I thought I'd add a bit about aircraft stability that hasn't really been discussed.

Dihedral effect - the effect of a sideslip causing a rolling moment in the opposite direction - is stabalizing. It is very important in the design of full-scale aircraft because it allows the pilot to control roll with the rudder if the aileron controls fail. However, high lateral stability means increased dutch-roll (The aircraft rolling and yawing back and forth at a given frequency). Therefore, selecting the dihedral/anhedral angle must be done as a trade off between lateral and dutch-roll stability.

Based on the original question posted, it looks like you want to perform a completely flat turn without using aileron mixing. To do this you would have to reduce the dihedral effect of the aircraft to zero, essentially making the aircraft neutrally stable. There are many factors that contribute to the dihedral effect of the aircraft. The primary contributor is the wing, of which three parameters contribute: dihedral angle, sweep angle, and wing placement on fuselage. The vertical tail also contributes a certain amount if its aerodynamic center is above the aircraft c.g.. So, to get zero dihedral effect you could design a mid wing aircraft with no wing or h. tail dihedral, with the v. tail extending above and below the aircraft c.g..

Or you could simply add wing anhedral until the net dihedral effect of the aircraft is zero. You can determine the dihedral effect through calculations (I suggest using Dr. Jan Roskam's books here) or you can do it through testing. If you put too much anhedral in, however, the aircarft will become unstable and a small sideslip will quickly turn into a huge sideslip angle. In addition to this, the rudder will cause a small rolling moment away from the turn directing if the rudder a.c. is above the aircraft c.g..

While, roll-yaw coupling may seem like a problem, it is one of the primary effects that makes aircraft controllable. All I'm saying is be careful what you ask for.

In terms of Britbrat's post about dihedral moving the aircraft aerodynamic center, that is completely false. Wing anhedral is destabilizing in the lateral mode, but has no effect on the longitudinal mode. I find that when we talk about aircraft stability, most people think of simply longitudinal stability and where the c.g. is relative to the a.c., but wing dihedral affects the lateral stability of the aircraft, which is just as important.