This is for your gy401, turn off your revo mix, put your rudder servo in the middle, put the gyro on heading hold( this means that you can see a steady light on your gyro gy401), fire it up. Don't touch the rudder trim!!! or you will turn off the heading hold if you did this by accident switch off the heli and turn it back on. This are all done with the transmitter on. Now fly it, if the tail will not hold steady then there must be something wrong on your set-up either on the gyro or your transmeter. get an experience pilot and have him set it up.