**Game Version number:** 1.6.0.54
**Modded?:** No
**Issue Description: **
Projectile wind drift calculation //seems// to have a fundamental physics error.
This will make not only the amount of wind drift incorrect, but also the nature of wind drift incorrect. Therefore wind drift will not match real life firing tables.
The problem is that wind drift currently seems to be modelled as the wind pushing on the side of the bullet (wrong!). The drag area of the side of the bullet seems to be approximated by 10x the frontal air drag ("Side Air Drag Scale"), as seen here:
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But the wind does not blow on the side of the bullet! That might be true for a stationary bullet, but airflow is relative, and a fired bullet is in motion.
When you combine the lateral component of airflow (for example, 15 m/s perpendicular wind) and the longitudinal component (for example, an 850 m/s muzzle velocity), you get an airflow that is coming almost straight back. The bullet's speed is so much higher than the wind's speed, that this means the airflow does not "see" the side of the bullet. See below:
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In this video, both the longitudinal and lateral components of airflow are shown, followed by the result when they are combined, the direction of the overall airflow (exaggerated). As you can see, it is acting mostly on the front of the bullet, rather than the side:
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But in addition to this, since the bullet is both aerodynamically and gyroscopically stable, this means the bullet also points into this airflow almost instantaneously, ie, in the direction of the wind (which to the bullet, is a very small angle).
This means that the "side area" is irrelevant for calculating the wind drift of the bullet. The frontal area is used, ie the "air drag" or "airFriction" value in ARMA, in ballistic calculators, the "ballistic coefficient". We do not need another value to find the wind drift.
Note that although it points into the airflow, the bullet does not fly in this direction because it has no longer has a propelling force (unlike an RPG-7 rocket).
However, since the airflow is now pointed off the firing axis by a small amount, the bullet's aerodynamic drag is now angled by a small amount. And that means the bullet is "dragged" off the centreline, which is what causes "wind drift".
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I did a quick test of what happens in game. Firing at a crate 301 m away with an M16A2, with a perpendicular wind of 15 m/s, according to real-life ballistic calculator, the wind drift should be approximately 97.8 cm.
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However the wind drift encountered in game was only about 21.6 cm, which is 4.5x less than the expected value. And consistent with the physics error described above.
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For example:
If I fire at M16A2 with a muzzle velocity of 930 m/s, and a full value crosswind of 5 m/s, the bullet velocity vector is still on the original firing axis, however the bullet's longitudinal orientation will be in line with the airflow, meaning off centreline by arctan(5/930) = 0.308 degrees, therefore the bullet's drag vector will be off centreline by the same 0.308 degrees, and the horizontal component of the bullet's drag will be tan(0.308 deg) = 0.00538 = 0.538% of the total bullet drag.
The way geometry works out means a bullet's wind drift can be calculated by the following equation:
D=(T-(R÷V))×W
Where D is the lateral wind drift, W is the lateral wind speed, R is the range, V is the muzzle velocity and T is the time of flight. So the wind drift is proportional to T-(R÷V).
See:
McCoy, R. L., Modern Exterior Ballistics, 2nd ed. Schiffer, 1998. pp 157-164
Litz, B., Applied Ballistics for Long Range Shooting, Applied Ballistics LLC, 2011. pp 68-70
Leupold, H. A., Wind Drift of Projectiles: A Ballistics Tutorial (ARL-TR-1124), Army Research Laboratory,1996.