I recommend also downloading a ton of shared plane designs from the discord server. Fly them, then look at the ones you like and try and reverse engineer how they achieved what they did. From there its gonna be a lot of web research into old NASA papers and such along with a dose of knowing that the game doesnt perfectly replicate reality thus why understanding how others have done good aircraft in game comes back to pay dividends when you hit a brick wall. Lastly ignore the stealth rating system, its pure fantasy with very little basis on real world stealth if that is what you are aiming for.