Except for Nytrunner... "crickets"
Its a shame because I figured out what RockSim calls Fn (Normal Force)... "Pitch Force" and it follows the AOA like is should, but notice the kink in the line at exactly where CNa stops being a constant and it appears to always be ~473 ft/s no matter which simulation is run
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I am not familiar with details of any of the popular sim tools. What this looks like, is some kind of compressibility correction applied to CN when the Mach number increases. At V=473 ft/sec you are around Mach 0.42, the Cn (and Cd-drag) compressibility corrections are applied gradually when you pass Mach 0.3, not just an on-off switch when you hit Mach 1. (The sim might be using the theoretical Prandtl-Glauert rule, perhaps?)
What is more concerning are the oscillations in the Pitch force (is really a moment or a torque that rotates the model about the c.g.) and wind angle of attack. If you get a wind angle of attack, you want the pitch force to restore you, and that is good static stability. But you don't want a pendulum effect where the pitch force and angle of attack continue to oscillate, you want dynamic stability. It appears that the oscillation does eventually damp out, but you might want to make some design changes to increase stability so that oscillation does not extend so far or the frequency of it slows down. That sim curve looks like the rocket is going over a washboard road. The sim curve is technically stable, but I would be curious as to the caliber measure of stability. Was there some wind event, or is the speed too slow off the rod that the rocket tilts? I don't know what you have going on in that particular sim to introduce those pitch oscillations.
I am not sure of the notation they are using, CNa might be the slope of the CN versus alpha curve, assuming a linear straight line near the low-alpha range. At zero-alpha, you have zero force, so CN would be zero at zero alpha, but the slope of the line from zero alpha to a small alpha < 10-deg would basically be constant.
Back to your original question, CN is used to help compute the location of the CP. The typical measure of stability for model rockets is the distance between the CG and the CP. When that distance is divided by the diameter of the rocket, this is called calibers. For a 1" dia rocket with 1-caliber stability, the CP is located 1" aft of the CG, for example. You want the Center of Gravity in front of the Center of Pressure. If you use the cardboard cutout method, then you are basically computing the CP location for an angle of attack of 90-deg, which is usually conservative. Barrowman equations compute CP location for a small alpha near zero degrees I think, right?
https://www.rocketmime.com/rockets/Barrowman.html
https://apogeerockets.com/education/downloads/Newsletter238.pdf