ORIGINAL: Magne
You have to place the fins matching the Aerodinamic center of the fin with the CoG of the plane; otherwise you'll get an extra moment around the yaw axis...
But then again, all the T-canalisers I have seen have been mounted behind the C of G, and their purpose is to an extent the same as these Dorsal and Ventral Fins.
The aerodynamic centre of pressure is normally not the same whether the model is flying the right way up or in knife edge. Is it not the location of the TOTAL aerodynamic centre of pressure that matters, including all the stick-on bits?
Magne
I think I see what you're saying. Having the fins away from the aerodynamic centre isn't necessarilly a bad thing. Just think of the stab or fin, the moment they produce is used to counter other moments and add stability. But these fins are designed to add lift in knife-edge, not for stability. By having them on the CoG they won't change the attitude (yaw) of the aircraft at different fin angles of attack (sideslip angles). All they do is add lift.
By having them say behind the CoG, you're actually adding in a sense to the size of the fin at the rear of the aircraft as it's adding stability as well as side area. In fact, if you were to put these fins far behind the CoG, so that the moment they produce during a side slip is large, you would have to increase the size of the rudder or it's throw. This is because the rudder will have to 'fight' the extra stability provided by the fins, which may be counter-productive for pattern.