Who says N stands for Naval? Especially with regard to airplanes.
As I said, with aircraft, the first letter represents the country, the second letter (if any) indicates what kind of aircraft (C for Commercial, X for Experimental, etc.)
For the purposes of this project, I'm going with what I think is the best interpretation of what we saw in the various Star Trek shows.
These registry codes do not translate into anything, any more than the real life counterparts do.
N = Federation registry
AR = Earth, Civilian Research
SP = Vulcan, Science
X = Experimental Prototype
CC = Starfleet Starship
CV = Starfleet Timeship
I'll probably cook up more as I feel they're needed.