ORIGINAL: paradigm
First of all, there is no such thing as aerodynamic dihedral. The direction of lift - up, down, or none - has nothing to do with dihedral effect. Wing, or horizontal tail dihedral adds lateral stability in the form of C_l_beta (or the derivative of rolling moment coefficient with respect to sideslip). When the a wing or h. tail has dihedral and is subjected to a sideslip, the lateral moving air creates a normal force on the wings. There is a component of this normal force upward on the right wing and downward on the left for dihedral and the opposite for for anhedral assuming the sideslip is positive (coming from the right). Therefore, the force couple created by dihedral will create a rolling moment to the left (away from sideslip - stabilizing), while anhedral will create a rolling moment to the right (into sideslip - destabilizing).
When I say that anhedral is always destabilizing, I mean in the lateral mode. As I said before, you could argue that adding anhedral increase the dutch-roll damping which increases directional stability, but that is always secondary to lateral stability.
And in terms of whether a horizontal tail creates a force upward or downward. For a longitudinally stable aircraft (c.g. is ahead of aircraft aerdynamic center) the horizontal tail will ALWAYS carry a downward force in straight and level flight.
In terms of stick fixed vs. stick free, you are correct in terms of stick-fixed being more stable longitudinally and directionally, but that doesn't mean that dutch roll can be ignored. Dutch roll is however much less restrictive in r/c aircraft because there are no passengers on board to get sick.
I was referring do dihedral effects in terms of positive roll correction with sideslip. A high-wing configuration with no geometric dihedral (or even slight geometric anhedral) still can show a positive roll stability effect. Semantics, perhaps.
According to Dr. Warren Phillips, horizontal tails do not have to lift negatively
at all times to avoid divergent static pitch stability. They usually do over most or all the approved CG envelop for most certificated light aircraft, but this is not absolutely necessary. There is a discussion of this very subject on and about page 351 of
Mechanics of Flight. For
efficiency, it is desireable for the tail to be generating zero lift at equilibrium. For meeting Part 23 certification requirements, the tail will usually lift negatively to provide the desired dynamic stability. So-called 3D models do not need (or want) positive dynamic pitch stability and can be designed for zero tail lift at equilibrium, and if the tail volume is great enough, a high degree of static stability
and control authority are achieved together. You can go to Amazon.com, look up the above book (by Phillips), and search inside to p. 351 to see what he has to say about it.
Also, I did not write that dutch-roll can or should be ignored. I
did write that stick-free considerations can be disregarded, since RC models do not operate stick-free.
(added qualification with emphasis)