MA, Turbulators, FOC penetration Effect, Etcetera

[Ed Ashby]

#5027

I got a couple of good questions on another website and thought that the replies were worth posting on Tradbow too. They cover a wide range of questions, and might be of help to someone out there who hasn’t asked the questions. You won’t have much trouble figuring out what the questions are. Individual names have been removed.

……., the Turbulator disrupts the air flow, which increases the pressure exerted by the air on the fletching. This increases the stabilizing effect of the fletching, which allows you to use a smaller amount of fletching to attain the same degree of stabilization in flight, but that is not the only purpose of the A&A fletching.

Using smaller fletching offers many benefits, and there’s a pretty comprehensive thread about the A&A fletching. Here’s the link. https://www.tradbow.com/members/cfmbb/messages.cfm?threadid=C8EA4FBD-1422-1DE9-ED1A1C909376B7B6

On an arrow that is already well tuned (correctly bare shaft tuned), using the minimum fletching required to overcome the wind shear created by the broadhead gives a higher FOC. As one approaches the level of Ultra-EFOC reducing shaft weight at the arrow’s rear becomes a major factor in the degree of FOC change you and attain.

Ultra-EFOC is (Study defined) as being 30% and above. The reason being, it’s really difficult to reach 30% and above FOC without making a concentrated effort to attain FOC.

Just putting high tip weight on most light weight shafts will get you into the EFOC range, but the higher the FOC gets the more increase in tip weight it takes to make the same degree of FOC increase. As Ultra-EFOC is approached it is more ‘productive’ to be reducing weight at the shaft’s rear than adding additional tip weight. That’s because the rear leaver arm (the distance from the nock to the balance point) gets longer as the FOC increases, and the forward lever arm, from the arrow’s front to the balance point) gets shorter. The longer the leaver arm the more force a given amount of weight will exert.

As for close shots, a well tuned Ultra-EFOC bare shaft will recover from paradox incredable rapidly; it will certainly be showing straight impact into the target within 3 yards, and usually at even closer ranges. By adding a broadhead and then the minimum fletching required to overcome the broadhead’s wind shear effect equally rapid recovery from paradon is retained. That’s one of the things I most like about EFOC and Ultra-EFOC arrows; they show faster paradox recovery than normal or high FOC arrows, giving greater penetration on close range shots.

As for longer range shots, because EFOC/Ultra-EFOC arrows recover from paradox much more rapidly than normal/high FOC arrows, and the smaller fletching required to stabalize the broadhead in flight has less surface area, they have less drag. The rapid paradox recovery means less of the bow-derived energy of the arrow is wasted by paradox. As the arrow proceeds downrange, the smaller fletching not only has less drag it drains less energy when any flight correction is required. Because the EFOC/Ultra-EFOC arrow has a longer ‘rear leaver’ (or rear stearing arm, if you prefer) the fletching requires less ‘applied force’ to overcome any flight instability required by such things as the broadhead’s wind shear and changes in air flow/pressure caused by changes in wind direction. All this retained arrow energy is now used to produce ‘productive work’; higher retained arrow velocity and arrow force.

If you take 2 broadhead-tipped arrows that are identical in all external dimensions (except for the fletching, as the lower FOC arrow will require more fletching to overcome the broadhead’s wind shear), and of equal mass (weight) and equally well tuned but having a large difference in FOC and shoot them at longer ranges you will see a very noticable difference in trajectory. The arrow having very high FOC will shoot noticable flatter. This is because of the additional ‘useful energy’ the arrow has attained from the bow and retained as it traveled downrange. Make up 2 such arrows yourself; one at normal FOC and one at EFOC and try that simple test at 40 meters or so. The difference in trajectory is very noticable.

In the Study’s testing, all test shots are conducted within 30 minutes of the animal expiring. That’s because early testing showed a difference between the results from shots taken on very fresh tissues and those taken after the animal had been dead for a longer period of time. Starting in 1982 I began to keep a seperate detailed database of each big game, bow killed animal. This information is used as a cross reference between the results observed on the test shots and the outcomes observed on real hunting shots. While the bow killed database has only 629 animals, whereas the test shot database(s) contains thousands, there is excellent correlation between the shot outcomes between the test shots and the actual bow kills. I wish I had started that bow killed database 25 years sooner, with my first deer. It would be very interesting to have the performance of those early arrow setups to also compare against.

Hope that helps a bit,

Ed
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…., the degree of centershot a bow has is a HUGE factor in the dynamic spine required for the arrow to bare shaft tune correctly. I’ve found much the same results as you show. I have one (far from centershot) 85# bow that tuned with a 40-55 shaft, with 415 grains of point weight.

There are a lot of factors that affect what dynamic spine your bow will require. Each is an individual, but altering the degree of centershot is a major tuning factor that you can use to your advantage when trying to develop EFOC and Ultra-EFOC arrows.

……., as to my own hunting arrow setup, the first consistantly measurable increase in real-tissue penetration between arrows of otherwise equal dimensions shows up at 19% FOC. All else equal between 2 arrows, as the difference in FOC increases above 19%, penetration increases at what appears to be an ever increasing rate of penetration gain per one % change in FOC. In other words, the penetration gain per 1% FOC increase gets greater the higher the FOC gets.

I had very good field success on big game for years with arrows in the 800 grain range at just over 19% FOC; all from fairly heavy draw weight bows. After getting into the EFOC/Ultra-EFOC testing and seeing the results I immediately changed the setup of my ‘serious hunting arrows’ to an EFOC setup, and am now moving them into an Ultra-EFOC setup.

Unlike some, I am not content to take the position that the arrow setup that has worked satisfactorily for me for years is ‘good enough’. There are so many things that can happen when one is shooting at an animal. Regardless of how good a shot one is, and how hard one tries to make a perfect shot on each animal, under actual hunting conditions it’s impossible to predetermine exactly where your arrow is going to impact, and what you MIGHT be asking your arrow to do if you are to successfully recover the animal. No hunting arrow can ever work ‘too good’. Whenever the testing shows me that I can improve on the terminal performance of my hunting arrow in any way I will do so. For hunting I want the very best performing arrow setup I can possibly be using; the one that gives me the highest possible chance of success, regardless of what the hit turns out to be. In bowhunting there is no such thing as overkill.

Ed
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MA stands for “Mechanical Advantage”. The MA of a “machine” is the ratio of the ‘force input’ to the ‘force output’. (Note that this is a ratio of “Force”, not a ratio of Kinetic Energy; yet another reason why KE isn’t useful for predicting the ‘work’ your arrow can accomplish – i.e. the degree of penetration.)

Your broadhead is a ‘simple machine’, comprised of a series of inclined planes. Your broadhead comprises a part of the larger ‘simple machine’; your arrow. Each has its own MA. The approximate MA of a broadhead is relatively simple to calculate. The overall MA of your arrow is more difficult to calculate.

The MA of a simple machine tells you how much the “applied force” is multiplied by use of that particular ‘machine’. A MA of 3.0 multiplies the applied force by a factor of three, meaning it allows the applied force of your arrow to do 3 times that amount of “work”, when applied against a given ‘resistance load’. A word of caution: Don’t confuse what some term a “3 to 1 ratio” broadhead with the broadhead’s MA; not the same thing.

Hope that helps,

Ed
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Woops, forgot the second part of your question. Here’s what the testing indicates:

(1) At equal arrow weight and equal external arrow dimensions, the higher the FOC the greater the penetration.

(2) At equal arrow weight and equal external arrow dimensions, the higher the arrow’s FOC the greater the degree of penetration gain PER EACH 1% OF FOC INCREASE.

(3) At equal external arrow dimensions and equal amounts of FOC, but with differing arrow weights, the greater the arrow weight the greater the average outcome penetration. The difference in average outcome penetration, percentage wise, will be PROPORTIONAL to the percentage of increase in the arrow’s force. (Force is measuered by the arrow’s momentum. An arrow’s ‘energy’ is not a measure of the arrow’s force.) For example: At equal external arrow dimensions and equal amounts of FOC, but with differing arrow weights, the percent increase in the arrow’s momentum is closely equivalent the percent of increase in the average outcome penetration; when measured across a large number of shots into real tissues. Often the percentage increase in average penetration will slightly exceed the increase in arrow force. Why? The increased arrow force is being applied by a ‘simple machine’, which can multiply the arrow’s force increase.

BTW: Not every broadhead on the market has a MA exceeding 1.0, though most traditional broadheads do. A broadhead with a MA below 1.0 REDUCES the ‘work output’ an arrow carrying a given amount of force can do. When measured across a large number of shots into real tissues, increasing arrow force by a given amount, WHILE USING A BROADHEAD WITH A MA BELOW 1.0, results in an average penetration increase SIGNIFICANTLY LESS than the percentage of increase in arrow force.

Ed
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…….., the mechanical advantage of a wedge depends on the ratio of its length to its thickness. Where a short wedge with a wide angle does the job faster, it requires more force than a long wedge with a smaller angle. For the purposes of simple comparison of one broadhead to another, and assuming similar profiles for the ferrules on both broadheads, you can just use the MA of the main blade, which is a wedge. I borrowed the directions for the MA of a wedge from a University of Arkansas web site.

“The mechanical advantage of a wedge can be found by dividing the length of either slope (S) by the thickness (T) of the big end. As an example, assume that the length of the slope is 10 inches and the thickness is 4 inches. The mechanical advantage is equal to 10/4 or 2 1/2. As with the inclined plane, the mechanical advantage gained by using a wedge requires a corresponding increase in distance.” Note that that the “Slope” is the length ALONG ONE EDGE of the wedge. The “Thickness” would be the width of the broadhead.

Here’s what the Study data indicates about the FOC effect on average outcome penetration into tissues, expressed differently.With arrows having equal external dimensions (same broadhead, same shaft diameter and material), equal quality of arrow flight and total arrow integrity (remembering that FOC’s measurable penetration effect does not show up until a FOC of 19% is reached):

(1) Having a greater degree of FOC allows a lighter arrow to equal the penetration of a heavier arrow having a lesser amount of FOC. How much difference in weight there can be becomes a function of both the amount of FOC difference between the two arrows and the starting point for the FOC change (the FOC of the heavier, lower FOC arrow). That’s because the rate of penetration gain becomes greater the higher the FOC gets. In other words, a change from 30% to 31% FOC yields a greater penetration increase than does a change from 20% to 21% FOC, or from 26% to 27%. The upcoming 2008 Update, Part 6 will have an analysis of what the data suggest is the degree of change that can be achieved by Ultra-EFOC, as well as a brief review of what a similar analysis of the 2007 testing EFOC data indicated.

(2) When arrow FOC is equal but arrow weight is different the arrow having the greater weight will have greater penetration.

You definitely want the diameter of your arrow’s shaft to be less than the diameter of the broadhead’s ferrule. All else equal between two arrows, the difference in average outcome penetration between a shaft diameter smaller then the broadhead’s ferrule and a shaft diameter greater than the diameter of the broadhead’s ferrule is 40%. That’s HUGE!

When all else is equal between two arrows, a tapered shaft out penetrates a parallel shaft, and a parallel shaft out penetrates a barrel tapered shaft. Here’s where you can read more about this.

https://www.tradbow.com/members/249.cfm

https://www.tradbow.com/members/228.cfm

https://www.tradbow.com/members/247.cfm

Hope that helps,

Ed

[sapcut]

#36776

On an arrow that is already well tuned (correctly bare shaft tuned), using the minimum fletching required to overcome the wind shear created by the broadhead gives a higher FOC. As one approaches the level of Ultra-EFOC reducing shaft weight at the arrow’s rear becomes a major factor in the degree of FOC change you and attain.

But….your leaving FOC on the table. Using the bareshaft tuned arrow above, why not add a little more weight (50 gr.) to the front making it a little weak, then add slightly larger fletching (2-3 gr.). The end result is higher FOC and tuned perfectly with fletching. I think that is the way to maximize the FOC.

To me, the bareshaft tuning is almost irrelevant when the end result will be a fletched arrow.

Richie

[Ed Ashby]

#37604

sapcut wrote:

On an arrow that is already well tuned (correctly bare shaft tuned), using the minimum fletching required to overcome the wind shear created by the broadhead gives a higher FOC. As one approaches the level of Ultra-EFOC reducing shaft weight at the arrow’s rear becomes a major factor in the degree of FOC change you and attain.

Using the bareshaft tuned arrow above, why not add a little more weight (50 gr.) to the front making it a little weak, then add slightly larger fletching (2-3 gr.). The end result is higher FOC and tuned perfectly with fletching. I think that is the way to maximize the FOC.

Richie

Richie, for me a “correctly bare shaft tuned” shaft will already be showing a very slight amount of ‘weak’ dynamic spine; because addition of the fletching is going to have a slight stiffening effect on the dynamic spine. If additional weight is added to the already tuned bare shaft you will be dealing with a greater degree of weak dynamic spine. I try to get the shaft tuning to a point that I can use the minimum amount of fletching possible; just enough to overcome the broadhead’s wind shear effect (and whatever release errors I introduce into the equation). This results in the highest FOC I can attain on that particular, “well tuned” shaft setup.

If one adds additional tip weight to an already “correctly tuned” bare shaft setup then an excessively weak dynamic spine situation is created. Using additional fletching will overcome this, but in doing so some degree of the arrow’s efficiency of flight is lost; mostly in paradox recovery.

Hope that clarifies,

Ed

[sapcut]

#37901

Richie, for me a “correctly bare shaft tuned” shaft will already be showing a very slight amount of ‘weak’ dynamic spine;..

Yea, that is what I’m talking about. Makes perfect sense to me.
With my three 4″ low profile banana feathers I may be loosing some speed that may be measureable compared to smaller feathers. But… due to their size and very little extra weight they do allow for the stabilization of more weight up from which allows for a few more percentage points of FOC…..which I strive for.

Richie

[Ed Ashby]

#38034

sapcut wrote: But… due to their size and very little extra weight they do allow for the stabilization of more weight up from which allows for a few more percentage points of FOC…..which I strive for.

Richie

What’s important is that the DYNAMIC SPINE of your arrow be correct to counteract the amount of point weight you are using; whatever that amount might be. You’re balancing the shaft’s dynamic spine to the point weight you are using AND to both your individual bow and your personal shooting technique. That’s the entire goal of bare shaft tuning; to get perfect arrow flight (on a field point tipped arrow) FROM YOUR BOW when shot by YOU, and WITHOUT ANY FLETCHING masking the arrow’s flight characteristics.

If a given shaft is correctly bare shaft tuned (with a field point) it should shoot every bit as accurately as it does when you add fletching. If the tuning is correct and you then add additional point weight the tuning will no longer be correct. Yes, you can increase fletching size/surface area and get the shaft to shoot accurately, but the shaft is no longer tuned.

When you add fletching it has a STIFFENING EFFECT on the arrow’s flight, but it DOES NOT actually stiffen the spine. What you are doing is adding excess fletching to mask the ‘weak spine’ flight characteristic of the shaft. (When I speak of bare shaft tuning so that there is a ‘slight weak spine impact’ I’m refering to no more than two to three inches off of dead center, left to right, at 40 meters; 44 yards; no more than that.)

With enough fletching area you can get a very poorly tuned arrow to ‘shoot accurately’; hitting your target; but the arrow will have an excess amount of paradox, and there will be an extended time of shaft vibration (oscillation) as the arrow travels down range. That has a marked, detremental effect on penetration. Even a modest increase in shaft oscillation at the time of impact severely decreases penetration. At all practical hunting distances, any extra penetration gains you are achieving by using excess fletching area to give a spine stiffening EFFECT (to overcome the shaft’s excessively weak dynamic spine) will, in all probability, be more than offset by the increased resistance caused by the increased shaft oscillation at impact.

When two EFOC arrows are both properly tuned, the arrow with higher FOC shows more penetration than an arrow with lower FOC precisely because it has less shaft oscillation, which reduces shaft drag. This is probably better explained in:

https://www.tradbow.com/members/256.cfm

It’s important to remember that perfect arrow flight is one of the three prime requirements that EVERY hunting arrow should have (total arrow integrity, perfect arrow flight and a broadhead that’s truly sharp). Perfect arrow flight is the ‘enabler’ for all of the other penetration enhancing factors. Anything less than perfect tuning of your arrow will have a detrimental effect on the amount of penetration gain you obtain from each additional penetration enhancing feature you add, including increased FOC.

It is only after you have achieved perfect arrow flight that the arrow’s degree of FOC comes into play. A poorly tuned EFOC/Ultra-EFOC arrow won’t penetrate as well as a perfectly tuned arrow with normal FOC. Somewhere above ‘poorly tuned’, and short of “perfectly tuned”, the EFOC/Ultra-EFOC will begin to exceed the penetration of the perfectly tuned normal/high FOC arrow. Exactly where depends on the quality of the arrow’s flight.

Hope that helps clear up the difference,

Ed

[sapcut]

#38441

To simplify…If my Ultra-EFOC arrow flys straight with feathers and broadhead does that mean it is “tuned” adequately for shooting an animal and expecting maximum penetration? Or is there another aspect or dimension that is being masked by the “straight” shooting arrow that will reduce penetration?

Richie

[Ed Ashby]

#42338

Richie, unless the arrow is tuned for as near perfect flight as you can get without fletching then the fletching is having to overcome some degree of unnecessary irregularity in arrow flight. The most damaging (penetration wise) flight irregularity would be a greater degree and duration of shaft oscillation. If your arrow is correctly bare shaft tuned then shaft oscillation will be at a minimum, and the fletching does not have to ‘fight’ to overcome shaft oscillation.

Use enough fletching and even a poorly tuned shaft will shoot ‘straight’, but the fletching is having to exert great pressure to overcome both the shaft’s tendency to deviate and the excess shaft oscillations the use of an incorrect dynamic spine caused.

Hope that’s stated understandable,

Ed

[sapcut]

#42510

I understand. I guess I will need to take a little weight off the front and settle for 32-33% FOC instead of 34-36% FOC.

[Ed Ashby]

#42618

That’s about where I’m at so far, 32%.

Ed

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