bow efficiency

[enewman]

just information.

i did this paper because a pro is telling people that when you add weight to an arrow it loses efficiency. he states it takes energy away from a bow to be able to push a heavy arrow.

https://1drv.ms/b/s!AtVqxLy9AZcGkhrP8LhN1FtrS_c1

[Lightning48]

Thanks for the info Newman.

[Bullseye07]

What pro would that be?

[Dry Bones]

I believe this "pro" should meet Rick Barbee.
High FOC makes my longbow perform terrific.

[treestand]

Very interesting paper. It would be great if how manufacturers ran their bows through your test. I believe this would reveal some interesting differences in efficiency.

[Rat]

just information.

i did this paper because a pro is telling people that when you add weight to an arrow it loses efficiency. he states it takes energy away from a bow to be able to push a heavy arrow.

https://1drv.ms/b/s!AtVqxLy9AZcGkhrP8LhN1FtrS_c1

First law of thermodynamics (conservation of energy): Energy can neither be created nor destroyed, it only be transferred or changed into another form of energy. (that's pretty close anyway)

If the bow itself isn't changed in anyway, the amount of stored energy stays the same.

What can change is the amount of energy transferred (kinetic energy) and not lost due to vibration, friction and hysteresis.

So, technically speaking, the bow is neither more efficient nor less efficient, it just is. What becomes more efficient is the USE of the energy. IOW, more energy is put to good use in propelling the arrow, instead of blowing up the bow.

In reality, we are making the arrow more efficient at doing the job it was built for due to the larger input of energy (because less energy is lost in other areas). The bow itself is the same efficiency whether it is dry fired or shooting an arrow of any weight.

Do the math on that enewman!

[treestand]

First law of thermodynamics (conservation of energy): Energy can neither be created nor destroyed, it only be transferred or changed into another form of energy. (that's pretty close anyway)

If the bow itself isn't changed in anyway, the amount of stored energy stays the same.

What can change is the amount of energy transferred (kinetic energy) and not lost due to vibration, friction and hysteresis.

So, technically speaking, the bow is neither more efficient nor less efficient, it just is. What becomes more efficient is the USE of the energy. IOW, more energy is put to good use in propelling the arrow, instead of blowing up the bow.

In reality, we are making the arrow more efficient at doing the job it was built for due to the larger input of energy (because less energy is lost in other areas). The bow itself is the same efficiency whether it is dry fired or shooting an arrow of any weight.

Do the math on that enewman!

Depends how you define the system. If you define it only as the bow, you are correct. If the system is the bow and the arrow, then the systems efficiency, defined as the percent of energy transferred to the arrow, does vary with arrow weight. You basically said this in a different way

[Rat]

Depends how you define the system. If you define it only as the bow, you are correct. If the system is the bow and the arrow, then the systems efficiency, defined as the percent of energy transferred to the arrow, does vary with arrow weight. You basically said this in a different way

This is an interesting thought experiment...

If the measure of a bows efficiency is how much (as a percentage) of it's energy is transferred to the arrow, ho how we define that?

Energy in equals energy out.

So, let's say we have a bow that produces 100 units of energy:
80 of those units go to the arrow
10 of those units go to friction
10 of those units go to vibration

Then, let's say we do something to the arrow (make it heavier), but we still only have a bow that produces 100 units of energy:
85 of those units go to the arrow
10 of those units go to friction
5 of those units go to vibration

So, we reduced the vibration amount by 5 units and added it tot he arrow; but does this really increase the efficiency of the arrow?

The efficiency of the arrow is defined by it's physical characteristics, cutting surfaces, shaft size, fletching type, etc. IOW, everything that creates drag.

I don't think an arrows efficiency changes as it's Ke is increased (as I stated earlier), I think it just has more energy so it will do more work; but at the same efficiency.

I think the only way to increase the efficiency in an arrow is by changing a physical characteristic, like the broadhead. Going from a mechanical to a two blade cut on contact would increase the efficiency of the arrow due to the lower friction (drag) of the cut on contact head. Efficiency is how much work can be done with the same amount of energy.

Higher energy doesn't make something more efficient: a 747 has much higher energy (thrust) than a Honda Civic, yet the Civic is much more fuel efficient.

I think we must define what "efficient" is in order to compare apples to apples.

If "bow efficiency" is defines as the percentage of the bow's energy that is transferred to the arrow, which is Kenetic Energy (not the arrow's Kenetic energy, the transferred energy from the bow) how do we measure that across arrows with a different mass?

We can't use the Ke of the arrow, that is has a variable (arrow mass) that is not dependent on the bow. IOW, we need to be able to tell the difference in transferred energy as it is NOT related to the increased mass of the arrow.

That would tell us if the higher mass arrow was really more efficient (taking more transferred energy from the bow).

I don't know how to do that but I think enewman's methodology is flawed as he is just calculating the Ke of the arrow without knowing if it is related to the increased mass or a more efficient transfer of energy from the bow.

I do think enewman is on the right track though; I just don't know the math to prove it.

I'm not arguing, I'm just trying to figure it out in layman's terms...

And I believe the hypothesis to be true, a heavier arrow will result in more transfer of energy from the bow; resulting in more efficient use of the bow's energy (a more efficient bow, as we've defined it). I just can't prove it.

[enewman]

First law of thermodynamics (conservation of energy): Energy can neither be created nor destroyed, it only be transferred or changed into another form of energy. (that's pretty close anyway)

If the bow itself isn't changed in anyway, the amount of stored energy stays the same.

What can change is the amount of energy transferred (kinetic energy) and not lost due to vibration, friction and hysteresis.

So, technically speaking, the bow is neither more efficient nor less efficient, it just is. What becomes more efficient is the USE of the energy. IOW, more energy is put to good use in propelling the arrow, instead of blowing up the bow.

In reality, we are making the arrow more efficient at doing the job it was built for due to the larger input of energy (because less energy is lost in other areas). The bow itself is the same efficiency whether it is dry fired or shooting an arrow of any weight.

Do the math on that enewman!

your wording is funny to me. maybe saying the same thing.

yes. the energy amount produced by the bow is the same. its where the energy is applied. to the arrow. when an arrow is increased in weight the arrow did not gain efficiency. the arrow has no energy till it leaves the bow. so the arrows energy comes 100% from the bow. this means if a measurement of ke increases any at all. there was a transfer of energy with an increase. again that comes from the bow.

now we could truly test, but not with any test equipment I have. we would have to have a way to test vibration.

[Rat]

your wording is funny to me. maybe saying the same thing.

yes. the energy amount produced by the bow is the same. its where the energy is applied. to the arrow. when an arrow is increased in weight the arrow did not gain efficiency. the arrow has no energy till it leaves the bow. so the arrows energy comes 100% from the bow. this means if a measurement of ke increases any at all. there was a transfer of energy with an increase. again that comes from the bow.

now we could truly test, but not with any test equipment I have. we would have to have a way to test vibration.

This is true only if the weight of the arrow doesn't change. If you shoot a heavier arrow, how do you know the increase in Ke isn't due to the increased mass only?

[treestand]

Conservation of energy is satisfied if the system is defined to be the bow and the arrow. Define efficiency as that percentage of the bow's energy transferred to the arrow. The arrow's KE is easily calculated from measured speed and measured weight. Vary the weight, measure the speed as the OP did and you can calculate the relative change in efficiency. The absolute efficiency can not be easily measured but the change is clear. Plot the data and you would see that increasing the weight all the way to infinity would continue to increase bow efficiency but the ability to measure the speed quickly becomes impossible. In truth, measuring arrow speed a few feet in front of the bow doesn't capture all of energy transferred by the bow as you have losses from the instant it leaves the string.

[enewman]

This is true only if the weight of the arrow doesn't change. If you shoot a heavier arrow, how do you know the increase in Ke isn't due to the increased mass only?

The heavier arrow is the reason for ke increase.

[enewman]

Conservation of energy is satisfied if the system is defined to be the bow and the arrow. Define efficiency as that percentage of the bow's energy transferred to the arrow. The arrow's KE is easily calculated from measured speed and measured weight. Vary the weight, measure the speed as the OP did and you can calculate the relative change in efficiency. The absolute efficiency can not be easily measured but the change is clear. Plot the data and you would see that increasing the weight all the way to infinity would continue to increase bow efficiency but the ability to measure the speed quickly becomes impossible. In truth, measuring arrow speed a few feet in front of the bow doesn't capture all of energy transferred by the bow as you have losses from the instant it leaves the string.

I think a vibration measurement might show it. I’m tryimg to see how i can do this with out spending money. If I had a pezzo Twitter and a scope I might be able to see it

[enewman]

This is an interesting thought experiment...

If the measure of a bows efficiency is how much (as a percentage) of it's energy is transferred to the arrow, ho how we define that?

Energy in equals energy out.

So, let's say we have a bow that produces 100 units of energy:
80 of those units go to the arrow
10 of those units go to friction
10 of those units go to vibration

Then, let's say we do something to the arrow (make it heavier), but we still only have a bow that produces 100 units of energy:
85 of those units go to the arrow
10 of those units go to friction
5 of those units go to vibration

So, we reduced the vibration amount by 5 units and added it tot he arrow; but does this really increase the efficiency of the arrow?

The efficiency of the arrow is defined by it's physical characteristics, cutting surfaces, shaft size, fletching type, etc. IOW, everything that creates drag.

I don't think an arrows efficiency changes as it's Ke is increased (as I stated earlier), I think it just has more energy so it will do more work; but at the same efficiency.

I think the only way to increase the efficiency in an arrow is by changing a physical characteristic, like the broadhead. Going from a mechanical to a two blade cut on contact would increase the efficiency of the arrow due to the lower friction (drag) of the cut on contact head. Efficiency is how much work can be done with the same amount of energy.

Higher energy doesn't make something more efficient: a 747 has much higher energy (thrust) than a Honda Civic, yet the Civic is much more fuel efficient.

I think we must define what "efficient" is in order to compare apples to apples.

If "bow efficiency" is defines as the percentage of the bow's energy that is transferred to the arrow, which is Kenetic Energy (not the arrow's Kenetic energy, the transferred energy from the bow) how do we measure that across arrows with a different mass?

We can't use the Ke of the arrow, that is has a variable (arrow mass) that is not dependent on the bow. IOW, we need to be able to tell the difference in transferred energy as it is NOT related to the increased mass of the arrow.

That would tell us if the higher mass arrow was really more efficient (taking more transferred energy from the bow).

I don't know how to do that but I think enewman's methodology is flawed as he is just calculating the Ke of the arrow without knowing if it is related to the increased mass or a more efficient transfer of energy from the bow.

I do think enewman is on the right track though; I just don't know the math to prove it.

I'm not arguing, I'm just trying to figure it out in layman's terms...

And I believe the hypothesis to be true, a heavier arrow will result in more transfer of energy from the bow; resulting in more efficient use of the bow's energy (a more efficient bow, as we've defined it). I just can't prove it.

Ok. I just read this post from you. Yes I agree. The only test we have to see if more energy from the bow is being transferred to the arrow. Is to shoot the arrow and get the fps and calculate it. This type of reading is 100% due to weight gain in arrow.

We know if we take a 400 gr arrow and a 500gr arrow the ke will be higher. Why is this. Less energy is wasted into bow as vibration. And transferred to the arrow

Now is this the best test. I do not think so, I’m looking into it. At this time it is the best test we have, or that I know of.

[Rat]

Conservation of energy is satisfied if the system is defined to be the bow and the arrow. Define efficiency as that percentage of the bow's energy transferred to the arrow. The arrow's KE is easily calculated from measured speed and measured weight. Vary the weight, measure the speed as the OP did and you can calculate the relative change in efficiency. The absolute efficiency can not be easily measured but the change is clear. Plot the data and you would see that increasing the weight all the way to infinity would continue to increase bow efficiency but the ability to measure the speed quickly becomes impossible. In truth, measuring arrow speed a few feet in front of the bow doesn't capture all of energy transferred by the bow as you have losses from the instant it leaves the string.

How do you know if the increased Ke of the heavier arrow is due to an increase in mass or higher bow efficiency; or, what percentage of the Ke increase is due to one or the other?

You say the change is clear, but to me the only change that is clear is that Ke increases with mass.

So, if I had a 2 ton arrow, drew my bow and hit the release (assuming a 70 lb holding weight) I would transfer 100% of the energy of the bow to the arrow. String doesn't move, limbs don't flex, no vibration, no movement of the arrow.

Is this what you mean by "increasing the weight all the way to infinity"? And is this a 100% efficient bow now?

So, just to be clear on our definition: When we say "bow efficiency" we are talking about the percentage of energy transferred to the arrow and not lost to vibration, friction etc. Not arrow efficiency and not bow efficiency, but the efficient transfer of energy.

Again, I think y'all are correct; I think the transfer of energy is more efficient with a heavier arrow. I just want to understand the mechanics better.