Author Topic: FT reticle point of impact  (Read 6329 times)

gvp

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FT reticle point of impact
« on: April 30, 2013, 10:26:57 PM »
Not tested in real world yet ...


admin

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Re: FT reticle point of impact
« Reply #1 on: May 05, 2013, 12:31:49 PM »
Cool, nice graph!

When I was involved with Fieldtarget, I found it quite hard to compare measured velocity data of HN FTT pellets with the values of chairgun (BfX did not exist then). What is the situation now?

« Last Edit: May 05, 2013, 12:36:49 PM by admin »

admin

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Re: FT reticle point of impact
« Reply #2 on: May 05, 2013, 12:49:00 PM »
Some other remarks:
did you know that bfx is able to convert most of its outputs, e.g. =bfx_d("cm";.... and bfx_d("moa";..
i do not understand your rationale behind the formula used to calculate the POI?
« Last Edit: May 05, 2013, 01:08:09 PM by admin »

gvp

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Re: FT reticle point of impact
« Reply #3 on: May 05, 2013, 10:53:42 PM »
Dear colleague (I'm a physicist too),

"""""""i do not understand your rationale behind the formula used to calculate the POI?"""""""
I send you a new version of my file and the explanation.
I tried the formula =BfX_Zx() (see comments at cell H4) but I couldn't use inclination.
When the inclination is zero I have the same results with your BfX_Zx() formula.
Can I use another BfX formula in order to have a result with an inclination ?

"""""""did you know that bfx is able to convert most of its outputs, e.g. =bfx_d("cm";.... and bfx_d("moa";.."""""""
I discovered it later :-) Very usefull !!!

"""""""When I was involved with Fieldtarget, I found it quite hard to compare measured velocity data of HN FTT pellets with the values of chairgun (BfX did not exist then). What is the situation now?"""""""
I'm not sure I understand your question. Can you be more detailed ? you have measured velocity data of HN FTT pellets in different distances and you wanted to compare your values with chairgun ?

Some more to tell/ask you ..
1. as you can see I use a custom drag table which I took it from chairgun (GA profile) (black background cells).
if pressure, temperature or humidity change are these values change or remain the same ? (logically they change) ...
... so in which temp, humid, press conditions must these values be measured, in order BfX make right calculations ?

ps
later on, I changed some of these values with the red ones, right next to them, in order to have the same results between chairgun and BfX. I used solver for this.

2. in your help file you wrote "Cd=BfX_Cd(v0; df=GP)".
I think it would be more user friendly if you write "Cd=BfX_Cd(v; df=GP)" because Cd change with velocity, and vo is just the muzzle velocity.

Thank you for your project !!!

admin

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Re: FT reticle point of impact
« Reply #4 on: May 07, 2013, 01:05:45 PM »
please have a look at bfx_Ze( ...   Z as a function of elevation

as I have no time know I will respond later to the other issues...

admin

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Re: FT reticle point of impact
« Reply #5 on: May 07, 2013, 09:52:32 PM »

Can I use another BfX formula in order to have a result with an inclination ?
-------------> Yes BfX_Ze

if pressure, temperature or humidity change are these values change or remain the same ? (logically they change) ...
... so in which temp, humid, press conditions must these values be measured, in order BfX make right calculations ?
------------->
BfX assumes T=15 [C]; P=101324.6 [Pa]; H=0.0 [fraction of 1]
these qualities modify the air density and that modifies the ballistic coeficient:
air density = BfX_Ad(T=15 [C]; P=101324.6 [Pa]; H=0.0 [fraction of 1]) [kg/m^3]

Whether one use custom drag functions or standard ones, one deals with the air density effects with BfX_C, e.g.

=BfX_Vx(800;200;1*BfX_C(10;765;"mmHg";78;"%H");dt)

In the above example the ballistic coeficient is 1 (because the custom drag function used requires it) but modified because of the density effects.

2. in your help file you wrote "Cd=BfX_Cd(v0; df=GP)".
I think it would be more user friendly if you write "Cd=BfX_Cd(v; df=GP)" because Cd change with velocity, and vo is just the muzzle velocity.

-----> good point!


gvp

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Re: FT reticle point of impact
« Reply #6 on: May 11, 2013, 11:42:05 PM »
new version ... checked with
chairgun 4.0.8
plus
http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi
and results are almost perfect.

In drag function cell @"Ballistics" and "BC from drop" sheets you can use GA for chairgun diabolo pellet drag function.

mman

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Re: FT reticle point of impact
« Reply #7 on: May 17, 2013, 04:03:13 PM »
1. as you can see I use a custom drag table which I took it from chairgun (GA profile) (black background cells).
if pressure, temperature or humidity change are these values change or remain the same ? (logically they change) ...
... so in which temp, humid, press conditions must these values be measured, in order BfX make right calculations ?
Air density correction (p, T or H) is is not included in drag function but the drag is defined for fixed air density, usually at ICAO conditions (1013,25 hPa, 15 C, 0 %). Air density correction is then made for drag afterwards at calculation. This is the way to define drag functions so that drag only depends on mach number of the projectile. This is how it works in Bfx and other ballistics programs.

As far as I know Chairgun fits pellets to a drag function A which is constant as a function of mach number. This is not a good drag function profile to fit any projectile. Drag of every projectile changes wildly close to speed of sound. If your gun shoots pellets at speed of 280 m/s or faster drag function A (chairgun) is not a good fit. I tried to explain this to the programmer of chairgun but I guess he didn't understand what I meant. Or is this problem corrected recently?

Anyway when I use my airguns for longer ranges I measure muzzle and downrange velocities and fit the data to G1 or RA4 drag function. Or even make my own drag function as now it is possible, thanks to Robert for his update to BfX!
« Last Edit: May 17, 2013, 04:07:25 PM by mman »

admin

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Re: FT reticle point of impact
« Reply #8 on: May 17, 2013, 08:51:07 PM »
Mman is right, the last time (2010?) I looked at chairgun, it had a constant drag function, which I copied into BfX.

I remember a version of Chairgun that showed the formulas used and told that the source was  Steve from NY. The formulas are the Siacci ones (yust like Pesja and BfX did) for constant drag function A. For non constant A one has to do al the math and physics Pesja and I did. Quite a hurdle for many I would suppose.

Give the rather wild shapes of many pellets, a constant A is probably the best guestimate at velocities below 280m/s, as mman indicated - and  Steve from NY probably would have thought the same.

gvp

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Re: FT reticle point of impact
« Reply #9 on: May 18, 2013, 11:23:32 AM »
As far as I know Chairgun fits pellets to a drag function A which is constant as a function of mach number. This is not a good drag function profile to fit any projectile. Drag of every projectile changes wildly close to speed of sound. If your gun shoots pellets at speed of 280 m/s or faster drag function A (chairgun) is not a good fit. I tried to explain this to the programmer of chairgun but I guess he didn't understand what I meant. Or is this problem corrected recently?

This is corrected ... at least in chairgun 4.0.8
see attached photo

as I said I compared my excel file v22 with chairgun (with GA drag) and JBM (with G1 drag) and I'm satisfied with the results.
The only problem I noticed was that chairgun POI differs from BfX and JBM when I change the temp from 20C (see JBM-CHAIRGUN-BfX.xlsx).
Anyway I use chairgun now only for printing my turret scope tape ...  :) :)
« Last Edit: May 19, 2013, 01:32:27 AM by gvp »

mman

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Re: FT reticle point of impact
« Reply #10 on: May 27, 2013, 08:34:20 PM »
Thanks for info gvp. That's nice update for chairgun. Perhaps I give it a try. And impressive work with your own spreadsheet.

gvp

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Re: FT reticle point of impact
« Reply #11 on: May 27, 2013, 10:22:21 PM »
Thanks for info gvp. That's nice update for chairgun. Perhaps I give it a try. And impressive work with your own spreadsheet.

Try my latest version 27 (I've included YOUR impressive work)
 ;) ;) ;)


mman

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Re: FT reticle point of impact
« Reply #12 on: May 28, 2013, 05:45:58 PM »
Nice! Isn't it fun to make your own calculator? Commercial versions always lack something or you would like to do something differently.

Is there also options for these mentioned on "about" sheet:

Wind weighting factors (McCoy)
Effect of atmospheric conditions to vertical zero range
Possibility to use horizontal zero to compensate spin drift for short ranges


gvp

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Re: FT reticle point of impact
« Reply #13 on: May 29, 2013, 02:51:53 PM »
Nice! Isn't it fun to make your own calculator? Commercial versions always lack something or you would like to do something differently.

yes you are right !!!
The only thing that I want now is to print my scope's elevation turret tape ... (see attachment)
Now I do it via chairgun ... do you have any other idea ? (excel ... other free software ? ... maybe i'll give it a try via excel's graphs)

Is there also options for these mentioned on "about" sheet:

Wind weighting factors (McCoy)
Effect of atmospheric conditions to vertical zero range
Possibility to use horizontal zero to compensate spin drift for short ranges

I'm not sure what you mean by "Is there also options for these mentioned on "about" sheet:"
the "about" term in my workbook is inaccurate ... I just copied your post and paste it on a sheet in order to remember the factors ...
The term "about" came to my mind out of the blue  :P :P :P so I wrote it ...

If you mean that I must include also the remain 3 factors .... may I'll do it when I find some time ...

mman can you please give some info about the formulas you used for
Spin drift (Litz)
Vertical and horizontal Coriolis (Litz)
Up/downhill shooting (Litz's improved riflemans rule)
Range wind drift (McCoy)
Aerodynamic jump aka crosswind vertical drift (Litz)

you google it or the formulas are written in a book ?

Thank you again ...
« Last Edit: May 29, 2013, 02:53:30 PM by gvp »

mman

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Re: FT reticle point of impact
« Reply #14 on: May 29, 2013, 04:15:31 PM »
If you mean that I must include also the remain 3 factors .... may I'll do it when I find some time ...
Yeah that's what I asked, except that you of course do what ever you want.

The only thing that I want now is to print my scope's elevation turret tape


Do you mean something like this? I have made excel tools for this purpose. Not so easy to use but you don't have to make these very often. If you give me your email address I can send them to you.

you google it or the formulas are written in a book ?
Reference (author of the book) reads in brackets after the phenomenon.
Litz refers to this book:
http://www.appliedballisticsllc.com/Book.htm
And McCoy to this:
http://www.amazon.com/Modern-Exterior-Ballistics-Symmetric-Projectiles/dp/0764307207
If there is no brackets then its "my own formula". But these ones are not very complicated.