Wind Drift

[Randy J.]

Cg

I believe center of gravity does have some effect on "drag over" and one reason the custom bullets we use are better than factory. If CG is not Center of mass then the bullet has a wobble (great technical term ) in relation to line of flight and will provide more area for the wind to affect as it is trying to nose into the wind. It's effect may be small relative to the net displacement but it does have some effect. Randy J.

 

[Cheechako]

It's kinda funny, don't you think? We're talking about this stuff in very technical terms (and some not so technical) but if you didn't notice that the tail on your #3 flag dropped just as you were about to jerk the trigger it's all for naught.

Ray

 

[steve stanley]

Ray, terrain is the culprit I think but who knows, but it does happen thats for sure. steve

 

[alinwa]

I believe center of gravity does have some effect on "drag over" and one reason the custom bullets we use are better than factory. If CG is not Center of mass then the bullet has a wobble (great technical term ) in relation to line of flight and will provide more area for the wind to affect as it is trying to nose into the wind. It's effect may be small relative to the net displacement but it does have some effect. Randy J.

Well actually, NO....

CG displacement from CF DOES produce dispersion but it DOES NOT "in effect present a larger frontal area" because it's moving forward 8-18times faster than it's rotating. Same goes for yaw, it doesn't actually drop the BC. Another way to put it...... custom bullets will GROUP better due to improved balance but they will not exhibit less wind drift.

al

 

[Randy J.]

Alinwa

Are you saying that a less stable bullet will have the same drift due to the wind as one with "perfect" stability? Randy J.

 

[alinwa]

Are you saying that a less stable bullet will have the same drift due to the wind as one with "perfect" stability? Randy J.

Yes.

In fact the less stable bullet will show less wind drift if anything.

al

 

[K Hope]

Al

CG displacement from CF DOES produce dispersion but it DOES NOT "in effect present a larger frontal area"

What about CG displacement of the Center of mass?

Ken

 

[alinwa]

What about CG displacement of the Center of mass?

Ken

If you drew a picture of it then YES, in the picture it would look "fatter," like a flower with petals.

But it isn't

It's moving forward too fast per revolution.

It's kinda' like looking at the pictorial depictions of yaw as it damps out.... or the video that Brian Litz recently posted. In the picture it looks wicked fat, but it ain't.

al

 

[K Hope]

Weight and Balance

I apologize for being so far from my original question but, appreciate the direction.

It's moving forward too fast per revolution.

Sorry for always coming back to this CG question but somehow I keep trying to connect an aircrafts aerodynamics to a bullets. Early in a pilots training he learns of CG and weight and balance (sometimes faster than wanted) and how a disproportional imbalance is Velocity relative and can quite quickly translate into destabilization.
Somehow I thought that was what was happening with the center of mass being moved more forward from a short range to a long range or vice versa. I understand the bullet is moving fast and turning but is it not also slowly falling and slowly destabilizing.

Ken

 

[alinwa]

Ok,

In the bullet the rotational stability is virtually unrelated to the forward velocity. Typically a stable bullet is rotating at 240,000-300,000rpms and over the time of flight of the bullet it doesn't really lose any stability. Spin anything up to 240,000rpms and fly it for only 1-3 seconds......... it ain't got time to lose many rev's.

In other words, it loses quite a lot of velocity but very little in the way of rotational stability, it gets MORE stable as it slows. Even if it were to slow to a complete stop it'd still have rotational stability.

al

 

[Pete Wass]

I've shot a lot of 30 cal TNT's

over the years and have always found them to shoot as well as "Hand Made" bullete once they were segregated by ogive length. For that reason I don't place a lot of reliance on this whole CG thing. Lead is Lead is Lead. I don't know about the jackets but have NEVER had any of them do anything wild when I have shot them. I once weighed a bunch of them and shot them by weight groups and only found a difference in the three or four that were 3 gr or more light but I wasn't measuring the ogive length back then so I don't know how much that mattered. I suspect the light ones would have a longer ogive.

I have often used them for foulers, keeping them segregated and find that the Hand Rolled go into the same hole on the sighter as the TNT's do. I shoot a lot of custom bullets but have won a number of matches using TNT's.

 

[alinwa]

over the years and have always found them to shoot as well as "Hand Made" bullete once they were segregated by ogive length. For that reason I don't place a lot of reliance on this whole CG thing. Lead is Lead is Lead. I don't know about the jackets but have NEVER had any of them do anything wild when I have shot them. I once weighed a bunch of them and shot them by weight groups and only found a difference in the three or four that were 1gr or more light.

I have often used them for foulers, keeping them segregated and find that the Hand Rolled go into the same hole on the sighter as the TNT's do. I shoot a lot of custom bullets but have won a number of matches using TNT's.

Pete this is fascinating....

I've got three wicked acc'rate 30HBR rigs and I've always looked for a cheap fun bullet..... and these thangs SHOOT?

ogive len'th eh???

Cool!

gonna' see what this does on a squeerrel!!

thanx

al

 

[Pete Wass]

If you jam hard

the length of the ogive doesn't matter I don't think. The hard jam takes care of it fer ya but I don't like tuning that way or to be faced with pulling a bullet if I have to open "er" up.

 

[K Hope]

More Questions

Like a friend once told me; all this information is giving me a sore head.


Is Spin Drift not by definition a form of instability caused by the location of the center of pressure in respect to the CG of long range bullets?


Is Pete’s bullet segregation not a form of quality control on the placement of the CG in order to maintain a consistent stable resistance of the mass in the barrel?

Ken

 

[alinwa]

Like a friend once told me; all this information is giving me a sore head.


Is Spin Drift not by definition a form of instability caused by the location of the center of pressure in respect to the CG of long range bullets?


Is Pete’s bullet segregation not a form of quality control on the placement of the CG in order to maintain a consistent stable resistance of the mass in the barrel?

Ken

Nope and NOPE!!



"Spin Drift" is something a LOT more tricky........ and it will give you a headache! Here's a morsel to chew on.....

If two identical bullets are launched at identical velocities but different ROTATIONAL velocities... In other words one from say a 14" twist and one from an 8" twist. The bullet with the higher rotation will show a little MORE "spin drift". "Spin drift" is a function of wind. It's the same weird play of forces which causes leftward-drifting bullets to impact HIGH and rightward-drifting bullets LOW at the same yardage. Look at any wind chart to see this. With a highly accurate (Bench Rest accurate) rifle if you shoot a series of groups in a switchy wind and if you just hold center, the group formed will be tilted, not flat. The bullets "blown left" will impact high and visey varsey.... Here's a link to a thread which discusses this effect.

http://www.benchrest.com/forums/showthread.php?t=54001&highlight=wind+charts


NOW...... once you've digested this set of data then you can apply it to "spin drift".....

In perfectly calm air a bullet fired from a right-hand twist will drift to the RIGHT. This is because it IS being subjected to a "WIND" even though it's not windy. It's falling, accelerating downward at a rate of 32ft/sec/sec. And THIS is the reason that the effect is exponentially GREATER as range increases.......as the bullet falls further the "wind" against it's underside increases in force.... hence the increased rightward drift.

(((( Note here that although I've used the accepted "drift" this IS NOT really a drift function, it's a DRAG function. A STABLE bullet tries it's best to center on the wind vector just like a top on a table, it "stands up" on the wind vector that it sees. But it also precesses continually which results in the nose constantly fighting to escape at 90degrees AHEAD of the direction of rotation. This results in a NOSE DOWN attitude for a bullet flying in a R-to-L wind, a NOSE UP attitude for a L-to-R wind and a NOSE LEFT attitude for a bullet dropping through still air..... And you won't get this explanation from any ballistics text of which I'm aware ))))

So take it for what it's worth....

This is GOLD man.....



gottaheadache yet???


Ohh yeahhhh, and NOPE to Pete's deep seating thang.... the reason Pete seats deep (repeat .... the reason Pete seats deep...5 time FAST!) is to standardize the initial pressure curve. Super Accurate BR Rifles are "tuned" for accuracy by playing with the seating depth at the lands. By jamming HARD you eliminate or temper this variable.

al

 

[HuskerP7M8]

A little experiment I conducted one day in the middle of an open field at two targets in opposite directions with a combined wind of 13mph.

Gyroscopic Drift

 

[K Hope]

Al

Sorry to disagree and these are the reasons why:

Spin drift: Aerodynamic forces are similar to that around a wing ( turbulence ). One slow side one fast side. The forces on the slow side ( less drag ) are replaced by the bullet. As you stated this lift is proportional to the spin rate. From here the bullet takes on a oscillating or coning path. These forces result in a statically stable ( mass ) but a dynamically ( aerodynamic ) unstable bullet.


Seating Hard: I believe we are close to the same thinking on this one. Hard jam bullets with varying ogive placements result in a CG ( mass ) placement inconsistencies; that increase the risk of mis-eliminating or tempering the variable.


Please note: This reasoning is no doubt flawed and is based solely on my interpretations of others; however, your clarifications are most welcome.


Ken

 

[Pete Wass]

I knew there was a good reason

Sorry to disagree and these are the reasons why:

Spin drift: Aerodynamic forces are similar to that around a wing ( turbulence ). One slow side one fast side. The forces on the slow side ( less drag ) are replaced by the bullet. As you stated this lift is proportional to the spin rate. From here the bullet takes on a oscillating or coning path. These forces result in a statically stable ( mass ) but a dynamically ( aerodynamic ) unstable bullet.


Seating Hard: I believe we are close to the same thinking on this one. Hard jam bullets with varying ogive placements result in a CG ( mass ) placement inconsistencies; that increase the risk of mis-eliminating or tempering the variable.


Please note: This reasoning is no doubt flawed and is based solely on my interpretations of others; however, your clarifications are most welcome.


Ken

I quit the hard seating to go to exact OAL's meaured close to where the bullet touches the lands. I guess my sorting has given me something I didn't suspect but from what you have said, a likely benefit. I am assuming
Like bullets are likely to perform alike, aye?

 

[alinwa]

Sorry to disagree and these are the reasons why:

Spin drift: Aerodynamic forces are similar to that around a wing ( turbulence ). One slow side one fast side. The forces on the slow side ( less drag ) are replaced by the bullet. As you stated this lift is proportional to the spin rate. From here the bullet takes on a oscillating or coning path. These forces result in a statically stable ( mass ) but a dynamically ( aerodynamic ) unstable bullet.


Seating Hard: I believe we are close to the same thinking on this one. Hard jam bullets with varying ogive placements result in a CG ( mass ) placement inconsistencies; that increase the risk of mis-eliminating or tempering the variable.


Please note: This reasoning is no doubt flawed and is based solely on my interpretations of others; however, your clarifications are most welcome.


Ken

Yes, we disagree.... profoundly.




al

 

[Wilbur]

Bottom line

I write this as if asked for a "bottom line" when I know full well a bottom line is likely unwelcome to this lively discussion of wind and how it blows our bullets. Speaking from 100 to 300 yard experience and in a competitive sense only, If you find yourself dependent on knowledge of bullet physics in a perceived wind your goose is cooked. A lifetime of study would result in a best guess decision. That would be fine if everyone else was guessing but they are not.

At the shorter ranges where we can see a bullet hole on the target, a sighter shot will calculate the wind drift as quickly and with greater accuracy than the most powerful computer. Watch the clock and use the sighter shot such that you are never, ever guessing.