The gyro reverse switch probably needs to be reversed.
If the gyro reverse is correct:
When the tail moves left, the gyro will push it right until the turn is reversed.
When the tail moves right, the gyro will push it left until the turn is reversed.
If the gyro reverse is incorrect:
When the tail moves left, the gyro will push it left some more until the turn is reversed. Since the gyro cannot reverse a left turn by pushing the tail left, the tail will keep spinning left forever.
When the tail moves right, the gyro will push it right until the turn is reversed. Since the gyro can't reverse a right turn by pushing the tail right, the tail will keep spinning right forever.
The gyro may be too close to the ESC. On a non-micro heli, the gyro should be at least four inches away from the ESC.
Tail belt may be twisted around. Removing the tail rotor assembly from the tail boom and make sure the tail drive belt isn't twisted.
Tail wag occurs because the heading hold gyro cannot precisely control the thrust of the tail rotor. This can be caused by several problems:
Inconsistent tail rotor speed
If the tail rotor speed is not consistent, then the gyro will have difficulty controlling the tail rotor thrust. This is unavoidable on fixed pitch helicopters to some extent. This may be caused by:
Tail belt may be loose
Main gear or other gear may be missing a tooth
Visually inspect the teeth of all gears and ensure no teeth are missing.
Tail rotor blade pitch resolution problem
For example, the heading hold gyro may want the tail blade pitch at 10.4 degrees. If there is a problem, and the tail blade pitch can only be at 10.2 and 10.6 degrees, then the heading hold gyro will alternate the blade pitch between 10.2 and 10.6 degrees. This will cause the tail to wag back and forth.
This tail rotor pitch problem may be caused by:
Excessive play in the tail rotor blade pitch control system.
A little play is acceptable in the control system, but you should strive to minimize the play.
Tail rotor pitch slider may be sticking to the tail shaft
This is usually caused by a dirty tail rotor shaft and/or dirty pitch slider. The tail rotor shaft can be wiped with a tissue, and the inside of the tail pitch slider can be cleaned by using a cotton swab.
Do NOT use oil to lubricate the tail rotor shaft because it will attract dirt and become sticky. Use powdered graphite instead.
Tail blade grip bearings may be dirty, damaged, or loctited to the screw.
Tail blade grip screw may be bent.
Tail servo has too much leverage on the tail pitch control lever.
You can increase the effective resolution of the tail servo at the cost of decreasing the travel speed which may reduce or eliminate the tail wag. To do this. you can move the tail pitch linkage IN one hole on the tail servo arm or OUT one hole on the tail pitch control lever.
Tail servo may not have enough resolution
This HS-50 servo, in particular, does not work very well with a heading hold gyro due to the servo slop.
Gyro or tail servo mounting problems
Gyro not mounted securely to frame. Use only the supplied gyro tape or 1/8" (3mm) thick 3M foam tape. Do NOT use velcro!
Servo not mounted securely to the frame or tail boom mount.
Check for loose or stripped screws. Also, rubber servo grommets can cause wag when used on tail servo mounts.
ESC with poor governor mode
Some ESCs have a badly programmed governor mode, and the motor speed will fluctuate when the governor mode is enabled.
This occurs because the gyro will apply extra tail pitch which increases the load on the motor, which causes it to slow down. The governor mode will apply more power to the motor, which causes the motor speed (and tail rotor speed) to increases, which causes the tail to overshoot the proper position. The same problem occurs when the tail pitch is decreased, and the tail overshoots the correct position again.
Basically, the ESC governor mode and heading hold gyro can go into a vicious cycle which causes the tail to wag. This is known to happen on Align ESCs and Castle Creations ESCs with older firmware versions.
Gyro gain too high. Smaller helis usually require lower gain settings.
Worn tail servo.
Metal geared servos tend to wear quickly when used for tail rotor pitch control.
Gyro gain too high. Smaller helis usually require lower gain settings.
Tail servo may be missing a tooth on a gear from previous crash damage.
Tail servo may not have enough torque to move the tail pitch lever smoothly.
Tail rotor may be too large (micro helis only)
This is a common problem when a GWS IPS motor with a 3x2 prop is used. Trimming the propeller by a little may help.
This could be caused by radio glitches. See Section 24.5, “Glitching”.
The one-way bearing in the autorotation hub may be loose.
Tail servo responds properly when tail swings one direction but tail servo fails to respond in other direction.
The gyro may be damaged.
Tail holds fine at the beginning of a flight, but starts to drift as the battery discharges. This is normal for a yaw rate gyro. Heading hold gyros do not have this problem.
The heli tail runs even at low throttle when using a heading hold gyro and a tail motor. This is normal. The heading hold gyro does not output zero throttle unless it receives a full left rudder stick. This is why you need to hold full left rudder to initialize the tail ESC.
Tail servo slowly moves in one direction (creeps) when heli is on the ground with heading hold gyro. This is normal. Don't worry about it.
Tail servo travel is unequal on both sides of the neutral point with heading hold gyro. This is normal. You should set the servo travel in the non-HH mode and the heading hold mode will work automatically.
Helicopter has a tail motor, and has insufficient tail thrust to counter main rotor torque. Tail prop may be mounted backwards. Examine the prop airfoil - the prop should be mounted so the airfoil will gradually increase pitch as the air moves past the rotor.
This is correct. With a heading hold gyro, the transmitter EPA controls the pirouette rate. The tail servo travel must be set using the LIMIT adjustment on the gyro. If the gyro does not have this feature, then the tail servo travel cannot be set, and you must move the tail servo linkage to a different servo horn hole to increase or decrease the linkage travel.
The transmitter rudder channel reverse function does not change the servo rotation direction with a heading hold gyro. It only changes the yaw direction command to the gyro.
The tail servo rotation direction can only be fixed by using the reverse switch on the gyro, or if the gyro has no reverse switch (such as the CSM LW200 gyro), it must be mounted upside down.
See Section 28.1.7, “How gyros work” for more information.
This can be caused by three problems:
With non-Futaba gyros such as the CSM LW200 or Ikarus Profi gyro, the transmitter rudder channel neutral point will need to be matched with the gyro's expectation of the neutral point, because these gyros do not calibrate the neutral point when the gyro is powered up. Use the transmitter subtrim to center the rudder channel. It may help to set the rudder channel subtrim to a smaller increment.
With a Futaba gyro, this problem is usually caused by bumping and/or moving the helicopter while the gyro is initializing. Do not bump or move the helicopter when connecting the helicopter battery.
Excessive vibration can cause a heading hold gyro to "drift" and slowly lose the correct position.
Also, the ESC will generate an electrical spike when the battery is connected, and if the gyro wires are close to the ESC or motor, this will disrupt the gyro calibration of the rudder neutral point. The solution is to route the gyro wires away from the ESC and motor wires.