Aerodynamic jump

Discussion in 'Main Message Board' started by SGK, Jul 15, 2019.

  1. SGK

    SGK Silver $$ Contributor

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    After a bit of frustration attempting to use Ballistics AE to examine the impact of a constant wind on a Hornady EDL-M 52 grain bullet I decided to play around with Hornady's app. The impact of even slight winds like 3-5mph at just 100 yards was informative. But I was particularly puzzled by what Hornady have labelled "aerodynamic jump" which affects their "come up" calculation/results. Specifically, the elevation adjustment inverted depending on whether the wind was from 9 o'clock or 3 o'clock. As an example, with the bullet in question, a 5mph wind and muzzle velocity of 3000fps (just picked at random for now) the calculator generates a come up of +/- 0.37" (0.1 mrad) depending on whether the wind is from 9 or 3 o'clock. I'm surprised at the magnitude of the elevation adjustment needed and why it is positive (bullet otherwise strikes low) for a 9 o'clock wind and negative for a 3 o'clock wind. Can someone enlighten me as to the ballistics here?

    (It was enlightening to learn that a 3mph 9 o'clock wind would kick this bullet a half inch high right.)
     
  2. Ggmac

    Ggmac Gold $$ Contributor

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    It's reversed for a left twist barrel if I remember correctly.
    Look at coriolis effect as another shooting / sighting correction .
     
  3. Ccrider

    Ccrider Gold $$ Contributor

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    I am sure bullet spin direction has much to do with this. You see the same relationship with headwind v Tailwinds.
     
  4. 47WillysGuy

    47WillysGuy

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    Quote; “I'm surprised at the magnitude of the elevation adjustment needed and why it is positive (bullet otherwise strikes low) for a 9 o'clock wind and negative for a 3 o'clock wind. Can someone enlighten me as to the ballistics here?” Quote end.

    As I understand, a full value left blowing wind pushes a right spinning bullet down. A full value right blowing wind tends lift a right spinning bullet. How much lift or drop depends on wind speed. Still trying to figure all that out, then throw in 1/4 and 1/2 value winds.
     
  5. Tommie

    Tommie Gold $$ Contributor

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    Very interesting. Is the wind affecting the spin drift upwards/downwards or is this a separate phenomenon?
     
  6. Dusty Stevens

    Dusty Stevens COVFEFE- Thread Derail Crew Gold $$ Contributor

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    Its just the way a spinning bullet climbs
     
  7. SGK

    SGK Silver $$ Contributor

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    You guys can play around with the app - it's free. The aerodynamic jump is zero for head and tailing winds. While I'm confident it is spin related, their "spin drift" item isn't affected by the wind value. The interesting thing is that a swirling 3mph wind, which would be barely noticed/felt, could lead to almost a full MOA of spread on the target. 0.37R/0.22D vs 0.37L/0.22U for this bullet (albeit its muzzle velocity is likely higher than the 3000fps I had plugged in).
     
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  8. Dusty Stevens

    Dusty Stevens COVFEFE- Thread Derail Crew Gold $$ Contributor

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    Thats why it takes a real good sport to shoot ftr with a 223
     
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  9. SGK

    SGK Silver $$ Contributor

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    Yeah and it has got me really thinking seriously about wind flags. Although when I look at the video that was posted in another thread I think I wouldn't have a clue what to do if they start moving in all directions. I'm also thinking about all the crap I'm going to get at my local range as I lug out my Shotmarker, then wind flags, then Shadetree rest etc. If I don't put 5-10 rounds in the same hole I'll never hear the end of it. "All the gear and no idea" as they say.
     
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  10. Ccrider

    Ccrider Gold $$ Contributor

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    I don’t know about a 223, but head and Tailwinds definitely move my 6ppc.
     
  11. SGK

    SGK Silver $$ Contributor

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    Just to clarify I meant the aerodynamic jump figure in the calculator is zero for these
     
    Last edited: Jul 16, 2019
  12. RGRobinett

    RGRobinett Silver $$ Contributor

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    The free-spinning bullet is a gyro - it must obey the laws of precession: L-R wind gyroscopic precession is in addition to gravity; R-L wind precession reduces gravity's effect . . . but gravity always wins!:eek: That is why, in a R/L wind, shots lost to the left (even in a L-R wind) do not exhibit as much "vertical" (High left) as shots lost to the right (LOW right). All that stated, presuming a R twist - it would reverse in L twist. The degree of precession is also tied to spin (velocity/twist) rate.:eek: Look up a copy of RIFLE ACCURACY FACTS, by Harold Vaughn. ;) RG


    OOOps - had to correct the initial R-L.:D
     
    Last edited: Jul 16, 2019
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  13. SGK

    SGK Silver $$ Contributor

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  14. damoncali

    damoncali Bullet Maker Silver $$ Contributor

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    Jump flips direction with spin. Right twist will jump high on a wind from the right, and low on a wind from the left. The more spin, the more jump you get. The longer the bullet, the more jump you get. The shorter the range, the more jump *as an angle* you get.

    It’s caused by a bullet tipping (in this case, because of the wind) and the resulting gyroscopic effects that cause the bullet to point 90 degrees away from the direction you might expect. (A sideways tip sends the bullet up/down).

    There’s a lot more complexity in the details, but that’s the gist.
     
  15. damoncali

    damoncali Bullet Maker Silver $$ Contributor

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  16. Tommie

    Tommie Gold $$ Contributor

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    Another HUGE ballistics lesson. The more I learn, the more I realize how much there still is to be discovered. I love this stuff!
     
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  17. SGK

    SGK Silver $$ Contributor

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    I find it interesting even for load development. There are those who like to do load development at ranges like 200 yards and up, looking for lack of vertical dispersion as a sign of a 'node'. Even ignoring normal shot dispersion for a given powder charge and seating depth combination which I don't believe is merely in the vertical plane, we can add in vertical dispersion caused by small winds and changes thereto. Take a 168 gr 30 cal bullet with muzzle velocity of 2700 fps (1:10 twist). If I am using the app correctly, a full value wind of just 5mph from the right causes a change in come up of 0.67" (with 100 yard zero) at 200 yards. In other words, without an elevation adjustment specifically for this very mild wind the bullet will, all else being equal, hit left (2.68", a quarter inch of which is normal spin drift vs the 100 yard zero) and high to the tune of 1.34". It would seem even more so to me that those looking for "waterlines" on target as a sign of reloading nodes are relying on very weak foundations.

    (Take a target with shots hitting perfectly on a horizontal plane - no vertical. If there were even slight changeable winds did you adjust for aerodynamic jump? If there really was no wind at all, why the lateral dispersion and was it merely happenstance that the dispersion was horizontal rather than vertical or any point in between? The lack of vertical dispersion on target is likely merely chance and quickly dispelled with higher shot count/bigger sample size.)
     
  18. dave@aDave

    dave@aDave Gold $$ Contributor

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    I hope that I am remembering this correctly.
    Didn't they have trouble with air currents in the Houston warehouse?
     
  19. wedgy

    wedgy Gold $$ Contributor

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    windvector-half.gif Basic idea
     

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