What factors determine the direction of the resultant force vector?
Yes, the resulting force is not perpendicular to the wing's chord but can point either way, slightly forward and backward. The very first polar charts which were published show this nicely, because those plots were true polar plots and used the wing chord as the reference orientation for 0°.



Illustration on page 196 of Otto Lilienthal's "Der Vogelflug als Grundlage der Fliegekunst" showing measurements on model wings as polar plots (source). Note the left part: Here the forces are plotted in the wing section's coordinate system.

The N/A relationship was covered by the work of Edward C. Polhamus, who did a lot of research on this at NACA Langley and published several papers with equations for calculating the leading edge thrust, the force which helps to tilt the resultant force vector forward.