musings on dynamic properties of barreled arrows

[joachimM]

I'm starting to think that for flight arrows (static) spine as we typically measure it, is much less important than we often think (or at least, than I thought).

When shot, the arrow extremities oscillate around the centre of pressure of the arrow. A barreled arrow has less mass at the extremities. which means less terminal and proximal inertia. so for the same static spine, a barreled arrow oscillates less and less heavily than a non-barreled arrow. meaning less drag during flight.
I think the barrel shape is most important for this aspect, than for mass saving purposes, but how much barrel-shape is needed for ideal tuning to the bow?

I think we might find the solution in tillering arrows. A non-barreled arrow on a spine-scale will show most bending at the centre, least at the extremities. My gut feeling tells me we need arrows that bend evenly when spined, in a slightly elliptical tiller shape. This should give higher frequency oscillations that return more quickly to a static shape, even at rather low static spine weights. 

Basically, it would reflect the difference between a bow with and one without hand shock.

Any thoughts?

[JNystrom]

Makes sense! Really simple but good idea. The barrel form might actually come close to that elliptical bent shape, but it's interesting to understand more why. I will pay attention to this when I start building arrows for our competition in may!

[Badger]

     I agree with you, I like to tiller an arrow to bend most a little behind center.

[willie]

I'm starting to think that for flight arrows (static) spine as we typically measure it, is much less important than we often think (or at least, than I thought)

Joaquim, have you seen the buckling test used by atlatl dart makers? It seems to work better for darts, than spining. It might be interesting to give that buckling test to proven performing arrows.

This should give higher frequency oscillations that return more quickly to a static shape

I found some quite primitive dart designs that seem to utilize quicker cycles, and have been impressed with how they fly. Higher frequencies seem to attenuate better in many types of systems.

[JNystrom]

If oscillation is what we want to improve, shouldn't the optimal shape be symmetrical? In other words, pile and nock end equally tapered and balance point in middle.

My resent best shot was with this kind of arrow profile. It's weird how the turkish flight arrows have balance point back and uneven barrel...

[Del the cat]

I think it's all very fine margins...my mate JT shot one of my flight ELBs on Sunday, very hot, head wind... about 50 yards short , yet he was getting back a full draw where last time he was short drawing.
It's very hard to change one variable without effecting the others.
Im sure we are all shooting barrelled arrows and just an extra 0.2mm diameter in the middle can add stiffness, but o you taper sharply at the ends and leave a good few inches in the middle fatter or tape all the way up to the thickest point?
It's all very subtle. Fat at the back gives a poor FOC... or is it poor? Hickman says you want 10% FOC but the Turkish arrows are behind centre. Do you need more mass and more FOC in a head wind?
So many questions... hopefully a bit of variation in the arrows and I may stumble across the best match to the bow.
I've just made one a whisker fatter in the middle but more tapered at the ends so it's prob dynamically stiffer, I've got the balance point about 1.5" front of centre... weighed it and the scale read 190 gn!!!    My instincts kicked in and I though... that can't be right!! So I got another flight arrow and weighed that 275gn... re-weighed the new one 260gn, that's more believable.
Maybe I hadn't let the scale zero.
Prob' with an ELB flight bow is the width at the grip.... just how low a brace dare I go for max powerstroke without risking snapped arrows?
That's what keep us playin' this game 
Del

[JNystrom]

Good choice is just to make as many different weight/spine/balance point arrows that you possibly can and you will find a good one from the lot. But who am i to give you tips, you Del are still a lot of ahead of me in distance. I'm coming! 

Assuming your arrow is somewhere around 28-30" and for a 80+ pound bow, even 260gn is really light. But i'm not sure, i myself have been shooting too stiff arrows for a while! Hah.

[willie]

If oscillation is what we want to improve, shouldn't the optimal shape be symmetrical? In other words, pile and nock end equally tapered and balance point in middle.

Do we want to improve the oscillations or eliminate it?   I am hoping that reducing or canceling the vibrations created at release will let the arrow fly straighter and further.

[Del the cat]

@ Willie.
There will always be oscillation (especially with an ELB where the arrow has to flex round the bow more than a bow with a cut away shelf).
I think we need to help the oscillation to be as small as is feasible and to die down as quickly as possible.
As usual improving one property often worsens another 
Mind that's more easily said than done...
Maybe a tubular arrow with the hole down the middle filled with porridge or molasses would damp out the vibration 
If it didn't work you could always snack on it 
Del

[willie]

Del,
of course the flex is needed at the loose unless centershot, but how to best settle the arrow after it clears the bow is the question. Joachim noted in the opening post that more mass in the ends seems detrimental to quick reduction. He also postulated that....

My gut feeling tells me we need arrows that bend evenly when spined, in a slightly elliptical tiller shape. This should give higher frequency oscillations that return more quickly to a static shape, even at rather low static spine weights.

I agree that higher freq vibes can be attenuated easier. A question in my mind is, can an assymetrical bend set up a tendency for two different harmonics to hopefully self cancel faster?

[Del the cat]

Analyse this! 
https://www.youtube.com/watch?v=SOcrM7FMQSI
Del

[avcase]

Analyse this! 
https://www.youtube.com/watch?v=SOcrM7FMQSI
Del

It looks like it could benefit from a little higher spine so the arrow nock will better clear the handle as the arrow leaves the string.

Alan

[Comalforge]

Del,
of course the flex is needed at the loose unless centershot, but how to best settle the arrow after it clears the bow is the question. Joachim noted in the opening post that more mass in the ends seems detrimental to quick reduction. He also postulated that....

My gut feeling tells me we need arrows that bend evenly when spined, in a slightly elliptical tiller shape. This should give higher frequency oscillations that return more quickly to a static shape, even at rather low static spine weights.

I agree that higher freq vibes can be attenuated easier. A question in my mind is, can an assymetrical bend set up a tendency for two different harmonics to hopefully self cancel faster?

Intuitively, it might make sense to place the largest diameter 2/3 from the nock, 1/3 from the tip.  Taper to the same diameter but at different taper rates.

[willie]

Intuitively, it might make sense to place the largest diameter 2/3 from the nock, 1/3 from the tip.  Taper to the same diameter but at different taper rates.

yes  an assymetrical taper. When considering similar ideas, I realized that one might have to weight the nock end somehow if FOC was too positive.
I did read about some turkish flight arrows that had lead in the nock end. I can not remember the reference, but it makes more sense with such a profile.

[Badger]

   I think asymmetrical taper has established itself as far as shape as being the way to go. Spine and FOC we don't seem to have conclusive results in yet. My belief is that a bow shooting 235 fps should be hitting 430 yards consistently even with 200 grain arrows if all things are right. So a 350 yard shot even though a good shot would not necessarily reflect a best design. I have hit 380 yards with a mediocre bow yet tend to hover around the 300 mark with much faster bows. This has to be due to arrow shelf design and or arrow design and possibly archers release.