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Calculating BC from velocity difference.. can anyone provide me with a formula please?

Discussion in 'Anything Airgun Related' started by cloverleaf, Jan 5, 2014.

  1. cloverleaf

    cloverleaf Super Moderator Staff Member Mod/Admin

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    I suspect I already know the answer :p

    I have a chrono on the way with a view towards doing some range testing with various pellet types at some point in the future.. obviously a large part of this will be calculating the Ballistic Coefficients (BC) of each pellet.

    I know of various sites and bits of software that will calculate the BC for you; however I'd prefer to have access to the formula so that the calcs can be integrated into the spreadsheet that will inevitably be constructed to house the results :D

    Any thoughts or pointers much appreciated :)
     
  2. Stevie Darling

    Stevie Darling Sexual tyrannosaurus

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    I think si pittaway did a lengthy video on youtube about calculating BC:)
     
  3. cloverleaf

    cloverleaf Super Moderator Staff Member Mod/Admin

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    Thanks - had a look (video here) and the method used is based on pellet drop at range and the use of Chairgun to calculate the BC.

    What I'm really after is the formula to calculate BC from velocity difference. Ta for the thought, though :)
     
    Last edited: Jan 5, 2014
  4. ratman60

    ratman60 Pro Poster

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    hello, another thread to look forward to reading
     
  5. Stevie Darling

    Stevie Darling Sexual tyrannosaurus

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  6. cloverleaf

    cloverleaf Super Moderator Staff Member Mod/Admin

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    Thanks - don't hold your breath, though :p


    Ta - I like the JBM site and have used it a fair bit in the past :) If all else fails I'll be using the bit you link to for my calculations of BC; however it would obviously be more convenient to get it all done in one spreadsheet if possible.
     
  7. Stevie Darling

    Stevie Darling Sexual tyrannosaurus

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    My physics is a bit rusty, but I can see where your coming from:)
     
  8. terry1001

    terry1001 Major Poster

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    I suspect that ballisticboy is the man to ask for this, might be worth a message as he doesn't seem to post that often.
     
  9. Rabbit Sniper

    Rabbit Sniper Member

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    I applied calculus to the equation for BC in Wikipedia, and played about with it a little. Hope it's useful.


    d( BC ) = BC *( 2 / V) * d ( V )

    Where...

    BC - ballistic coef.
    d( BC ) = some change in ballistic coef. due to velocity change
    V - velocity
    d( V) = some change in velocity.
     
  10. Ballisticboy

    Ballisticboy Posting Addict

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    There are as many equations and definitions for BC as there are learned experts which is one reason why I hate them so much. :mad: However, much of the easily available software still uses them for pellet calculations though they are uniquely unsuited for pellets. Sorry, rant over. The easiest method for calculating the BC and the one I think used in Chairgun or at least in its early versions is BC= range in yards/8000/ LN(V1/V). Should you want I can supply a spreadsheet which will work out Cd and BC from measured velocities at known ranges. Just make sure when you measure your velocities there is no wind blowing and preferably use two calibrated chronos a known distance apart.
     
  11. r10hunter

    r10hunter Honorary Member

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    When you calculate a BC is it always done at a certain altitude, temperature, barometric pressure, etc. Or does that not have enough influence on pellets as they are so short range???
    Cheers Andy
     
  12. Ballisticboy

    Ballisticboy Posting Addict

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    Compared to all the other assumptions being made (constant Cd etc) and the small changes in altitude etc in the UK then no, over the short distances in air gunning there will be little difference.
     
  13. terry1001

    terry1001 Major Poster

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    I thought that you would have the answer :up:
     

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