigh the same On a bare shaft. Point + the feather weight on a fletched shaft. That arrow would not know which end to fall back to earth first. I think I have a headache now. >>>----> Ken

Well, don’t take that tooooo literally… ;)

But don’t at least some of the flight shooters use razor blades for fletchings?

I agree with you on your point about the arrow not knowing which end should fall to earth first, though. So that’s why it makes sense that they want to stay close to neutral. If you have a big fletching on one end with a (relatively speaking) very low terminal velocity and a point on the other with a much higher terminal velocity, then your arrow is going to be under constant correction for “nock high” flight, which bleeds velocity. I’m not ready to say how much I think high FOC changes that equation but in principle…. if one end of the spectrum is approaching the optimum then the other and must be approaching the opposite….

It isn't -0- front of center. It is an arrow that balances at the mid point of the shaft, or as close as it can be.

Why would an arrow that has a high FOC be considered as more forgiving than any other configuration?

If a HFOC arrow started out off course would it not be plausible to think that it would be less likely to straighten out because of the high concentration of weight on the front which was pointed in the wrong direction to begin with ?

Well…. I don’t think so… Because it seems that whatever amount of drag your fletching is creating (which would ordinarily keep your arrow tracking straight) would have a lot more leverage with the high FOC, and less mass back to keep in line behind the point.

So (at least as far as I’ve been able to puzzle it through!) A high FOC arrow would be more forgiving for the same reason that a flu flu is. Just seems to take less feather to get there.

I think zero FOC is unattainable. I don’t have any info from anyone shooting zero FOC.

I have talked to Easton engineers about FOC ( industry show) many years ago said they have tested this extensively to come up with their FOC recommendations (8%-16%) for best overall arrow flight. Sure a little more of a little less works.

And yes, the better your form and release, the wider the spectrum of what tip weights will tune. I do believe there is an advantage to staying in that middling Easton recommended range as its more forgiving of a bad release and poor form. It seems my best flying arrows are in the high teens for FOC.

Feathers create drag to keep the nock end of the arrow behind the point, as in correcting for the flexing of the shaft. They don't change the direction of the arrow to go where we want it to. If that were so we would never miss. Sorry but they are not guided missiles. Continue your thoughts to make a - FOC or BOC Put the point weight on the shaft next to the nock. It won't go far before it is sideways & flips over. Bare shafting will speed it up. I guess bare shafting will point out any error. When your arrows get near sideways & break the shaft when they hit, they are telling you something. You are a long ways from a proper FOC. A long ways from tuned. >>>----> Ken

Funny thing, but of all of the points that could be debated on this thread, somehow the attainability of “0%“ FOC was not something I expected to have come up.

My point is (more simply) that I think that I could probably be tuned better than I am, but it’s difficult to detect whatever issues I may have because they are masked so successfully by the influence of the FOC.

So if I were to set myself up with a thick-walled aluminum arrow and relatively light point weight to shift the center of balance as far back as I reasonably could, is deflection simply deflection, or is the “correct” dynamic spine going to depend on how you achieve it?

I suppose I could also go with weight tubes inside of a carbon and get to the same result? But then I would have to use a different spine class for the carbon, so I’m still back to whether the dynamic spine requirement is an absolute for each bow (in terms of how much flex you put on that shaft to clear the riser) or if It Depends.

Carbon, has an advantage anyway, so it doesn't need to be so heavy to attain good penetration. It resists flexing on the shot and it also corrects quickly, then it resists flexing when it contacts an animal so less energy is needed to penetrate versus wood or aluminum. So actually, weighing them down may well be counterproductive. I have some I can shoot very accurately and with near perfect flight with just 145 grains out front not counting the 20 grain aluminum insert. Not a heap of FOC but I'm betting they will go through any deer like a hot knife through butter, and that's with my 48# recurve.

I probably don't understand your question as I am not nearly as technically minded. Dynamic spine is dynamic spine not the same as static spine. You can achieve it by weakening or stiffening the shaft. You can do the same thing by increasing or decreasing the point weight.(don't work for static spine) AS you know 8 to 10% is about optimum for speed, point on flight, & penetration. You can increase FOC for a harder to stop momentum at the cost of speed. You can decrease it for a little more speed at the cost of penetration. Do you want to flight shoot,shoot through steel barrels,hunt thinned skinned game or thick skin heavy game. Depending on your choice pick your weight of arrow & FOC It has been proved many times You can't go wrong with 10% It is a real good average & will kill anything in this country.>>>---> Ken

I can tell you only what I have found out. I could shoot 207 fps out of my 45# limbs and my 40# limbs but too often for me I would get a flyer. Those arrows had a 5 % FOC. When

I added enough insert weight to get 11 % FOC I did not get that occasional flyer. For me

that was more forgiving. Another thing I found out when I had my riser cut more past

center I coud shoot 400, 480 and 500 spine arrows. That is also what I call more

forgiving.

I don’t think carbon resists flexing anymore than anything else of the same spine rating. (Casting aside for the moment the substantial discrepancies and how such things are calculated…).

But yes, carbon arrows certainly do appear to come out of oscillation more quickly than wood, certainly… I have seen the Ken Beck tuning videos with arrows swimming all the way to the mark.

@Ken - I certainly am not having any issues with my arrows flying sideways. They actually seem to be flying straighter than they have any right to, which is what got me wondering about this in the first place.

Corax, read the studies and experiments done with Olympic javelins and your questions will be answered.

Orrrrr…. WILL they? LOL

It’s interesting to think about, because on the one hand they don’t get pushed from behind… But on the other hand I’ll bet that they do go through a period of oscillation because the release on them has to be sort of like plucking a big string and setting up a vibration end to end. And then I suppose the ballistic advantages of having the javelin’s center of mass at a particular point along its length have probably been pretty well figured out. So that addresses the maximum cast aspect of the question, I guess…

I think what I ought to do is run the arrows which are “working” for me through Stu’s calculator and see what the dynamic spine calculation is for each. Then maybe work up a couple other solutions that should be close at the conventional FOC and dial those in with the bare-shafting. Then figure out what I need to do to tune the higher- FOC arrows to the same dynamic rating and see what happens then…

Bruce makes a good point about throwing javelins in the Olympics. Most javelin ranges are inside the running track portion of the stadium. My understanding is a number of years ago when the throwing distance of Javelins was increasing, a real concern developed about the safety of runners on the track when the javelins were being thrown. Lots of work done on how to limit the distance and the winning solution was apparently to increase the FOC.

Larry Hatfield has posted several times on this and other sites, that flight shooters learned a long time ago the best distance and accuracy results came from FOC’s less than 7-10%. And usually much less.

So our archery history teaches most if not all we need to know about FOC if we but choose to study, and believe the results of many skilled folks who preceded us.

Bruce makes a good point about throwing javelins in the Olympics. Most javelin ranges are inside the running track portion of the stadium. My understanding is a number of years ago when the throwing distance of Javelins was increasing, a real concern developed about the safety of runners on the track when the javelins were being thrown. Lots of work done on how to limit the distance and the winning solution was apparently to increase the FOC.

Larry Hatfield has posted several times on this and other sites, that flight shooters learned a long time ago the best distance and accuracy results came from FOC’s less than 7-10%. And usually much less.

So our archery history teaches most if not all we need to know about FOC if we but choose to study, and believe the results of many skilled folks who preceded us.

Bruce makes a good point about throwing javelins in the Olympics. Most javelin ranges are inside the running track portion of the stadium. My understanding is a number of years ago when the throwing distance of Javelins was increasing, a real concern developed about the safety of runners on the track when the javelins were being thrown. Lots of work done on how to limit the distance and the winning solution was apparently to increase the FOC.

Larry Hatfield has posted several times on this and other sites, that flight shooters learned a long time ago the best distance and accuracy results came from FOC’s less than 7-10%. And usually much less.

So our archery history teaches most if not all we need to know about FOC if we but choose to study, and believe the results of many skilled folks who preceded us.

C-l -

Kinda basic Physics, you want (need) more weight at the front of the arrow than at the tail, and more air resistance and the tail than at the front.

In both cases, once you have "enough" to do the job, or a combination that does the job (stabilizing the arrow), then any more head weight or air resistance at the other end, hinders performance on some level.

How much does any of that matter? Kinda depends on the distance you're shooting and the accuracy/precision you expect. Sorry to break this to some guys here, but a 6" group at 20 yards doesn't require much in the way of equipment or tuning.

Viper out.

Ouch Viper, I usta think “paper plate” accuracy at twenty was a high mark.

Just something to mess with the 0 FOC discussion.

Years ago I played with flu-flu's a lot. At one point I took some 6 fletch 4" full height feathered shafts and just used a broadhead insert for a tip. I'd guess a 50 grain tip (insert and adapter) wasn't too far off from near zero FOC with 6 feathers and a nock at the other end.

I know the feathers had serious drag, but with nearly no point weight they barely made it 15 yards.

Informative reading https://www.sciencedirect.com/science/article/pii/S18777058110099 70

Arrow Aerodynamics The aerodynamics of arrow flight is really complicated. At launch the arrow experiences a 500 G acceleration causing all sort of complex superimposed motions - vibrational, rotational and sideways. These all impact arrow aerodynamics but ultimately distill down to just two important characteristics - arrow drag and directional stability. Arrow aerodynamics is briefly covered here to convey a feeling for the issues without going into excessive detail. The detail may still look excessive, but be assured, it is greatly simplified! The Arrow An arrow is an assembly of parts, each designed for a specific function. Most bow energy is transferred as kinetic energy in the forward motion of the arrow. However, a small proportion of this energy is miss-directed causing the arrow to have a combination of: o a bending resonance in one or more modes, o moving slightly sideways, o rotating about the center of gravity, During launch the nock is assumed to move in a straight line, but this is rarely the case. Imbalance of limbs and cams, off-center shot bow, miss-alignments of components, vibration and balance, archer-form, bow twist and release issues. All these combine to make it difficult to predict the precise condition of the arrow as it leaves the string. This means the initial conditions for the aerodynamic calculations are also vague. Axis System When describing an arrow's flight, we need to share a common terminology and positional reference system. The axis system we use has its origin at the arrow's center of gravity (CoG), the x- axis aligned with the air-stream, the y-axis horizontal and at right-angles to the air-stream, and the z-axis at right-angles to the other two axis: Arrow axis system showing the yaw and pitching planes. Note that the axis system aligns air-stream, not the arrow. The diagram shows the arrow aligned with the air-stream. The arrow's angle of attack or offset from air-stream is the angle between the arrow's longitudinal (spin) axis and the x- axis. Note that in arrow aerodynamics there is no concept of roll. Arrow spin is assumed but largely ignored except for the induced drag during spin-up. Archery has a long tradition of specialist terms for various arrow flight characteristics but they can be vague or ambiguous. To help resolve this, aeronautical terms are used where appropriate.

Drag Drag is the aerodynamic force that acts to slow an arrow in the direction of travel. It is proportional to the frontal area, the square of the speed (assuming turbulent flow) and the aerodynamic form expressed as the drag coefficient. The standard drag equation is: FD = 1/2 ? v2 CD A The details of this are covered below. Despite its sleek appearance, the typical arrow's aerodynamic form is not particularly good. The drag can be estimated from our knowledge aerodynamics, but presently no one can claim high precision from theory. Direct or indirect measurement is required for reasonable accuracy. Lift Lift is the force that keeps an aeroplane in the air. But it not only does that, it provides the force needed to turn an aeroplane when banking. It is closely related to drag but at right-angles to the direction of travel. (In reality lift and drag are components of a single force acting on the arrow, that we arbitrarily choose to describe this force). The standard lift equation is similar to the drag equation: FL = 1/2 ? v2 CL A where the lift coefficient CL is proportional to the angle of attack for small angles. The details of this are covered below. A well launched and stable arrow will not experience lift, however should the arrow fish-tail (yaw) or porpoise (pitch) lift is generated in the direction of the angle of attack. The magnitude of this lift is not large, but sufficient to result in path deviations and the drag it induces slows the arrow. It is lift on the fletching that provides most of the arrow's orientation correction. It is interesting to note that a bare shaft with an angle of attack will experience lift and hence path deviation in flight. Center of Pressure (CoP) The center of pressure is the point on the arrows axis where the aerodynamic forces are deemed to be balanced and applied. This point may move a little in flight depending on flight speed and angle of attack, potentially allowing a marginally stable arrow to become unstable. Larger fletching will move the CoP backward and broadhead blades will move it forward. For stability the CoP should always be behind the center of gravity otherwise the arrow will oscillate or even reverse orientation during flight with significant deviation of flight path. See Measurements, Tips and Tricks to find out how to determine CoP. Front of Center (FoC) Front of Center indicates the location of the Center of Gravity (CoG) of an arrow expressed as a percentage of arrow length forward of the arrow center. Typical FoC values range from 7% to 18%. Usually the standard length (nock valley to front of shaft) is used, although in reality the aerodynamic length would be more appropriate. See Measurements, Tips and Tricks to find out how to determine CoP Fletching Fletching significantly impacts an arrow's aerodynamics. Its principal purpose is to move the center of pressure rearward for improved stability. Other functions include inducing spin and aesthetics. Generally, fletches provide drag and lift, both of which provide a correcting moment to rotated the arrow into alignment with the air stream. The lift provides the most efficient stabilizing action as the correcting force and associated induced drag is only generated when required. (Note that some flu-flu fletches provide little or no lift, only drag, so are deliberately less efficient in this regard). The sum of the lift and drag components gives the correcting moment Mc : Mc = L d + D d tan( ? ) where d is the distance between the CoM and the CoP of the fletches. Also, both the lift L and drag D forces are approximately proportional to the angle of attack ?. The optimal proportion of drag to lift is an interesting question with no definitive answer. Most modern fletching ensures lift is the major corrective force as it is generated only when required, reducing drag on the arrow when aligned to the air stream. Over sized low drag fletches can cause overshoot and even continuous oscillation due to excessive correction and low dampening. Straight Offset Fletches Straight offset fletches are a simple compromise to the slightly more aerodynamically efficient helical fletch. With these fletches the offset angle is the same at all radii, with the result that when spinning, the actual angle of attack will increase with distance from the arrow surface. This is because the velocity vector due to the spin increase with distance from the arrow spin axis. The result is higher drag at the steady state speed because there will be a slight negative pitch at the inner part of the fletch working against the slight positive pitch at the fletch tips. Helical Fletches Helical fletches are notionally more aerodynamically correct in the sense that the angle of attack is the same at all distances from the arrow surface (at the steady state spin rate). The result is the steady state spinning drag is more or less the same as if there were not offset with straight fletches. Helical fletches are suitable for any type of archery, but in general for most archers the small reduction in drag is probably not worth the additional complexity in attaching then to the shaft. There seem to be persistent reports of helically fletched arrows dropping a little more than straight offset fletches of a similar size and shape. The only explanation would appear to be incorrect design or mounting that generates higher drag. Flu-Flu Fletching Flu-flu fletching is the aerodynamic opposite to the helical fletching. Flu-flu fletches provide high drag and are designed to reduce an arrows range and are particularly useful for aerial shots. These fletches may be adaptive in that the initial launch acceleration and drag may deform them to a relatively low drag shape and then their natural resilience restores their higher drag form after a short time. Good flu-flu fletches provide high initial speed then abruptly appear to drop as the high drag kicks in. Flu-flu fletching tends to lack the fineness of standard fletches, however their effectiveness - that is accuracy at short ranges and the suddenness of the drop - depends on the material and arrangement. Achieving the adaptiveness involves selecting a material of the right mass and stiffness so the form recovery time is in the order of 200 ms. A heavier point may be required to counter the extra mass and maintain the FoC in the acceptable range. Spin Spin is the rotation of an arrow about its longitudinal axis. Typical spin speeds are 500 to 4,000 rpm and is not particularly critical. The spin is generated by a slight angular offset, typically 0.5° to 4°, of the fletching which creates a wind turbine effect. Spin is used to: o gyroscopically stabilise, o ensure the arrow's aerodynamic profile is averaged out over a short travel distance, o provide increased drag in early flight, o possibly improve grouping in a cross wind. Spin speed is directly proportional fletch angle and flight speed and independent of the number of fletches. However, spin is more complex than it may first appear. Gyroscope Effect Interestingly the gyroscopic effect on the arrow is minimal and can safely be ignored for most purposes. When and if it does come into play, its precession effect is more negative than helpful. It is only mentioned here as it is often the first thing that comes to mind. It is far more important with gun ballistics where the spin may be 100x faster. Averaging Asymmetry An arrow's longitudinal profile is variable due to fletching, nock and boardheads if used. Spinning the arrow eliminates a directional bias on a yawing or pitching arrow by averaging out these variations. This is particularly desirable for two blade broadheads. Time Dependent Drag During spin-up, a little forward kinetic energy is transferred into rotary kinetic energy with some short term induced drag. As the arrow approaches its steady spin speed, the drag drops back to a value similar to that at zero offset for helical fletches but a slightly greater value for straight offset fletches. This is a most convenient characteristic as the extra drag occurs when maximum yawing and pitching is likely to occur so has a useful dampening effect. Three parameters determine this effect: o Fletch area determines the spin torque for a given offset. A larger total area will produce more torque, achieve spin speed quicker, so will present higher initial drag that will drop off more quickly. o Fletch offset determines steady state spin speed and the spin torque available. Higher offsets will have higher spin speeds and higher initial drag. o Arrow spin moment of inertia will determine how quickly an arrow can accelerate its spin with the applied torque. Thus, there may be some value in tuning this effect for best dampening and minimum drag (arrow drop). It is possible that as an arrow slows, some angular momentum could be converted back to forward momentum by thrust from a propeller effect! Magnus Effect Due to the Magnus effect a spinning shaft that is at an angle to the air stream will experience a small lateral force called the Kutta-Joukowski lift. It is referred to as a lift because it is equivalent to an aerofoil. This is the curved path effect seen with spinning balls in most ball based sports. The Kutta-Joukowski force is approximated: Fm = w r2 ? v L where w is the spin rate in revolutions per second (rps), r the arrow radius, ? the air density, v the air velocity and L the effective exposed length. In practice this force is small and only comes into play with significant fishtailing or porpoising. Spin Interactions If the spin rate (or the fletch sweep rate) is similar to the arrow's lateral resonance frequency (40 to 150 Hz) then possible cross-coupling effects may sustain or even enhance the vibration resulting in an abrupt change in drag. This would be a good reason to limit the spin speed. Broadhead users should be aware that higher spin speeds will increase drag unless the broadhead has similar angular offset to the fletches. A one sided sharpen bevel on the broadhead will help a little at lower spin speeds.

Effect on Grouping It has been reported that cross wind grouping is improved with spin. The rational for this observation may be that the cross- wind drag (and hence wind sensitivity) is dependent on orientation about the longitudinal axis. Slow spin would then widen the group. Nock Typically, the nock can be shown to contribute 10% of the arrow's drag. This is higher than one might expect and is due to the suction effect. It would seem to be a good candidate for drag reduction, however, its is not easy. The simplest approach is the taper the shaft to the smallest practical nock. Point In some ways the point is the easiest part of an arrow to model as it operates in clean air and is usually a simple shape. Normal target points contribute less than 5% of an arrow's drag, but can determine boundary layer formation (see below). Shaft The shaft contributes overwhelmingly to an arrow's drag. In general, the thinner the better, especially with barrelling. Turbulent or Laminar Flow A significant complication in arrow aerodynamics is the laminar- to-turbulent air flow transition. Various studies have shown this transition is not well defined and certainly not well understood. Wind tunnel tests have shown flow can be laminar with an angle of attack less than 2 degrees. However, it is widely accepted that for normally launched arrows, the flow is always turbulent. The reality is probably in between, with a transition to turbulent flow occurring at some distance down the shaft. Further, the transition from lamina to turbulent and back to lamina can show some hysteresis in the transition speed. It seems turbulent flow is induced at launch by arrow vibration and / or initial off axis movement. If a perfect release on a center shot bow is achieved (which is everyone's goal) then it is possible that laminar flow is established. Such a perfect launch would then be susceptible to air turbulence inducing turbulent flow at any point in its flight with the result of a change of drag and hence grouping. A solution would be to ensure turbulent flow by introducing boundary layer energisers or surface roughness (like golf ball dimples?) near the point. Boundary Layer Like all flying objects, arrows develop a boundary layer of slower laminar or turbulent air, that usually grows in thickness along the shaft. A typical thickness may be 10 mm at the nock for a perfect arrow flight (i.e. no resonance, pitching or yawing). This means the fletches will be significantly immersed in this boundary layer. This in turn means the fletching and shaft should be analyzed as one - but that is very complex. Non-ideal motion of the arrow will also significantly modify the boundary layer geometry with time, and it is this effect that makes precision arrow aerodynamics such a difficult subject. Vibration Launching an arrow induces significant vibration into arrows shaft. While many modes and harmonics are present, the fundamental lateral mode remains dominant with a decay rate measured in seconds. The amplitude may be as high as 30 mm peak to peak at the nock of the arrow. The amplitude at the point is generally less as it is constrained by the concentrated mass of the point. The impact of this vibration on drag is difficult to predict, but it is likely to be small as the oscillation involves alternate drag and thrust quarter cycles. A greater effect may well be to ensure turbulent flow over the full length of the arrow. The arrow's fundamental resonant frequency is closely related to the arrow's dynamic spine. It is typically in the 40 to 150 Hz range. You can get a feel for the resonance by holding the arrow vertically between the thumb and forefinger at about a 20% down from the nock and flick the shaft at its midpoint. You will feel a vibration last for several seconds. By moving you hold point up or down the resonance may be more pronounced and last longer. Your hold point is then close to a null, where your fingers can have very little dampening effect. Invert the arrow and repeat - and you will find the second null closer to the point due to the anchoring effect of the point’s mass. In both cases you will feel the same resonant frequency - it is the arrow's fundamental. Stability An arrow's stability is the result of net effect of the above factors. A fundamental necessity is the CoP must always be behind the center of gravity to create a positive stability. The relative distance of the CoP behind the CoG determines the magnitude of the corrective moment. Neutral Stability and Bare Shaft Flight A fletchless arrow has the CoP very close to the CoG with the result of neutral stability. A neutrally stable arrow is assumed to have no corrective forces acting on the arrow, so on leaving the bow any angular offset or angular rotation will be maintained to become obvious. The bare arrow will still experience drag and lift. If the is no angle of attack, the drag will be reduced and there will be no lift. If there is some angle of attack, there will additional induced rag and some lift at right angle to the air stream in the direction of the angle of attack. If an arrow leaves with angular rotation speed, the rotation will continue, so the angle of attack will increase down range. The resultant lift will then cause a deviation from the expected ballistic path. Sufficient angular rotation speed can cause the arrow to completely reverse its orientation. It should be noted that a broadhead arrow with undersized fletching can result in neutral of even negative stability. This can be dangerous as the larger plan area can result in significantly increased lift and a rogue flight path. Stability of Fletched Shafts Fletches move the CoP rearward. This invites the question "what is the optimal fletch arrangement?" In general, the aim would be for rapid correction of an arrow launched with angular offset and /or angular rotation speed and minimum path deviation in the process. The fletches are part of a feedback system where a proportional force is applied to correct an error. Mc = L d + D d tan( ? ) which for small a (±10°) can be approximated to: Mc = Kc d ? where Kc is a catch all constant of proportionality. The corrective moment acts against the arrow's moment of inertia, so the relative magnitudes determines the speed of correction. Without dampening the arrow will oscillate - the corrective moment will cause overshoot, which will be corrected only to overshoot again and again. This is very similar to a swinging pendulum. So what is going to dampen this oscillation? Well, not much! The oscillation is likely to continue through to target with minimal amplitude reduction. Typically, the dampening time constant is 0.5 to 2 seconds for the amplitude to drop to one third. The best solution is to minimize the oscillation at launch by tuning out the angular offset and the rotation from the start. Feather fletching is said to provide dampening; however, the author suspects higher drag is the reason. The aim of the arrow design then is to minimize the path swing as the arrow fishtails or porpoises. This swing is proportional to the arrow's lift coefficient, oscillation amplitude and inversely proportional to its frequency. Broadhead Stability With the addition wing area of a broadhead, the CoP will move forward. This must be compensated for with a larger fletch area.

The Aerodynamic Model We have an arrow with a complex set of motions on a ballistic trajectory. The aerodynamics are non-trivial and most analysis make significant simplifications. Despite the simplifications useful results are still possible. Drag Interestingly, there is no completely satisfactory drag equation for arrows. One can argue that there are at least three components to drag: o pressure drag proportional to sectional area, o frictional drag proportional to skin area and o induced drag proportional to plan area and angle of attack These are all relatively easily evaluated. However, there are other sources of drag that are usually either ignored or somehow bundle with the above. These include drag due to arrow vibration, lamina to turbulent transitions, spin and fletch deformation.

The commonly used drag equation is: FD = 1/2 ? v2 CD A where FD is the drag force ? is the air density v air velocity CD the drag coefficient and A is a reference area. This assumes turbulent flow over most of the arrow. The reference area A can be a bit fuzzy. Just how it is defined depends on the conventions of the application field. For aircraft it is typically wing area. For bullets and cars, it is the net frontal sectional area. In archery there is no well-established convention, however when dealing with arrow components, each component is treated using the most appropriate method. For a fully assembled arrow we use the shaft's maximum frontal sectional area. For a particular arrow component and atmosphere FD = K v2 where K an all encompassing constant. All this seems simple, but wait, the detail is hidden in CD which is a function of the mysterious Reynolds Number** which in turn is also a function of velocity. By definition the Reynolds number is: Re = ? v L / ? = v L / ? where ? is the dynamic or shear viscosity ? is the kinematic viscosity = ? / ? L is the characteristic length The L term is interesting in that it is proportional to size, and provides an understanding of the effect of changing the size (but not the shape) of an arrow. In practice, the Reynolds number, drag coefficient and the resultant drag force of each component (point, shaft, fletches and nock) of the arrow are calculated at the launch speed. These drag components are added for the total drag and the arrow's composite drag coefficient calculated. It is this composite drag coefficient that is used by the ballistics engine to calculate the actual drag at different air speeds. From a practical point of view, this approach is found to be the best of the options, but it is not perfect. The main limitations are that it fails to consider the aerodynamic interaction between the components and it makes no attempt to model lamina to turbulent transitions. However, a significant advantage is that the impact of changes to components can be determined with useful precision. Calibration To link theory to the reality, it is best to calibrate the system with real flight data. This allows us to compensate for the known and unknown limitations of the theory. The arrow's composite drag equation is slightly modified by a proportionality factor determined by calibration: FD = 1/2 ? v2 CD A k where k is the proportionality factor and A is the arrow is the shaft's cross sectional area. To calculate k, the arrow's speed needs to be measured at several different times after launch. This is most simply done indirectly by measuring arrow drop for several different ranges. In FlyingSticks this can be obtained from sight data or by direct drop measurement in a process called Reverse Ballistics. FlyingSticks hides the k factor from the user as it is seen as an internal calibration factor. For individual ballistic trajectory evaluation, the drag equation is reduced to: FD = K v2 where K combines all the non-varying values into a single constant for a particular flight: K = 1/2 ? CD A k For area A, FlyingSticks uses the maximum shaft sectional area, so for a high performance arrow this might yield a CD ~= 2.1. However, if the net sectional area (i.e. shaft + fletches + broadhead) were to have been used a lower CD ~= 0.9 would be expected. The advantage in using the shaft sectional area is that aerodynamic inefficiencies are more intuitively expressed as a higher CD. For example, a high performance arrow fitted with a LARP point may have a CD of ~45, but if the net sectional area were used the CD would change little from ~0.9 even though the drag would have increased greatly. Drag Due to Vibration In reviewing the published literature, it appears no one has seriously studied the aerodynamics of a bent arrow, little alone a vibrating arrow. We have assumed a vibrating arrow has increased drag due to an increased in effective sectional area. Spin-up Drag Spin drag is modelled as induced drag from the fletch rotational torque "lift" that decays exponentially with time as the spin reaches steady state. It uses the standard lift equation applied to each fletch, as if were a wing immersed in the shaft's boundary layer. However, arrow vibration will periodically move some fletches into clean air, changing their lift and drag characteristics. This latter effect in not modelled. The exponential decay rate is calculated from the fletch lift and the arrow's spin moment of inertia and presented to the user as the spin time constant. Lamina to Turbulent Flow Transition Conventional understanding from wind tunnel testing indicates a typical arrow is likely to experience lamina flow for Re < 10,000 and turbulent flow for Re > 22,000 and in between, some arrangement where the transition point progressively moves long the shaft towards the point. In practice it seems the flow is always turbulent. There are likely to be several factors at work. Firstly, it appears that the lamina to turbulent flow transition along the arrow shaft can be very critical on point shape. Secondly, a finger release or non-center-shot bow will induce significant lateral vibration that almost certainly induces turbulent flow. A flight with lamina flow will experience significantly lower drag, but the probability of attaining this state is low so best avoided entirely by NOT having a streamlined point and even adding some surface roughness. There are some conditions when lamina flow might be reliably achieved - such as a well tuned, low speed center-fire bow with thin stiff arrows using a mechanical release in still air. This approach could be useful for indoor rounds.

Sensitivity Analysis From a knowledge of the components in drag equation it is possible to conduct a sensitivity analysis to indicate the likely drag impact of making a change to the arrow's geometry and the environment. This impact is expressed as a change at the target (in offset, strike velocity and penetration) and the sight's range-tape.

Lift Lift is an important force in arrow flight when yawing or pitching occurs for several reasons: 1. lift generated by the fletching provides a correction moment to "stabilize" the arrow, 2. lift experienced by the arrow causes the arrow to deviate from its ballistic path in a dampened oscillatory fashion, 3. additional drag (induced drag) while generating lift. The lift equation is similar to the drag equation: FL = 1/2 ? v2 CL A where the lift coefficient is typically a linear function of angle of attack. Generating lift also generates induced drag that must be added to normal drag.

** Note on characteristic length L and Reynolds number: The drag equation's area term L and the Reynolds number are intimately related. The Reynolds number is primarily used for comparing similar shapes but of different sizes or different speeds or in different fluids. In other words, the flow over similar bodies with the same Reynolds number will be very similar. The important issue is to be consistent in the way L is defined for the purpose it is used, hence a degree of fuzziness is acceptable!

Godfrey Daniels felipe, now thats an explanation

I'm not smart enough to even understand half of the technology. I have a basic concept of bare shaft tuning. I shoot high FOC (28%) at low poundage (for now). I also don't take shots hunting further than 15 yds.

And all perfectly sensible.

For a 0 FOC experiment we will afix only a nock to each end of a shaft and shoot it (fits in "here, hold my beer" catagory)

Looks like I’d better read that article on the original site… lost formatting is a horror show.

I hear you, Viper; and that’s what got me started down this line of thought in the first place… call me crazy, but I’d like to do better than most probably aspire to, simply because I believe it’s possible. Should be, considering (I gather) that it used to be fairly common.

But I don’t know that these sell-correcting arrows are probably gonna get me there… I think they can be made to be right, but now I’m not sure I trust them to tell me how…..

And if it turns out that they flatten my trajectory a bit at longer range, then I will be pretty happy about that. Or maybe not them, but the target spec ones perhaps… I do think the higher FOC version might still be The Ticket for hunting - especially once I am good and sure that they are really Dialed and not just telling me what I want to hear…

Good stuff felipe!

That clearly explains that a bow shot arrow is NOT analogous to a missile, javelin, bullet or anything else- an arrow is unique.

I personally dont like to get bogged down in “ laminar flow” - grin,…. I just tune for good aero flight and use a decent weight arrow- done.

All saying basically the same thing with less or more & different words. Corax I know you are not having tuning trouble. That was a reference in history & bare shaft tuning where you busted a few shafts.

Bare shaft tuning does tell all if you listen.

No free lunch If you stray too far from 8 to 10% you are going to give up something. Your choice speed or momentum. 600 & 400 can be the same dynamatic spine with the correct weight up front. It just takes more weight for the stiffer spine.

You are right Static spine is basically the same for different materials but the recovery rate is different. You might think of fast or slow tips on fishing rods. Well this has been fun. >>>----> Ken

C-l -

Your thinking is correct, but I have a simpler take on it. One "stunt" I used to pull (and made my better students do) was to shoot a full match with only bare shafts.

Once tuned, the bare shafts act like a silent coach (if you know how to read the results) and when they could stay within 5 points or so of their "fletched" scores, they were good to go.

The bottom line is that WE are always the limiting factor, and once people realize that, the improvements begin.

Viper out.

"The bottom line is that WE are always the limiting factor, and once people realize that, the improvements begin."

Yessir, that sure is true.

I’m going to agree 100% that We are the limiting factor, but to your point on a silent coach… I just don’t think high FOC arrows are all that demanding as coaches go, and maybe I need to be pushed a bit…

So for my thought experiment… First thing I did was to plug-in the arrows that I have been using and see how much they differ, and it’s only a couple of pounds either way, so that makes me feel a bit better about being able to make them both work. You would have to make a pretty serious investment in wood arrows to get much closer.

Then I ran a bunch of alternatives through the Calculator and I was able to come up with a couple of arrows in the 10%-12% FOC bracket with the same spine that I’ve been shooting through the Bamboo Viper, which I’m using as my standard of reference because its specs are built into the calculator.

I guess I can take some comfort in the fact that one of the permutations is exactly where I ended up with that bow when I first tuned it for aluminum using 2bears’ paper sack method: an 1816 finishing at 7.9 GPP and 12.3% FOC. If I were to push the envelope with an 1813 that ends up at 10.8% FOC, I am down to 6.6 GPP. Which I suppose would certainly flatten my trajectory at 70 yards compared to what I’ve been using, FOC or not. Maybe I could get less FOC, but 6.6 GPP seems to be light enough.

Correct link to the article above (sorry), I also have it in "Word" format. Easier to digest with correct formatting and included drawings. Will take your understanding and tuning of flyingsticks to a new level.

https://www.flyingsticks.com.au/book/aerodynamics.html#Drag:

C-l-

Again, I'll take the more neanderthal approach. A number of years ago, I set up two arrows with a compromised tune between them. One was with a heavier head (higher FOC and larger helical fletch vs a more std weight head and straight vanes. The difference in scores @ 20 yards were well within statistical error.

Regarding 70 yards (OK, I use meters, for obvious reasons), I can group (score) as well (if not better) with aluminums @ 7 something percent as I can with carbons at nearly double that FOC. The trajectory difference is, however noticeable - due to the total arrow weight (about 1/4" on my sight bar).

I remember one Olympic coach a while back saying FOC meant Freakin' (or something to that effect) Over Complicated.

And for the record, I'm not buying the whole "penetration" thing. Like draw weight, if you don't have enought, nothing you do is going to change that, besides using more weight and once you have enough, any more is kinda useless.

Viper out.

“ I set up two arrows with a compromised tune between them. ”

You see, that’s what I feel like I have already done, and that’s not what I’m after. What I’m trying to do is get at least one arrow tuned as precisely as I am capable of tuning it, but I can’t shake the notion that the higher FOC carbons are glossing over a few things (and to be clear, mine are in the 20%-25% range - nothing extreme, I’d venture).

I was looking over some pics I’d taken at the 40-yard butt, by the way, and confirmed that there’s a limit to how far they’ll stay down the middle; between 30 and 40, there’s some Bad Juju that happens with all of them save one…. Which is a 500 + 200 and (if I am not mistaken), that one has a weight-tube in it. That one will track down tbe middle past 50. So the lighter, faster one reads weak and the slower, heavier one…. doesn’t… which makes perfect sense.

Maybe if I were smarter, I would just figure out what weight tube is in there and stick that in all of them, but that’s going to put me over my target GPP. And I would be unhappy, I’m sure, if I were to increase the amount of arc in my trajectory, as I really feel that I have more than enough already.

At least for the Viper I have some ACCs to mess with, but they need some parts…

Well, FWIW, I think there’s an argument for high FOC arrows penetrating a bit better than conventional, especially when the broadhead is redirected by uneven resistance (one side v the other) upon impact.

Just because the closer the center of gravity to the point, the less likely it is to overtake it upon impact, at which point the arrow is just slapping.

I'm pretty sure one can make an argument for nearly anything they want to if they invest enough energy in it.

Question bwing does it matter?

The answer to that is I individual. :)

Well, I’m not gonna second-guess Todd’s Moose-Hunting arrah….

It appears to have been satisfactory.

It indeed was without question. Now that said, we don't have the results of a similar shot with a same weight arrow, shot from the same bow of a different configuration. So, there is nothing to compare it to.

And yes...Bowmania and I still visit and discuss/debate this stiff.

We all have a setup that we have conviction and confidence in.

I’ve seen it many times with very experienced guys. It might not be the scientifically or physically best setup….. but they know it, shoot it extremely well and its a killer.

I’m a fact and science based guy not susceptible to the confirmation biases endemic in our sport. That said, if you shoot a bow or pistol like its an extension of your arm… forget all the noise on websites and just go kill stuff.

But given the widespread consensus that carbons penetrate better than Al or wood…

To what can we ascribe that? Is it the same benefit if you go lighter spine, add weight tubes, and use lighter points, or does putting the CG at the mid-point (give or take a smidge) and weighting the length of the shaft cause it to wallow like a woodie or Alumalog?

Some days I wish I could afford a camera that would run at about 1000 frames/second…

@Beendare - you forgot shotguns!

My brother is left eye dominant but shoots right handed, and he has a custom built side-by-side. Woe betide any fool who tries to shoot that gun right-eyed or left- handed!!

But I’ve seen him roll big, Nebraska roosters at 40 yards with an improved cylinder, and I’ve seen him bag 7 doves on 5 shells.

With a 20-ga.

WHO DOES THAT???

LOL

"wide spread consensus"

That's what it is. I'm not aware of any actual testing comparing wood and carbon of the same spine, diameter and configuration to see which penetrates best.

Carbon shafts recover from oscillation faster than aluminum- I’ve seen studies and slo mo vids on that.

I think saying it penetrates better is misapplied logic.

Penetration is a combination of many factors…but the material the arrow is made from is a less important factor than say arrow energy, arrow weight, etc.

Some using friction based targets claim the smoother aluminum shafts penetrate better…but in an animal with a slippery (bloody) makeup, the friction on the surface of the shaft is less of a factor.

Like I insinuated earlier, there are important factors we should probably be focused on…and then there are the hair splitters.

"You see, that’s what I feel like I have already done, and that’s not what I’m after. What I’m trying to do is get at least one arrow tuned as precisely as I am capable of tuning it, but I can’t shake the notion that the higher FOC carbons are glossing over a few things"

What's the distance you are trying to tune an arrow? Because if you tune a bareshaft 40yards and over your arrow will have a "normal" FOC. I was never capable to build an arrow with + 28 FOC- I think this is what gets into HFOC - and capable to make it land with the fletched ones at 40 yards. And all arrows I shoot with hunting weight bows and 9-11gpp end in the 21-23 FOC.

To answer to the challenge from the 1st topic: If you can bareshaft at 40 yards, high FOC is covering nothing.

If you have already registered, please

sign in now

For new registrations

Click Here