FOC, center of pressure and performance

[DC]

Theoretically, if the dry fire speed is the same, then then will both only shoot a 0 grain arrow at the same speed. As the arrow weight increases the bow that stores more energy will take the lead. But that's only my 2cts worth.

[joachimM]

dry fire speed isn't about theoretical 0 grains arrows, it's the speed at which reducing arrow mass doesn't yield higher initial velocity. It may be that the dry fire speed of two bows is identical, one with 250 grain arrows, the other at 350 grain arrow. The heavier arrow will fly farther, because of its higher energy content (energy = mass times velocity squared).
With the same bow design, identical dry fire speed at higher arrow mass requires a larger bow, hence scaling up the bow.

[Badger]

dry fire speed isn't about theoretical 0 grains arrows, it's the speed at which reducing arrow mass doesn't yield higher initial velocity. It may be that the dry fire speed of two bows is identical, one with 250 grain arrows, the other at 350 grain arrow. The heavier arrow will fly farther, because of its higher energy content (energy = mass times velocity squared).
With the same bow design, identical dry fire speed at higher arrow mass requires a larger bow, hence scaling up the bow.

  Dry fire speed is just that, dry fire speed, there is no lower limit where they quit getting faster as you lighten the arrow. If there is close to one it is way below an practical arrow weight. I have tested them down below 100 grains and they keep getting faster.

[Badger]

Theoretically, if the dry fire speed is the same, then then will both only shoot a 0 grain arrow at the same speed. As the arrow weight increases the bow that stores more energy will take the lead. But that's only my 2cts worth.

   Energy storage isn't the big factor when dealing with light arrows, efficiency is the big one. Higher energy storage along with efficiency is a good thing especially when it allows you to shoot a heavier arrow at a desired velocity. As arrow weights get lighter the gap in speed between high energy storers and highly efficient bows starts narrowing. Eventually the more efficient bow will overtake the higher energy storing bow unless they are just too far apart to begin with.

[joachimM]

It seems the point I was trying to make is missed here, as it's not about dry fire speed per sé. It's that, IMO, a heavy arow shot at the same initial velocity will fly farther than a light arrow, because, unlike bow and arrow dimensions, air density (and thus drag) cannot be scaled up or down, it remains the same.

But it's close to the obvious, since we already know heavy flight bows shoot farther than light flight bows...

This said, can you explain, Steve, how you think larger bows and more energy storage will aid flight shooting?

[Badger]

  This is a good conversation, I am going to pour my heart out archery fashion.
One of the reasons I am thinking of going toward longer bows, longer arrows and more energy storage is basically to compensate for inadequacies in my arrows and release techniques for clean arrow flight. I am not so sure about the longer arrow but I know heavier arrows carry much better and are more forgiving. The greater potential I believe is still in the shorter arrows.

    One thing that I have only recently appreciated the importance of in recent years is our ability to lower hysteresis in wood bows. The lighter you arrow gets the more significant this becomes. Normally a very well built self bow with a little reflex would usually hit around 172 fps at 10 grains per pound, by controlling set and lowering the hysteresis that number can suddenly jump into the 180's with a bow of identical unstrung profiles. This won't happen every time but if you get everything right you will get some of these super shooters. When it comes to light arrow shooting the lower hysteresis is even more significant and can add over 30 fps to a 200 grain arrow, that might give you 100 yards. I am pretty sure 50# wood bows will eventually hit 450 yards. With the longer arrows and bows I am hoping for shots in the 375 to 380 yard range. Finding a place to practice often, at least a dozen times a year I think would help immensely. 

[willie]

THE ONES YOU CAN SEE LEAVE  vs. THE ONES YOU CAN'T
Badgers observation and comment seems sensible. The short reason is of course, "aerodynamics". The links posted earlier seem to categorize drag values as being highly dependent on whether the airflow past an arrow is considered "laminar" or "turbulent".  For a look at the desirable low drag laminar flow around a point, see the pics in
 
http://sci-hub.bz/10.1177/1754337111430569
 
Also of note is the assertion that a typical arrow shaft has air flow that is neither laminar nor turbulent, but transitional, (primarily because of its diameter to length ratio).  Although airflow may start out as laminar, there appears to be an increasing amount of drag caused by growing turbulence, the further down the arrow shaft you look.  The distance from the point that this turbulence begins, and when it occurs during the arrow's flight could be enough to separate the winners from the losers.

The tradeoff/strategy Badger mentions,  heavier arrows vs lighter arrow, certainly could be viable on account of the ability to launch a heavier stiffer arrow more consistently. Yaw or angle of attack is a large predictor of point drag when the flow is considered laminar. In this study, after about 3 degrees, it is essentially the same as if the flow was turbulent.            See graph 7

https://www.researchgate.net/publication/241124001_Aerodynamic_properties_of_an_arrow_Influence_of_point_shape_on_the_boundary_layer_transition

For whatever reasons turbulence forms (and there are a few more like surface smoothness, wind gusts, etc.),  It is unclear (to me) how easy or hard it is for turbulence to subside, once the smoother flow has been disrupted.

[avcase]

WIllie,
I haven’t run across that article before. That’s a good one that I will have to read through carefully. I thought the comment about the observation of arrow vibration induced by aerodynamic turbulence was interesting.

I have found a trend that shorter arrows do better than longer arrows with my footbow. These also have a very smooth and polished outer surface. In this case, the arrows are released with a mechanical release, which eliminates a lot of the tuning issues that come with a finger release.

On the other hand, the split cane arrows I posted about earlier are far from being smooth or polished.

Thanks!

[Badger]

  I am starting to think a key design feature on a flight arrow might be in it's ability to recover as fast as possible from release. I think Allen had some discussion on this a few years back. A 150 grain arrow can easily leave a 50# wood bow at 250 fps but will not tolerate any sideways motion without loosing most of its velocity. What qualities enhance rapid recovery from slight inconsistencies in release.

[Del the cat]

I think that it's the speed of the arrow rather than the poundage of the bow that determines the spine. In order to get to say, 180 fps the arrow has to accelerate at a certain rate. There is more involved than just bow draw weight. I believe that if you have a 70# bow that has a dry fire speed that is the same as a 30# bow the same spine arrow will work in both.

Exactly... it's acceleration that flexes the arrow. I've had 50# spine arrows shot from 120# warbow no prob'...
Mind when I mentioned it on a FB warbow thread I had some bloke who couldn't understand it calling me an idiot  . It's sad 'cos the guy didn't even realize how little he knew !
Del

[Del the cat]

  I am starting to think a key design feature on a flight arrow might be in it's ability to recover as fast as possible from release. I think Allen had some discussion on this a few years back. A 150 grain arrow can easily leave a 50# wood bow at 250 fps but will not tolerate any sideways motion without loosing most of its velocity. What qualities enhance rapid recovery from slight inconsistencies in release.

Yeah, it must be springy enough to recover quickly but not to resonate and keep oscillating, so it needs damping... maybe bamboo stuffed with Christmas pudding?
Del

[willie]

maybe bamboo stuffed with Christmas pudding?

Have not tried the pudding yet Del, but I agree about the oscillations and dampening.  A  thread in the arrow section has a few links to interesting footage of arrows and darts in flight, focusing on those ocillations.
(the recent discussion and links start with Reply #26)    http://www.primitivearcher.com/smf/index.php/topic,61394.0.html

I have been experimenting with primitive tapers on Atlatl darts. . The tapers I have tried are copied from darts found in melting ice fields in Yukon Territory that are between 3200 and 4500 years old. A good dart release is not easy, on account of the pronounced lateral displacement of the nock that an atlatl spur creates. Soft spines are required, stiff darts do not fly well at all, and tapering can change the location and timing of the oscillations quite a bit.

[Badger]

I have thought about this same thing with the darts, I am not that good with darts but played around with them some and was amazed at how much difference the proper spine makes. I was able to throw a little over 100 yards with a not so hard throw once I got my spine figured out. You could visibly see it oscillate. Intuitively I would think that once they are on the right path the oscillations don't really help but there was no question that my softer spines took off and flew better. I think it was easier for a beginner to get a clean throw.

[avcase]

Arrow flex is always a necessary evil with finger release and degree of non-center shot. A wiggling arrow experiencing a lot of excess drag in the early part of its flight, so it’s best to find ways to avoid as much as possible.  When it comes to damping our vibrations, I don’t know what may work best. Big fletching helps, but adds lots of drag.

Once the vibration damps out, the air flow boundary layer should start to reattach and become laminar toward the front couple of inches of a polished arrow.

My best arrows are higher density at the point and nock, and light and stiff in the center. It seems like this should be worse for a shot that leaves the bow misaligned, because the arrow has high inertia for the fletching to overcome.  However, this higher inertia should be better if we are talking about a “floater”. 

Alan

[willie]

Alan, are you still describing your bamboo hex shaft arrows, where the outer skin on the bamboo is presumably denser than the interior?

It seems like this should be worse for a shot that leaves the bow misaligned, because the arrow has high inertia for the fletching to overcome.

The inertial force causing yaw?

I have been giving some thought to similar mass distributions, ie, heavier footings, both front and rear, or both, on some designs I hope to make soon.