First off, this is not the common "how do I prevent spikes", "change response time" etc. I'm not working on a flight model for any aircraft, I'm just looking for knowledge.

I know that 1505 is involved in determining new N2 values from present ones. In other words, delta_CN2 is somehow calculated from dThr/dt and the current CN2. (dThr is the difference in throttle position)

Also, since dCN2/dt changes smoothly, I also assume there's a d2CN2/d2t involved.

Does anyone know the equation for calculating the next CN2 given dThr/dt and 1505?