This is the analysis that I have been attempting to generate here:
"First Thought: Most shooters think that wind drift is caused by the wind impacting on the side of the bullet and thus applies a force that pushes the bullet downwind. That idea is incorrect for a spin stabilized, statically unstable projectile i.e. bullets we fire from our rifles that have a gyroscopic stability factor greater than 1. If that is how you thought of wind drift then eliminate that thought from your mind because it is just plain wrong at any velocity or Mach number, for any bullet caliber, nose shape or base shape, whether there are grease rings, cannelures, or not, deep or shallow rifling marks, jacket surface roughness or any combination of those characteristics of a bullet that the human mind can think of.
Second Thought: Wind drift is caused by the air drag force on the bullet. It is that simple. That might or might not sound strange but it is absolutely correct. I know many of you know the shape of the drag coefficient curves versus Mach number. At high Mach numbers for centerfire rifles, say Mach 4, the drag coefficient, Cd, increases slowly as the Mach number is reduced. As the Mach number approaches 1, Cd increases more rapidly until it reaches a maximum just above Mach 1. As the Mach number decreases further, Cd drops off a cliff decreasing rapidly to about Mach 0.8 to 0.6 where is levels off. The drag force is equal to bullet velocity squared times Cd. Thus you can see that the force that causes wind drift will decrease dramatically as Mach number drops below 1.
So how does the drag force cause wind drift? A bullet is a gyroscope and when it exits the barrel into a cross wind flow the wind initially is impacting the bullet from a slight angle off the direction the bullet is heading. That creates a moment that the gyroscope resists by turning its long axis parallel to the oncoming air flow. If you are firing a bullet at 900 f/s and there is a wind of 30 f/s from 9:00 o'clock then from the bullets point of view the air flow is coming from a direction just 1.91 degrees from the left of the centerline of the bore axis. Thus the bullets long axis turns 1.91 degrees to the left fairly quickly while its center of gravity is moving straight ahead initially. The air flow is parallel to the long axis of the bullet and there is no air impact on the side of the bullet. Now, say the drag force on the bullet at one instant is 1 pound. The 1 pound force is acting point on to the bullet thus it is acting at an angle of 1.91 degrees from the direction the bullet center of gravity is traveling. Thus the wind drift force is the component of the drag force acting normal to the direction the bullet is traveling. Thus the wind drift force is tan(1.91 deg.)x1 lb. = 0.033 pounds. Now the wind drift force is constantly changing because the drag force is constantly changing and the angle the long axis of the bullet is pointing is constantly changing.
If any of you are pilots you know if you are flying east and there is a prevailing wind from the northeast, the attitude of your plane is slightly to the north of east but you are actually traveling east. It is the same situation for a bullet but in the case of the airplane the thrust is exactly equal to the drag and thus there is no wind drift. In the case of a fin stabilized rocket the same thing happens. The fins cause the rocket to turn such that the air flow is parallel to the long axis of the rocket but in the case of rockets the thrust is normally greater than drag and a negative wind drift occurs. That is........ the rocket drifts into the wind."
:grin:
