ORIGINAL: shoey721
Here's what we're doing on our new 37% Yak Pull Pull Rudder, for what it's worth.
We've studied the Ackerman principle, and studied the SWB diagrams. We're using a slightly modified "Example B" from SWB. We have an offset SWB Servo Arm which is about 4 1/4 inches across from cable to cable. It is offset 5/16 of an inch to the rear. On the rudder we have a Control Horn which measures 4 3/4 inches across from cable to cable. Cables will be crossed. The modifier will be: The control horn is offset to the rear about 1/8," per Mr. Ackerman, to help keep both cables under tension all the time. We made the control horn slightly longer than the servo horn so that we can observe the amount of rudder throw we will get, and then be able to "dial it in" by rotating the connectors on the ends inwards for more throw.
I think you'll find the offset of the control arm rearward is not required. Ackerman has been introduced by the servo arm in SWB example "B" (5/16") adding another 1/8" will bring the total calculated rear offset to 7/16". The SWB example is calculated for a specific amount of rearward offset based on arm dimensions and the rudder control arm being centered over the hinge line..
Your proposed arm lengths 4.25-4.75 @
60* servo angle will net 50* rudder throw.
50* servo angle will net 43* rudder throw.
Your unequal arm lengths may create a problem with slack too. A calculated fixed value will minimize slack, any other value will provide for same.