Attention all ballisticians

G

Gene Beggs

Active member
That should bring 'em outta' the woodwork; huh? ;)

The recent thread, "For You Ballisticians" initiated by Richard Fast (Model14) was most interesting and informative. :) The number of views suggests there is great interest in the subject of crosswinds and their effect on bullets.

Thanks to those who joined the conversation and unselfishly shared their knowledge, I now have a better understanding of crosswinds. I am especially grateful to Lisa Spendlove. Thank you Lisa! I hope you will stay with us because you have a wealth of knowledge to share and a wonderful way of explaining it! :) I am also impressed with the company you keep! ;)

In the previously mentioned thread, we pretty well beat to death the subject of crosswinds, but unless I missed it :rolleyes: no one said anything about headwinds and tailwinds. We are fortunate to have with us some very talented and knowledgeable individuals. Let's continue with our discussion; shall we? :) For starters,,

Edited later. I thought it best to remove my reference to 100 yards and leave the yardage and scenarios to the experts.


Gene Beggs
 
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Welll, I ain't a ballistician but when has THAT ever stopped me! :D

Hi Gene,


Depends on terrain. Some ranges it'll go high, some low.

Since this is only 15fps I'd say that generally the true flat-ground difference is lost in the noise, lower than variation due to Extreme Spread.

TECHnically I guess the bullet SHOULD hit low by factor equal to a 15fps velocity loss...... but it doesn't. :)

maybe this'll help get the ball rollin'

al
 
100 yards...why play this game at pistol range??? Let's go back to our "Grand" so we can get a little height to that trajectory and make this interesting! :p

With that said, Al that was a damn fine anwser...how many times any of you ever seen a flat head or tailwind?? Exactly flat??? If you measure that wind at 3 feet then again at 20 feet what is the difference in the same "wind"? What is happening in between those layers of wind???

Let the games begin! :D
 
Gene,

Okay, I’ll bite again.

In my opinion, this is one of those questions that is ALMOST impossible to answer, because it is ask in an ambiguous way.

The question stated is “We zeroed our rifle to hit dead on point of aim at 100 yards in calm conditions. An hour later, winds are 12:00 o'clock at 10 mph. Will point of impact change? Which way? Up or down?”

So is the question “What will happen in this particular instance?”, or is the question “how does a headwind affect the trajectory of a bullet in flight?”

Answering the first question “What will happen in this particular instance?? Is difficult with respect to the fact that (as Tony put it) in the real world there is no such thing as a true head wind. There is almost ALWAYS a vertical and/or crosswind component (just like there are virtually no such thing as a true crosswind). Add to this the other factors that affect drop and or dispersion from point of aim (including coriolis, magnus, spin-drift, poisson drift, and equilibrium yaw), and it is hard to “predict” one way or another what will happen on the target. Will the bullet strike differently than if there were no headwind? Absolutely! But to predict EXACTLY where and TO WHAT DEGREE the headwind causes the differing bullet strike is virtually impossible. Mostly because all of the factors work together--some are compounding the dispersion, some work to negate it.

To answer the question “How does a headwind affect the trajectory of a bullet in flight?” The answer is a little more clear. In the absence of ALL other factors a headwind will cause the bullet to strike low on the target because it extended the amount of time the bullet was exposed to the gravity of the earth before reaching the target range.

Unlike wind “drift”, true ballistic drop is a function of how long the bullet is in flight. When comparing the strike of the bullet in a “no wind” situation to the strike of a bullet in “headwind only” situation the fact that is takes slightly longer for the bullet to get to the target equates to gravity has a little bit longer to pull the bullet toward the ground.

To test this, fire the bullet in the absence of gravity. A bullet fired in the absence of gravity and no wind (again ignoring all the other effects listed above) will continue to coast in a straight line until it’s forward velocity becomes zero. Then it will just hang there.
If you fired another bullet in the same conditions, except with a headwind, it would STILL just continue in a straight line, it would just come to stop before reaching the same distance as the one fired in no wind and would take longer to get to any distance shared by the bullet fired in no wind.

If you fire another bullet in the same conditions, except with a tailwind, it would STILL just continue in a straight line, but it would bump into the bullet fired in the headwind before coming to a stop, and take less time to get to any intermediate distance shared by either of the other two bullets.

So, when adding gravity to the equation (just like every bullet fired on the planet), the less time a bullet requires to reach a specific distance equates to less drop due to gravity, and the more time a bullet spends reaching a specific distance equates to more drop due to gravity.

All other things being equal, a bullet fired into a headwind will drop more than one fired in a tailwind, or in no wind.

Unfortunately, usually all other things are NOT equal.

Let’ see what alinwa and Tony have to say.

Lisa
 
(Written before I saw Lisa's response)

Using the airplane (my trusty C182) analogy again, when a pure headwind is encountered the indicated airspeed remains the same and the groundspeed, as measured by the GPS in the plane, slows. The drag on the airplane is strictly a function of the indicated airspeed (fact) and a constant overall coefficient of drag (similar to the shape factor used in the BC number); so the airplane never knows about, or physically feels, that headwind (the drag stays the same, as if there was no headwindwind).

I say the same goes for the bullet, except, of course, the bullet is not powered and is constantly slowing down due to drag and dropping due to gravity.

The instant the bullet leaves the muzzle the bullet ground speed (what a chrony would measure) is 10 mph less than it normally would be (with no headwind), and stays that way so long as the headwind stays at 10 mph. Therefore .... (do I dare reach this conclusion :eek:), the bullet will be in the air longer than normal before it reaches the target. That being said, it drops more and hits lower (not by much, but some).

As with crosswinds and BC's, I will keep an open mind on this. My airplane is not a non-powered, spin stabilized bullet, so my conclusion may well be wrong. ;)

Richard
 
I'd shoot a couple on the sighter. That's the only way you'll know for sure.:rolleyes:;):cool:

Ray
 
I think a compelling reason Tony

Tony Shankle said:
100 yards...why play this game at pistol range??? Let's go back to our "Grand" so we can get a little height to that trajectory and make this interesting! :p

With that said, Al that was a damn fine anwser...how many times any of you ever seen a flat head or tailwind?? Exactly flat??? If you measure that wind at 3 feet then again at 20 feet what is the difference in the same "wind"? What is happening in between those layers of wind???

Let the games begin! :D
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Is the majority of us here DO play this game @ 100 yds MOST of the time. That is why it is important for us to know what happens there.
 
My real 2 cents this time.

A head or tail wind is just like a crosswind except that it will usually have two components because of terrain. Horizontal and vertical. To determine the total deflection you'd have to seperate the components in order to determine their individual deflection.

The deflection as a result of the horizontal component is simple math. A simple headwind would cause more drag, the bullet would be slowed, and it would drop more. The reverse is true for a tailwind - less drag, more velocity, higher impact. Unless the wind speed is very high, a headwind or tailwind results in only a small deflection.

For the vertical component the deflection can be measured in the same way it is measured for a crosswind. The nose of the bullet will turn into the wind an amount that is just about equal to what results from a crosswind of the same speed.

There is also a small deflection caused by the bullet "tilting" to follow it's trajectory. And, a crosswind also produces a small vertical deflection and a head or tail wind produces a small horizontal deflection. Fortunately, these added deflections are small and usually "lost" in the bigger picture.

Crosswinds can have a big vertical component but most ranges are constructed such that it is small. But if you are at a range that has large vertical components for all winds, and those winds are blowing, you're gonna need a bigger target.:cool:

This is what makes 1000 yard shooting such a challenge. And why there are sighters between each record string.

JMHO

Ray
 
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Pete, I was trying to rub Gene a bit for fun plus i believe that since the problems at long range are magnified it helps one understand better at shorter ranges. But, you are correct that we should look at 100 yards. Probably 1000 too plus a fictional senerio of a perfect flat straight on headwind. So, three senerios...

1st up, the perfect flat headwind where you can just run the numbers (you math guys jump in here). Lisa as well as the likes of Vaughn and Rinker would agree that in that perfect world that 12OC wind will lower the impact point and the 6OC tailwind will raise the impact. Vaughn uses an example of a 6mm 68 grain bullet at 200 yards in a 20mph headwind is .017" low with a 0.4 msec difference in flight time. He said the tailwind is the reverse with the same numbers.

But you will notice that Lisa, Vaughn and Rinker all quickly issue a "Caution" after mentioned this perfect condition. Rinker says it will constantly fluctuate which will cause your groups to be bigger than expected. Vaughn talks of "downwash" off the roof and berms at his range.

Yesterday after work I had worked up some test loads to try. I would shoot one shot, drive to the 1000 yard target to make sure I was on paper then drive back and pound down 5 more shots (I was testing different bullets so I had to make sure they were on paper). I had nothing but a headwind showing on any of my range flags and it ranged from 3 to 7 mph on the kestrel. I tried my best to shoot the conditions but to my surprise the first shot point of impact was much different (a full MOA at least) than the 5 shot group. In just the time that it took me to drive down and back something changed about that wind that I could not see. Not just once but every time I did this exercise. Groups were larger too...in all directions from my Sunday test (twice the size).

I tell this tale of woe because i believe we often confuse ourselves by thinking of this wind as "flat and even" in lieu of what it really is on the range. That is why it has haunted us riflemen (and ladies) for so long...it is not what it appears to be.

Now lets shoot a shot. It leaves with some intial yaw from cant or blast gas and even some nuation not to mention the largest part of its arc on it's way downrange...is that nose slightly above the trajectory...you bet it is...is that nose moving around...you bet it is...then WHAM, it is slapped in the face by that 12OC wind at 10mph. A wind mind you that is moving itself both left and right and up and down. We know when it swings it hits that COP and we beat that horse enough in the other thread. But, does this delay our stabilization? Does it retard that spin arc at different rates? With that nose high and to the right does it actually hit that bullet different than we imagine? Some believe that the cushion underneath the nose in a compressed flowfield is different, does this headwind effect that cushion???

Lots of things to discuss and I can't wait to see this thread develop.
 
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...is that nose slightly above the trajectory...you bet it is...[/QUOTE said:
Tony

I have to say I'd disagree with that unless you can convince me otherwise.

The bullet's rotation causes its longitudinal axis to stay almost perfectly parallel to the velocity vector. That's what causes a bullet to follow its trajectory rather than hitting the target in a nose up attitude. Remember on the other thread, I gave the example of an artillery projectile. Much more exaggerated than a bullet but the same principle.

Ray
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For fun I thought I would run it through Exbal and see the numbers. Remember that Rinker said it was the same numbers in reverse.

6mm Berger 105gr BT at 3205 at 1000 yards

0 wind TOF 1.2883 elevation -224.8

5mph wind at 12OC TOF 1.2917 elevation -226.0

5mph wind at 6OC TOF 1.2849 elevation -223.5

So with a base line of 0 the tailwind would impact 1.3" high and the headwind would impact 1.2" low. That's a SD of 1.25" and ES of 2.5"...that alone tells me there is much more going on in the field! :(
 
Cheechako said:
Tony

I have to say I'd disagree with that unless you can convince me otherwise.

The bullet's rotation causes its longitudinal axis to stay almost perfectly parallel to the velocity vector. That's what causes a bullet to follow its trajectory rather than hitting the target in a nose up attitude. Remember on the other thread, I gave the example of an artillery projectile. Much more exaggerated than a bullet but the same principle.

Ray
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Gravity pulls the bullet in an increasing and accelerating downward path. Bullets take on a constant trajectory attitude with the nose pointing a little to the right (right twist) and a little nose high...equilibrium yaw. The gyroscopic cycloidal action is more and more down and toward the direction of rotation. Air pressure presses against the bullet on the front end but a little less on the underside (told you we would get to that cushion and the pictures of the flowfield Al) rather than dead straight. The pressure is ahead of the COG which tries to force the nose up but good ole gyroscopic acton causes the nose to arc down and to the right. Without that gyroscopic effect the air pressure would pitch the nose up and it would tumble but the side effect is spin drift.

If that nose isn't up then there is no gravity and no spin drift. It is not necessarilly above the horizon and it does drop as does the trajectory curve. But...although it is slight it is above the line of trajectory. Remember gravity works on the COG not the COP.

I do a poor job of explaining in this format so please forgive me in advance. Maybe Lisa or Al will jump in and wordsmith this thing on up! :)
 
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I got no smithery to offer :)

Like I said, IMO it's like a 15fps difference in velocity when shooting 100yds. I can't visualize a difference between a 10mph headwind and a 10mph velocity change.

This is just a knee-jerk response though. And I notice that it doesn't seem to agree with Vaughn... :(

Now as you get out to 1000 it get's more sticky, the effect becomes more pronounced as velocity drops. The 15fps is only 1/2 of one percent of the total velocity initially, but by the 1000yd mark it's worth between 1% and 2% ......depending on BC. So even in a perfect world it won't be 3moa difference in drop but more like 5-7inches.

OPINIONSby

:)

al
 
Tony Shankle said:
Air pressure presses against the bullet on the front end but a little less on the underside (told you we would get to that cushion and the pictures of the flowfield Al)


)
Click to expand...

So here I THINK what you're saying is simply that the difference in "cushion thickness" is due to the attitude of the bullet referred to as "yaw of repose"........ correct?


(You're losing me though with the "little less on the underside..." I'll go look but as I recall the THINNER cushion is on the under side?)

And yaw of repose is nose high.......

IMO it's "a little MORE on the underside...."

What did I miss?

I'll go look at the pic again....

al
 
Originally posted by Ray:

" A simple headwind would cause more drag, the bullet would be slowed, and it would drop more. The reverse is true for a tailwind - less drag, more velocity, higher impact. "

Ray,

The above is not correct. Headwinds and tail winds do not cause more or less drag. Please reread Lisa's and my posts.

Richard
 
Good for you,,,,,

model14 said:
Originally posted by Ray:

" A simple headwind would cause more drag, the bullet would be slowed, and it would drop more. The reverse is true for a tailwind - less drag, more velocity, higher impact. "

Ray,

The above is not correct. Headwinds and tail winds do not cause more or less drag. Please reread Lisa's and my posts.

Richard
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,,,,Richard. I was hoping someone would pick up on Ray's misconception. I'm trying to keep my mouth shut as much as possible. I'm trying on for size the "Shultz Policy." Remember Shultz, the German prison gaurd at Stalag 13 on Hogan's Heros; his famous response to anything was, "I know nothing."
 
Richard & Gene

Let me quote directly from William McDonald and Ted Almgren, ballisticians at Sierra Bullets:

". . . in the case of a headwind acting alone and blowing from the target toward the shooter, the speed of the bullet relative to the air would be greater than it would be if the air were still. Then, the drag on the bullet would be higher, and the bullet would travel slower relative to the ground and drop more than it would if the air were still. On the other hand, for a tailwind acting alone and blowing from the shooter toward the target, the speed of the bullet relative to the air would be less than it would be if the air were still. Then, the drag on the bullet would be lower, and the bullet would travel faster relative to the ground and drop less than it would if the air were still. . ."

Calculating the effect of head and tail winds is basic high school math. Not a lot of room for misconceptions. :rolleyes:

What am I missing???:confused::confused: Or, maybe a better question would be, what are McDonald & Almgren missing?

Ray
 
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Cheechako said:
Richard & Gene

Let me quote directly from William McDonald and Ted Almgren, ballisticians at Sierra Bullets:

". . . in the case of a headwind acting alone and blowing from the target toward the shooter, the speed of the bullet relative to the air would be greater than it would be if the air were still. Then, the drag on the bullet would be higher, and the bullet would travel slower relative to the ground and drop more than it would if the air were still. On the other hand, for a tailwind acting alone and blowing from the shooter toward the target, the speed of the bullet relative to the air would be less than it would be if the air were still. Then, the drag on the bullet would be lower, and the bullet would travel faster relative to the ground and drop less than it would if the air were still. . ."

Calculating the effect of head and tail winds is basic high school math. Not a lot of room for misconceptions. :rolleyes:

What am I missing???:confused::confused: Or, maybe a better question would be, what are McDonald & Almgren missing?

Ray
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Ray, maybe I'm wrong. The older I get the more often that is the case. I'll let someone else explain. I'll bet Lisa can make it clear to both of us.

Gene Beggs
 
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