The tail servo for a helicopter needs to be very fast to respond quickly to small random tail movements. The tail servo should have a specification of 0.12 sec / 60 degrees of travel or better.
For the Corona, Logo 10, and ECO 8/16, some usable tail servos are:
JR DS368 (do not use in DS mode, will burn out)
Volz Speed-Maxx XP (DS mode compat, will be released about Oct 2003)
Multiplex Micro BB Speed (DS mode compat)
Hitec HS-5245MG (DS mode compat)
Hobby Electonics HDS-577 (DS mode compat)
The Multiplex Micro BB Speed is slightly larger than the HS-81/DS368 so some modification may be required to use it.
For the Logo 10 or larger, the Futaba S9253 is very popular used with the GY401, although the S9253 is rather heavy for the Logo 10.
Some servo mounts will allow you to use high-speed micro servos for the tail pitch servo, such as the Precision Model Products tail servo mount for the ECO 8/16.
For micro helis such as the Hornet, Shogun, and T-rex which use tail servos, some usable tail servos are:
Hobby Electronics HDS-877 (DS mode compat)
The Hitec HS-50 is not recommended for a tail servo because it has excessive slop in the gears and may cause tail wag when used with a heading hold gyro."
The metal gear servos are not recommended for the tail because the metal gear servos wear faster and have more backlash than plastic gear servos, which will result in less precise tail control (with the exception of the Volz Speed-Maxx XP which is specifically designed for tail pitch control)
The slower digital servos (such as the JR DS368, 0.21 sec/60 degrees) do not work well as a tail servo unless the stock servo arms are replaced by extra-long servo arms (Du-bro, Servo City, etc). This sacrifices some precision for extra speed.
JR does not recommend the DS3421 for tail gyros because the servo motor is too small to handle the frequent movement required.
Gyros are sensitive to temperature changes. If you car is warm, the weather is cold and you take your heli out of the car and immediately try to fly, the gyro will not work well. You should allow a few minutes for the gyro temperature to settle before flying.