What is the correct definition of "spanwise" and "chordwise" with a swept wing? What is the correct definition of &quot;spanwise&quot; and &quot;chordwise&quot; with a swept wing?  For example, in the context of &quot;spanwise airflow&quot; and &quot;chordwise airflow&quot;?
a) spanwise is from root to tip, generally following the quarter-chord line, and chordwise is the direction that gives the shortest distance from leading edge to trailing edge, very roughly perpendicular to the quarter-chord line.  I.e., roughly as illustrated in this related question (either diagram): Where is the spanwise flow? How does the span wise flow point the air towards the wingtip?
b) spanwise is perpendicular to the aircraft's longitudinal axis, and chordwise is parallel to the aircraft's longitudinal axis.
c) either of the above definitions could be considered to be correct, depending on the context
d) other
Not meant to be strictly multiple choice, those options are just offered as food for thought-- if one of the choices is almost right but needs some tweaking, please elaborate--
Among other things, the intent is to understand whether the diagrams linked in option a), exactly as drawn, really illustrate any net spanwise flow at all.
b) is a correct description of both .
&quot;Spanwise&quot; is perpendicular to the aircraft's longitudinal axis because it does not relate to the width of the wings, but to the span of the aircraft. which includes both the wings and the fuselage. Consider that the wings of most aircraft and all birds, don't really have a clear &quot;root&quot; where  the wing ends and the torso/fuselage begins.
&quot;Chordwise&quot; is parallel to the aircraft's longitudinal axis because that is the way the air flows. Any other angle would not make sense. It might be of interest on some specific airplanes, but otherwise the only thing that matters is the general airflow, which is parallel to the aircraft's longitudinal axis for cruise flight in most any aircraft.
Swept wings, be they aiming forward or aiming back, have a characteristic that can't be defined by either span or chord. The same goes for delta wings for which every chord over the span of the wing is different.