I presume you are talking about the 47in Edge.

You must have a 70 oz in or better servo on the rudder, IMHO digitals are now so cheap if you are buying servos buying analog is a waste of money. Most pilots will notice a difference in how crisp the control response is with decent digitals.

I use Futaba and the servo no I am using for 3D is S3152. Running on either 4.8 or 6v would be OK for you.

You can find 3152s under $30 if you look around.