Bullet BC and wind drift, wrapup.

M

model14

Guest
Here is the way I now view bullet behavior in a cross wind:

If a spin stabilized bullet encounters a cross wind, the difference in drag force on the upwind and downwind side of the bullet, will result in the bullet turning into the apparent wind to the point where the drag forces become equal.

The effect the wind has on bullet drift is strictly a function of BC (sectional density and form/shape factor) and lag (the time of flight difference caused by bullet slowdown).

A "side profile" BC need not be considered because the apparent wind is acting along the longitudinal axis of the bullet.

The weight of the bullet is already taken into account by the BC and the muzzle velocity is part of measuring lag time.

Therefore, the highest BC bullet will always have the least drift in a cross wind.

The highest BC bullet may not be the most accurate bullet to shoot.

There, I think I have it in a nutshell.

Thanks all for your patience.

Richard
 
Richard

Four comments.

1. It's not wind drift. It's wind deflection. Drift is the bullet drifting right or left due to the rotation. i.e. right hand vs left hand twist.

2. The bullet does not turn into the wind. It turns to follow the wind.

3. Bullet weight has very little to do with BC. It's all in the shape.

4. High BC bullets can be very accurate. Just like low BC bullets can be very inaccurate. Apparant accuracy is probably more a function of the barrel, load, and shooter.

JMHO

Ray
 
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The main force on the bullet is its momentum, in the direction that it was initially fired. Its change in direction from this initial (three dimensional) heading, is always modified by gravity, and may also be modified by the wind. For purposes of determining what the change in heading ,from the influence of wind, is, we may use vector analysis, and although this is a useful tool, it leads to some misconceptions, for one that the vector that represents the sum of the forces acting on the bullet somehow magically becomes something real in nature, a new force. It does not.
If we go back to Mann, and read a little farther, we discover that, according to him, the best evidence available at the time, indicated that the maximum that a spin stabilized bullet can turn, pivoting on its center of mass, is six degrees from its heading. Beyond that, the bullet would become unstable, and tumble. From that we may deduce that the real wind does "see" the side of the bullet since it is not restricted as to where it may originate. Additionally, although the misalignment of a bullets axis from its heading may influence what happens, to a small degree, it is not what initiates or sustains wind caused movement of the bullet, coming closer to being an additional, related outcome. Witness the round ball. As to which way the nose of a bullet makes a less than half degree twitch, I am have not seen a picture that proves it one way or the other, and given what we do know about the magnitude of other gyrations that the bullet normally makes on its way to target, perhaps it does not matter.
 
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model14,

Don't worry, be happy. You get it.

Boyd just defies physics............immovable mass.

Greg
 
Experiment for Boyd

Boyd,

If you want to see it with your own eyes, gather up one of your wind flags and a big Estees model rocket. Load that rocket with the lowest powered, longest duration motor you can get. Wait for a windy day and get back from it as far as you can at 90 degrees from the direction of the wind. Have someone set it off for you when you're in good position to observe. Do not attach a round ball to this rocket.

If you can't see that rocket fly upwind while accelerating and then drift downwind while pointing upwind while decelerating after burnout and before apogee then I will buy a set of wind flags of my very own as you have suggested to improve my shooting per your council.

Get back to us on this, please.

Waiting in anticipation and budgeting for the future I remain,

Greg
 
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Hey Greg,
Feel free to dissect my post, but take it point by point and be specific. Saying that someone that you disagree with is a flat earther does neither. I grow tired of reading about exterior ballistics with "is likes" that don't fit, not to say that you have done that. BTW do you know why Mann said that bullets are not blown or deflected? Hint, it has nothing to do with physics or mathematics. BTW, as you undoubtedly know, none of this is required to calculate wind drift, and very little of what has been expounded is from hard data, just arguments about different imaginings of what is happening in a situation that is difficult to observe directly. In my earlier posts I gave examples of published material written by specialists that was at variance with what others had posted. Make your points.
 
Boyd,

You have stated that you didn't believe a rocket would accelerate upwind. You want something that you can observe easily. This is something that you can observe easily. So observe.

Greg
 
Greg,
I will have to look over my previous posts to find the one where I said that a rocket would not fly upwind. If I said exactly that than I will stand corrected.

What I think that I said was that for those who work backwards from wind drift being proportional to lag time, to if there were no lag time, there would be no wind drift, I was merely indicating that even with a "negative lag time" such as while a rocket's motor is causing it to accelerate from zero speed, or an airplane that is held at a constant speed (hence without lag time) that lacking control input, wind still influences trajectory.

As to which way the falling rocket turns, I would suspect that as a matter of economics and safety that a parachute would be involved, and that if the rocket was able to pivot into the wind that the point of the chute's attachment might not be at either end of the rocket but somewhere near the middle, possibly near the center of mass. If that is the case then the area of the stabilizing fins would have the center of pressure back of the center of mass (the opposite from a bullet) in which case, I would expect it to "weathervane" to an attitude with the nose into the wind. I lack your experience in these matters, so please correct any errors in my assumptions. Also, I really appreciate the use of a concrete verifiable example.

As to wind flags, I would have thought that an intelligent fellow such as yourself would already have made some provision to be able to "look at the wind" while shooting, whether it be sticks and ribbons or the fancier sort. I will make one exception, if your competition shooting is done within sports where only a range flag or two are allowed, I can see where you might want to practice under match conditions. Even so, for evaluating ammunition and/or equipment I would think that additional information would be useful.
 
Boyd,

I spent a lot of time crafting an elegant and informed (you'll have to trust me on this) line by line response to your post to have it timed out and unresponsive to reloading. So, just getcha a hobby rocket and a good wind and see for yourself. I shoot long range and do look at range flags (doesn't mean I can read them), just not my own. The only ones I know that makes much use of personal flags at long range are Hall, Davis, Wagner, Isenhour....maybe I'm making your point. But try the rocket thing. You'll be glad you did.

Greg
 
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Boyd Allen
If you have the time try a search for Dopppler Radar of a bullets flight as it shows exactly what is happening.If you don't find what your looking for let me know.
Waterboy aka Lynn
 
Greg,
I would have genuinely liked to read your response. Next time compose in a word processing program, and then cut and paste into the composition box for a new post on this thread.

As to the rocket, ballistics aside, sounds like a lot of fun. Reminds me of a college science fair at what was then called Central State College in Edmond,OK. It was around 1958 or 9 and the Russians had put up Sputnik late in 57. I was in the sixth grade on the lab school on campus. It was amazing. There were all sorts of elaborately crafted hand made rockets, some close to a foot in diameter, some multistage, little animals had bee launched and recovered. It was magic for technically interested boy.

Lynn,
Thanks for the tip. If I need help, I WILL ask.

Boyd
 
Cheechako said:
Richard

Four comments.

1. It's not wind drift. It's wind deflection. Drift is the bullet drifting right or left due to the rotation. i.e. right hand vs left hand twist.

2. The bullet does not turn into the wind. It turns to follow the wind.

3. Bullet weight has very little to do with BC. It's all in the shape.

4. High BC bullets can be very accurate. Just like low BC bullets can be very inaccurate. Apparant accuracy is probably more a function of the barrel, load, and shooter.

JMHO

Ray
Click to expand...

Ray,

1. Technically you are correct. However, all of the people I shoot with use the term drift when discussing wind deflection. The drift due to gyroscopic precession is not a big topic in Grass Lake, Michigan ;)

2. Being a sailor and pilot, when I turn a boat or airplane toward the wind (coming into my face), I am turning into the wind. If I want to follow the wind, I turn downwind and get a nice boost in ground speed. A bullet turns into the wind also. If you want to say it follows the wind that is okay, but few will understand what you are saying.

3. Bullet weight has everything to do with BC! Sectional density is bullet weight divided by cross sectional area. From a least affected by the wind standpoint, A long thin heavy bullet with a sharp nose ( large numbered form factor) will beat out a light bullet with a sharp nose everyday.

4. True.

Richard
 
Boyd Allen said:
The main force on the bullet is its momentum, in the direction that it was initially fired. Its change in direction from this initial (three dimensional) heading, is always modified by gravity, and may also be modified by the wind. For purposes of determining what the change in heading ,from the influence of wind, is, we may use vector analysis, and although this is a useful tool, it leads to some misconceptions, for one that the vector that represents the sum of the forces acting on the bullet somehow magically becomes something real in nature, a new force. It does not.
If we go back to Mann, and read a little farther, we discover that, according to him, the best evidence available at the time, indicated that the maximum that a spin stabilized bullet can turn, pivoting on its center of mass, is six degrees from its heading. Beyond that, the bullet would become unstable, and tumble. From that we may deduce that the real wind does "see" the side of the bullet since it is not restricted as to where it may originate. Additionally, although the misalignment of a bullets axis from its heading may influence what happens, to a small degree, it is not what initiates or sustains wind caused movement of the bullet, coming closer to being an additional, related outcome. Witness the round ball. As to which way the nose of a bullet makes a less than half degree twitch, I am have not seen a picture that proves it one way or the other, and given what we do know about the magnitude of other gyrations that the bullet normally makes on its way to target, perhaps it does not matter.
Click to expand...


Boyd,
Momentum is not a force, it is a measure of inertia (First Law). There need not be acceleration associated with momentum. Newton would not like your statement. ;)

Vector analysis of the bullets and wind velocity yields a resultant vector which gives us the mathematical ability to determine the forces acting on a moving body in a cross wind. Real or not, it yields the correct results. The number "2" is not real, but we call it a real number, as opposed to being an irrational number.

Having a maximum of 6 degrees turn for a stabilized bullet may be true but may not be germane to our discussion. A 2000 fps bullet is traveling 1370 mph. By my quick calculation (I don't have a trig function on my calculator), the cross wind would have to be 100 mph or better (a lot better) to cause the bullet to turn 6 degrees.

I agree none of this has anything to do with hitting the target (although a high BC in a cross wind sure helps), some of us are of a mind mind that we just have to know the why's and wherefore's of things that happen around us.:)

Fair winds and followings seas.

Richard
 
model14 said:
Ray,

1. Technically you are correct. However, all of the people I shoot with use the term drift when discussing wind deflection. The drift due to gyroscopic precession is not a big topic in Grass Lake, Michigan ;)

2. Being a sailor and pilot, when I turn a boat or airplane toward the wind (coming into my face), I am turning into the wind. If I want to follow the wind, I turn downwind and get a nice boost in ground speed. A bullet turns into the wind also. If you want to say it follows the wind that is okay, but few will understand what you are saying.

3. Bullet weight has everything to do with BC! Sectional density is bullet weight divided by cross sectional area. From a least affected by the wind standpoint, A long thin heavy bullet with a sharp nose ( large numbered form factor) will beat out a light bullet with a sharp nose everyday.

4. True.

Richard
Click to expand...

Richard

1. "Technically" correct is good enough for me. If you call a duck a goat, it doesn't make it a goat. If you shoot 1000 yards in Grass Lake, drift should be a topic to discuss since it has a profound effect at the target. 1 MOA or more to be exact.

2. A bullet's nose will turn to follow the wind. No clearer way to say it. If, as you say, it turns into the wind, what causes it to be deflected? Surely your not going to say that it is pushed sideways.

3. BC is shape. If those two bullets that you described have the same BC, travelling at the same velocity, they will be deflected equally regardless of their caliber, weight, or sectional density.

Ray
 
Ray,
How does a round ball turn to follow the wind? Surely it has to be pushed sideways. If that is the case, why not an elongated bullet? I guess that it all gets down to what you mean by pushed. Which ever way the bullet turns in response to a crosswind, it can't be to the actual heading of a 90 degree cross wind, and so, the side of the bullet has to be "seeing" the actual wind. It is the addition of this relatively small force to the bullets momentum (Notice that I didn't call it a force.) that causes the bullet to deviate from its original heading. I guess that the point of this whole thread is how the force of the wind is applied to the bullet.
 
Boyd

If a bullet is only pushed to the side and the nose does not turn to follow the wind then it would resume it's original direction once it is past the wind. If you are shooting 1000 yards and there is a 1 MOA crosswind from the muzzle out to 100 yards, and then calm from 200 yards to the target, are you saying that the wind deflection at the target will only be 1"? I wish it were so. That's why the wind closest to the muzzle is the most important. Once the bullet turns to follow the wind that's the direction it will continue on until changed by other factors.

Ray
 
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Ray,
No, I am not. Once the wind has imparted a sideways motion, the bullet will have to go through its own slowing down in that direction, from atmospheric friction, in a similar fashion to its loss of forward velocity. I should also add that this is what I think. Knowing has a higher standard of proof.
 
Cheechako said:
Richard

1. "Technically" correct is good enough for me. If you call a duck a goat, it doesn't make it a goat. If you shoot 1000 yards in Grass Lake, drift should be a topic to discuss since it has a profound effect at the target. 1 MOA or more to be exact.

2. A bullet's nose will turn to follow the wind. No clearer way to say it. If, as you say, it turns into the wind, what causes it to be deflected? Surely your not going to say that it is pushed sideways.

3. BC is shape. If those two bullets that you described have the same BC, travelling at the same velocity, they will be deflected equally regardless of their caliber, weight, or sectional density.

Ray
Click to expand...

Ray,

Please explain "follow the wind". Are you saying if the wind is from the right the bullet turns left?

When the bullet turns into the wind (facing the direction from which the relative wind comes), it drifts because the whole mass of air in which it is moving has a component of velocity from the cross wind. From the bullet's standpoint, once it has turned to eliminate the difference in drag on each side, it no longer is subject to a cross wind on the bullets surface. It is just moving in a media which is moving in the direction of the cross wind.

BC is shape and weight! Your analogy with the two bullets is correct because the weight has already been factored into the BC.

Richard
 
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