It will really depend on the aircraft. If it is a Euro, 8411 or similar as this is a huge rudder. The torque is really required to stop any flutter that might start if did not get the support right. I have heard of one EURO that ended up with rudder flutter and the model was almost lost. If it is a T33, a smaller servo could work. I would not consider anything less than an 8411(or similar for the Hitec, Multiplex, Futaba, etc fans!!!!!!!) on any flying surface on a turbine jet as this is a small price to pay if you look what the total investment is!

My Kangeroo flys with 4 8411's on the wings!

Zane