Tail moment is different things to different people who build model airplanes. But to the aero industry, it's the force the tail has based on it's size and it's leverage. You figure out the area of the tail and multiply that by how far back it is, and you have a value to compare relative to other areasXdistances. And it's power is absolutely tied directly to the airspeed it sees. And in fact, there are two of them. There's the horizontal tail and the vertical tail. It's significant that there really isn't much written about the fin/rudder even having "tail moment". Why? Maybe because it's sort of an on and off thing. It is pretty worthless until it's up to its flying speed, and once there, is suddenly as effective as it needs to be. And has enough power from then on.

Keep in mind that those things have zero force unless there is air movement. And the force is relative to the airflow over the surface. So when a Cub for example starts to taxi, the tires start out with the possibility of causing extremely more effect than any of the surfaces and keep that edge until airspeed has been built up. If you look at the battle between the tires trying to yaw the model and the fin/rudder trying to keep the airplane straight, it's not a fair fight. A Cub's fin/rudder really don't come online until the airplane has some speed. And one wheel can hit a bump or hole and create almost absolute STOPPING force. Until the wing helps keep the wheels out of the holes that fin/rudder is a fairly lame force.

We get a lot of our taxi problems when the fin/rudder aren't really flying yet.