airplane propellers are twistied because they have two sources of velocity components ... the plane's speed (out of the plane of rotation) and the in-plane circular component (due to the spinning propeller).
helicopter blades are probably less twisted because they are (i guess) less sensitive to the velocity component due to the vehicle's motion.
I would expect that prop/rotor twist is so that the each slice of the blade from the hub out flies at an angle of attack that optimizes L/D, or at least that this is a consideration.
For a hovering helicopter and a prop in forward flight, your simple vector diagram will look the same.
If V[sub]infinity[/sub] is the same across the rotor/prop span, omega is the rotational speed, r the radius, I would expect to find r*omega/V[sub]infinity[/sub]to be a constant ratio across the span. I don't see how a duct would make much of a difference in this respect, except that the hub may be large relative the to the duct. In this case r[sub]tip[/sub]/r[sub]root[/sub] may be close enough to 1 that the twist is hard to notice.