A curve line is always smooth by nature ;)
So your object is smooth by essence
As nurbs are tensile surfaces between curves your object is also naturaly smooth:)

Now you can change the form of the "profil" as you want

Or just select your middle line and use function Transform/ fillet with 0.3 by example
So jonction between 2 wings part will be softy :)
ps Michael show you a more elaborated process ;)