### Author Topic: For which atmospheric conditions did Pejsa derive his drag functions  (Read 12981 times)

• Full Member
• Posts: 237
##### For which atmospheric conditions did Pejsa derive his drag functions
« on: February 17, 2011, 12:46:44 AM »
Delved deep in the issue, Pejsa was not clear about it. However on page 76 in his book he stated that he adjusted his A function to equal the Mayevski a function at 2600fps.

this means that he uses the Ingalls conditions P=30inchHg, T=60F, humidity=67%.

The (ICAO air density)/(Ingals air density)=1.00371. Although the effects will be minor to the point of being neglible, I will adapt Pejsa's drag calculations in BfX to ICAO. Furthermore, I will add Mayevski Ingalls (under ICAO conditions) drag functions.

« Last Edit: February 17, 2011, 10:54:54 AM by admin »

#### ThunderDownUnder

• Guest
##### Re: For which atmospheric conditions did Pejsa derive his drag functions
« Reply #1 on: February 17, 2011, 11:45:26 AM »
Interesting research and all good news for BfX! Let us know when we need to update the add-in. I really think within 12 months your BfX functions will be in use by the sporting shooting community world wide. Your continuous improvements are much appreciated.

Ian

• Full Member
• Posts: 237
##### Re: For which atmospheric conditions did Pejsa derive his drag functions
« Reply #2 on: February 21, 2011, 10:38:04 PM »
I used several atmospheric density calculators on the net - they give different answers! It seemed that, in 2011, it is not clear how to compute the density of air! I know now also why people mention pressure, temperature and humidity and not density - it is not trivial to compute. I researched more and found an article called

"Revised formula for the density of moist air (CIPM-2007), Metrologia 45 (2008) 149–155"

I have a prototype implementation of it. Soon I will release a BfX function that calculates the air density and expand BfX_c to include humidity as an option. (humidity effects, however are very very small).

(CIPM = International Committee for Weights and Measures)

The calculations confirm the earlier calculations menitionned in this thread for the density for which Ingalls tables are valid, as well as its relative density to icao.

#### ThunderDownUnder

• Guest
##### Re: For which atmospheric conditions did Pejsa derive his drag functions
« Reply #3 on: February 26, 2011, 04:42:53 AM »
Do you plan on adding atmospheric condition parameters to your 'Bullet Path and Elevation Functions', etc, (BfX_Zx and BfX_Ze)? I guess new function names would be needed e.g. BfX_Zxa, a for atmospheric. Thinking about this, it work make more sense to simply correct the ballistic coefficient by applying BfX_C to it within the existing functions.

I'm looking forward to making up a range chart using these functions when you have the update available and checking them against Berger Bullets ballistic calculator. I guess if humidity does not make much difference then the existing functions should be very accurate already. I'll make up some tables and check!

Ian
« Last Edit: February 26, 2011, 05:13:03 AM by ThunderDownUnder »

• Full Member
• Posts: 237
##### Re: For which atmospheric conditions did Pejsa derive his drag functions
« Reply #4 on: February 26, 2011, 09:08:59 AM »
Indeed BfX_C is modified. I will put the update online immediately, then your weekend is still long enough to play with it.

Have also a look at gettingstarted. Furthermore look at bfx_ad.

I am in a hurry, one of the shooting friends is coming and we will go at a match at 10.00.

• Full Member
• Posts: 237
##### Re: For which atmospheric conditions did Pejsa derive his drag functions
« Reply #5 on: February 26, 2011, 09:33:30 AM »
I'm looking forward to making up a range chart using these functions when you have the update available and checking them against Berger Bullets ballistic calculator. I guess if humidity does not make much difference then the existing functions should be very accurate already. I'll make up some tables and check!

After the match, I will look!

#### ThunderDownUnder

• Guest
##### Re: For which atmospheric conditions did Pejsa derive his drag functions
« Reply #6 on: February 26, 2011, 12:29:04 PM »
I used your new BfX_C function in the calculation for trajectory, drift and velocity in a Range Chart application. I then checked it against Berger Bullets ballistic calculator (Bryan Litz designed program) and it is damn near perfect when using the same data including temp, pressure and humidity. Your BfX functions are really good, precise and accurate.

The new "GettingStarted" spreadsheet is now so much better and easier to understand. The use of the functions and the units can be easily understood in the well displayed example spreadsheets. It is amazing how many different units can be converted within the functions. This is a big update and really needs the new add-in and spreadsheet to be downloaded by all who are using it.

Ian
« Last Edit: March 01, 2011, 08:51:29 AM by ThunderDownUnder »

• Full Member
• Posts: 237
##### Re: For which atmospheric conditions did Pejsa derive his drag functions
« Reply #7 on: February 26, 2011, 09:12:21 PM »
Thx, I am flattered!

In there you used BfX_C(\$B\$5;\$B\$6;"inHg";\$B\$7/100)

This formula contains \$B\$7/100 because you expressed the humidity in %, for instance 50 and by default BfX_C expects a number from zero to 1. Hence you divided 50 by 100. However, I am not sure if you saw the "%H" qualifier that would have allowed you to write BfX_C(\$B\$5;\$B\$6;"inHg";\$B\$7;"%H")

Robert.

#### ThunderDownUnder

• Guest
##### Re: For which atmospheric conditions did Pejsa derive his drag functions
« Reply #8 on: February 27, 2011, 10:18:13 AM »
I added "%H" as suggested for use in the spreadsheet and its been uploaded to the website. Thanks for the suggestion, I seem to have overlooked the functions ability to convert to almost anything. BfX functions have great versatility with unit conversion, a much appreciated quality that you have included for no extra cost

When I get time I will explore the BfX_AD function and see if I can use it somewhere!

Ian

#### ThunderDownUnder

• Guest
##### Re: For which atmospheric conditions did Pejsa derive his drag functions
« Reply #9 on: March 05, 2011, 01:15:23 AM »

Ian

• Full Member
• Posts: 237
##### Re: For which atmospheric conditions did Pejsa derive his drag functions
« Reply #10 on: March 05, 2011, 08:22:58 AM »
Thanks for putting some life in this forum!

After the match (60 shots; 100m small bore 30x and 30x big bore), this evening I will have a better look. With BfX_U you can use the unit also as input parameter - your ballistic calculator becomes unit independent.

#### ThunderDownUnder

• Guest
##### Re: For which atmospheric conditions did Pejsa derive his drag functions
« Reply #11 on: March 05, 2011, 10:23:28 AM »
Thanks for putting some life in this forum!

After the match (60 shots; 100m small bore 30x and 30x big bore), this evening I will have a better look. With BfX_U you can use the unit also as input parameter - your ballistic calculator becomes unit independent.

I really think that that once people realise how useful and easy BfX functions are to use we will see many spreadsheets posted here to share. All the thanks goes to you Robert, for your ballistic add-on!

"With BfX_U you can use the unit also as input parameter - your ballistic calculator becomes unit independent." If you get a chance could you put up a sample use of this concept?

Ian

#### ThunderDownUnder

• Guest
##### Re: For which atmospheric conditions did Pejsa derive his drag functions
« Reply #12 on: March 05, 2011, 10:49:03 PM »
Thank you for the email with attached example spreadsheet. Whats the famous cliche? "its easy when you know how" I will use the name method of applying different units of measure in all my future spreadsheets.

I shot in 300 metre F-Open yesterday using my 22BR (90gr Berger VLD's @ 3160fps) in light wind with many direction reversals and velocity changes. The wind changes were probably at max 1 moa. Out of a possible score of 60 (USA 100) in the second round I scored 58.7X (USA 98.7X). The little 22BR in a 10Kg rifle has very little recoil and with slightly better ballistics than a 6BR. Couple that with my much improved understanding of ballistics I still could not manage better than two points off of 60.

In long distance shooting once you have ballistics covered it all comes down to wind reading. It seems the challenge of wind reading and conditions is the one variable that will never be able to be dropped into a spreadsheet and I wouldn't have it any other way.

Ian

• Full Member
• Posts: 237
##### Re: For which atmospheric conditions did Pejsa derive his drag functions
« Reply #13 on: March 05, 2011, 11:42:16 PM »
I was't happy with my own match. I shot a mere 567 of 600. 30 shots with small bore, 30 with a big bore, both at 100m indoor, iron sights, shooting jacket. I am still puzzled how that could happen - a too busy week, causing a dimnished sharpness? My friend shot 585 - that is better.

Next week I'll shoot my service rifle - outdoors 100m, iron sight shooting jacket. Wind is no problem but the light is.

#### mman

• Guest
##### Re: For which atmospheric conditions did Pejsa derive his drag functions
« Reply #14 on: May 08, 2011, 07:23:02 PM »
I used several atmospheric density calculators on the net - they give different answers! It seemed that, in 2011, it is not clear how to compute the density of air! I know now also why people mention pressure, temperature and humidity and not density - it is not trivial to compute. I researched more and found an article called

"Revised formula for the density of moist air (CIPM-2007), Metrologia 45 (2008) 149–155"

I have a prototype implementation of it. Soon I will release a BfX function that calculates the air density and expand BfX_c to include humidity as an option. (humidity effects, however are very very small).

(CIPM = International Committee for Weights and Measures)

The calculations confirm the earlier calculations menitionned in this thread for the density for which Ingalls tables are valid, as well as its relative density to icao.

This air density issue is interesting.. I compared my own air density calculator results to bfx results. There seems to be some differences.. Error between these results is highest when temperature and humidity are high. Actually difference can be as much as few percents... I've always thought that air density for ICAO conditions is 1.225 but with your new formulas it seems to be 1.226. I don't know what is right but my calculator is in line with quick target 3.2 and very close to litz's PMBS.

Here is direct comparison in excel file:

http://dl.dropbox.com/u/12618884/Air%20density%20calculation.xlsx
« Last Edit: May 08, 2011, 07:43:42 PM by mman »