https://wiki.blender.org/index.php/User:Fade/Doc:2.6/Manual/3D_interaction/Transform_Control/Pivot_Point/3D_Cursor i had that same problem. you have to set the 3d cursor to the selected object (the rotor hub) using the hotkey shift s, then you use the hot key . to set that as the pivot point, and then you can duplicate and rotate along the z axis (i think z..)
