Calculating drop with head or tail wind?

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by RockyMtnMT, Sep 21, 2009.

  1. RockyMtnMT

    RockyMtnMT Official LRH Sponsor

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    Hey all,

    Anyone have a formula for calculating drop with head or tail wind? Or just some good insight.

    Thanks,

    Steve
     
  2. Varminator 911

    Varminator 911 Well-Known Member

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    I think there is a good reason this isn't discussed, because it will have very little effect on the flight of a high BC bullet. But hey, that's just my hunch.
     

  3. trebark

    trebark Well-Known Member

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    Just ran a test with Exbal. Used generic 308 ballistics (175grain SMK BC = 505, 2600fps). With 10mph wind from 12 o'clock and 6 o'clock. This changed the scope adjustment by .5moa at 1k.
     
  4. elkaholic

    elkaholic Well-Known Member

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    Where a head or tail wind does have an effect is when it is up or down slope (especially a steep slope).......Rich
     
  5. Michael Eichele

    Michael Eichele Well-Known Member

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    The example below is not technically 100% correct but it is so close it is scary.

    You can find the FPS of the wind by converting it from MPH. Add or subtract the FPS of the wind (subtract for a head wind, add for a tail wind) from your true FPS and you will be within a small fraction of an inch at 1K with a 10 MPH wind. The conversion number is 1.467

    MPH * 1.467 = Feet Per Second.

    PS:

    Steve, that excell file I sent you corrects for head winds and tail winds.
     
  6. RockyMtnMT

    RockyMtnMT Official LRH Sponsor

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    Thank you Michael,

    If a wind is not straight on to the path of the bullet, does it have more surface area to act on than when it is straight on? Such as a two o'clock wind.

    The winds that we dealt with yesterday were very turbulent and it may be as suggested earlier that we had a down draft we were dealing with. That and all but one of the rifles that we had on the outing were in one stage or another of development. The one rifle that had a proven load did not perform predictably vertically either.

    Seems to me that long range shooting works better when it is calm.:D

    Thanks all for the info.

    Steve
     
  7. Michael Eichele

    Michael Eichele Well-Known Member

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    Mathematically with a wind other than 6 or 12 O'clock a bullet will have a measure of up or down varience as well as a sideways varience. Of course there is always the vertical component of crosswinds but for simplicity sake lets forget about the minor technical issues.

    You ask a good question. To the letter of physics, I cannot offer you an honest answer. Mathematically I can help you.

    That said, if you had a proven load not acting normal vertical, I would ask if you accounted for ALL of the enviornmental factors such as temp, humidity, pressure. Raw pressure that is and NOT altitude. Also, was the powder hotter or colder than normal? Were you able to accuratey judge the windage at locations other than the firing line and the target?

    What was the distance fired? How 'proven' is the load? How well have you documented the velocity of the load (ie: how many sessions have you chronied the load in different temps)??

    There is a reason your load did not perform as expected. There is a reason AND an answer. You just have to perform the proccess of elimination tactic to find the answer. Be honest!
     
    Last edited: Sep 22, 2009
  8. RockyMtnMT

    RockyMtnMT Official LRH Sponsor

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    The honest answer is... Not enough accuracy in all the elements to know for sure where the bullet would hit. We use altitude for the pressure, temp was good at 50 to 55 deg. Wind for us at this stage of the game is nothing more than our best guess. I have always saved the long range shooting for calm weather.

    Thanks again,

    Steve
     
  9. Michael Eichele

    Michael Eichele Well-Known Member

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    What was the distance?
     
  10. RockyMtnMT

    RockyMtnMT Official LRH Sponsor

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    At 575yrds my calcs worked out well. Moved to 1038yrds and had a tough go. 1 out of three was a good shot. Other two went low in the dirt.

    Steve
     
  11. LouBoyd

    LouBoyd Well-Known Member

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    Chapter 7 of Robert L. McCoy's "Modern Exterior Ballistics" gives the equations for head and tail winds. It's simply the amount that gravitational drop changes due to the increase or decrease in the time of flight of the bullet to the target,. The effect increases with more than the square of the distance to the target, so it can be significant at long range.

    Most commercial ballistics programs are based on McCoy's work and included the calculations. To see the effect just run the programs and look at the effect. Also try it with the wind angle a few degrees offset to see what's important.
     
    Last edited: Sep 22, 2009
  12. MontanaRifleman

    MontanaRifleman Well-Known Member

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    Been thinking about this a little and a 20 MPH wind is eqivilent about 30 fps, which will rsult in about a 2 1/2" difference in drop. I dont think the head wind itself is the cause. It may be the buffeting action of the wind with up and down drafts. But still doesn't seem like it would have a big effect. Maybe it was enough to cause some slight instability and those bullets are right on the stability edge with your 10 twist. This might be a dumb question, but any chance your muzzle was being slightly moved by the wind?

    We will probably never figure this out :rolleyes: :cool:
     
  13. RockyMtnMT

    RockyMtnMT Official LRH Sponsor

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    I am more willing to blame my inabilities then some quirk of nature. Just got my mind to wondering about the effects. I am going to lean more towards form problems with trying to send a bullet between gusts.

    Steve