[fargle]

> I guess there is a ton of knowledge about previous planes floating around. You don't do it from scratch, do you ?

Well, derivative designs bring their own problems. Also a double edged sword. Making significant mods to a complex system is often even more difficult. There's a lot of analogy with software and computing. Take 737-Max for example. Obviously boeing royally screwed that up - but mainly because their business people underestimated the difficulty and tried to patch over it and take short cuts. That itself is a hard problem: a commercial company does eventually have to make a profit.

And yes, absolutely there are new greenfield designs. If you need a Cessna 170, you don't usually start from scratch. But one reason to start from scratch is that you are pushing the envelope even further because of new materials, new requirements, etc. So more problems.