Discussion in 'Technical Articles - Discussion' started by ADMIN, Sep 17, 2011.

# A Scientific Basis For Evaluating Variable Crosswinds

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This is a thread for discussion of the article, A Scientific Basis For Evaluating Variable Crosswinds, By Paul Carter. Here you can ask questions or make comments about the article.

2. ### MikecrWell-Known Member

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I agree in a general sense with this illustration and results of near-vs-far wind.
But it can be taken further.

Wind drift is not really about TOF, but of T-LAG and drift angles.
And that's where the boat scenario is better explained. It's only while the boat is deccelerating (lagging) that drift occurs, and the higher that decceleration the higher the drift angle that is set.

It often appears that a very fast bullet drifts less in wind. But this seems otherwise after observing a very heavy(high BC), yet slower bullet, produce less downrange drift.
If you took a very low BC bullet/going fast -vs- a very high BC bullet/going slow, both with the same TOF by 1kyd, and both exposed to the same near wind, the high BC bullet would win over the low BC bullet -by 1kyd.
This, even though the fast bullet had lower TOF while exposed to near wind.
It drifts more because it is deccelerating more.
Also both bullets deccelerate the most nearer the muzzle than the target, and that's why the drift angles are influenced most up close -vs- far downrange(where TOF constantly increases).

Just sayin

3. ### vendetta333Well-Known Member

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the argument of low velocity/ high bc vs. high velocity/low bc is not a scientific argument. you are changing two variables at one time.

4. ### MikecrWell-Known Member

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It was just a general example to illustrate my contention that wind drift is more about TLag than TOF.

TOF is only a factor in determining TLag, and it's TLag that actually is a factor in wind drift.
The decceleration that occurs due to drag causes the time lag which wind speed is applied to.
Wind drift(in inches) is: 17.6*CrossWindSpeed(mph)*TLag(sec)
TLag = TOF-TOFvac
A higher BC bullet deccelerates at a lower rate than a lower BC bullet regardless of velocity(provided the BC relationships hold).
So at any two points in flight, the higher BC bullet produces lower TLag, and is therefore less affected by crosswind between those points.

It's a perspective that readers need to consider before concluding that they could reduce wind drift by going with lighter bullets at higher speeds.
While they would reduce TOF, they could in the same move -increase TLag(due to higher drag) -increasing wind drift sensitivity.

When wind drift is truly the focus of concern, so is TLag.

5. ### Ben22New Member

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yup he is right both variables are quite different to each other.......

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7. ### vendetta333Well-Known Member

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i just feel that people in general make many claims based on "experiments" that aren't experiments due to changing more than one variable at a time. the reply you posted cleared everything up, was just wanting to check with you. not trying to be a smartass, just a major pet peev of mine. thanx for the info. much helped me.

8. ### trappers sonWell-Known Member

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what would happen if your variables changed.... this experiment is true for the given variables but could the factors' influence change in significance if the experimental parameters were changed.... such as having a 7mm pushing 180 hybrids close to 3300 in wind that was 40 miles per/hour....
I have not crunched the numbers but I would expect that the higher bc and time of flight to the first five hundred would reduce influence of wind effects at the muzzle when the velocity of the bullet is faster and bucks the wind more in comparison to your experimental scenarios, and the later condition of comparison (0 wind 1st 500, 40 wind 500-100) would have more influence due to the slower velocity of the bullet, its reduced bc and the greater influence of the wind upon the projectile outside of the consideration of the point of influence and distance from target. I don't know if my scenario would actually reduce the magnitude of influence that early wind farthest from target would affect bullet trajectory or challenge the wholeness of your statements about early influences always having more impact on bullet path in comparison to later ones closer to target but I bet if someone was creative enough and wasted enough of their time, they could do the math and find scenarios that contradict your rule of thumb in specific or unique scenarios by changing factors such as wind speed, bc, bullet velocity, distance to target, variation of land features at varied distances etc etc ...... I believe your hypothesis is accurate but I think for every law there is an exception....

9. ### Boom13ANew Member

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Because range(feet)/MV (fps) = TOFvac I think the original article is correct. He just didn't use the term TLag.

10. ### MikecrWell-Known Member

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Or TLag = TOF-TOFvac
Or Wind drift(in inches) is: 17.6*CrossWindSpeed(mph)*TLag(sec)

11. ### Rich CoyleWell-Known Member

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"but I think for every law there is an exception...."

If it's a law, there are no! exceptions.

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I would like to add. As your round slows down in flight the BC ever so slightly increases. To put this in to point I will use some data from the book being released soon. Take a 6.5mm 150gr BTHP round. Above 3000fps the G7BC is .303 once it falls below 3000fps it changes to .306, once it drops below 2000fps it changes to .314, below 1500fps it is at .322 and starts to curve off from there. This of course isnt a perfect fall off from point to point, but is a curve. As this BC changes, so does the effect your wind has on the round.

Now I would also point to a research article done by one of our engineers here at Applied Ballistics. You can read this article here http://www.nvisti.com/wp-content/uploads/2014/06/NVDOC1403-Wind.pdf but it explains when, and where the wind is the most critical and how when it changes its effects change. The wind at the shooters location has the most effect on the bullets trajectory. Now as you increase in range (using the example from the article) that effect starts to level out. But if you are shooting at 200 or 1000 the wind still has around 50% of its effect from the first third of the bullets flight path.

This is a direct quote from the study "If we break each range into thirds, how much do those segments contribute to the overall effect of the wind? Figure 3 shows that breakdown: at 200m, the first third accounts for 56% of the influence, the middle contributes 33%, with the third closest to the target roughly 12%. As the distance increases to 1000m, the influence of the first third drops to 44%, the middle third increases to 39%, and the last third grows to 17%. The front portion of the trajectory obviously plays the most significant role in determining the bulletâ€™s deflection, but with increasing range, the middle section needs to be considered as well."

I would bring the formulas in to this thread, however they are in the article if you wish to give them a go! Just to complicate things a little As the bullet slows down, the BC increases in which case the winds effect on the round changes, so at each step along the flight path (each time the fps changes) the winds effect changes, and you end up with a formula that becomes extremely complicated and long. Or you can let us do the solution for you!

One last thing to add, driving a heavier round, at a higher velocity helps to reduce the wind deflection.

Applied Ballistics Support Staff
doc.beech@appliedballisticsllc.com

13. ### MikecrWell-Known Member

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You've either invented a bullet like no other in existence, or you should read whatever book again -but right side up next time.
BC does not go up with decreasing velocity, it goes down(inverse of it's drag coefficient).

The paper referenced has some misconceptions as well. It keeps going back to time of flight(TOF) while departing from Time Lag (TLag). And it took a lot of effort to do that given that McCoy's time lag formula was actually discussed, and then for some reason departed from, in the article.

You should start over.