I see no reason to have anything larger than a mini servo on the rudder of an airplane that size. An 8321 is a fantastic servo, but useless in that application unless you just need the weight. If it makes you feel better, a 9411 would have a little stronger gear train with it being a larger servo. My Axiome 70 has DS3517s on the elevator and ailerons and a DS3421 on the rudder. Once this white crap stops falling, I'll get out and fly it.