addressing red, green or blue planes for rotation I succeed now thanks of this illustrative post (https://sites.google.com/site/sketchupsage/sages/gully-foyle/draw-vertical).
Now what to do if the object to rotate is not in an orthogonal direction and is to rotate perpendiculary to the plane of this object which is not orthogonal either? Like the wings of a fly or a rotor blade of an helicopter…
Seems to be a little “academical”. The solution could teach a lot of SU strategies.
My workaround -not elegant at all- is to raise lines for orientation.