Pitch Change Verses Flap Change:
Delta3 angle of +0º. ~ Pitch angle is not changed.
Delta3 angle of +1º to +44º. ~ Pitch angle change is less than flap angle.
Delta3 angle of +45º. ~ Pitch angle change equals flap angle.
Delta3 angle between +46º and +89º. ~ Pitch angle change is greater than flap angle.
Positive and Negative delta3:
"Note that positive coupling δ3 >0 represents negative feedback, decreasing the blade pitch for a flap increase." ~ [Source ~ HT p.239]. This is the common usage.
Re tandem configuration "..speed stability ...... differential pitch-flap coupling (positive on the front rotor and negative on the rear rotor)." ~ [Source ~ HT p.850] This implies that positive delta3 causes an increase of the flap to decrease the pitch and negative delta3 causes an increase of the flap to increase the pitch. In other words, if the reference flapping or teetering hinge is normal to the span of the blade, then rotating this hinge in the opposite direction to that of the rotor's rotation will result in positive delta3. Conversely, rotating this hinge in the same direction as that of the rotor's rotation will result in negative delta3.
To me, having upward flap increases the pitch sounds scary, since this increase in pitch will want to increase the flap even more.
Dave