#21

Quote from: admin on December 07, 2015, 03:44:22 PM

Pejsa makes two assumptions to derive his formulas:

1) at a given mach interval the drag function can be approached by a power of the mach number g(m)=a*m^b a and b are real numbers. With the custom drag funtions BfX determines for each interval a and b. Then applies Pejsas method. Numerical integration assumes a constant drag value for a given interval.

2) a flat fire approximation

ad 1) if the mach interval is small enough Pejsa method, and the software has to sum the effects over many intervals, Pejsa method becomes is equivalent to a numerical integration, probably equalling the accuracy of a runge kutta integration. BfX has the ability to use a measured drag function, e.g. the ones of lapua. BfX results matches here the ones of a 3dof model, see the workbooks mman posted.

ad 2) a 3dof is able to go beyond the flat fire approximation

BfX is all about generating tables. However, Pejsa publishes many simple formulas which one can use with an electronic calculator are sufficiently accurate for most purposes. In that case it is much faster than 3dof.

Robert, thanks for the reply. But how would you compare Pejsa with PM beyond the flat fire approximation and the treatment of drag? I understand that BfX has many improvements over the traditional method. Perhaps I haven't got a full grasp of your text.

#22

Pejsa makes two assumptions to derive his formulas:

1) at a given mach interval the drag function can be approached by a power of the mach number g(m)=a*m^b a and b are real numbers. With the custom drag funtions BfX determines for each interval a and b. Then applies Pejsas method. Numerical integration assumes a constant drag value for a given interval.

2) a flat fire approximation

ad 1) if the mach interval is small enough Pejsa method, and the software has to sum the effects over many intervals, Pejsa method becomes is equivalent to a numerical integration, probably equalling the accuracy of a runge kutta integration. BfX has the ability to use a measured drag function, e.g. the ones of lapua. BfX results matches here the ones of a 3dof model, see the workbooks mman posted.

ad 2) a 3dof is able to go beyond the flat fire approximation

BfX is all about generating tables. However, Pejsa publishes many simple formulas which one can use with an electronic calculator are sufficiently accurate for most purposes. In that case it is much faster than 3dof.

1) at a given mach interval the drag function can be approached by a power of the mach number g(m)=a*m^b a and b are real numbers. With the custom drag funtions BfX determines for each interval a and b. Then applies Pejsas method. Numerical integration assumes a constant drag value for a given interval.

2) a flat fire approximation

ad 1) if the mach interval is small enough Pejsa method, and the software has to sum the effects over many intervals, Pejsa method becomes is equivalent to a numerical integration, probably equalling the accuracy of a runge kutta integration. BfX has the ability to use a measured drag function, e.g. the ones of lapua. BfX results matches here the ones of a 3dof model, see the workbooks mman posted.

ad 2) a 3dof is able to go beyond the flat fire approximation

BfX is all about generating tables. However, Pejsa publishes many simple formulas which one can use with an electronic calculator are sufficiently accurate for most purposes. In that case it is much faster than 3dof.

#23

Folks, I have a friend asking for the main differences between Pejsa and Point Mass (3DOF), which are in your opinion those?

Besides the traditional answer, of a closed form vs a numerical approach, the slope (N) I'd like to have a summary of the differences and strengths of both if possible.

thanks in advance for the help.

Besides the traditional answer, of a closed form vs a numerical approach, the slope (N) I'd like to have a summary of the differences and strengths of both if possible.

thanks in advance for the help.

#24

I have updated BfX to support Excel 2015.

Fixed also a bug that might prevented it to support Excel 2013. I cannot test this...

Fixed also a bug that might prevented it to support Excel 2013. I cannot test this...

#25

Dear all,

It is getting a mess. Let us collect what we have produced during the years.

can you post your best spreadsheets and a small description here, status 2015?

Some of them I will put them on the download page of www.bfxyz.nl

It is getting a mess. Let us collect what we have produced during the years.

can you post your best spreadsheets and a small description here, status 2015?

Some of them I will put them on the download page of www.bfxyz.nl

#26

#27

... nice to have ...

http://www.mylabradar.com/#Home

http://www.mylabradar.com/#Home

#28

I can't give you direct references I'm afraid but I have seen some of them along the way. For me those are not very interesting since most are not that accurate except perhaps limited projectile shapes.

Let's see if robert can do a better job with his new project.

Let's see if robert can do a better job with his new project.

#29

Quote from: mman on January 08, 2015, 04:21:25 PM

375CT,

There are lot's of analytical and numerical ways to estimate Cd directly. However the method I just told you is one of the most accurate and still very simple.

If you don't know the actual BC of your bullet use measurements and drawings from Litz's book to estimate form factor and calculate BC from it (I have done this numerous times and usually get the BC right with the accuracy of 3-4 % which is better than I can manage with CFD). All you need for this method is good reference bullet. When you have BC then just use G-drag curve as shown before to calculate Cd.

Mman, thanks again for your kind reply.

Are you aware of any other method, besides McDrag to compute Cd?

#30

375CT,

There are lot's of analytical and numerical ways to estimate Cd directly. However the method I just told you is one of the most accurate and still very simple.

If you don't know the actual BC of your bullet use measurements and drawings from Litz's book to estimate form factor and calculate BC from it (I have done this numerous times and usually get the BC right with the accuracy of 3-4 % which is better than I can manage with CFD). All you need for this method is good reference bullet. When you have BC then just use G-drag curve as shown before to calculate Cd.

There are lot's of analytical and numerical ways to estimate Cd directly. However the method I just told you is one of the most accurate and still very simple.

If you don't know the actual BC of your bullet use measurements and drawings from Litz's book to estimate form factor and calculate BC from it (I have done this numerous times and usually get the BC right with the accuracy of 3-4 % which is better than I can manage with CFD). All you need for this method is good reference bullet. When you have BC then just use G-drag curve as shown before to calculate Cd.