Windage

Discussion in 'General Airgun Chat' started by poison dwarf, Aug 26, 2013.

  1. poison dwarf

    poison dwarf BANNED

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    ammanford
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    tondu /quarry
    Given the same power output (say 11.5 ft/lbs) does anyone know if there is any significant difference in the amount of wind deviation for .177, .20, .22 and .25 pellets. Is there a "magic formula" to do a calculation?
     
  2. blackscale

    blackscale Member

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    I think you have to look up the ballistic coefficient of each pellet (might be in chairgun already!) :D
     
  3. dia6olo

    dia6olo Member

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    Oct 29, 2009
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    Leigh, Lancashire
    Indeed Chairgun has WSP (wind susceptibility factor) data in in the pellet database, the lower the number the less drift in the wind :)
     
  4. Tench

    Tench WHFTA World Champion 2016.

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    From experience i have learnt that there are basically no rules here, there are some but usually there are also exceptions, A light pellet usually takes more wind than a heavier one, a fast pellet usually takes less wind than a slower one, but those 2 rules contradict each other, throw in different rifling designs and twist rates of barrels and you have another set of variables. It basically comes down to each individual pellet and barrel combination will behave differently to another so get testing!
    I reached a conclusion that twist rate needs to be matched to a pellets ability to maintain spin (gyroscopic stability) to find the best resistance to being moved by the wind. But you can spin a pellet too fast, in a slower twist barrel the 10.3gn jsb heavy does a very good job of staying on target in the wind, fire this pellet out of a faster twist barrel and it retains too much rotational speed down range resulting in the Magnus effect in winds above about 10mph, this causes the pellet to rise when the wind comes form the left and fall when it comes from the right giving a diagonal shift in the wind rather than an horizontal one. The diagonal shift down is greater than up due to being assisted by gravity too. This pellet is unusual in its design, it has a wiide waist so a lot of its mass is away from the rotational centre giving it more "flywheel" effect than a conventional diablo helping maintaining its rotation. It is listed in the CG data base as having the lowest wind factor of all the ones listed regardless of calibre but spin it too fast and weird stuff happens.
    2 barrels of the same twist rate but different manufacurer can also behave differently due to the imprint of the rifling on the pellet having an effect on the BC and the rate of degradation of the spin.
    I also believe barrel length has an influence here, a shorter barrel allows a higher reg pressure resulting in a more severe air blast which can give an higher BC, maybe due to inflating the skirt of the pellet more? Higher pressure behind the pellet may also be why some springers appear to take less wind than pcp's.

    So as you can see, i have done a lot of testing with many barrels, had allsorts of results and come to the conclusion that there are no hard and fast rules that can be applied across the board.

    If we could fire all of the different pellets at the same velodity we may find some basic rules but as we get varying velocity with different pellet weights due to using energy as the constant it becomes very difficult or impossible to do so.
     

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