For you ballisticians.

[model14]

I keep reading that a bullet with the higher BC will experience less wind drift regardless of bullet weight, assuming the same downrange velocity profile. Does that take into account the inertia of the higher weight bullet? For a given cross wind, the heavier bullet will not be accelerated across the flight path by the force of the cross wind as much as the lighter bullet (remember Newton?), and will therefor have less wind drift. Also, BC is measured with no wind component and all drag is strictly along the straight line flight path, so any effects of cross wind are not taken into account. Maybe a cross wind BC would be appropriate, but that would be a B**** to measure . Any enlightenment would be appreciated.
M14

 

[Gene Beggs]

Sorry to be the one to tell you,,

I keep reading that a bullet with the higher BC will experience less wind drift regardless of bullet weight, assuming the same downrange velocity profile. Does that take into account the inertia of the higher weight bullet? For a given cross wind, the heavier bullet will not be accelerated across the flight path by the force of the cross wind as much as the lighter bullet (remember Newton?), and will therefor have less wind drift. Also, BC is measured with no wind component and all drag is strictly along the straight line flight path, so any effects of cross wind are not taken into account. Maybe a cross wind BC would be appropriate, but that would be a B**** to measure . Any enlightenment would be appreciated.
M14

but you have some serious misconceptions about the effects of wind on bullets in flight.

There was a lengthy discussion about this very subject on the 1000 yard forum some time ago that will help you understand. I hope Alinwa will step up to the plate and take this one on; I just do not have the energy.

Gene Beggs

 

[Tony Shankle]

The question is flawed. Since mass is part of the calcaulation for determining BC it is already accounted for and is part of the equation.

Since drag (not the wind pushing the bullet) is also the issue with cross wind, the same theory (and formula) is applied. A 105 non VLD with a BC of .500 at 3150 fps has 3.25 MOA drift at 1000 yds with a 5 mph crosswind compared to a 105 VLD with a BC of .540 at the same speed and distance with 3.0 MOA. Remember that a bullet turns into that cross wind so drag is major compentent in that case too.

Ultimately I only believe what I see in my own testing. People like to talk BC in theory but it is a highly flawed subject. Speed, air density, which drag model, doppler, twist, land engraving, published vs non published and twenty other variables jump into my mind. For me, published BC is a starting point, something to put in my Ballistic Program to START TESTING in the field. I have driven myself crazy trying to match actual BC data to computer data while making data cards only to go out the next day and have it all change again (slightly but still a change). Also, BC will vary greatly within the same box of bullets...not something you will hear the manufacturer tell you when you buy them!

It is one of those areas of exact science with so many variables that arriving at a constant in testing is impossible...yes, even with a doppler you will get different results on different days. For me, it also one of those subjects that the more I study the less I know.

 

[PEI Rob]

Here ya go, enjoy http://benchrest.com/forums/showthread.php?t=43279

 

[Louis Boyd]

All that's needed to determine crosswind deflection X is:
Distance to the target. D
The crosswind velocity. W
The muzzle velocity. M
The actual time of flight to the target T

The equation is X= W * (T - (D/M))

Units only need to be consistent.

This is called Didion's equation, first published in 1857. It works for all fin or spin stabilized projectiles even if they're powered (like unguided rockets).

Projectile weight, BC (or drag functions) and atmospheric density are used in ballistics programs to calculate the time of flight instead of measuring it, but measuring time of flight directly is practical.

Using Didion's equation you can accurately determine what the wind deflection of a particular projectile for an arbitrary crosswind speed would be by shooting across three chronograph screens with no other information about the the projectile. You don't need to measure the wind speed or direction unless it's extreme, then small corrections are needed. Screens 1 and 2 are placed near the muzzle to measure M. T is measured from screen 1 to 3 which is set at distance D downrange.

See "Modern Exterior Ballistics" by Robert L. McCoy, chapter 7 for a full explanation.
This simple equation is used in just about all ballistics programs which calculate crosswind deflection. Most available programs are based on McCoy's work.

 

[model14]

Here ya go, enjoy http://benchrest.com/forums/showthread.php?t=43279

I read as much of the 11 pages as I could (about 4), before I realized I was getting even more confused
I disagree with quite a bit of what was being said, especially the constant denial of a side drift force component on the bullet which follows the laws of physics. I will do my best to explain my position on this.

I am a pilot and I fly my own Cessna 182. When the airplane wheels leave the ground in a cross-wind, and I leave my feet off of the rudder pedals, two things happen:

1. Because the center of pressure of the fuselage and vertical stabilizer and rudder. is behind the center of gravity (big rudder and long fuselage section), the aircraft turns some into the wind. It can't turn more than a few degrees into the wind because the forward motion of the airplane down the runway creates a strong wind relative to the rudder and fuselage which is trying to keep the airplane going straight down the runway. This wind is much stronger than most any cross wind (unless you have a hurricane force cross wind), so the aircraft only crabs a small amount. This turning into the wind on its' own (no pilot action) is not near enough of a crab angle to prevent the aircraft from drifting in the direction of the cross wind. This leads to the second item.

2. Unless the pilot takes action to either lower a wing into the cross wind or put in a lot of rudder into the wind (actually you should do both), the aircraft drifts sideways with the wind. This drift occurs because there is a component of force due to the air molecules striking the side of the airplane. The amount of force is proportional to the velocity squared, the air density and the amount of area exposed to the cross wind molecules.

Because of this, and because the airplane has mass, the airplane is accelerated in drift and follows Newton's second law (f=ma). Because the aircraft is accelerating in drift, its' drift velocity will increase until the force causing the acceleration equals the side drift drag force. The drag force will be determined by the same equation as the drift force, except that a drag coefficient is thrown in as a multiplier. The drag coefficient is very much determined by the shape of the aircraft where the molecules strike the fuselage and rudder sides and attempt to continue on across the top and bottom structures. Needless to say, we are talking a very big drag coefficient here, compared to the drag coefficient which applies to a head on aircraft view.

From this I can only conclude that a bullet in flight experiences the same physical forces and follows the same laws of classical physics as my airplane does. Yes, a bullet does turn a little into the wind (and I mean a little), but it also drifts sideways to the intended flight path (a lot) because of the cross wind force on the side profile of the bullet.

We have no way of knowing what the drag coefficient for this cross wind is for any given bullet, and I propose that it is relatively unrelated to the advertised and/or measured BC of a given bullet. The BC talked about for a given bullet is measured along a straight line to the target and will only change slightly in a cross wind due to the small turn into the wind. The amount a bullet will turn into the wind is much more complex a problem than an airplane because of the spin of the bullet causing gyroscopic effects which counter a turn into the wind, etc.

I believe there does exist for every bullet a BC for drift that is independent of the bullet BC for no wind flight. I don't know what it is and I don't know how to measure it, but I am convinced it exists, and I am also convinced a heavier bullet will drift less in a cross wind than a lighter bullet (given both bullets have the same side area and same drag coefficients for the side area, and given the same time of flight (Newton's 2nd law again). It is easy to make two bullets that meet this criteria by just using different density materials. From this a “drift BC” could be applied to the bullets just like a regular BC is applied to forward motion, and it would give us a true measure of how well the bullet bucks the wind.

For now, it is agreed by most everyone that a higher BC bullet has less wind drift than a lower BC bullet, given the same time of flight. What is not even close to being agreed upon is exactly why that is true. But don’t tell me the cross wind doesn’t exert a force on the side of the bullet, or that the bullet doesn’t drift, but turns into the wind!

 

[Gene Beggs]

Patience my friend,,

I read as much of the 11 pages as I could (about 4), before I realized I was getting even more confused
I disagree with quite a bit of what was being said, especially the constant denial of a side drift force component on the bullet which follows the laws of physics. I will do my best to explain my position on this.

I am a pilot and I fly my own Cessna 182. When the airplane wheels leave the ground in a cross-wind, and I leave my feet off of the rudder pedals, two things happen:

1. Because the center of pressure of the fuselage and vertical stabilizer and rudder. is behind the center of gravity (big rudder and long fuselage section), the aircraft turns some into the wind. It can't turn more than a few degrees into the wind because the forward motion of the airplane down the runway creates a strong wind relative to the rudder and fuselage which is trying to keep the airplane going straight down the runway. This wind is much stronger than most any cross wind (unless you have a hurricane force cross wind), so the aircraft only crabs a small amount. This turning into the wind on its' own (no pilot action) is not near enough of a crab angle to prevent the aircraft from drifting in the direction of the cross wind. This leads to the second item.

2. Unless the pilot takes action to either lower a wing into the cross wind or put in a lot of rudder into the wind (actually you should do both), the aircraft drifts sideways with the wind. This drift occurs because there is a component of force due to the air molecules striking the side of the airplane. The amount of force is proportional to the velocity squared, the air density and the amount of area exposed to the cross wind molecules.

Because of this, and because the airplane has mass, the airplane is accelerated in drift and follows Newton's second law (f=ma). Because the aircraft is accelerating in drift, its' drift velocity will increase until the force causing the acceleration equals the side drift drag force. The drag force will be determined by the same equation as the drift force, except that a drag coefficient is thrown in as a multiplier. The drag coefficient is very much determined by the shape of the aircraft where the molecules strike the fuselage and rudder sides and attempt to continue on across the top and bottom structures. Needless to say, we are talking a very big drag coefficient here, compared to the drag coefficient which applies to a head on aircraft view.

From this I can only conclude that a bullet in flight experiences the same physical forces and follows the same laws of classical physics as my airplane does. Yes, a bullet does turn a little into the wind (and I mean a little), but it also drifts sideways to the intended flight path (a lot) because of the cross wind force on the side profile of the bullet.

We have no way of knowing what the drag coefficient for this cross wind is for any given bullet, and I propose that it is relatively unrelated to the advertised and/or measured BC of a given bullet. The BC talked about for a given bullet is measured along a straight line to the target and will only change slightly in a cross wind due to the small turn into the wind. The amount a bullet will turn into the wind is much more complex a problem than an airplane because of the spin of the bullet causing gyroscopic effects which counter a turn into the wind, etc.

I believe there does exist for every bullet a BC for drift that is independent of the bullet BC for no wind flight. I don't know what it is and I don't know how to measure it, but I am convinced it exists, and I am also convinced a heavier bullet will drift less in a cross wind than a lighter bullet (given both bullets have the same side area and same drag coefficients for the side area, and given the same time of flight (Newton's 2nd law again). It is easy to make two bullets that meet this criteria by just using different density materials. From this a “drift BC” could be applied to the bullets just like a regular BC is applied to forward motion, and it would give us a true measure of how well the bullet bucks the wind.

For now, it is agreed by most everyone that a higher BC bullet has less wind drift than a lower BC bullet, given the same time of flight. What is not even close to being agreed upon is exactly why that is true. But don’t tell me the cross wind doesn’t exert a force on the side of the bullet, or that the bullet doesn’t drift, but turns into the wind!

,,the light will come on eventually and you will truly understand how wind affects bullets in flight. It's not as simple as it looks.

So you are a pilot? Good, that will help. I've flown a bit myself.

If you will carefully study, with an open mind, the thread that PEI Rob so graciously pointed out, you will come to have a completely different understanding of how wind affects bullets and aircraft in flight.

Please,,, read it all; it gets better, trust me. I'll give you a hint; the wind does not blow on the side of your 182 in flight once the wheels leave the ground and the wind does not blow on the side of a bullet in flight.

Trust me, you are in for an exciting study!

Oh, and by the way; why not use your real name so we know who we are talking to?

Gene Beggs

 

[mikecr]

Time of Flight is the key.
Lower BC bullets slow more(higher lag time) than higher BC bullets. More TIME in the wind..

Seems to me assumed(and I'm sure tested to death), that the area presented by a bullet to wind is plenty sufficient to be fully affected by it. And mass accounted for plays a role in TOF.

You can have two bullets with same BC, but of different mass, -and drifting differently.
They just present different form factors(different drag w/resp to a standard).
If I were choosing the bullet between them with least wind drift, I would favor less drag over more mass.

Someday, a genius will come up with a way to effect drift in a free falling object independent of time. I think bullets truly will be 'flying' from that point.

 

[tobybradshaw]

I read as much of the 11 pages as I could (about 4), before I realized I was getting even more confused
...

Gene is right. Keep reading the to the end of the other thread, and you will better understand the effect of wind on a bullet's flight.

Since you're a pilot, here's a somewhat analogous situation that also is a bit counterintuitive:

You make a round trip in your airplane, once in calm air and on a different day you make the same round trip with a 10kt headwind on the outbound leg, and a 10kt tailwind on the return leg. Which (if either) trip takes less time, and why?

Toby Bradshaw
baywingdb@comcast.net

 

[Louis Boyd]

But don’t tell me the cross wind doesn’t exert a force on the side of the bullet, or that the bullet doesn’t drift, but turns into the wind!

Obviously there are forces perpendicular to the bore axis or a bullet could not fly in other than a straight line, but they do not come form the wind blowing on one side of the bullet.
They do come from the bullet aligning itself to fly into the oncoming air. What your missing is that does not require a separate BC to calculate. It's exactly the same forces which cause drag and slow the bullet's forward velocity. The energy to turn and deflect the bullet is a negligibly small part of the energy being shed by the bullet pushing the air molecules out of the way in it's forward flight. The wind deflection is locked to the same oncoming air drag which sheds energy. That making its calculation or direct measurement relatively simple. Didion's equation works even if it isn't intuitive at first glance. There are more complex methods using fluid dynamics and a six degree of freedom model which calculate precession and yaw and how that affects airflow around the bullet but they don't give better practical results.

You came here with questions and you're getting good answers. At least think about them before rejecting them.

And it is not true that higher BC bullets always have lower wind defection. Whether they do or not depends also depends on the muzzle velocity of each bullet and to a lesser extent how well the bullet shapes fit the BC models.

 

[Tony Shankle]

Gee, I'm glad I didn't already say that in post #3!

While Didion's Approximation is a staple and I am sure works great for you short range guys, it is not the end all formula. Setting up skyscreens (if someone actually had an accurate set) at distance is not practical at distance. Also, Didion relied on a constant drag coefficient which may work at 100 yards but changes a few times on it way to 1000. He also ignored real life conditions such as elevation angle and as you mentioned, precession and yaw.

I am certainly not arguring that his work is a good place to start...I just don't think it should be the only tool in your toolbox.

 

[Donald]

This kind of thread makes my brain hurt. Or at least ache. I just don't care. All I want to know is how much do I hold off when the tails are standing straight out horizontal. Crap, I have enough junk to take to the range now. I don't need a laptop and all the other stuff that goes with it.

Donald

 

[Louis Boyd]

Gee, I'm glad I didn't already say that in post #3!

While Didion's Approximation is a staple and I am sure works great for you short range guys, it is not the end all formula. Setting up skyscreens (if someone actually had an accurate set) at distance is not practical at distance. Also, Didion relied on a constant drag coefficient which may work at 100 yards but changes a few times on it way to 1000. He also ignored real life conditions such as elevation angle and as you mentioned, precession and yaw.

I am certainly not arguring that his work is a good place to start...I just don't think it should be the only tool in your toolbox.

Certainly Didion's equation has limitations. The equation does however allow using integral methods with any coefficient it uses being variable over the path. No ballistics programs I've seen bother to do that, though most provide output in incremental steps. It only address the deflection caused directly by drag not accounting for any of the effects related to the bullet having a spinning surface. It also doesn't account for bullet jump, the initial turning of the bullets spin axis which is not instantaneous. Of course it's main limitation is that the actual cross wind velocity over the trajectory is never know with high precision, but that's not the fault of th equation.

All computer models are approximations leaving out some physical phenomina.. If an additional component can be added to give better >usable< accuracy it gets added. But that may also require knowing additional input conditions which are not typically available. The most commonly recognized limitation is not knowing what the wind conditions are other than at the shooters location. Even there most meters don't have better than two digit resolution. It makes little sense to worry about subtle effects if you don't know what the wind vectors are along the trajectory.

Sure, if better tools are available I'm interested. I'd spend good money for a scintillation anemometer which can measure downrange crosswind in two axes. They exist but I've yet to see one suitable to hand carry in the field. Until then I don't think I'm limited by existing computer programs.

Measuring time of flight at a distance isn't as difficult as you seem to think. An infrared photo detector can "see" the impact and resolve the rise time of the flash to a few microseconds. The only downrange equipment needed is a hard target which will splatter the bullet set at a known distance. The time resolution for the distant target is not as critical as for measuring the muzzle velocity with a chrongraph. If you can't trust your sky screens to give accurate muzzle velocities getting the time of flight won't be of much use.

 

[model14]

To all,
I promise to read all 11 pages with an open mind. You guys are definitely getting my attention. It is obvious I don't understand bullet (or my airplane's) flight in a cross wind

Believe me, when I shoot my F Class .260 at our 600 yard range I don't ponder the whys of bullet drift, but I do enjoy trying to understand it when I am home. In fact most of the time I don't hold off for the wind and find I do better than when I am trying to guess what the wind is doing (we have a lot of high trees blocking the wind from both sides all of the way down range, so wind is not usually a big factor).

Thanks for the inputs and patience.

Richard Fast
Grass Lake, MIchigan

 

[model14]

Gene is right. Keep reading the to the end of the other thread, and you will better understand the effect of wind on a bullet's flight.

Since you're a pilot, here's a somewhat analogous situation that also is a bit counterintuitive:

You make a round trip in your airplane, once in calm air and on a different day you make the same round trip with a 10kt headwind on the outbound leg, and a 10kt tailwind on the return leg. Which (if either) trip takes less time, and why?

Toby Bradshaw
baywingdb@comcast.net

Toby,
I am also a flight instructor and your question is one often asked of novice pilots. The trip with the headwind and tailwind takes longer because you are exposed to the headwind for a longer time than you are exposed to the tailwind. However, I am not sure why this is analogous to our discussion on bullet drift. Once I figure what is really going on with wind drift, you can bet I am going to have some fun with my flight instructor buddies
Richard

 

[Gene Beggs]

Atta' boy Richard!

To all,
I promise to read all 11 pages with an open mind. You guys are definitely getting my attention. It is obvious I don't understand bullet (or my airplane's) flight in a cross wind

Believe me, when I shoot my F Class .260 at our 600 yard range I don't ponder the whys of bullet drift, but I do enjoy trying to understand it when I am home. In fact most of the time I don't hold off for the wind and find I do better than when I am trying to guess what the wind is doing (we have a lot of high trees blocking the wind from both sides all of the way down range, so wind is not usually a big factor).

Thanks for the inputs and patience.

Richard Fast
Grass Lake, Michigan

That's the right mind set!

With the exception of flying, rifle accuracy and exterior ballistics have been the most fascinating subjects I have encountered. As you will soon discover, flying and shooting have a lot in common.

Wind! What is it? Wind is simply movement of the airmass relative to the earth's surface. How it affects airborne objects is a fascinating study. You're in for some eye-opening discoveries!

Oh,, and by the way, once you fully understand how wind affects your bullet in flight, will this help you shoot better? NO! Sorry to say, but no it won't.

There are many world class shooters who still believe crosswinds 'blow on the side' of the bullet. And from a shooter's standpoint, that's okay. It really doesn't matter whether or not we thoroughly understand HOW wind drift works but only that it does and how to correct for it.

Later,

Gene Beggs

 

[tobybradshaw]

Toby,
The trip with the headwind and tailwind takes longer because you are exposed to the headwind for a longer time than you are exposed to the tailwind. However, I am not sure why this is analogous to our discussion on bullet drift.

The analogy isn't wonderful, but in both cases it is "lag time" that matters, where the standard of reference is no change in velocity. The bullet in a crosswind decelerates more rapidly (relative to the straight-line path to the target), similar to the way the airplane in a headwind spends proportionately more time "going slow" than it can ever regain in the tailwind "going fast."

On second thought, just skip to Greg Culpeper's post #89 in the "Wind effects [sic] what exactly?" thread.

Toby Bradshaw
baywingdb@comcast.net

 

[benenglish]

Time of Flight is the key.
Lower BC bullets slow more(higher lag time) than higher BC bullets. More TIME in the wind.

Darn it. Now I'm going to be forced to go read that thread y'all are referencing. Why? Because I always thought that time of flight was completely unimportant. I thought that wind drift was a function of rate of velocity loss. I thought that explained why the same bullet in the same conditions launched at 1700 fps will drift more in a crosswind than it will if you launch it at 850 fps.

I wonder how far I can get through that thread before the nightcap sets in?

 

[mikecr]

Well declaring what happens, and understanding how, are two completely different things.
I concede that I don't understand wind drift, and will read the wind thread when I get a week to do so..

 

[tobybradshaw]

Because I always thought that time of flight was completely unimportant. I thought that wind drift was a function of rate of velocity loss.

Have a look at the Didion approximation that Louis posted in this thread. Note that wind deflection does depend on time of flight, but wind deflection isn't directly proportional to time of flight.

Wind deflection is directly proportional to lag time -- the difference between actual time of flight and the theoretical time of flight in the absence of drag.

I thought that explained why the same bullet in the same conditions launched at 1700 fps will drift more in a crosswind than it will if you launch it at 850 fps.

I hope not, because this isn't the case. For the same projectile, wind deflection is inversely proportional to muzzle velocity (and it's probably simpler to use conditions that avoid transonic flight). Check it out:

http://saxtech.eu/Ballistik/Didion/Didion.htm

This jibes with experience -- using the same bullet, there is less wind deflection with hotter loads. (The exception I'm aware of is high velocity vs. subsonic .22RF, but I understand that this has to do with increased drag in the transonic range for the high velocity projectiles that have a supersonic muzzle velocity.)

Toby Bradshaw
baywingdb@comcast.net