Forces and moments

The shadowgraphs have shown that the flowfield in the vicinity of a bullet most generally consists of laminar and turbulent regions. The flowfield depends in particular on the velocity at which the bullet moves, the shape of the bullet and the roughness of its surface, to mention just the most important factors. The flowfield obviously changes tremendously, as the velocity drops below the speed of sound, which is about 1115 ft/s (340 m/s) at standard atmosphere conditions.

The mathematical equations, by means of which the flowfield parameters (for example pressure and flowfield velocity at each location) could be determined are well known to the physicist. However, with the help of powerful computers, numeric solutions to these equations have been found up to now for very specific configurations only.

Because of these restrictions, ballisticians all over the world consider bullet motion in the atmosphere by disregarding the specific characteristics of the flowfield and apply a simplified viewpoint: the flowfield is characterized by the forces and moments affecting the body. Generally those forces and moments must be determined experimentally, and this is done by shooting experiments and wind tunnel tests.

Generally, a body moving through the atmosphere is affected by a variety of forces. Some of those forces are mass forces, which apply at the CG (center of gravity) of the body and depend on the body mass and the mass distribution. A second group of forces is called aerodynamic forces. These forces result from the interaction of the flowfield with the bullet and depend on the shape and surface roughness of the body. Some aerodynamic forces depend on either yaw or spin or both. A summary of the most important forces affecting a bullet's motion through the atmosphere is shown in the table. However, the following discussion will be restricted only to drag, lift and the Magnus force.

Table: Forces, affecting a bullet's movement through the air

Remarks:

(1) The acceleration of gravity depends on the degree of latitude.

(2) The Coriolis force and the centrifugal force automatically arise from the fact that the earth is not resting, but rotates about an axis.

(3) The name lift suggests an upward directed force, which is true for a climbing airplane. However, the direction of the lift force depends on the orientation of the yaw angle (see later). Thus a better word for lift force could be cross-wind force.

Wind force and overturning moment

Now let us consider the most general case of a bullet having a yaw angle. By saying so, the ballistician means that the direction of motion of the bullet' s CG deviates from the direction into which the bullet's axis of symmetry points. Innumerable experimental observations have shown that an initial yaw angle is principally unavoidable and is caused by perturbations like barrel vibrations and muzzle blast disturbances.

For such a bullet, the pressure differences at the bullet's surface result in a force, which is called the wind force. The wind force seems to apply at the center of pressure of the wind force (CPW), which, for spin-stabilized bullets, is located in front of the CG. The location of the CPW is by no means stationary and shifts as the flowfield changes. The figure schematically shows the wind force F₁, which applies at its center of pressure CPW.

As shown in the following figure, it is possible to add two forces to the wind force, having the same magnitude as the wind force but opposite directions. If one let those two forces attack at the CG, these two forces obviously do not have any effect on the bullet as they mutually neutralize.

Now let us consider the two forces F₁ and F₂. It can be shown that this couple is a free vector, which is called the aerodynamic moment of the wind force or, for short, the overturning moment M_W. The overturning moment tries to rotate the bullet around an axis, which passes through the CG and is perpendicular to the bullet's axis of form, just as indicated in the figure.

We want to summarize: The wind force, which applies at the center of pressure, can be substituted by a force of the same magnitude and direction plus a moment. The force applies at the CG, the moment turns the bullet about an axis running through the CG.

This is a general rule of classical mechanics (see any elementary physics textbook) and applies for any force that attacks at a point different from the CG of a rigid body.

You may proceed one step further and split the force, which applies at the CG, into a force which is antiparallel to the direction of movement of the CG plus a force, which is perpendicular to this direction. The first force is said to be the drag force F_D or simply drag, the other force is the lift force F_L or lift for short. Obviously, in the absence of a yaw angle, the wind force reduces to the drag.

So far, we have explained the forces, which compose the wind force and the overturning moment, but we haven' t dealt with their effects.

Drag and lift apply at the CG and simply affect the motion of the CG. Of course, the drag retards this motion. The effects of the lift force will be met later.

Obviously, the overturning moment tends to increase the yaw angle, and one could expect that the bullet starts tumbling and become unstable. This indeed can be observed when firing bullets from an unrifled barrel. However, at this point, as we consider spinning projectiles, the gyroscopic effect comes into the scene, causing an unbelievable effect.

The gyroscopic effect can be explained and derived from general rules of physics and can be verified by applying mathematics. For the moment we simply believe what can be observed: due to the gyroscopic effect, the bullet' s longitudinal axis moves aside into the direction of the overturning moment, just as indicated by the arrow in the figure.

As the global outcome of the gyroscopic effect, the bullet's axis of symmetry thus would move on a cone's surface, with the velocity vector indicating the axis of the cone. This movement is often called precession. However, a more recent nomenclature defines this motion as the slow mode oscillation.

To complicate everything even more, the true motion of a spin-stabilized bullet is much more complex. A fast oscillation superposes the slow oscillation. However, we will return to this point later.

Magnus force and Magnus moment

Generally, the wind force is the dominant aerodynamic force. However, there are numerous other smaller forces but we want to consider only the Magnus force, which turns out to be very important for bullet stability.

With respect to the figure, we imagine to look at a bullet from the rear. Suppose that the bullet has right-handed twist, as indicated by the two arrows. We additionally assume the presence of an angle of yaw. The bullet's longitudinal axis should be inclined to the left, just as indicated in the previous drawings.

Due to this inclination, the flowfield velocity has a component perpendicular to the bullet's axis of symmetry, which we call vn.

However, because of the bullet's spin, the flowfield turns out to become asymmetric. Molecules of the air stream adhere to the bullet's surface. Air stream velocity and the rotational velocity of the body add at point B and subtract at point A. Thus one can observe a lower flowfield velocity at A and a higher streaming velocity at B. However, according to Bernoulli's rule, a higher streaming velocity corresponds with a lower pressure and a lower velocity with a higher pressure. Thus, there is a pressure difference, which results in a downward directed force, which is said to be the Magnus force F_M (Heinrich Gustav Magnus, *1802, 1870; German physicist).

This explains, why the Magnus force, as far as flying bullets are concerned, requires spin as well as an angle of yaw, otherwise this force vanishes.

If one considers the whole surface of a bullet, one finds a total Magnus force, which applies at its center of pressure CPM (see figure). The center of pressure of the Magnus force varies as a function of the flowfield structure and can be located behind, as well as in front of the CG. The magnitude of the Magnus force is considerably smaller than the magnitude of the wind force. However, the associated moment, the discussion of which follows, is of considerable importance for bullet stability.

You can repeat the steps that were followed after the discussion of the wind force. Again, you can substitute the Magnus force applying at its CP by an equivalent force, applying at the CG, plus a moment, which is said to be the Magnus moment M_M. This moment tends to turn the body about an axis perpendicular to its axis of symmetry, just as shown in the figure.

However, the gyroscopic effect also applies for the Magnus force. Remember that due to the gyroscopic effect, the bullet's nose moves into the direction of the associated moment. With respect to the conditions shown in the figure, the Magnus force thus would have a stabilizing effect, as it tends to decrease the yaw angle, because the bullet's axis will be moved opposite to the direction of the yaw angle.

A similar examination shows that the Magnus force has a destabilizing effect and increases the yaw angle, if its center of pressure is located in front of the CG. Later, this observation will become very important, as we will meet a dynamically unstable bullet, the instability of which is caused by this effect.

Two arms model of yawing motion

We have now finished to discuss the most important forces and aerodynamic moments affecting a bullet's motion, but so far we haven't seen how the resulting movement looks like. For the moment we are not interested in the trajectory itself (the translational movement of the body), but we want to concentrate on the body's rotation about the CG.

The yawing motion of a spin-stabilized bullet, resulting from the sum of all aerodynamic moments can be modeled as a superposition of a fast and a slow mode oscillation and can most easily be explained and understood by means of a two arms model (see reference [4]).

Imagine to look at the bullet from the rear as shown in the figure. Let the slow mode arm CG to A rotate about the CG with the slow mode frequency. Consequently point A moves on a circle around the center of gravity.

Let the fast mode arm A to T rotate about A with the fast mode frequency. Then T moves on a circle around point A. T is the bullet's tip and the connecting line of CG and T is the bullet's longitudinal axis.

This simple model adequately describes the yawing motion, if one additionally considers that the fast mode frequency exceeds the slow mode frequency, and the arm lengths of the slow mode and the fast mode are, for a stable bullet, continuously shortened.

With respect to the figure imagine to look at a bullet approaching an observer's eyes. Then the bullet's tip moves on a spiral-like (also described as helical) path as indicated in the drawing, while the CG remains attached to the center of the circle. The bullet's tip periodically returns back to the tangent to the trajectory. If this occurs, the yaw angle becomes a minimum.