dave,
thanks for the tip re winding direction.
that type of thinking suits my kiss method and capability.
keep safe,
bruce.
Coriolis Effect - practical considerations
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bruce moulds
- Posts: 2900
- Joined: Sun Jun 19, 2005 4:07 pm
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DaveMc
- Posts: 1453
- Joined: Mon Aug 22, 2011 6:33 pm
No worries Bruce. Same for me - it has to be simple on the mound or errors will occur!!
Ian - Magnus Force is definitely one of the very minor effects and really should be ignored. I guess Bryan has left it out of his book for this reason - it really has a negligible effect on the long range target shooter. Vaughn disputed its often misconceived use and to quote twice from McCoy "Magnus Force acting on a spinning projectile is usually small enough to be neglected" (p37). and "Free-flight spark range tests have never been able to measure this force or moment; they produce such insignificant effects on trajectory that even a high precision spark range facility cannot detect them" (p39)
It acts in the opposite direction to the movement we see here and I looked back through some of our records and did some quick calculations - it seems immeasurable in the field in our testing too as we actually do see no decay in the vertical velocity component induced by the original gyroscopic moment (or aerodynamic jump) over 1000 yards of testing. Of course our testing methods have such significant vertical variation that to try and decipher such small differences becomes impossible.
The interesting thing that although Magnus force can be small the Magnus moment apparently can cause some serious dynamic stability issues. The Magnus moment normally occurs behind the centre of gravity. McCoy described this effect as "cubic, bi-cubic or even quintic" (to the power of 4). Perhaps one of the explanations as to why some projectiles really go to pieces in transonic flight when there is "any" wind around. "It is usually the nonlinear Magnus moment coefficient that dominates the behavior of spin stabilized projectiles" (p287).
What does this mean to us??? Providing we stay above the transonic zone very little. If we enter into the transonic region then it can have a major effect on the dynamic stability of the projectile.
Ian - Magnus Force is definitely one of the very minor effects and really should be ignored. I guess Bryan has left it out of his book for this reason - it really has a negligible effect on the long range target shooter. Vaughn disputed its often misconceived use and to quote twice from McCoy "Magnus Force acting on a spinning projectile is usually small enough to be neglected" (p37). and "Free-flight spark range tests have never been able to measure this force or moment; they produce such insignificant effects on trajectory that even a high precision spark range facility cannot detect them" (p39)
It acts in the opposite direction to the movement we see here and I looked back through some of our records and did some quick calculations - it seems immeasurable in the field in our testing too as we actually do see no decay in the vertical velocity component induced by the original gyroscopic moment (or aerodynamic jump) over 1000 yards of testing. Of course our testing methods have such significant vertical variation that to try and decipher such small differences becomes impossible.
The interesting thing that although Magnus force can be small the Magnus moment apparently can cause some serious dynamic stability issues. The Magnus moment normally occurs behind the centre of gravity. McCoy described this effect as "cubic, bi-cubic or even quintic" (to the power of 4). Perhaps one of the explanations as to why some projectiles really go to pieces in transonic flight when there is "any" wind around. "It is usually the nonlinear Magnus moment coefficient that dominates the behavior of spin stabilized projectiles" (p287).
What does this mean to us??? Providing we stay above the transonic zone very little. If we enter into the transonic region then it can have a major effect on the dynamic stability of the projectile.
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bartman007
- Posts: 921
- Joined: Thu Aug 13, 2009 10:06 pm
- Location: Gippsland
Williada,
I like your practical exercise for use with team shooting. It would be a great way of comparing shooters, and their gear. And may assist in team selection.
For me, it has taken till this year to take account of elevation changes due to wind strength/direction changes. In the past, I have simply forced myself to stay as close to the center as possible and take advantage of the 1 MOA (typically) in vertical that is available.
From the comments that Ian and Dave have made, it would seem that a good rule of thumb is 1/3 MOA for every 10 mph.
This is very important when the direction changes (goes down the range), where you can lose 10 -> 15 mph in one fowl swoop. You have also lost 0.5 MOA in elevation which accounts for shots lost at all 4 corners of the Center! You can get a 1:30 4:30 7:30 or 10:30 shot, with shooters typically putting it down to a dodgy round or dirty barrel. Knowing this should help the shooter account for elevation and strength, and get 3 oclock or 9 oclock 6's instead.
Great feedback guys!
I like your practical exercise for use with team shooting. It would be a great way of comparing shooters, and their gear. And may assist in team selection.
For me, it has taken till this year to take account of elevation changes due to wind strength/direction changes. In the past, I have simply forced myself to stay as close to the center as possible and take advantage of the 1 MOA (typically) in vertical that is available.
From the comments that Ian and Dave have made, it would seem that a good rule of thumb is 1/3 MOA for every 10 mph.
This is very important when the direction changes (goes down the range), where you can lose 10 -> 15 mph in one fowl swoop. You have also lost 0.5 MOA in elevation which accounts for shots lost at all 4 corners of the Center! You can get a 1:30 4:30 7:30 or 10:30 shot, with shooters typically putting it down to a dodgy round or dirty barrel. Knowing this should help the shooter account for elevation and strength, and get 3 oclock or 9 oclock 6's instead.
Great feedback guys!
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bruce moulds
- Posts: 2900
- Joined: Sun Jun 19, 2005 4:07 pm
dave,
the 175 gn 30 cal sierra has always had a good name for trans and subsonic stability with match rifle shooters. there could be others, but that one comes to mind.
creedmoor shooters (800, 900, & 1000 yd) are shit acared of what they call windshear, which seems to be a localized wind condition that the bullet goes through.
their bullets are subsonic at those ranges.
certain nose shapes seem to be better than others, and it was assumed until recently that more mass in the nose reduced this problem.
it now appears that a lower drag nose is an answer, but not a spitzer.
the old metford bullet was proven through experimentation to be stable in these conditions. it had a round nose bigger than 50% of calibre, but less than 60%, and a secant ogive.
your comments re stability in wind have great interest in relation to this.
keep safe,
bruce.
the 175 gn 30 cal sierra has always had a good name for trans and subsonic stability with match rifle shooters. there could be others, but that one comes to mind.
creedmoor shooters (800, 900, & 1000 yd) are shit acared of what they call windshear, which seems to be a localized wind condition that the bullet goes through.
their bullets are subsonic at those ranges.
certain nose shapes seem to be better than others, and it was assumed until recently that more mass in the nose reduced this problem.
it now appears that a lower drag nose is an answer, but not a spitzer.
the old metford bullet was proven through experimentation to be stable in these conditions. it had a round nose bigger than 50% of calibre, but less than 60%, and a secant ogive.
your comments re stability in wind have great interest in relation to this.
keep safe,
bruce.
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DaveMc
- Posts: 1453
- Joined: Mon Aug 22, 2011 6:33 pm
Bruce - I was going to come back to this.
The 168 Sierra was renown for dynamic instability and as you say the 175 less so (although often blamed on the steep boat tail of the 168)
A couple of years ago at the NRAA queens I watched keenly as a few of the top standard shooters switched to Sierra 155 and were shooting unbelievable scores. Conditions were mild generally - then we hit the 900 and 1000 yard day (as always Brisbane knows when to turn it on) and a stiff switching headwind kicked in. With the 7mm I was going backwards and forwards 3 minutes each way but when I finished I watched a couple of these guys shoot the 1000 yards. They really did a magnificent job of wind reading as it would switch 6 minutes each way every 20-30 seconds. Without naming names one particular shooter would wind on 6 minutes left go to take the shot and then shake his head and wind back the other way 5-6 minutes then stop- shake his head again and repeat. It was great to watch but terrifying to shoot in! Basically starting in the flags at the butt and rolling down the range - never really sitting still and flicking back the other way.
Anyway the long and the short of it was the wind reading was great but groups opened up hugely (not with the 7mm though and not just left to right slope either). The discussion in the bar that night was the feeling that the 155's start to "surf" the wind at 1000 yards. An interesting analogy and trashed by some but interesting nonetheless. Talk was the 155 HBC did not do this to the same extent.
There is a fair bit of difference in the design of these bullets and I could "imagine" the centre of gravity being further back relative to bullet length on the HBC as you describe. Our long high BC bullets in 7mm and 30 cal are well known to be dynamically quite stable.
The hypothesis from this I guess is - a long, light nose might be harder to gyroscopically stabilize against normal aerodynamic lift moments (overturning forces) that are generally in front of the centre of gravity because there is more lift generated in front (or cg relatively further back). BUT conversely should become more dynamically stable as the centre of gravity moves further back towards the magnus force moment.
The 168 Sierra was renown for dynamic instability and as you say the 175 less so (although often blamed on the steep boat tail of the 168)
A couple of years ago at the NRAA queens I watched keenly as a few of the top standard shooters switched to Sierra 155 and were shooting unbelievable scores. Conditions were mild generally - then we hit the 900 and 1000 yard day (as always Brisbane knows when to turn it on) and a stiff switching headwind kicked in. With the 7mm I was going backwards and forwards 3 minutes each way but when I finished I watched a couple of these guys shoot the 1000 yards. They really did a magnificent job of wind reading as it would switch 6 minutes each way every 20-30 seconds. Without naming names one particular shooter would wind on 6 minutes left go to take the shot and then shake his head and wind back the other way 5-6 minutes then stop- shake his head again and repeat. It was great to watch but terrifying to shoot in! Basically starting in the flags at the butt and rolling down the range - never really sitting still and flicking back the other way.
Anyway the long and the short of it was the wind reading was great but groups opened up hugely (not with the 7mm though and not just left to right slope either). The discussion in the bar that night was the feeling that the 155's start to "surf" the wind at 1000 yards. An interesting analogy and trashed by some but interesting nonetheless. Talk was the 155 HBC did not do this to the same extent.
There is a fair bit of difference in the design of these bullets and I could "imagine" the centre of gravity being further back relative to bullet length on the HBC as you describe. Our long high BC bullets in 7mm and 30 cal are well known to be dynamically quite stable.
The hypothesis from this I guess is - a long, light nose might be harder to gyroscopically stabilize against normal aerodynamic lift moments (overturning forces) that are generally in front of the centre of gravity because there is more lift generated in front (or cg relatively further back). BUT conversely should become more dynamically stable as the centre of gravity moves further back towards the magnus force moment.
Last edited by DaveMc on Tue Feb 25, 2014 12:07 pm, edited 2 times in total.
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williada
- Posts: 969
- Joined: Wed Jul 22, 2009 12:37 am
Some years ago I was doing some experiments with a dozen barrels from a machine rest at 1000 yards. Barrels were six pairs of the same dimension i.e. land and groove and twist rate. The experiment was nominally to get an edge with .308 Winchester at 1000 yards with factory ammunition. This cartridge was marginal at the 1000 yards and one of the fears was the dreaded transonic range. The rifling twist rate was the biggest factor in negating its presence seeing the barrels were restricted to 30 inches by a management decision. Needless to say some barrels I cut back from 34 inches progressively for my own interest were better due to marginally increased velocity.
The pressure in barrels as we all know is immense and it only takes minor changes in mechanics to have a significant change in outcome. It’s like a skier or an ice skater getting an edge and arse up they go. It’s a fine line sometimes. So my interest extended those mechanical factors that influenced the bullets flight. At that time Sierra, having corresponded with them did not have the information or sophisticated ballistic programs that used 6 degrees of freedom to explore uncertainty. So the next best thing was to suck it and see. Litz has taken this knowledge further and published. Before, it was a trade secret. Those nuances matter because the centres are getting progressively smaller. There is so much information now I don’t know where people are going to get time to shoot because of what they have to digest. It’s becoming a hospital without patients. On the other hand we don’t have to know how a microwave works to use it. Just blame the coach. He or she has to lift their game now, not the shooter.
From an international perspective, the barrels of sophisticated international fullbore teams have been made since the time of Fulton in the UK, early last century to take into account barometric pressure and humidity at the competition venue, and the biggest factor is twist rate. However, if varying twist rates are used by team members and different calibres it makes it harder for coaches to work out the net effects of each shooter and transfer that knowledge to the next shooter. The bullet stability argument has been settled but the finer points now are the drift factors emanating from the spin. Otherwise it’s a game of sighters in tough conditions and a loss of points because of Murphy’s Law. In a way, even though F Class is so developed, that the human challenge of reading the conditions and making sight alterations is coming into play from what I can see, as has been the case for full bore shooters for years, where target size tested the margins of the equipment used and the art became managing a large group. “F Open Class” shooting sounds very challenging. Just might get back into shooting even though I my own mechanicals need a far bit of oil these days because brain speed has diminished sharply.
Post script for Baartman007. Over a decade ago, I was using .5 minute for 10 mph based on observations with .308 Win. It was close enough then. Ian and Dave have probably refined this now. Good work fellas.
The pressure in barrels as we all know is immense and it only takes minor changes in mechanics to have a significant change in outcome. It’s like a skier or an ice skater getting an edge and arse up they go. It’s a fine line sometimes. So my interest extended those mechanical factors that influenced the bullets flight. At that time Sierra, having corresponded with them did not have the information or sophisticated ballistic programs that used 6 degrees of freedom to explore uncertainty. So the next best thing was to suck it and see. Litz has taken this knowledge further and published. Before, it was a trade secret. Those nuances matter because the centres are getting progressively smaller. There is so much information now I don’t know where people are going to get time to shoot because of what they have to digest. It’s becoming a hospital without patients. On the other hand we don’t have to know how a microwave works to use it. Just blame the coach. He or she has to lift their game now, not the shooter.
From an international perspective, the barrels of sophisticated international fullbore teams have been made since the time of Fulton in the UK, early last century to take into account barometric pressure and humidity at the competition venue, and the biggest factor is twist rate. However, if varying twist rates are used by team members and different calibres it makes it harder for coaches to work out the net effects of each shooter and transfer that knowledge to the next shooter. The bullet stability argument has been settled but the finer points now are the drift factors emanating from the spin. Otherwise it’s a game of sighters in tough conditions and a loss of points because of Murphy’s Law. In a way, even though F Class is so developed, that the human challenge of reading the conditions and making sight alterations is coming into play from what I can see, as has been the case for full bore shooters for years, where target size tested the margins of the equipment used and the art became managing a large group. “F Open Class” shooting sounds very challenging. Just might get back into shooting even though I my own mechanicals need a far bit of oil these days because brain speed has diminished sharply.
Post script for Baartman007. Over a decade ago, I was using .5 minute for 10 mph based on observations with .308 Win. It was close enough then. Ian and Dave have probably refined this now. Good work fellas.
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