The basic issue is that polygons have only one visible face. Flip a polygon over you don't see it. Sometimes a setting on a program can make a polygon APPEAR to be visilble on both sides, but that's an illusion, once imported into fsx, you will only see one side of the poly.
The easiest way to model the inside of your cylinder is to copy (clone) the existing polys, and then "invert" them. Now you've double the number of polys, with half facing out, and half facing in.
That's odd - I replied to your post on Saturday, but my reply seems to have disappeared !
Anyway, I followed your guidance, and my cylinders now look fine, inside and out. Easy when you know how ! I'd spotted that 'flat' polygons only have one visible face, just hadn't made the connection with cylinders etc.
1. make a cylinder. with or w/o the ends and for now let's make sure it is Y orientation. btw- give this part a name, not just cylinder. let's say ... pipe.
2. map your texture you have chosen front and back.
3. do a copy/paste - you now have pipe and pipe.1 AND your current part is pipe.1, and it has the texture also.
4. go to poly mode and then choose poly on your top menu and choose flip all selected.
BINGO - you now have a pipe with the inside also!!!!
extra- I strongly suggest ( your current part is still pipe.1........... hit scale button - the black X and make the pipe.1 ever-so-slightly smaller than the outside. Can explain later.
Now- if you want to.......... hit your spacebar the hit back arrow for previous part and hit spacebar then hit the J key on keyboard. This joins the two parts!!!!!
Stick a fork in ya.... you're done! btw- connect with me and others I our "family" on Skype. We help each other a ton! I am Robert Lacy on Skype.... If you do a search a ton of us will show up. I AM THE ONE IN IONE, WASHINGTON. aka jyarddog Notice I gave you step by step right? I am a retired teacher.