Wondering about what generates the spin drift? Curious to know more about the yaw of repose? In this article we will examine the reason behind this behaviors of the bullet in flight.
In Bullet Trajectory part 2, we found that spin drift is a deflection generated by the gyroscopic motion of the bullet. The direction of the deflection is toward the direction of the spin, and the amount of deflection is dependent on bullet length, flight time and, not surprisingly, spin rate. Its effect is noticeable, and must be taken into account, starting around 500yds. To make an example, for a 175gr SMK bullet, fired at 2700fps from a .308, with a barrel twist rate of 1:12”, the amount of spin drift would be about 1½ in at 500yds and about 9 in at 1000yds. But why does the bullet’s spin generate a deflection in the trajectory?
As we discussed in the previous article about dynamic stability, when it exits the barrel, the bullet wobbles because of gyroscopic precession. When the wobbling is dumped out, and the bullet is dynamically stabilized, the bullet longitudinal axis no longer points in the direction it’s travelling, but instead has a yaw angle, called yaw of repose (or equilibrium yaw), toward the direction of the spin. The bullet is actually skidding along the trajectory, with the center of gravity following the trajectory and the tip pointed to the right (for righteous spin bullets) and slightly upward. The incoming air pushing on the left side of the bullet causes it to drift.
The yaw of repose should be (I’ve found contrasting opinions about this) another result of the phenomenon of gyroscopic precession. Indeed, when an external force is applied toward the axis of spin of a spinning object, the object doesn’t react in the direction of the applied force, but perpendicularly, at 90° in the direction of the spinning. Once again, we can observe this phenomenon with our spinning top example. If you push the toy on one side while it’s spinning, it won’t react in the direction of your pressure, but at 90° in the direction in which the toy is spinning.
The bullet, as we have seen in the bullet shape article, tends to fly with its axis parallel to the line of departure, forming an angle (the angle of attack) with the axis of movement (that is, the trajectory). This means that the air flow is actually coming from below the projectile. The bullet, behaving like a weathervane, tends to point its nose in the direction of the incoming air. Because of the phenomenon described before, this downward force generates a yaw in the direction of the spin, resulting in the yaw of repose and, consequently, in the spin drift.
In the next article I’ll talk about a very important long range shooting topic: the passage of the bullet through the transonic region. The transonic region range is considered by many shooters as the limit range for an accurate shot. If you want to discover the reason, stay tuned!
