Opps!! I was wrong.
The last four lines of the previous post, as quoted below, were OK. They explain the relationship (ratio) between pitch and flap.
"Delta-3 angle of 0º. ~ Pitch angle is not changed.
Delta-3 angle of 1º to 44º. ~ Pitch angle change is less than flap angle.
Delta-3 angle of 45º. ~ Pitch angle change equals flap angle.
Delta-3 angle between 46º and 89º. ~ Pitch angle change is greater than flap angle."
But, the first two lines of the post, qualifying 'positive' and 'negative', were very wrong.
The following two quotations discuss positive and negative delta3.
[Source ~ Helicopter Theory p.239] ~ "Note that positive coupling [delta3 >0] represents negative feedback, decreasing the blade pitch for a flap increase."
[Source ~ me
] ~ This is the common usage.
[Source ~ Helicopter Theory p.850] ~ Discussion about speed stability on the tandem configuration "..... differential pitch-flap coupling (positive on the front rotor and negative on the rear rotor)."
[Source ~ me
] ~ This implies that positive delta3 will result in upward flapping causing the pitch to increase, and negative delta3 will result in upward flapping causing the pitch to decrease. 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 negative delta3. Conversely, rotating this hinge in the same direction as that of the rotor's rotation will result in positive delta3.
Since "positive coupling [delta3 >0] represents negative feedback"
, it appears that any discussion of 'positive' or 'negative' must depend on whether one is talking about the direction of rotation of the delta3 angle or talking about the feedback resulting from the delta3.
To me, having upward flap increase the pitch sounds scary, since this increase in pitch will want to increase the flap even more.