Changing local axis / Rotating around a line

I’m modeling a 737 200 series, and I’m happy with my model and now I’m going to add shape keys or a single bone for the flaps, rudder and stabilizer. I want to rotate the rudder around the selected line in the image. I’m a little hazy on how to either change my local axis, or whether I can just explicitly state the the line as the rotation axis, through a vertex group or something. I Googled for about an hour an most of the explanations seem outdated. What’s my best approach here?

What do you actually want, just a still or for animation ?
For a still you can select an edge on the line and create a custom transform

Or add a bone aligned to the edge and rotate the bone around its local axis under “Custom orientations”

EDIT: Woops, too bad “Post Quick Reply” doesn’t have preview option :smiley:

That’ll work. Thanks. And it’s for an animation.