If I were making a flight sim in UE4 (let’s pretend it’s not a crazy idea for an indie game) these are the things I would do:
First thing you should do is nail down the scale - don’t worry if you are doing it right/wrong, you will have to redo everything anyways.
I don’t care about real world units. You are really looking for feel. Does the world feel big or small? Then adjust.
You can scale terrain actors. I would do this instead of tiled worlds at least for now. Start with one. You get that working perfect, then go for tiled worlds.
You don’t need crazy ground collision- this is a good thing.
Bad thing- you don’t really have any way of engineering your level for visibility except for distance culling (maybe later you can do something with a cloud layer, etc.)
Get your pawn feeling like your game. Attach a rough cockpit to the camera. Play with speeds. Fly around your level, tweak, tweak, tweak.
Stop worrying and just do. You will never ship this game, trust me, it’s about learning at this point. It’s like riding a bike. You won’t learn by trying to figure out all the things that will go wrong. Once you know the concept (peddle with your feet) you just have to do it.