## For which atmospheric conditions did Pejsa derive his drag functions

Started by admin, February 17, 2011, 12:46:44 AM

well, it proves the point that it is not trivial to compute.

I stick with my implementation of "Revised formula for the density of moist air (CIPM-2007), Metrologia 45 (2008) 149–155".

Back to QT. I have no clue how the radar measurements are done and interpreted. I have the following issue.

Under which elevations the bullets are fired?  How does one know the air density corrections?

(and  How does one know the properties for v0 close to 0?

If one fires a bullet at various v0, how does one correct for various yaw related effects on bc?)

#### mman

#16
Quote from: admin on May 08, 2011, 09:52:09 PM
well, it proves the point that it is not trivial to compute.

I stick with my implementation of "Revised formula for the density of moist air (CIPM-2007), Metrologia 45 (2008) 149–155".

Yeah, I haven't read the article yet, but it may even be the best formula so far. Still this arises questions about the accuracy of trajectory calculations. If we can't even calculate air density more accurate than that, how one can except that bcs are measured for the accuracy of 1 %. For example Litz claims that his measurements are in those limits. I'll look into this when I have more time.

Quote from: admin on May 08, 2011, 09:52:09 PM
Back to QT. I have no clue how the radar measurements are done and interpreted. I have the following issue.

Under which elevations the bullets are fired?  How does one know the air density corrections?

I'm not sure if I got your point... Is it that you are concerned about the accuracy of measurements or something else? If I'm not wrong it seems to be that most error in air density happens when humidity changes a lot. This is a weakness of ICAO standard. If measured bcs are corrected to ICAO conditions, there is always a huge variation in humidity. When in ICAO condition's humidity is 0%, in real world it is nearly always much higher. I think the pressure isn't so much of a problem. It can be directly measured and that counts out the elevation issue, at least if trajectory isn't very high. It also turns out that those equations pretty much agree in pressure effect to air density.

Quote from: admin on May 08, 2011, 09:52:09 PM
(and  How does one know the properties for v0 close to 0?

If one fires a bullet at various v0, how does one correct for various yaw related effects on bc?)
Yaw effect to drag is insignificant.. And when it isn't aerodynamic jump or unstability will destroy the groups anyways..

the radar measurement... if the bullet went up a few kilometres, how do you know the conditions.... balloons?

#### mman

#18
Aa okay... That's a good point. Lapua's ballisticians do not make very much noise about what they do, they might still answer if someone would ask them..

#### RidderMare

Dear Robert,

The real beauty of the Pejsa model is that you can model your own dragfunction, tailored to your own data.
Do I understand it correctly that your Pejsa function is fixed to the retardation coefficient rate of 0.5?
If that is the case, it is not possible to model your own drag function any more.
Would it be possible to modify the retardation coefficient rate in Bfx, and if not, would it be possible to incorporate that in the code? I would be much appreciated if you could.

Best regards,
AL

Al,

The retardation coeficient is not used in BfX. I use Pejsa's drag function, what Pejsa calls in his book "a far superior drag function". What a retardation coeficient and slope factor allows is to create your own drag function. During my holidays I will see how I could allow users of BfX to supply their own drag functions/tables.

In the mean time you might consider adapting the ballistic coefficient. If you give me your data I will show you how to do it. On the otherhand you can follow a procedure I have published in one of the downloads to do so.

Robert

#### RidderMare

Dear Robert,
Thanks for your offer. However I know how to use the Pejsa model and know how to use the retardation coefficient, but it will be great if it can be used with BfX. For me it will most certainly add value (and maybe for a few others as well ;-)).
Al

... nevertheless I am quite curious now, can you give me an example of what you are doing?

#### RidderMare

Dear Robert,
I will post an example later this month. Its holiday season here in Belgium and my kids don't allow me to sit behind the computer a lot.
Regards,
AL