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#12151222 - 07/17/17 JBM Ballistics Question  
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Hondo64d Online content
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Hondo64d  Online Content
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My understanding is that the JBM range card function uses density altitude. My understanding is also that the temperature inputs are for transonic calculations. Density altitude is pressure altitude corrected for temperature, and, to a minute extent, humidity. It is a measure of air density. 4000 DA is 4000 DA whether the temperature is 0F or 120F. What is confusing is in a given DA column in JBM, the comeups actually increase with temperature. Only by .2 mils in the most extreme conditions, but there nonetheless. That would indicate that that at higher temps in any given DA, the air density increase. This is completely counter to the definition of DA. Can someone explain this to me?

Thanks,

John


If my people, who are called by my name, will humble themselves and pray and seek my face and turn from their wicked ways, then I will hear from heaven, and I will forgive their sin and will heal their land. 2 Chronicles 7:14
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#12151597 - 07/17/17 Re: JBM Ballistics Question [Re: Hondo64d]  
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AH64guy Offline
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Originally Posted by Hondo64d
4000 DA is 4000 DA whether the temperature is 0F or 120F. ... That would indicate that that at higher temps in any given DA, the air density increase.


Not exactly, the performance of the bullet was usually calculated in a given set of conditions for the BC, with a minimal impact of the DA in the equation.

You are making corrections with the Density Altitude for the air conditions the bullet will be fired into, not just the physical altitude and ambient temps for the location of the shot.

The DA is calculated for the factors of barometric pressure, the altitude, current temperature, and the humidity - all which influence air density, and cause the bullet to act as if the altitude were different.

The temperature frequently has a significant impact on the DA, you are seeing the increase in corrections because the given set of conditions you are looking at will cause the bullet to act as if it was being fired at a lower elevation/temperature.

This is much harder to explain in shooting terms, than pilotage...as a long time helo guy, we spent hours calculating the effects of the DA on our aircraft performance, by time of day, location at start/during/end of mission, and the load plan for the flight.

But it was much the same, we had plan where the aircraft "thought" it was flying, more than just the current altitude/temperature.

Last edited by AH64guy; 07/17/17.
#12151700 - 07/17/17 Re: JBM Ballistics Question [Re: Hondo64d]  
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Hondo64d Online content
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Hondo64d  Online Content
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John


If my people, who are called by my name, will humble themselves and pray and seek my face and turn from their wicked ways, then I will hear from heaven, and I will forgive their sin and will heal their land. 2 Chronicles 7:14
#12151744 - 07/17/17 Re: JBM Ballistics Question [Re: Hondo64d]  
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rcamuglia Offline
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According to Ballistic AE (which uses JBM as the ballistics calculator), output will be more accurate if you set up your Kestrel for absolute pressure (station pressure) and individually input the figures for Humidity, Temperature, and Pressure. This is what I do. I also know those who use DA.


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