You would have to see the interface to understand, and I'll post a screenshot, but you could basically have one Aircraft Projects forum, then prefixes of, say, Pre-war and Post-war, and then at the top you can select to view all, just pre-war, just post-war. You could even merge all "Projects" into one forum, then have prefixes of "Aircraft", "Missile", "Army",Navy","Space".
I feel the current forum is too split into tiny parts, so I'm experimenting with different middle grounds between Facebook group style mess of everything in one place, and the plethora of separate sections we currently have, some of which are rarely visited.