## Recent posts

#41
##### Ballistics / Re: How to calculate Cd
Last post by meccastreisand - September 24, 2014, 11:49:10 PM
Quote from: 375CT on September 23, 2014, 08:37:13 AM
Quote from: meccastreisand on September 22, 2014, 10:23:23 PM
I think I have basically what you're looking for buried in some hidden tabs on my Ballistic_XLR workbook.

you can download it at Ballisticxlr.com. If you need detailed info on it you can get in touch with me there too.

So, you say that from downrange velocities, you can calculate Cd (for any G function) ? Am I right?

Thanks in advance for any help!
When you click on the excel file at the download share it'll show you an image of the spreadsheet. At the top will be an open link and an arrow pointing downward that will download it for you.

#42
##### Ballistics / Re: How to calculate Cd
Last post by 375CT - September 23, 2014, 09:13:23 PM
Quote from: admin on September 23, 2014, 01:21:22 PM
I think I have missed this request.

http://www.bfxyz.nl/pics/bcfromvelocitysimplelarge.PNG
It uses velocity measurements at several distances, yet one can apply the same method to a single measurement.
Yet the more measurements (at the same or different disatances) the better the results/

Alternatively you might post your data and I reply with an excel document that does the job.

Robert, thanks for the graph. I don't have any measurements yet, but if you can tell me how to do the spreadsheet I can try. Any help, much apprecitated for sure!
#43
##### Ballistics / Re: How to calculate Cd
Last post by admin - September 23, 2014, 01:21:22 PM
I think I have missed this request.

http://www.bfxyz.nl/pics/bcfromvelocitysimplelarge.PNG
It uses velocity measurements at several distances, yet one can apply the same method to a single measurement.
Yet the more measurements (at the same or different disatances) the better the results/

Alternatively you might post your data and I reply with an excel document that does the job.
#44
##### Ballistics / Re: How to calculate Cd
Last post by 375CT - September 23, 2014, 08:37:13 AM
Quote from: meccastreisand on September 22, 2014, 10:23:23 PM
I think I have basically what you're looking for buried in some hidden tabs on my Ballistic_XLR workbook.

you can download it at Ballisticxlr.com. If you need detailed info on it you can get in touch with me there too.

So, you say that from downrange velocities, you can calculate Cd (for any G function) ? Am I right?

Thanks in advance for any help!
#45
##### Worksheets / Re: Getting Started Workbook -...
Last post by meccastreisand - September 22, 2014, 11:45:22 PM
For my purposes as a recreational shooter altitude matters only insofar as I may climb or descend while out in my area of operations. It might be hunting or just poking at steel but altitude still has to be accounted for in my shooting solutions occasionally.

My own Excel spreadsheet (ballisticxlr) has extra data on it that's meant to deal with variations of air temp, air pressure, ammo temp, etc... for that reason.
#46
##### Ballistics / Re: How to calculate Cd
Last post by meccastreisand - September 22, 2014, 10:23:23 PM
I think I have basically what you're looking for buried in some hidden tabs on my Ballistic_XLR workbook.

you can download it at Ballisticxlr.com. If you need detailed info on it you can get in touch with me there too.
#47
##### Ballistics / How to calculate Cd
Last post by 375CT - September 22, 2014, 08:31:54 PM
Folks, I wonder if someone has the equations to compute Cd (coeff of drag) from observed downrange velocities.

I know how to do that with BCs, now I'd like to know how to do the same for Cd. Any help is much appreciated in advance.
#48
##### Worksheets / Re: Getting Started Workbook -...
Last post by admin - September 20, 2014, 11:05:02 PM
Years ago I was puzzling with these matters, should altitude be a paremeter?
Yet it is much easier to measure pressure and calculate the density from it. Hence I removed the altitude parameter.

Clearly one can study the sensitivity of a path to atmospheric conditions via BfX_C. Yet, in al those years I have used BfX, I seldom used it. And, if so, mostly for rather academic purposes, to develop an intuition.

Some of the guys on the forum, shooting long range in Australia and Finland, at extreme temperatures, might be devoted users.

Measuring pressure certainly does not require devices that exceed 20 Euro's. For that price I have a electronic weather station over here (not measuring wind speed)

Furthermore, I think that the influence of temperature on the burning of the powder might cause more deviation  than through density fluctuations. However, I am not able to do te calculations. The aformementioned forum users keep their amunition in a  coolbox. In Finland to keep them warm, in Australia to keep them cool

#49
##### General discussion / Re: Newbie problems installing
Last post by admin - September 20, 2014, 10:42:55 PM
Thanks for the compliments,

Fine that you managed to install. I certainly wouldn't be able to help you with XP and Office 2003 anymore.

Robert
#50
##### Worksheets / Re: Getting Started Workbook -...
Last post by DTV Student - September 18, 2014, 10:22:22 PM
Hi Folks,

I think I am beginning to understand why "Altitude" is not part of the parameters for the BfX_AD and BfX_C functions.  I think it is because the barometric pressure input is expected to be an absolute pressure, not a pressure that has been adjusted to sea level conditions.  I suspect the absolute barometric pressure input is often measured at the rifle range with one of those expensive portable "Kestrel" weather stations.

I do not have access to that sort of hardware, so I rely on weather reports to supply the temperature, barometric pressure and percent relative humidity.  I try to pick a weather stations close to my rifle range or close to my favorite hunting areas.  I wrote some VBA code last summer to sort of "undo" barometric pressure adjusted to sea level conditions as part of my version of an "Air Density" function.  Most of the formulas were collected from Wikipedia.   The "undo" code is contained in a few lines in the middle of following:

Public Function AirDensity(Altitude, BarometericPressure, RelativeHumidity, Temperature)
Const ProcedureName = "AirDensity"
On Error GoTo ProcedureErrorTrap

Dim Alt As Double                               ' Altitude Above Sea Level (input units = feet)
Dim BP As Double                               ' Barometeric Pressure (input units = inches Hg)
Dim RH As Double                               ' Relative Humidity (input units = percent)
Dim TempF As Double                          ' Ambient Temperature (input units = degrees F)

Dim TempC As Double                          ' Temperature converted to degrees Celcius
Dim TempK As Double                           ' Temperature converted to degrees Kelvin
Dim ReportedPressure As Double           ' Reported pressure (inches Hg) at a given altitude
Dim ExpectedPressure As Double           ' Expected pressure (inches Hg) at a given altitude
Dim PressureAdjustment As Double        ' Adjustment (inches Hg) for sea level conditions
Dim ObservedPressure As Double          ' Observed total absolute pressure (inches Hg)

Dim Pta As Double                                ' Total Absolute Pressure (Pascals)
Dim Pda As Double                               ' Partial Pressure of Dry Air (Pascals)
Dim Pwv As Double                               ' Partial Pressure of Water Vapor (Pascals)

Dim AirD As Double                              ' Air Density (kg/m^3)

Alt = Altitude
BP = BarometericPressure
RH = RelativeHumidity
TempF = Temperature
TempC = (TempF - 32) / 1.8                                     ' Units = degrees Celcius
TempK = TempC + 273.15                                        ' Units = degrees Kelvin

' A weather station observes the actual total absolute pressure, adjusts this pressure to
' what it would be at sea level, and then reports the result as the atmospheric pressure.
' We need to reverse this process to estimate the observed total absolute pressure.

ReportedPressure = BP
ExpectedPressure = 29.92126 * (TempK / (TempK + (-0.0019812 * Alt))) _
^ ((32.17405 * 28.9644) / (89494.596 * (-0.0019812)))
ObservedPressure = ReportedPressure - PressureAdjustment         ' Units = inches mercury

Pta = ObservedPressure * (101325 / 29.92)                                  ' Units = Pascals
Pwv = RH * (6.1078 * 10 ^ ((7.5 * TempC) / (237.3 + TempC)))    ' Units = Pascals
Pda = Pta - Pwv                                              ' Units = Pascals

AirD = (Pda / (287.05 * TempK)) + (Pwv / (461.495 * TempK))      ' Units = kilograms/cubic meter

'MsgBox "TempF = " & TempF & vbCrLf _
& "TempC = " & TempC & vbCrLf _
& "TempK = " & TempK & vbCrLf & vbCrLf _
& "ReportedPressure = " & Format(ReportedPressure, "0.000") & " (inches Hg)" & vbCrLf _
& "ExpectedPressure = " & Format(ExpectedPressure, "0.000") & " (inches Hg)" & vbCrLf _
& "PressureAdjustment = " & Format(PressureAdjustment, "0.000") & " (inches Hg)" & vbCrLf _
& "ObservedPressure = " & Format(ObservedPressure, "0.000") & " (inches Hg)" & vbCrLf & vbCrLf _
& "Pta = " & Pta & " (Pascals)" & vbCrLf _
& "Pwv = " & Pwv & " (Pascals)" & vbCrLf _
& "Pda = " & Pda & " (Pascals)" & vbCrLf & vbCrLf _
& "AirD = " & AirD & " (kg/m^3)" & vbCrLf _
, , "De-Bugging Air Density Calculations"

AirDensity = AirD
Exit Function

ProcedureExit:
Exit Function

ProcedureErrorTrap:
Select Case Err.Number
Case Else
MsgBox "ProcedureName = " & ProcedureName & vbCrLf & vbCrLf _
& "Err.Number = " & Err.Number & vbCrLf & vbCrLf _
& "Err.Description = " & Err.Description, vbCritical, "We Have An Error..."
Resume ProcedureExit
End Select
End Function

Thank you,
DTV Student