Advanced Marksmanship Use of Density Altitude in Ballistic Calculations

[Coloradocop]

Okay, so I'm a licensed pilot, and I'm already pretty familiar with density altitude, and the impact it has on flying. I've recently noticed that a lot of folks have started using density altitude in ballistic calculations, which makes a lot of sense to me. Since density altitude accounts for non-standard temperature and pressure, it essentially allows you to have one figure from which you can account for altitude, barometric pressure, and temperature.

But, in looking at some drop charts that have been developed using density altitude, I can't help but notice that these charts (at least the ones I've seen) include different temperature ranges. Since DA already accounts for the aerodynamic effects of various temperatures, this raises a question for me:

<span style="font-weight: bold">Why are various temperature ranges shown in drop charts that were built off of density altitude?
</span>

My *guess* is that these temperature ranges are possibly being used to account for the effect that temperature variation has on the ammunition itself, rather than the flight of the bullet through the air (in other words, perhaps it is accounting for the fact that a bullet exiting a cartridge at 10F may not be moving as fast as the bullet from the same cartridge at 90F). But, that's merely a guess.

Can anyone help clarify this issue as far as ballistics are concerned? Since I live in a state where shooting can theoretically occur at nearly plus or minus 2 miles in vertical elevation, I think incorporating DA into my ballistic charts would prove useful.

 

[elephantrider]

Re: Use of Density Altitude in Ballistic Calculations

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Coloradocop</div><div class="ubbcode-body">
<span style="font-weight: bold">Why are various temperature ranges shown in drop charts that were built off of density altitude?
</span>

My *guess* is that these temperature ranges are possibly being used to account for the effect that temperature variation has on the ammunition itself, rather than the flight of the bullet through the air (in other words, perhaps it is accounting for the fact that a bullet exiting a cartridge at 10F may not be moving as fast as the bullet from the same cartridge at 90F). But, that's merely a guess.

Can anyone help clarify this issue as far as ballistics are concerned? Since I live in a state where shooting can theoretically occur at nearly plus or minus 2 miles in vertical elevation, I think incorporating DA into my ballistic charts would prove useful. </div></div>

Your guess is correct, so you have basically answered your own question there.

Here are some useful articles on understanding and use of atmospherics. A lot of it may be review for you, but the application part should help out.

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/References/

 

[doubletap66]

Re: Use of Density Altitude in Ballistic Calculations


The shooter app instructions say that temperature is needed to compute the speed of sound, which is needed for an accurate ballistic solution.

 

[waste_knot]

Re: Use of Density Altitude in Ballistic Calculations

In your first paragraph you are confusing density altitude with standard atmosphere.

 

[TimK]

Re: Use of Density Altitude in Ballistic Calculations

I've been wondering the same thing. Ballistic AE asks for both the DA and the temp. I tried 0°F and 100°F in that spot and ran the numbers. The dope is identical. It's obviously not being used in the calculations. I think it may be used to determine when the bullet goes transonic.

 

[TimK]

Re: Use of Density Altitude in Ballistic Calculations

Just went back and checked. One of the outputs from the software is the speed of sound, and it did change by 100fps with the two temperatures.

So, it doesn't affect the drop, just the calculation of the speed of sound and the range at which the bullet goes transonic.

 

[elephantrider]

Re: Use of Density Altitude in Ballistic Calculations

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Tim K</div><div class="ubbcode-body">Just went back and checked. One of the outputs from the software is the speed of sound, and it did change by 100fps with the two temperatures.

So, it doesn't affect the drop, just the calculation of the speed of sound and the range at which the bullet goes transonic.

</div></div>

DA should already have temp. incorporated into it as part of the DA calculation. Since you are manually entering DA instead of pressure, altitude, temp, etc then it sems your calculator just uses the temp. input for speed of sound as you state. I guess it assumes you have already accounted for temp. in the DA you entered. Temp. does affect DA and resulting drop calculations significantly. If anyone is at all confused about how DA is calculated I recommend going to the link I posted above and reading the article entitled 'Manual Calculation of Density Altitude." Good reading

 

[Coloradocop]

Re: Use of Density Altitude in Ballistic Calculations

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: waste_knot</div><div class="ubbcode-body">In your first paragraph you are confusing density altitude with standard atmosphere. </div></div>

Respectfully, I disagree with what you are saying regarding the definition of DA and its relationship to a standard atmosphere. Density altitude is more formally described as pressure altitude corrected for non-standard temperatures. Pressure altitude is the altitude that you read off of your altimeter (indicated) in aircraft operations, when the altimeter is set to 29.92 inches (which is the standard atmospheric pressure). As such, your "pressure altitude" at any given location will vary depending upon atmospheric pressure changes (changes away from the standard atmosphere in either direction). Therefore, since "Density Altitude" is derived from that figure, DA does take into account the effects of a non-standard atmosphere.


From the FAA:

QUOTE:

"<span style="font-style: italic">Pressure Altitude is the indicated altitude when an altimeter is set to 29.92 in Hg (1013 hPa in other parts of the world). It is primarily used in aircraft performance calculations and in high-altitude flight.
• Density Altitude is formally defined as “pressure altitude corrected for nonstandard temperature variations.


High, Hot, and Humid

High density altitude corresponds to reduced air density and thus to reduced aircraft performance. There are three important factors that contribute to high density altitude:

1. Altitude. The higher the altitude, the less dense the air. At airports in higher elevations, such as those in the western United States, high temperatures sometimes have such an effect on density altitude that safe operations are impossible. In such conditions, operations between midmorning and midafternoon can become extremely hazardous. Even at lower elevations, aircraft performance can become marginal and it may be necessary to reduce aircraft gross weight for safe operations.

2. Temperature. The warmer the air, the less dense it is. When the temperature rises above the standard temperature for a particular place, the density of the air in that location is reduced, and the density altitude increases. Therefore, it is advisable, when performance is in question, to schedule operations during the cool hours of the day (early morning or late afternoon) when forecast temperatures are not expected to rise above normal. Early morning and late evening are sometimes better for both departure and arrival.

3. Humidity. Humidity is not generally considered a major factor in density altitude computations because the effect of humidity is related to engine power rather than aerodynamic efficiency. At high ambient tempera- tures, the atmosphere can retain a high water vapor content. For example, at 96 oF, the water vapor content of the air can be eight (8) times as great as it is at 42 oF. High density altitude and high humidity do not always go hand in hand. If high humidity does exist, however, it is wise to add 10 percent to your computed takeoff distance and anticipate a reduced climb rate.”
</span>
https://www.faasafety.gov/files/gslac/li...DensityAltitude[hi-res]%20branded.pdf



Now, admittedly, in ballistics the effects of high DA is quite the opposite of what it is in aircraft operations: at higher density altitudes we can get more "performance" from our bullets, whereas high density altitudes make aircraft operations less safe.

 

[Coloradocop]

Re: Use of Density Altitude in Ballistic Calculations

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Tim K</div><div class="ubbcode-body">Just went back and checked. One of the outputs from the software is the speed of sound, and it did change by 100fps with the two temperatures.

So, it doesn't affect the drop, just the calculation of the speed of sound and the range at which the bullet goes transonic.

</div></div>

Interesting. I didn't consider that as a possibility, but it certainly might explain why the charts require multiple temperature inputs despite the use of DA in the calculation of the chart.