An unusual request

[Rasped]

Hello people

I'm a game developer, and I need help.

I need a "good enough" way to calculate bullet drag. The change in velocity of the bullet due to drag.
I have tried and tried to get my head around the math, but I just can't make it fit. It has to be reasonably accurate, but not 100%.

Preferably I need someone to really cut it out for me with a couple of examples. And since I am working in the Metric system(the game engine only supports that), it would have to be metric.

What i've been trying to do is something along these lines:
dv=-k/m*v^2 dt

dv being delta velocity in m/s
k a constant of some sort, Cd*A... this is the one I have the most trouble with.
m is mass in kg
v is velocity in m/s
dt is delta time in seconds

The problem is that this produces really small numbers for me, and the speed remains too high for too long(rougly 1 second to half the speed of an AK47 is about right I believe?)

Can someone tell me if my formula is all wrong?

I really just need a formula I can use on G1 and G7 bullets... Metric system preferred.

I really hope you guys can help me out.
Thank you, shoot straight.

 

[rduckwor]

Google Bryan Litz. Drink until you no longer thirst.

 

[nosualc]

go to http://www.jbmballistics.com/ballistics/bibliography/bibliography.shtml

or http://balcomp.sourceforge.net for the GNU ballistics source (SourceForge).

Hope that helps

-nosualc

 

[Joe Salt]

Man I wish I could have taken all that math back in HighSchool, Just think how much better I would shoot! I could have really won some wood. Bryan Litz is probably the go to guy on that stuff.

Joe Salt

 

[CharlieNC]

What do you mean by "good enough", for what purpose? If you checkout the suggested sources I think you will find there is not really a simple alternative that will give you accurate results, especially since the calculations are based on BC which is the measured performance of the test bullet vs a standard bullet.

 

[Rasped]

What do you mean by "good enough", for what purpose? If you checkout the suggested sources I think you will find there is not really a simple alternative that will give you accurate results, especially since the calculations are based on BC which is the measured performance of the test bullet vs a standard bullet.

Good enough is... I don't want to change the BC every update to have it match the BC for the speed... if I can use the same BC over the entire flight(the change in BC is not too high) and still get a reasonable result, I would consider that "good enough".

As for the other replies, thank you, I will take a good look at those.

 

[Toby Bradshaw]

What i've been trying to do is something along these lines:
dv=-k/m*v^2 dt

...

The problem is that this produces really small numbers for me, and the speed remains too high for too long

That's probably because small arms projectile drag functions increase as the square of speed only at (unrealistically) low velocities, but increase with considerably larger exponents at projectile velocities typical of small arms. By using the squared velocity in your equation you have greatly underestimated drag.

The advice to read Bryan Litz is sound.

 

[watercam]

When Litz posts measured BCs for the bullets that he shoots he gives and average BC that would work for your purposes. In other words he gives a BC for 3000 fps and one for 1500 fps and offers an average for computation purposes.

 

[Quinc]

http://www.shooterready.com/lrsdemo.html

If you could make this on the android (must be 99.9% accurate) I would buy it. =)

 

[rvn1968]

Quinc,that is one of my favorite sites. Tom