Author Topic: Custom Drag curves  (Read 7183 times)

375CT

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Custom Drag curves
« on: November 25, 2013, 09:51:18 PM »
First of all, thanks for this forum! I truly hope to particpate as much as possible.

I've been reading wih much interest the topics on the "user defined" or "custom drag curves" and while I understand the use and rationale behind them, I still have two basic questions :

1) Once the custom curve is entered, how this program works with it?

Is it by taking the dataset and calculating a series of "banded BCs", corresponding to each bracketed Cd zone?

If not, how it works?

2) Does anybody compared the accuracy of the prediction yielded by a custom curve vs. a standard "G" curve using a well measured BC?
thanks in advance for any response.

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Re: Custom Drag curves
« Reply #1 on: November 25, 2013, 10:40:37 PM »
Hello on the forum!

The custom dragfunction is used instead of the standard ones, e.g. g7. For a customized drag function for a specific bullet, the ballistic coefficient (BC) is then 1 lb/in^2. Other dragfunctions do require a BC.

On page 20 of the GettingStarted workbook the custom drag functions are explained

One can (could?) obtain custom drag functions for Lapua bullets.
These drag functions where obtained bij analyzing radar data, probably via differentiating deacceleration as function of speed.

MMAN on this forum has the most experience in using customized dragfunctions. He has probably an idea how good they are. For my purposes using g7/g1 was good enough. Have a look at MMANs sophisticated ballistic calculators made with BfX on this forum.

You can make the comparison yourself quite easily. If one has a Lapua drag function one can with BfX compare its output (velocity as function of time/range) to the G7 one with ballistic coefficients from Bryans Book. Take care, at large ranges, say above 1000m, the elevation is such that BfX looses accuracy.





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Re: Custom Drag curves
« Reply #2 on: November 25, 2013, 10:43:10 PM »
You wrote

Is it by taking the dataset and calculating a series of "banded BCs", corresponding to each bracketed Cd zone?

No, the drag function allows to directly calculate the deacceleration.

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Re: Custom Drag curves
« Reply #3 on: November 25, 2013, 10:47:16 PM »
from an earlier post the following comparision

375CT

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Re: Custom Drag curves
« Reply #4 on: November 25, 2013, 11:51:48 PM »
You wrote

Is it by taking the dataset and calculating a series of "banded BCs", corresponding to each bracketed Cd zone?

No, the drag function allows to directly calculate the deacceleration.

Interesting, but in some ways I guess the dataseries needs to be somewhat parameterized or how the Pejsa's formulas can deal with Cd directly?

Same question for other programs using "custom" curves like JBM. Well, I'm curious on how things actually work!

Thanks for the welcome and replies, much appreciated!

375CT

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Re: Custom Drag curves
« Reply #5 on: November 26, 2013, 03:57:10 AM »
You wrote

Is it by taking the dataset and calculating a series of "banded BCs", corresponding to each bracketed Cd zone?

No, the drag function allows to directly calculate the deacceleration.

Robert,

What you meant is BfX uses :

1) F = V2/A

2) A as defined in the workbook image.

Am I right?

Still trying to understand how to load Lapua's Doppler data on BfX.

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Re: Custom Drag curves
« Reply #6 on: November 26, 2013, 11:30:14 PM »

I want to remember that Pejsa parametrized the drag function for four velocity regions (splines). He did that with four functions of the form cd=a*velocity^b, a and b being the parameters. This allows to derive formulas for velocity, drop etc as function of time and distance etc. The formulas depend on a and b.

What I did for the drag functions (e.g. G7)  that are included in BfX is that I parametrized them myself. However, the included drag functions require more velocity intervals for a good parametrisation. Typically 10 velocity regions were necessary and as much sets of a,b values.

The custom drag function, listing drag for a set of typically 40 velocity regimes, are parametrized automatically by BfX. BfX calculates the parameters a and b for each velocity interval of the supplied custom drag table. Instead of having only four parameter regions that Pejsa's drag function has, this use of custom drag tables yields typically  40 of such intervals.

But for our computers this is not a problem.

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Re: Custom Drag curves
« Reply #7 on: November 26, 2013, 11:40:11 PM »

Still trying to understand how to load Lapua's Doppler data on BfX.

have a look at MMANs ballistic calculator to see examples:

http://www.2shared.com/file/YEE1WaWl/Ballistic_calculator.html

375CT

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Re: Custom Drag curves
« Reply #8 on: November 27, 2013, 04:48:32 AM »

I want to remember that Pejsa parametrized the drag function for four velocity regions (splines). He did that with four functions of the form cd=a*velocity^b, a and b being the parameters. This allows to derive formulas for velocity, drop etc as function of time and distance etc. The formulas depend on a and b.

What I did for the drag functions (e.g. G7)  that are included in BfX is that I parametrized them myself. However, the included drag functions require more velocity intervals for a good parametrisation. Typically 10 velocity regions were necessary and as much sets of a,b values.

The custom drag function, listing drag for a set of typically 40 velocity regimes, are parametrized automatically by BfX. BfX calculates the parameters a and b for each velocity interval of the supplied custom drag table. Instead of having only four parameter regions that Pejsa's drag function has, this use of custom drag tables yields typically  40 of such intervals.

But for our computers this is not a problem.

Robert,

Thanks for the answer, now it's much clear to me how BfX works with custom curves. Thanks for that nice explanation.

Another question (if you don't mind) :

In the way Pejsa defines each drag zone Cd=a*velocity^b, how are the "a" and "b" coefficients calculated?

Is it as explained in the "Getting Started" image on Drag? Unfortunately I cannot find where except for this:

1) F = V^2/A

2) "A" as defined in the workbook image ??

Sorry for asking but I still need to relate what's in the workbook and Pejsa's book, because I'll like to calculate custom curves myself and any help on how to compute those "a" and "b" values is greatly appreciated.

375CT

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Re: Custom Drag curves
« Reply #9 on: November 27, 2013, 04:50:03 AM »

Still trying to understand how to load Lapua's Doppler data on BfX.

have a look at MMANs ballistic calculator to see examples:

http://www.2shared.com/file/YEE1WaWl/Ballistic_calculator.html

Taking a look at them righ now! :)

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Re: Custom Drag curves
« Reply #10 on: November 27, 2013, 05:24:43 PM »

Sorry for asking but I still need to relate what's in the workbook and Pejsa's book, because I'll like to calculate custom curves myself and any help on how to compute those "a" and "b" values is greatly appreciated.


here is the excel spreadsheet with which i determined the splines.

375CT

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Re: Custom Drag curves
« Reply #11 on: November 27, 2013, 10:47:17 PM »

Sorry for asking but I still need to relate what's in the workbook and Pejsa's book, because I'll like to calculate custom curves myself and any help on how to compute those "a" and "b" values is greatly appreciated.


here is the excel spreadsheet with which i determined the splines.

Robert thanks a lo again for your help. It's much appreciated! Now time to study the new WB in full detail.

Is the VBA code available somewhere? I'd like to take a look in order to add my own future work for a bigger shareable library.
« Last Edit: November 27, 2013, 10:51:30 PM by 375CT »

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Re: Custom Drag curves
« Reply #12 on: November 28, 2013, 12:07:13 AM »
The VBA code for the drag function fit is in the workbook, I just checked it. You have to switch on the developer tab in Excel to get access to it.

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Re: Custom Drag curves
« Reply #13 on: November 28, 2013, 12:48:24 AM »
Maybe you can present your function/measurements to the forum and then we can supply you with either the parametrisations, or a bc, or some comparisions (e.g. the special purpose drag function versus g7...).


375CT

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Re: Custom Drag curves
« Reply #14 on: November 28, 2013, 03:37:10 AM »
The VBA code for the drag function fit is in the workbook, I just checked it. You have to switch on the developer tab in Excel to get access to it.

Sorry for the confusion, you are right about that code, what I meant is the VBA code of BfX main routines.

I'm not skilled with Excel programming, but I do well in Visual Basic, and while VBA is not the same it's quie easy to port.

My goal is to make a public library of BfX functions on a higher language, to share with BfX community.

My idea also, is to port that code to other languages (C#, Java) as well as long as others are interested.

Well, just an idea that could make sense for the members.