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# Ballistic Program Inconsistencies

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#15
09-11-2009, 08:58 PM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 3,972
Re: Ballistic Program Inconsistencies

Mach is also affected by humidity.
With the problem Bryan posted, 3000f/s at ICAO is Mach ~2.69,
and the ICAOG7BC of his SMK is 2.30

Add 78% RH, and 3000f/s is a vapor adjusted Mach 2.56. The ICAO G7BC would drop a touch to 2.29 at that speed(2858f/s).
In reality though, the reduced air density with humidity closely offsets this Mach-to-CD lookup shift, and local G7BC remains at 2.30
Without accounting for 'Wet Mach' the G7BC might appear to go up with this humidity to 2.31
The beauty of G7 is that these errors amount to less than would be using G1.

But I wonder if actual Mach, or even accurate air density was accounted for when developing G-models.. Std Metro just seems too unlikely to me, and I think both McCoy and Pejsa generalized in areas less familiar to them.

We need a system allowing generation of custom Cd tables based on local testing.

Bryan, I've enjoyed your book so far, and thanks for the program
You the man

My math:
Speed of sound = (49.0223*(Tf+459.67)^0.5))
Dry Mach = Mv/Speed of sound
Vapor correction = 1+0.0014*(Rh-78)*(vapor pressure/29.92126)
Wet Mach = DryMach*VaporCorrection

Vapor Pressure = Rh/Saturation Pressure
Sat Pressure = 6.1078*(0.99999683+Tc*(-0.009082695+Tc*(0.0000787362+Tc*(-0.00000061118+Tc*(0.00000000438842+Tc*(-0.0000000000298839+Tc*(0.000000000000218744+Tc*(-1.78923E-15+Tc*(1.1112E-17+Tc*(-3.09946E-20))))))))))^-8
Temp Celsius = (Tf-32)*5/9)

Jeff, I've managed to patch math together that you may be missing. Email if interested.

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