This is my preferred servo for the T-Rex 500 as well as for the T-Rex 450 tail. I have tried many different servos and this one so far has the best holding power and consistency as a tail servo for small helis. On the T-Rex 500, it handles the cyclic like no other servo with lots of torque and adequate speed.

Futaba also make a small digital servo (s9257). It works very well also, but I prefer the JR DS3400G as it was designed for 6 volts thus eliminating the need for a step-down when used as a tail servo.

If you fly hard 3D, you need a servo capable of holding the tail without hesitation. The 3400G does so flawlessly when used with the appropriate gyro.

Servo Specs:

Torque:42 oz/in @ 4.8V, 51 oz/in @ 6V
Speed:.11 sec/60° @ 4.8v, .09 sec/60° @ 6v
Dimensions (WxLxH):.58″ x 1.30″ x 1.02″
Weight:1.04 oz