ORIGINAL: Skylane
With the center of pressure on the rudder above the aircraft CG there is an adverse roll induced by rudder deflection. That is, right rudder puts a force on the tail which pushes to the left. But, it pushes above the centerline of the fuselage so that there is an induced roll to the left. Right rudder > left roll and vice versa. Dihedral, on the other hand, induces a proverse roll. In the case of a yaw to the right, the dihedral induces a right roll. So, the idea is to balance the adverse roll induced by the rudder with the proverse roll caused by the dihedral. If the two are matched correctly, you get a pure yaw with no roll either way. That is the goal. There are clear interactions between the amount of dihedral and the design of the rudder. Also, as mentioned in another post, there can be an interaction with the anhedral in the stab (if any).
Jeff
Nicely put Jeff.
There has been some discussion on the subject at least in two or three other threads, one of them somewhat current. The theme is generally on the interaction and requirement of increased wing dihedral in models which have pronounced stab anhedral. Models with significant anhedral such as the Curare and the Tiporare have been discovered to require large amounts of wing dihedral for a more neutral pattern plane. On the other hand, models such as the Aurora (and several other Japanese designs) which have zero stab anhedral (or dihedral for that matter) and a more concentric wing position (i.e., placed closer to or on the thrust line) tend to have very little wing dihedral with a frontal design resembling that of the third diagram in PJ's post. Given the more symmetric frontal and lateral planform of the model, the airplane is presumably more neutral in either upward or inverted flight.
It has been mentioned in construction articles of models designed around the same time as the Curare (e.g., the Deception) that stab anhedral is to be avoided at all costs in a model designed around a flat top wing (i.e., the second drawing in PJ's sketch). No doubt a Deception with the typical flat top wing (as many American classics had at the time) would fly rather erratically with a stab built with considerable anhedral.
In short, how much dihedral is introduced in the construction of an SPA wing is dictated by the overall planform and moments of the model. Reducing dihedral might result in a more neutral model with similar flight characteristics whether upright or inverted but the model might also be inherently more unstable. Building a Curare wing with a flat top while retaining the pronounced stab anhedral turns out to not be such a good idea as tested and discovered by Steve.
PJ, what model do you have in mind for SPA?
David.