density altitude
- Thread starter cheek9075
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Why is Hillary wearing that Trump button?
I don't know that the BC changes at all, But the drag at different velocities does. Others can correct me if I"m wrong, but the banded BC's in the sierra manual are to account for the changing drag at different velocities. I read on here somewhere that a bullet sitting on your bench has the same BC as one flying to the target.
The BC has to change based on velocity, it's a given. But BC stays the same if a bullet can keep its velocity, but it can't. BC is a number that bullet makers say is correct. Air, thicker or thinner, creates drag, air changes everyday, every hour causing less or more drag causing bullets to drop differently on a given day. This is what I'm seeing, and need to account for it. Velocity is what it is (I think) speed based on a crono. BC is what it is, a number. The only thing that changes is weather, causing DA to change, which means less of more drag on the bullet.
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One more time, let something else do it for you.
https://kestrelmeters.com/products/kestrel-elite-weather-meter-with-applied-ballistics
This accounts for loss of speed as bullet flies.
It's on my list of "things to have" but for the time being, lack of funds, I have to make due with what I have. That is a phone (ballistic app) crono, range finder, and what I know and can understand.
Thanks for the input everyone. I'm not trying to over think this, just trying to know why it happens, and explain what I'm seeing. I'm not trying to reinvent anything here, Just trying to understand how/what happens .
An app on a phone will do it too if you stay on top of it throughout the day, I used to like AB, it has not been updated in sometime, other great ones. I find getting DA from the nearest race track used to work, they give predicted DA throughout the day.
DA is a poor 'shortcut' for external ballistics, because it's not actually a shortcut, and flat out wrong for ballistics.
For it, you need all three air density parameters(Temp, absPress, RH) anyway, so you should just enter those directly. And doing so is more accurate to ballistics then DA(even if DA is actually entered as correct). Also forget altitude, or standard air density at altitude,, worthless,, you won't ever be where that matches.
One factor in my contention about DA is humidity, which many tend to ignore,, don't do that.
Humidity does affect air density.
Humidity also affects Mach, and your standard drag coefficients are looked up/compared per mach# by ballistic software to internally work with relative BC. In other words, disregarding RH is more an error than just being off with air density solutions(affecting drag calcs). The error is also compounded with assigning of the wrong standard drag coefficients(again affecting drag calcs) due to errors in mach.
For it, you need all three air density parameters(Temp, absPress, RH) anyway, so you should just enter those directly. And doing so is more accurate to ballistics then DA(even if DA is actually entered as correct). Also forget altitude, or standard air density at altitude,, worthless,, you won't ever be where that matches.
One factor in my contention about DA is humidity, which many tend to ignore,, don't do that.
Humidity does affect air density.
Humidity also affects Mach, and your standard drag coefficients are looked up/compared per mach# by ballistic software to internally work with relative BC. In other words, disregarding RH is more an error than just being off with air density solutions(affecting drag calcs). The error is also compounded with assigning of the wrong standard drag coefficients(again affecting drag calcs) due to errors in mach.
Technically true; however, the effect on the speed of sound with changing humidity is so tiny as to be nearly non existent for the purposes of shooting. For our purposes, the speed of sound is almost entirely dependent on temperature.
Here's how small an effect RH has on the speed of sound. At sea level the effect going from completely dry air (0% RH) to 100% is .0035. At lower pressures the effect is a bit larger. The effect on the speed of sound doubles to .007 at 20,000 feet.
Some of us down South shoot at places which seem stuck on 100% humidity both summer and winter and I suppose some high altitude winter shooters get cracked lips 'cause it seems like their air is super dry at 0% RH. The truth is that the RH most of us experience is moderate and doesn't change all that much throughout the day; certainly not enough to make a significant change in the speed of sound when you compare the change with other errors built into out ballistics calculations.
To put it in perspective, lets say you input a RH of 25% but it's really 75%. That would be a wild miss in estimating humidity. Most folks can feel this kind of difference, but it would be an error similar in size to thinking you're shooting at an elevation of 900 feet but you find out your real elevation is 901.5 feet. Or thinking your MV is 2500 fps but it's really 2504 fps.
In other words, it ain't no big deal compared with other unknowns.
At ICAO SL standard, which is 59deg, 29.921inHg, 0%Rh, MACH1 = 1116fps
Add 100%Rh, and MACH1 = 1189fps, 73fps faster.
Multiply this error x2.5 for a bullet traveling at M2.5 ->
ICAO would be 2791fps
Add 100%Rh and M2.5 is 2974fps
183fps difference(73 x 2.5)
The difference that makes to calculated local BC (if humidity is not entered) ->
Beginning with a G1 ICAO BC of .500 at 2791fps(M2.5)
Add 100%Rh(actual) at 2791fps(now M2.346), and local BC(actual) goes to .497
That's BC going down to .494 because M# dropped + .003 add for lower air density.
So in this case, if .500 was entered for ICAO BC, and 100% Rh was present but not entered, actual BC would be .003 lower than entered.
You're right Mozella. Compared to all the other abstracts, RH is not a big deal.
I get too hung up in numbers..
Add 100%Rh, and MACH1 = 1189fps, 73fps faster.
Multiply this error x2.5 for a bullet traveling at M2.5 ->
ICAO would be 2791fps
Add 100%Rh and M2.5 is 2974fps
183fps difference(73 x 2.5)
The difference that makes to calculated local BC (if humidity is not entered) ->
Beginning with a G1 ICAO BC of .500 at 2791fps(M2.5)
Add 100%Rh(actual) at 2791fps(now M2.346), and local BC(actual) goes to .497
That's BC going down to .494 because M# dropped + .003 add for lower air density.
So in this case, if .500 was entered for ICAO BC, and 100% Rh was present but not entered, actual BC would be .003 lower than entered.
You're right Mozella. Compared to all the other abstracts, RH is not a big deal.
I get too hung up in numbers..
Interesting. My numbers were based on the physics associated with aerodynamics and I'm guessing yours came out of a ballistics calculator. Am I right? Anyhow, our numbers match up nicely coming at it from two different directions.
I shoot in very high humidity right on the Gulf Coast and if you use 100% you won't be far off most days. But the biggest worry when it comes to ballistic precision is getting a yellow fly bite just as you pull the trigger. You know your fellow competitor didn't stab you with an ice pick 'cause ice picks don't hurt that much. A yellow fly bite will definitely effect the point of impact.
Well, if they take off before you slap them, they probably aren't Mississippi Yellow Flies. For some reason, these flies are either stupid or slow. I'm betting slow, because they are somehow smart enough recognize the back of your knees every time. But even after you feel the bite, you can still slap them and kill them before they are gone. At least there is that satisfaction.
Of course there are countless other kinds of biting flies, some of which are very fast and difficult to slap. But these particular Yellow Flies are easily killed by slapping. Could your pest be a Deer Fly?
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