Range Report Making Ballistic Cards Using Density Altitude

[one shot ST]

Re: Making Ballistic Cards Using Density Altitude

Brad... just to be sure..

in the field zero altitude I've to write zero, if I want to use DA in the following altitude field..

 

[jbmillard]

Re: Making Ballistic Cards Using Density Altitude

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: davide</div><div class="ubbcode-body">Brad... just to be sure..

in the field zero altitude I've to write zero, if I want to use DA in the following altitude field..

</div></div>

The range card calculators are different than the regular trajectory calculators. They use altitude to calculate atmospheric density (they assume that the altitude input is a density altitude).

The "Zero Altitude" field is used to calculate the air density at zero conditions. The "Zero Temperature" field is used to calculate the muzzle velocity at zero. For example, my AR-10 load varies from 2750 ft/sec at freezing to 2800 ft/sec at about 80 degrees. I enter 2750 for low velocity, 2800 for high velocity and 32 and 80 for the low and high temps. Now the calculator "knows" what the velocity is for any temperature. (This is with RE-15 if anyone cares.)

The "Altitudes" field where you enter a bunch of altitudes is used to calculate the densities for multiple trajectories. The "Temperatures" field is where you enter a bunch of different temperatures. For each temperature, the velocity is calculated using the input low and high velocities and temps. The density is calculated using the input altitudes.

You need to enter values for both fields.

Brad

 

[one shot ST]

Re: Making Ballistic Cards Using Density Altitude

Thanks for replay....... is what I do, and data from JBM match esactly for example ballisic table created with FFS version IV.

in the zero altitude I put 0 with standard temperature for 0 density altitude or the density altitude that I have with the temperature for this density altitude.

 

[Brasscow]

Re: Making Ballistic Cards Using Density Altitude

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JBM</div><div class="ubbcode-body">Sorry, I've been pretty busy with work lately.

Brian nailed it. The speed of sound changes with temperature and my calculators account for that. You're just seeing it a little more clearly because you have multiple trajectories on one card.

The speed of sound is proportional to temperature (square root of temperature to first order). As temperature goes up, speed of sound goes up and mach number of the bullet falls (mach number = velocity / speed of sound). This puts you on a different part of the drag curve which has a little higher drag and you see a little bit more drop at the higher temperature (for the SAME density altitude which is the same air density). It's really pronounced here because the calculator uses the altitude for density so you have the same density for both temperatures. One can argue if that's realistic or not, but it is what it is.

Thanks!

Brad</div></div>

Brad,

Thank you for your response. Please forgive me for more questons - my background is not in mathematics or engineering. I'm finally on the road and able to digest this.

Would we see these same results if the drag function was based upon the velocity and air density only and not its relation to the speed of sound at a given temperature? Is using the mach number the best way to calculate the drag curve for real world results (i.e. mach does play a role in the drag)? I tried to answer these questions myself but I couldn't find anything definitive.

Thanks again!

 

[jbmillard]

Re: Making Ballistic Cards Using Density Altitude

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: brasscow</div><div class="ubbcode-body">

Brad,

Thank you for your response. Please forgive me for more questons - my background is not in mathematics or engineering. I'm finally on the road and able to digest this.

Would we see these same results if the drag function was based upon the velocity and air density only and not its relation to the speed of sound at a given temperature? Is using the mach number the best way to calculate the drag curve for real world results (i.e. mach does play a role in the drag)? I tried to answer these questions myself but I couldn't find anything definitive.

Thanks again! </div></div>

In compressible flow (air is compressible), the drag coefficient is a function of mach number. The drag function and BC are used to find the drag coefficient. Having a drag function as a function of velocity never made sense to me.

For more detail, there are a number of aerodynamics books that talk about this. I'm not an aerodynamics expert so I won't comment further...

Brad

 

[Brasscow]

Re: Making Ballistic Cards Using Density Altitude

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JBM</div><div class="ubbcode-body">In compressible flow (air is compressible), the drag coefficient is a function of mach number. The drag function and BC are used to find the drag coefficient. Having a drag function as a function of velocity never made sense to me.

For more detail, there are a number of aerodynamics books that talk about this. I'm not an aerodynamics expert so I won't comment further...

Brad
</div></div>

I suspected so since you calculated it that way, but nothing I read definitively said so. I don't think I could comprehend the level of discussion in an aerodynamics book, so I'll go off of what you said and let'r rip

.

Thanks again

 

[ggmanning]

Re: Making Ballistic Cards Using Density Altitude

Any tips on printing the table? I can only get part of it to print. It matches my Shooter program and field data very well.

 

[Brasscow]

Re: Making Ballistic Cards Using Density Altitude

Copy/paste the data to a spreadsheet and then print.

That or copy/paste it to here and bracket it in

and I'll do it and post a pdf you can print.

 

[ggmanning]

Re: Making Ballistic Cards Using Density Altitude

Thank you very much for the offer. I will send it to you by PM first of next week. I am spreadsheet illiterate.

 

[jbmillard]

Re: Making Ballistic Cards Using Density Altitude

I use FireFox and scale it appropriately (it's on the page setup tab from the File | Print menu).

Brad

 

[Bluejazz]

Re: Making Ballistic Cards Using Density Altitude

Tag for future reference

 

[SRSDriver]

Re: Making Ballistic Cards Using Density Altitude

I wanted to clarify one thing...

If I'm using the JBM card/ucard, the elevations I'm inputting are density altitudes, correct?

And my zero elevation should be set to zero?


And if I'm using the jbmtraj calculator, I enter the ICAO chart pressures/temperatures, and uncheck the correction boxes below in the atmosphere section?

Thanks in advance!

 

[jbmillard]

Re: Making Ballistic Cards Using Density Altitude

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: SRSDriver</div><div class="ubbcode-body">I wanted to clarify one thing...

If I'm using the JBM card/ucard, the elevations I'm inputting are density altitudes, correct?

And my zero elevation should be set to zero?


And if I'm using the jbmtraj calculator, I enter the ICAO chart pressures/temperatures, and uncheck the correction boxes below in the atmosphere section?

Thanks in advance!
</div></div>

"Elevation" has a different meaning for me, so I'll call it "Altitude" as listed on the inputs. Is that what you mean? If yes, then, yes, they are density altitudes. Your zero altitude should be set to the density altitude for your zero conditions.

As for the jbmtraj calculator, I think the answer is yes. It depends on what you're doing. If you don't want a standard atmosphere at altitude (you want to use the input pressure and temperature), then yes you uncheck the box "Std. Atmosphere at Altitude". If you pressure you enter is absolute, then uncheck "Pressure is Corrected".

Brad