This has appeared on a few forums, mainly in the UK, so some of you may have seen it before. I have tried to make it as simple and none technical as possible.
Quite often you will see someone mentioning stability, talking about how pellets go unstable as they slow down or are more accurate because they are more stable. But what is stability with regard to pellets, and how does it affect how a pellet flies? This post does not describe the actual mechanism for stability on a pellet, only the different types of stability and how they affect the pellet.
A stable pellet is not one which comes out of the barrel and keeps pointing in the same direction. A stable pellet is one which comes out of the barrel and attempts to point directly into the airflow. As the pellet flies along its trajectory, the direction of the airflow will change due to winds and the pellet being pulled towards the ground, but a stable pellet will try to change the way in which it is pointing to stay with the airflow (Figure 1).
Fig 1
An unstable pellet will not point in the direction of the airflow and may eventually tumble (Figure 2).
Fig 2
When a pellet is not facing into the airflow we say it is at an angle of yaw (Figure 3) and pellet stability is all about trying to control and reduce that angle of yaw.
Fig 3
There are two basic types of stability working on any projectile as it flies through the air. These are static and dynamic stability.
To try to explain the difference between the different types of stability, think about a weight hanging on the end of a piece of string (figure 4). If you do not touch the weight, it will just hang down under the string. This is its original position, the position it likes to sit in. This is equivalent to a stable pellet pointing directly into the airflow at zero yaw angle. If you pull the weight slightly to one side and let it go, if it is stable, it will swing back towards its original position. This is because the forces and moments produced by the weight and the string are trying to push the weight back to its original position. If the weight and string were an unstable system, then as soon as we release the weight, it would move away from its original position. This type of stability is called static stability.
Fig 4
When we have pulled our weight to one side and let it go, if it is stable it will move back towards its original resting position, but the first time it reaches its resting position, it does not stop but goes on the other side until it eventually stops and then returns towards its original position from the other direction. Our weight will keep doing this with each swing getting a little smaller until it eventually stops back in its original position. It does this because the weight and string are dynamically stable, so the size of the swing reduces each time until there is nothing left. If the weight had neutral dynamic stability, it would keep on going, with each swing being the same size as the one before. If the weight and the string were dynamically unstable, the swings would get bigger until eventually the weight would go in a complete circle even though it is statically stable.
As far as pellets are concerned there are actually three types of stability acting on them and affecting their flight as there are two types of static stability. If we just think of normal dome type diabolo pellets they are what we call aero/gyro stabilized, that is they rely on aerodynamic and gyroscopic methods for static stability. The third type is dynamic stability, which is needed to stop the pellet continuously yawing about its zero yaw position as it flies along the trajectory.
When we fire a pellet, it is highly unlikely that it will be pointing exactly in the direction of the air flow after it has left the barrel. This is due to many things including wind, barrel vibrations, pellet manufacturing problems etc. so it will usually have a yaw angle soon after it has left the barrel. The diagrams below illustrate the effects on the yaw angle for the different stability states. In each case, the vertical value is the angle of yaw in degrees and the horizontal is the range in yards. First (figure 5) we have a pellet which is both aerodynamically and gyroscopically unstable. That is, it is statically unstable.
Fig 5
Here, the pellet yaw angle will just increase until the pellet eventually faces backwards and tumbles.
Next (figure 6) is a pellet which is statically stable but dynamically unstable.
Fig 6
In this case the yaw swings through zero, but each swing gets bigger, and the pellet will eventually go sideways. Next (figure 7) is a pellet which is statically stable but dynamically neutrally stable. Here, the swings of the pellet through zero yaw are always the same.
Fig 7
Last (figure 8) is the situation we want, which is a pellet which is stable both statically and dynamically. The pellet swings through zero yaw, and each swing is smaller than its predecessor.
Fig 8
In reality most conventional pellets appear to be aerodynamically statically stable at speeds well below the speed of sound (1116.5ft/sec), that is, if we fired one from a smooth bore barrel it will continue to point in the direction of the trajectory. However, its aerodynamic static stability is marginal and at high speeds disappears completely. Also, no pellet, or any other projectile, is made completely symmetrical and any differences from one side of the pellet to the other will produce an aerodynamic side force which will cause it to try to fly on a curved path. To reduce the effects of any projectile asymmetry, pellets and most other aerodynamically statically stable projectiles are given some spin so that any side force is not pointing in the same direction all the time. This will make the pellet wobble a bit, but will not produce the curved flight path. The spin rate needed for this is very low, much less than is needed for gyroscopic stability.
Most barrels give pellets much higher spin rates than those needed to reduce side force effects. This is because, with the marginal aerodynamic stability, a degree of gyroscopic stability in addition to the aerodynamic stability is beneficial. This is why we say they are aero/gyro stabilized.
From the reported behaviour of pellets, they would seem to have pretty much neutral dynamic stability, possibly changing to dynamic instability if fired at high speeds for long ranges. The change to dynamic instability is probably due to the increase in pellet spin rate relative to the pellet forward speed as the pellet flies along its trajectory. This apparent increase in spin rate is due to the pellet losing forward speed much quicker than it loses spin, until it causes the pellet to become dynamically unstable. It appears to be the dynamic instability produced by the excess spin rate, which may lead to apparent spiralling and accuracy effects at longer ranges or at higher speeds. The pellets are still statically stable, in fact the gyroscopic stability has increased, but the dynamic instability is adversely affecting the pellet flight.
So next time you are shooting, just pay those little pellets some respect and marvel at the way they still manage to go through all the complications of the different stabilities and still hit a small target. Or forget about all the science and just get on and enjoy your shooting.
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