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ChairGun Comparison (long with lots of graphs)

Discussion in 'Anything Airgun Related' started by Ballisticboy, Jun 13, 2014.

  1. Ballisticboy

    Ballisticboy Posting Addict

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    Had cause to look at downloadable ballistic applications recently and it so happened that one of them claimed to be able to provide trajectories for pellets in a similar way to ChairGun. So I thought I would compare ChairGun with the other application.
    Now the equations for the trajectory of any projectile (except perhaps for one where the motor thrust exactly equals the drag) are too complex to be solved analytically so just about all trajectory models use the same system. A very short time is taken and the movement of the projectile in this short time is calculated assuming many of the variables stay constant. Successive short time steps are taken all along the trajectory to build up the total time of flight.
    This is fine but the assumption that the variables are constant in a short time step is not strictly correct so each time step gives a small error. This error tends to build up as you move through the trajectory. The more time steps you take the more accurate the answer should be until if you take an infinite number of steps the answer should be exact. Since it is impossible to take an infinite number of steps the models used for long range trajectories use what is called an ”integration routine”. This is basically a way of fiddling the result to make it more accurate.
    An integration routine makes the programming more complex and many of the models on the internet do not bother to include one. I am not sure about ChairGun as I have not seen the equations used in the latest version however the rival system here does not appear to use an integration routine. To be honest the amount of error you get over 50-60 yards in not using an integration routine is probably not worth the effort.
    The pellet I have mainly used for the comparisons is the .177 AA Field pellet as that is the one I have most data on from my own experiments and work. I have also compared a Beeman Lazer pellet at 6.5 grains as a more extreme example.
    The graph shows the difference between ChairGun and the other application called ISNIPE over 50 yards with a 35 yard zero.
    View attachment 98293
    [​IMG]

    Now I appreciate it is difficult to see anything on that chart so I have split it into two to show the differences more clearly.
    View attachment 98294
    [​IMG]
    View attachment 98295

    [​IMG]

    There are differences but they are not exactly huge, a maximum of 0.04 inches below 35 yards and 0.05 inches at 50 yards. Most of us will not notice that kind of error. The predicted velocities show bigger differences.
    View attachment 98296
    [​IMG]

    At 50 yards there is a difference of 14ft/sec but again, do we really care?
    The AAF is a fairly conventional pellet so if there are differences in the models for that how do they compare on a different design? The Beeman Lazer pellet is a fairly lightweight wadcutter design so how does that compare? Graphs below.
    View attachment 98297
    [​IMG]
    View attachment 98298

    [​IMG]

    The maximum differences are a bit bigger, 0.06 inches below 35 yards and 0.12 inches at 50 yards though I appreciate it is unlikely anyone would try to fire one of these to 50 yards. The pellet speeds are 22ft/sec different at 50 yards, ChairGun predicting the higher speed.
    Why are they so different? Based on a random check both sites use the same BC values when they both have the same pellet listed. However, ChairGun uses the GA drag function and ISNIPE uses the G1 function. They are similar but not the same so that should account for the difference. ChairGun can be made to use the G1 function instead of the GA function so if this is used the answers should be the same but they aren’t. Now ChairGun predicts more drop and a lower final speed at 50 yards than ISNIPE using the same BC and drag function. The differences are somewhere in the detailed programming of the models.
    The obvious question is which one is right? The answer is probably both of them. Because 50 yards is such a short range either is accurate enough for most purposes over that distance, it is only at much longer ranges that one may prove better than the other. However, I have compared both models to two of my own more complex models to see how they compare.
    The trajectory models I use do not use BC, they use purpose made drag functions and, in the case of the full six degree of freedom model, a full aerodynamic suite of coefficients. Now, no one who isn’t mildly insane is going to use a six degree of freedom model for a simple trajectory, but, in this case, I have used one (PelTraj) for comparison. I have also used a so called point mass model (PellPM) which is similar to the internet ones but which uses the purpose drag function and has an integration routine in it. The comparison is below.
    View attachment 98299
    [​IMG]
    View attachment 98300

    [​IMG]
    View attachment 98301

    [​IMG]
    View attachment 98302

    [​IMG]

    So ChairGun appears to be closest to the two more complex models if you want to be exact. The differences in trajectory height are really too small to be of significance to most people and don’t forget that none of the models will exactly recreate what happens to a pellet from your gun. So in the end either ChairGun or ISNIPE looks suitable for initially setting up your gun if you want to use a computer or phone app, at least out to 50 yards.
     
    Last edited: Jun 13, 2014
  2. mark112

    mark112 Engaging Member

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    Sorry but I don't seem to be able to see the graphs!
     
  3. Ballisticboy

    Ballisticboy Posting Addict

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    Hopefully now you can see something.
     
  4. GPConway

    GPConway Busy Member

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    I'm not a professional ballistician but that doesn't stop me from being confused. :)

    I understand that you're comparing Chairgun and iSNIPE on the G1 drag function but what are the "purpose drag function(s)" used with PelTraj and PellPM? Are they the same as each other? Are they (the programs) reliant on G1 drag functions too?
    If not - and they are instead derived from real data - would not the PelTraj/PellPM/GA comparison be more valid (at least for the AA Field)?
    Are we comparing apples with oranges?

    We both know that these particular differential equations cannot be solved analytically but the complexity to which you refer is surely down to real-world drag curve - which cannot be resolved analytically. Were it not for that, we could use closed-form expressions instead, couldn't we?

    George
     
  5. Ballisticboy

    Ballisticboy Posting Addict

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    As I said above, ChairGun uses the GA function which is slightly different to the G1 function. ChairGun can be made to use the G1 function but the models still do not agree despite using the same BC value. Modern trajectory models do not use any reference drag functions such as G1 as they can never be an exact match to the actual projectile drag function, only an approximation at best. The drag function shape for a pointed or wadcutter pellet is not the same shape as that of a round head pellet yet by using BC and a standard drag function the assumption is made that one drag function fits all. The BC only moves the entire drag curve up or down it does not change the fundamental shape. Making a purpose drag curve for each projectile design will change the shape as well as the values. Of course over 50 yards you will not see much difference, but over 25km there can be a big difference. As BC is not used projectile mass and reference diameter also have to be input.
    Again, over 50 yards, closed form expressions can be used for many things as the variations in the variables are small, I have myself used them on many occasions. For .22 at 12 foot pounds a constant Cd is not going to give a big error particularly for a heavy round head pellet. For .177 it is different as the speed will be well into the drag rise area at the start of the flight. For many projectiles drag is not the only thing changing in a time step, air density and wind direction being just two more amongst many.
    PelTraj uses the same basic drag curve as PelPM but it also has additional functions for yaw induced drag as well as lift etc, about 11 different aerodynamic functions if I remember properly. It is not a short job to create realistic values for all of them:(
     
  6. mark112

    mark112 Engaging Member

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    Hi,

    Firstly thank you for making the graphs now visible (mostly). As someone not fully understanding of ballistics (alright I get about 10% of it) it seems to me that for normal airgun ranges of up to say 50 yards these packages are a good indicator of trajectory. Please don't shoot me down in my naivity but if the various models (i.e. GA) are not quite appropriate can they not be adjusted to suit that particular pellet. Chairgun allows full adjustability of the shape of the curve when using a custom profile so is it not possible to tweak it and then save it as say AADF? Sorry to lower the tone of an obviously very tecnical thread :).

    Mark
     
  7. GPConway

    GPConway Busy Member

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    Thanks BB,
    I fully appreciate the stuff in your first two paragraphs but I'm really asking about the 'personalised' drag curve used by your PelTraj/PellPM software and the validity of comparing it to the G1 curve as used by iSnipe and Chairgun. Clearly, if we are comparing disparate drag curves, the resulting graphs are not particularly meaningful.
    Like I said before, apples and oranges. :)

    George
     
  8. terry1001

    terry1001 Major Poster

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    The message that I'm getting from the comparisons is that, for air rifle users, there is little to choose between the cheap/free applications and more rigorous solutions. Most people, I imagine, use Chairgun (or similar) to get a good idea of where to start before confirming the results by shooting. I use Hawke scopes and find the reticle charts useful so that I know roughly where I need to be before I start shooting. Clearly if you want to drop a 100kg projectile inside a tennis court sized area 30km away then exact analysis is essential but for me a quick and dirty solution at my fingertips.
    Well done to ballisticboy for all the effort.
     
  9. Ballisticboy

    Ballisticboy Posting Addict

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    If the purpose made curve was the same as the G1 curve I cannot see much point in comparing them. The purpose made curve should be the more accurate curve (unless I got it completely wrong in which case the artillery had better be careful!!) and hence it is fully valid to compare the generic curves represented by GA and G1 with it to see how accurate they are in theoretical modelling. GA is better as you would expect since it was designed for pellets. The work also shows the potential pitfalls of using BCs with the wrong drag function. The same BC should not be used with both GA and G1.
    Terry1001 yes you are quite right, the simple applications are quite suitable for starting the setting up of your rifle. As I said, only someone slightly mad would go to the trouble of getting the data for a six degree of freedom model for this use. ChairGun is perfectly adequate for normal airgun ranges. You will only use the more complex models if you are studying the detailed projectile motion over 50 yards. ChairGun of course cannot show you the yaw wave lengths or group size for a given pellet error, for that you need the other models and the input data to go with them. Even then you will need to "adjust" the values to match observed behavior.
    Mark yes you may well be correct, I have not tried to use personalised drag curves in ChairGun. However, if you do, you will need to change the BC as well since the form factor with the proper drag function will be 1 which will need a new BC value. You could approach it the other way and keep the BC the same but then you will need to fiddle the drag curves based on the form function which you have. It is probably not all worth it for most pellets and applications.
     
  10. GPConway

    GPConway Busy Member

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    At least we'd be comparing like with like. :) You've already stated that iSnipe and Chairgun give different results* using the same G1 drag function ...

    … but we've not yet seen how the PellPM method compares using the G1 drag function.

    *With regard to the differences:
    I've just run a little comparison between iSnipe (V4.0.5), Chairgun Pro (V4.1.7) and the JBM point-mass calculator using Muzzle Velocity=780ft/s, Zero Range=35 Yard, Scope Height=0.0", BC=0.021, G1 drag function and the same environmental conditions (20 degC, 29.95"Hg and 50%RH).
    At 100 Yards, the maximum velocity difference between the three programs is just 1.5 ft/s and the maximum poi spread 0.33".
    So how does the PellPM method compare?

    If you're using the curve derived for diabolo air gun pellets, they'd better be more than just careful! :)

    George
     

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