Crosswind effects on Vertical

Discussion in 'Practical Precision--PRS, NRL, ELR' started by xswanted, Feb 12, 2019.

  1. Ballisticboy

    Ballisticboy

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    The angle the bullet is pointing to is driven partially by the ratio of the wind speed and the bullet forward speed so as the bullet speed changes then that ratio and the angle will change slightly. An additional complication is produced by the aerodynamics also changing as the bullet slows down causing further changes in the bullet angle. The changes in angle are going to be small and gradual and their effects on the POI will get less as the bullet gets nearer to the target.
    I am not sure you would call it parabolic, more like a not quite straight line. Of course in assuming a constant percentage of down wind drift then a curved variation in jump with range is assumed.
     
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  2. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    More that any change in direction will have an associated jump. If the wind dies down half way to the target, the bullet will re-orient itself into the airfow by tipping a little. That tipping will have an associated “jump” that changes the trajectory, but it’s not a sudden thing like when the bullet leaves the muzzle, so the term can be confusing.

    If you assume constant wind, as most ballistic calculators do, this doesn’t happen because the airflow doesn’t change direction.
     
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  3. Meangreen

    Meangreen

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    If I understand Ballisticboy correctly, he indicates that it does change. Am I misunderstanding?

     
  4. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    The
    No, you got it. I wasn't exactly clear. I was talking about discrete changes in airflow due to varying wind.

    Edit: basically the angle of the airflow depends on the forward velocity and the wind. If you change either, the angle changes, and the bullet adapts. So saying that jump is linear with range is not *exactly* true, but it's closer to linear than it is to the nonlinear wind deflection that we're all familiar with.
     
    Last edited: Feb 21, 2019
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  5. seymour fish

    seymour fish

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  6. M14AMU

    M14AMU

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    Ballistics is kinda like being a brilliant medical Doctor in that....the more that is known the more difficult it becomes to offer a diagnosis!!
     
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  7. Meangreen

    Meangreen

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    Sometimes simplicity...in it's apparent imprecision, is what makes a concept most applicable.
     
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  8. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    Ain’t that the truth. It’s always important to keep the context in mind. So much of the interesting ballistic minutiae that we like to discuss on forums really isn’t very significant. It’s cool, but there are plenty of great shooters who either have no idea about it, or even have incorrect ideas.
     
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  9. Ballisticboy

    Ballisticboy

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    Knowing what I know never enabled me to hit anything, just gives me more excuses for why I missed.
     
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  10. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    It’s important to note that the spread of this knowledge puts pressure on manufacturers to up their game in terms of bullet design. Customers won’t demand something that they don’t know exists. Imagine the threads we’d see if nobody published BCs.

    I think Bob McCoy deserves a lot of credit for some of the advancements we’ve seen over the last 15+ years simply because he wrote the best readily available ballistics text in decades, which is a major effort that he did not have much incentive to undertake other than to preserve knowledge. I never knew him, but I wish I had, if only to thank him.
     
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  11. dsculley

    dsculley

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    To borrow an analogy from a friend of mine, when airplanes were first developed and flight became possible, engineers argued over what made airplanes fly: Was it an increase in pressure on the bottom of the wing? Was it a decrease in pressure on the top of the wing? Was it a combination? There was no consensus among "experts", but airplanes took off and flew anyway. Pilots didn't care why the plane flew as long as it did.

    Shooting can be the same way. There are sometimes many theories about why a particular phenomenon occurs, but none of that really matters to the shooter as long as he knows it does occur and how to compensate for it. For me, the 10% rule of thumb works quite well due to ease of use and repeatability!
     
  12. Ballisticboy

    Ballisticboy

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    Below is a plot (blue line) of the variation in the ratio of vertical crosswind effect to down wind drift for a typical 7.62mm bullet over a 1000 yards range modelled using a six degree of freedom model. The red line represents the 10% value. The bullet muzzle velocity was 2650ft/sec.
    Please do not take the exact values of the ratio as being for every bullet design and calibre. The ratio values will change for different bullets and rifle twist rates. I cannot remember where the bullet data came from, it may even be based on the data in McCoy's book hence I cannot vouch for its exact accuracy. However, I have carried out this exercise for many different bullets and projectile types and the shape of the curve always comes out the same. It is presented merely to show the comparison between the true variation in the ratio with range and the 10% assumption.

    verterror.jpg
     
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  13. Meangreen

    Meangreen

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    PRS, tactical shooters, hunters or any other discipline that shoots unknown distance, typically use a 100yd zero. So anything closer than 100yds is factored out.

    Basically, if you take the short steep 40% slope from 100yds, down to 10% at 400 yards, and the long shallow slope from 10% to 4% from 400yds to 1000yds, it looks like the angular adjustment averages out very close to 10% at the ranges past 500 yards where this effect might actually be a factor for consideration. It is unlikely that a shooter would be able to either dial or hold the difference.
     
    Last edited: Mar 11, 2019
  14. Lannister

    Lannister

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    I read somewhere as well that for a right twist barrel, a wind from the right (3:00) will tend to have the bullet impact slightly high and the reverse for a wind from the left (9:00). Can't recall where I read it though.
     
  15. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    Yep. That’s correct. The bullet goes 90 degrees to the wind, in the direction of the spin.
     
  16. Ballisticboy

    Ballisticboy

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    The direction depends on the aerodynamic stability of the bullet, aerodynamically stable projectiles go in the opposite direction to aerodynamically unstable projectiles. However, apart from airgun pellets, I don't really know of any gun launched high spin rate projectiles which are aerodynamically stable though undoubtedly there will be some somewhere.
     
  17. kzin

    kzin

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    The 6-7 and 12-1 jumps seem to indicate this is extremely out of scale.

    ?
     
  18. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    Very. I kind of hate that chart because it misleads new shooters into thinking that it’s universal or to scale. It’s much simpler to recognize that a wind from the right will push you up about x percent (where x depends on range and what you’re shooting). Head and tail wind are usually going to be very small effects.
     
  19. paulT

    paulT

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    Except in senarios when shooting at longer Ranges say from 1000y with tail wind switching from 5 oclock to 7 oclock left wind where vertical dispersion can be an eye opener.
     

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