Coriolis Effect Questions

Discussion in 'Long Range Hunting & Shooting' started by justgoto, Jul 10, 2010.

  1. justgoto

    justgoto Well-Known Member

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    I was trying to figure out the specifics on Coriolis Effect and keep finding conflicting information. I remember seeing an old thread in here where someone was wondering why their POI was off and it was determined that it was due to Coriolis Effect. I can't find that thread, and was pretty hard to follow because there was a lot of conflicting information in there also.

    So I was wondering if someone could provide a simple explanation and answer a few questions.

    (I use "drift" because I don't know a better word to use.)

    1.) A bullet will drift to the right in the northern hemisphere and drift left in the southern hemisphere, no matter what direction the shot is directed. Correct?

    2.) Assuming the above is correct, is the strength of drift equal shooting in all directions?

    3.) POI height; drifts low shooting West, high East?

    4.) There is less Coriolis Effect at the equator than there is at the polls. Is that true for both the right/left drift and the POI height?

    Sorry for all the questions, just trying to understand the Coriolis Effect better.
     
  2. phorwath

    phorwath Well-Known Member

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    Check out this Thread, starting at about post #84:
    http://www.longrangehunting.com/forums/f19/help-needed-lb3-0-338-bergers-57430/index12.html
     
    Last edited: Aug 2, 2010
  3. Eaglet

    Eaglet Well-Known Member

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    From the site

    Spin and Coriolis Drift

    I copied the following statement:

    I wouldn't be surprised if I'm miss understanding something!
     
  4. phorwath

    phorwath Well-Known Member

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    Eaglet,
    Between the time I posted my original reply (7/10/10) and today, August 22, 2010, I've spent hours researching this. I have concluded that the vertical deflection caused by Coriolis force is indeed at a maximum value at the equator, and null at both poles. This is also how Patagonia Ballistics LoadBase 3.0 Desktop software currently models vertical deflection due to Coriolis force. Your post/statement was, and currently is, correct - to the best of my knowledge, understanding, and belief.

    With this edit, I have deleted my original response as an effort to minimize confusion, and to provide correct information.
     
    Last edited: Aug 2, 2010
  5. RMulhern

    RMulhern Well-Known Member

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    Cost too damn much to travel to the equator and the Poles to shoot so I guess I'll just keep me Irish hinney here and burn up in North Louisiana!!
     
  6. tlk

    tlk Well-Known Member

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    Honest question: at what distance does all of this become material?
     
  7. phorwath

    phorwath Well-Known Member

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    That question would be a matter of opinion. If a target shooter or hunter can take some sighter shots before engaging the game or the competition, then the scope can be adjusted so all is good before taking the shots that count.

    Depends on your Latitude also. I hunt from 59* to 64* North Latitude, and Coriolis drift is a little greater here than at 40* North Latitude.

    When you're thrown into a hunting situation where there's not opportunity for sighter rounds down range, and first round hits are all that count, I consider the horizontal deflection of Coriolis to become large enough (~1.5 inches) to include in my dope at around the ~600 yard range. The vertical deflection component of Coriolis drift is of less magnitude than the horizontal component, and the vertical component may only reach the ~1.5 inches at more like ~1000 yds. Don't hold me to the exact values of drift. The ~ means approximately.

    Add the bullet's spin drift to Coriolis drift, and rightward horizontal deflection at 1000 yds can be ~9-10", in my real life experiences - which pretty closely match the Patagonia Ballistics LoadBase 3.0 predicted magnitudes of rightward drift.

    To put this in perspective, at 1000 yds, a steady & constant 2 mph crosswind will produce more drift than both Coriolis an Spin drift - combined. So a lot of guys that hunt in windy areas may not be able to distinguish - on target - the Coriolis and Spin Drift from the 5-10 mph Wind caused drift. The nice thing about Coriolis and Spin Drift is that their magnitude of drift can be doped for more easily than wind conditions - because they're a by-product of Latitude, direction of fire (azimuth), air density, etc. - factors which are less variable than wind, which can change direction and speed without a moment's notice.
     
    Last edited: Aug 2, 2010
  8. tlk

    tlk Well-Known Member

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    OK, so where can I get the basic lesson on this? Like, big crayons and red cheif tablets kind of lesson to start out with?
     
  9. phorwath

    phorwath Well-Known Member

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    It's taken me about 2 years to get my head comfortably around spin drift and coriolis drift, so if you can find a quick study on these topics, you'll be doing well. I learned a lot from Patagonia Ballistics software/program, LoadBase 3.0. In addition, I also researched and read on this Forum, and researched using the internet in general. The subjects are not very well understood by the average layman/shooter, so be careful about believing everything you read on this Forum, or on the internet at large.

    Bryan Litz wrote a book, is a Sponser of this Forum, and it's got a lot of good information in it. Bryan's member ID is 'bsl135'. If I knew of a quick & easy Level 101 course on this subject matter, I'd provide it. But I learned and came to understand the subject matter from a variety of sources.

    Good luck... It's easier to just understand when Coriolis drift comes into play, than it is to understand the theory behind it.
     
    Last edited: Jul 12, 2010
  10. angus-5024

    angus-5024 Well-Known Member

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    Litz's book is sold on this site. I've been putting off getting it for a while, but will probably get it soon.
     
  11. silvertip-co

    silvertip-co Well-Known Member

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    As to item 1... I think it may be true if the rifling is RH and spin is CW. If its LH rifled barrel and bullet spins CCW then it dont seem like its gonna drift right in northern hemi.
     
  12. phorwath

    phorwath Well-Known Member

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    Coriolis drift will drift the bullet to the right in the northern hemisphere no matter the direction of twist of the rifling. And Coriolis drift will cause a bullet fired in the southern hemisphere to drift leftward, no matter the direction of rifling twist. Coriolis drift is a separate cause of bullet drift from spin drift. Coriolis drift is a result of the earth's rotation. Spin drift is caused by the rotation of the bullet while traveling through the air.

    Spin drift will drift the bullet leftward with left-twist rifling, and rightward with right-twist rifling, independent of which hemisphere the bullets are fired in. Whether or not the bullet will ultimately drift right or left will be based on the summation of these two independent sources of drift - Coriolis drift & spin drift.

    Purchasing a barrel with left twist rifling and using it in the northern hemisphere would result in the leftward spin drift counteracting the rightward coriolis drift. The resulting horizontal drift could end up being pretty minimal, as the two sources of bullet drift would be in opposite directions.

    Almost all rifle barrels are right-twist. It might be possible to order a cut rifled aftermarket barrel with left-twist rifling without too much additional cost. Dunno. Never looked into it.

    Like I mentioned previously, learning about the magnitude and direction of horizontal deflection/drift associated with Coriolis and spin drift is much easier than understanding the theory behind each source of drift.
     
  13. MTBULLET

    MTBULLET Well-Known Member

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    about 6 yards further than your gun/cartridge can shoot !

    serioulsy, if you have time to figure this into your equation, the shot is to far.