Two arms model
The simple two arms model adequatly describes the yawing motion
of spinstabilized bullets, resulting from the action of all aerodynamic
moments.
The yawing motion can be understood as a superposition of a fast
and a slow oscillation, often called nutation and
precession.
Imagine to look at the bullet from the rear. The slow mode arm
CG to A should be hinged at the CG and rotates with the slow mode frequency.
Consequently A moves on a circle around the CG (the red circle).
The fast mode arm A to T, where T is the bullet´s tip, should be
hinged at A and rotates with the fast mode frequency. Thus, T rotates
on a circle about A. CG to T is the projection of the bullet´s longitudinal
axis.
An animation of the two arms model is shown in the figure below.
One additionally has to consider that the fast mode frequency exceeds
the slow mode frequency (which is true in the animation) and the arm lengths
CG to A and A to T are continuously shortened (which is not true
in the animation) if the bullet is dynamically stable.
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