Author Topic: FOC, center of pressure and performance  (Read 2323 times)

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Offline JNystrom

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FOC, center of pressure and performance
« on: September 08, 2017, 01:25:45 am »
I think the arrowhead discussion answered the questions, so let's make another topic.
Cp - center of pressure.
https://www.grc.nasa.gov/WWW/K-12/rocket/rktstab.html
Turkish arrows.
http://www.turkishculture.org/lifestyles/turkish-culture-portal/turkish-flight-arrows-554.htm?type=1

I don't have a very good method to analyze the center of pressure with my Primitive flight arrows. I do a little better job of this with my non-Primitive footbow arrows, by observing how they sink and shoot underwater. I can do this with arrows made of carbon and stainless steel components, but not with an arrow made of Primitive materials.

Probably the best advice I can recommend is to mark all your flight arrows with a permanent serial number and keep a log of how the arrows perform with different bows and shooting conditions. I observed some interesting things when I started doing this. First, I found that I had two or three arrows that were consistently the longest flyers, even if shot from radically different bows using radically different shooting styles.  For example, I have one or two arrows that I found were used to set records for Modern Longbows 35 & 50lb), English Longbows(35 & 50 lb), and Primitive bows (35 & 50lb). Second, the arrows that matched the classic Turkish flight arrow geometry with the maximum thickness in the rear 1/3rd of the arrow were never my best performers. I have better luck with this shape if shot backward with the point where the nock should be, and nock where the point should be. Third, my high FOC arrows fly very consistent, but have not been the farthest flyers for whatever reason. They do seem to drill a deeper hole in the ground, but that is about it.  Fourth, it is very important to pay a lot of attention to the fletching. Thick turkey feathers are the equivalent air-brakes.

Alan

Hope I am not going too far off topic, as this  is not much about points anymore, but there are some good observations being shared in this thread about aerodynamics that I would like to follow up on. There might even be some erroneous statements also.  I have to question my own assertation that the Cp moves in relation to the CG as the arrow slows down. The lift and drag forces that act at the Cp on the lever arm are reduced as velocity diminishes, thus lessening the ability of the arrow to restore itself to efficient flight, if disrupted,  but I cannot see why the length of the lever arm would have reason to change.
 
A few different questions have been on my mind.....

1.  what is the gliding effect that JNystrom mentions?

2. why did the reproduction turkish arrows that Alan made, fly better backwards?

3. why do high FOC arrows nose dive more at the end of the flight?

Offline JNystrom

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Re: FOC, center of pressure and performance
« Reply #1 on: September 08, 2017, 01:53:02 am »
Avcase,

I personally have just started to keep a log of all flight shooting. I mark the arrow serial number, weight, spine (deflection in 22" test, exact millimeters), length, balance point, thickness, fletching height and finally the distance arrow has travelled (every shot, not only the longest), the bow from which the arrow is shot.
Lots of info, so lets see how long do i have the patience to continue.

willie:

1) I may have just repeated others words, but i feel there is truth behind the speculation that back balanced arrow keeps itself in flying longer, than tip "heavy" arrow.
2) I haven't tested those 2" -FOC arrows of the Turks, but 1,5" -FOC arrows seemed to fly still ok. To the earlier question have i inspected any Turkish arrow, no way, i haven't seen any even reproduction of Turkish arrows. I live in Finland and well, for many reasons haven't been in any museum to check any. I have seen some of that style arrows made by a Finnish bowyer, with horn nock and pile, fletched with parchment.
3) Good guestion. Just because of gravity?

I don't know if you have visited this page, but it's full of nice information in terms of flight and it's aspects. http://www.tap46home.plus.com/mechanics/
For example the optimum calculated weight of the arrow, 60 grains???

Offline Marc St Louis

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Re: FOC, center of pressure and performance
« Reply #2 on: September 08, 2017, 04:34:19 am »
I think the arrowhead discussion answered the questions, so let's make another topic.
Cp - center of pressure.
https://www.grc.nasa.gov/WWW/K-12/rocket/rktstab.html
Turkish arrows.
http://www.turkishculture.org/lifestyles/turkish-culture-portal/turkish-flight-arrows-554.htm?type=1

I don't have a very good method to analyze the center of pressure with my Primitive flight arrows. I do a little better job of this with my non-Primitive footbow arrows, by observing how they sink and shoot underwater. I can do this with arrows made of carbon and stainless steel components, but not with an arrow made of Primitive materials.

Probably the best advice I can recommend is to mark all your flight arrows with a permanent serial number and keep a log of how the arrows perform with different bows and shooting conditions. I observed some interesting things when I started doing this. First, I found that I had two or three arrows that were consistently the longest flyers, even if shot from radically different bows using radically different shooting styles.  For example, I have one or two arrows that I found were used to set records for Modern Longbows 35 & 50lb), English Longbows(35 & 50 lb), and Primitive bows (35 & 50lb). Second, the arrows that matched the classic Turkish flight arrow geometry with the maximum thickness in the rear 1/3rd of the arrow were never my best performers. I have better luck with this shape if shot backward with the point where the nock should be, and nock where the point should be. Third, my high FOC arrows fly very consistent, but have not been the farthest flyers for whatever reason. They do seem to drill a deeper hole in the ground, but that is about it.  Fourth, it is very important to pay a lot of attention to the fletching. Thick turkey feathers are the equivalent air-brakes.

Alan

Hope I am not going too far off topic, as this  is not much about points anymore, but there are some good observations being shared in this thread about aerodynamics that I would like to follow up on. There might even be some erroneous statements also.  I have to question my own assertation that the Cp moves in relation to the CG as the arrow slows down. The lift and drag forces that act at the Cp on the lever arm are reduced as velocity diminishes, thus lessening the ability of the arrow to restore itself to efficient flight, if disrupted,  but I cannot see why the length of the lever arm would have reason to change.
 
A few different questions have been on my mind.....

1.  what is the gliding effect that JNystrom mentions?

2. why did the reproduction turkish arrows that Alan made, fly better backwards?

3. why do high FOC arrows nose dive more at the end of the flight?

Not sure how a rocket can be compared to an arrow since a rocket is under rear end propulsion while an arrow is in free flight
Home of heat-treating, Corbeil, On.  Canada

Marc@Ironwoodbowyer.com

Offline willie

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Re: FOC, center of pressure and performance
« Reply #3 on: September 08, 2017, 01:53:44 pm »
Thanks for opening a new thread, JN. Lots of good reading for the weekend in that last link.

Once in a while I have an arrow or dart shot that seems to just float exceptionally before it falls, catches air as some say. A statistical outlier to the normal expected range variation. I thought perhaps there is another factor to the equation I am missing.........  Lift that keeps the nose up? Allowing the trajectory to stay horizontal longer, in spite of the additional drag caused by the nose up attitude?

Alan, the arrows that fall short and dig in harder must land more vertical that the winning shots. What observations can you offer about the winners? Do they land on their sides or stick in the ground?



« Last Edit: September 08, 2017, 08:04:41 pm by willie »

Online Badger

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Re: FOC, center of pressure and performance
« Reply #4 on: September 08, 2017, 02:23:29 pm »
   Willie, very close to 100% of the arrows are stuck in the ground regardless of being a bad or good shot. Most of them are stuck at a higher degree than 45, some of the real bad shots are facing the wrong way.

Offline avcase

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Re: FOC, center of pressure and performance
« Reply #5 on: September 09, 2017, 05:03:31 pm »
A model rocket is a lot like an arrow in a lot of ways. The same physics are at work. A rocket requires a little more stabilization, but it will fly stable as long as the center of gravity is forward of the center of pressure.  There are a number of model rocket builders that have altitude competitions that resemble our flight archery competitions. The model rocket builders are also looking for that perfect balance of center of gravity to center of pressure. They need enough stability to keep the rocket on a straight upward path. An over-stable, point heavy rocket does not do as well because it tends to weathervane into every fluctuation of the wind currents that it passes through.

I do pay attention to the angle the arrows strike the ground. Every arrow seems to have a hard limit in how far it can be thrown. There comes a point where faster launch speeds from the bow makes little difference in how far the arrow can be thrown. If you start to notice the arrow starts landing at a very steep angle, like 75 or more degrees from horizontal, then this is a sign that the only way to progress is to work to improve the arrows.  I experienced this first hand a couple of years ago at one of our Salt Flats events. I launched about 20 arrows from different bows over a wide range of launch speeds, launch angles, and wind conditions, and every arrow landed within +/-15 yards distance of each other!  The arrows launched at high speeds went a little bit further, but landed almost vertically. The arrows launched at slower speeds landed at a little shallower angle, but still went nearly as far.  It was as if there was an invisible wall that these arrows were not allowed to cross. 

This led to a long process of trying to figure out how to break past this barrier. I was able to rule out atmospheric conditions by running into this same distance limit at completely different shoot locations and atmospheric conditions. I started playing around with arrow geometry and test shot my arrows alongside arrows made by past flight archers (arrows made by Harry Drake, Ike Hancock, and George Alavekiu). I found each of these groups of arrows had their own limits. Some were better than others. I considered an arrow topped out if it landed at an angle greater than 75 degrees. Faster launch speeds were a waste of time beyond this.

I even went as far as creating computer models of these flight arrows, and I used a fairly sophisticated model rocket program to shoot these virtual flight arrows at various launch velocities and angles. Funny thing is that these simulations showed the exact same symptoms that indicate the point of reaching the performance limits of a flight arrow.

The hard part is to figure out what to do to get the most out of an arrow. All I can say for certain is that there isn't a simple formula to make a great flight arrow. There are several factors that all interact with each other. I played around with several experiments to try to find some breakthrough. I tried an experiment shooting numerous arrows from a shooting machine that were identical weight, and identical nock and fletching geometry, but with different shaft geometry. Some were barreled with more thickness toward the rear, others barreled with more thickness to the front, others barreled with greatest thickness to the center.  About the only consistent result from this indicated that the barelled arrows with more thickness at the rear performed worse than the rest. Also, arrows with more radical barrel tapering performed worse than arrows that had less radical barrel tapering. But it is kind of hard to isolate if this performance difference is due to the geometry differences, or due to shifts in the center of gravity due to the shape differences.

There's much more to this, but this post is long enough already. I will say that I did manage to break through the performance limit that I ran into a couple of years ago, but I am also sure there is still much to learn. For example, I cannot figure out how Harry Drake shot as far as he did using the equipment he had. I've ruled out the bows. They were very good, but not magic. It had something to do with his arrows and how they were shot. Pictures indicate Drake's record footbow arrows landed at pretty exceptionally shallow angles at distances exceeding 1-mile, but I don't understand how this was achieved. Drake's arrows were barreled with more of the thickness toward the front, and this pushes the center of pressure toward the front of the arrow. Drake's footbow arrows did not have metal points, but had a metal nock and steel razor blade vanes, which pushes the center of gravity rearward.  My farthest flying arrows have similar properties, so maybe there is something to this. 

Alan

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Re: FOC, center of pressure and performance
« Reply #6 on: September 09, 2017, 05:49:41 pm »
  Very interesting post Allen, I enjoyed every bit of it. I also appreciate greatly the work you have done over the last decade and more. And congratulations on breaking the 1 mile barrier, that is a great achievement.

  Arrow knowledge is the key to great flight shooting. My experience matches yours to a tee, it doesn't seem to matter what launch speed some arrows take off, they seem to have an upper limit. The best comprehensive approach to arrow design I could come up with was to rent an apt. near the flats and spend about a year their making and shooting arrows. The few arrows we launch each year for distance are not enough to tell us much if anything. At this time I am favoring slight tapers from front to back, no opinion on spine I really need to work on that, I have had some good flyers from arrows I thought were weak but I have also had some good flyers from arrows I thought too stiff.
I could pull my hair out just thinking about arrows!

Offline Del the cat

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Re: FOC, center of pressure and performance
« Reply #7 on: September 10, 2017, 12:13:48 am »
Great thread from everyone, great post Alan
Del :)
(Mrs Cat has vetoed the wind tunnel in the living room  ;) )
Health warning, these posts may contain traces of nut.

Offline JNystrom

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Re: FOC, center of pressure and performance
« Reply #8 on: September 10, 2017, 06:43:12 am »
That's good idea to check the arrow angle at ground level! If the arrow angle changes quickly, shouldn't this be because of,
a) too much drag
b) too much FOC
?
In extreme, badminton ball leaves fast from the racket, but drops fast like hitting the imaginary wall. Because of drag and low mass.

Shouldn't one always use vellum vanes only if he want's to shoot far? They surely are thinner and have less drag. I don't have any yet, so i would be interested in trying paper vanes saturated with superglue, which should act similary.

Yep, it's excellent we have a discussion about flight arrows! Its always bows, bows.
« Last Edit: September 10, 2017, 06:47:42 am by JNystrom »

Offline BowEd

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Re: FOC, center of pressure and performance
« Reply #9 on: September 10, 2017, 06:57:16 am »
Great post fellas.As just an interested observer of the knowledge commented here way more then my experience for this competition but making my own arrows.The greater front of center degree I thought was mostly for better penetration reasons hunting without any concern for distance.With not as much concern for the extra stiffness needed on the shaft flight shooting too.
My related thoughts when making arrows with more front of center degree were of a javeline being thrown for farther flight though.A javeline is very front heavy.
Arrow making for this competition is far more involved then just making hunting arrows.
Beadman
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Re: FOC, center of pressure and performance
« Reply #10 on: September 10, 2017, 08:20:59 am »
   I have had my best distances, well past world record in the 50# classes with very average bows. Just by chance an arrow came out clean and flew well. The arrow is a huge part of the equation. I am convinced that the best strategy for working out the mysteries has a lot to do with shooting a lot more arrows of different designs at least until recognizable patterns start to emerge. We know drag is a bad thing but the jury still seems to be somewhat out on the perfect shape of an arrow and the best way to get an arrow out of a bow. I do believe we have a pretty good idea on these things that just needs some fine tuning. More shooting, shooting, shooting!

Offline avcase

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Re: FOC, center of pressure and performance
« Reply #11 on: September 10, 2017, 02:27:00 pm »
Steve,
I agree. There is no substitute for going out and shooting.  I often use the theoretical ideas to help target areas to explore with real world testing, and I use discoveries from real world testing to understand the limititations and improve the theoretical methods.

I feel there is a lot be learned about arrow flight. There really hasn't been a lot of research into the way we are trying to apply aerodynamics to a flight arrow. Especially where beneficial drag might be concerned. An arrow is not quite like a bullet or a rocket. An arrow is unlike fin stabililized armor piercing projectiles due to the lower velocity and due to the fact that we are not necessarily trying to punch as deep a hole as possible into the ground. Same goes for gravity bombs. We are able to learn a lot from these other applications, but there is much to be learned on our own as well.

Alan

Offline willie

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Re: FOC, center of pressure and performance
« Reply #12 on: September 10, 2017, 08:48:09 pm »
Alan and Steve,
the sharing of your experience(s) at the flight line and observations are appreciated, and make much food for thought.

"Especially where beneficial drag might be concerned." brings to mind some other innovative drag applications
 

1. some ideas seen in javelin development
Code: [Select]
http://www.symscape.com/blog/engineering-in-sports-javelin-throwing

2. a review of some primitive ideas used on harpoons (for extended glide underwater)
Code: [Select]
https://kayakgreenland1959.wordpress.com/look for winged harpoons about 4/5 down the page

Offline avcase

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Re: FOC, center of pressure and performance
« Reply #13 on: September 11, 2017, 10:28:22 pm »
I used to throw javelin in high school. They used to be less point-heavy than they are now.

I am not finding the info on the harpoon but I think seems pretty interesting. I suppose it isn't possible to post a direct link here.

There is a water toy called a toypedo that will provide hours of entertainment in the swimming pool and get you thinking about flight arrows.

There has been a lot of unresolved discussion about the potential of beneficial drag with flight arrows. Is it better to give the arrow a polished surface, or leave it somewhat rough?  I think it depends on a few different things. I can see how it might be beneficial to trip the boundary layer with some roughness on a longer arrow, but it with a very short arrow, it may be better to give it a highly polished surface in order to preserve as much laminar flow across the surface of the arrow as possible.  those that compete and win model rocket altitude contests go with a polished surface. I will take a look through my best natural material arrows and see if there are any common traits worth sharing. It has been awhile since I really looked through them closely.

Alan


Offline willie

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Re: FOC, center of pressure and performance
« Reply #14 on: September 12, 2017, 11:19:45 am »
https://kayakgreenland1959.wordpress.com/category/yak-1959-three/
about halfway down the page
his blog contains the same info in a few places, as one post seems to be an edit of another


In looking over article and tables from the Journal of the Society of Archer-Antiquaries, volume 4, 1961 article, I noticed that...........

Quote
"in some cases lead was inserted in a hole drilled in the bottom of the nock to bring the balance point........".

and that the balance point  of the sampled arrows always coincides with the maximum diameter, and occurs at 55% (plus/minus 1%) back from the tip. These arrows are very similar, almost too similar?

Has anyone seen sources of info on different/other turkish arrows that show some variety? or is the turkish design that consistent?
« Last Edit: September 12, 2017, 04:24:41 pm by willie »