I've got a brand new MCX and I noticed that when I put the fresh battery the heli tends to turn to the right. So I trim the rudder but as I fly and battery gets used up (but it is still perfectly fine and has enough energy for at least 5 minutes) the heli turns to the right slowly. So I trim the rudder in another direction. Then putting fresh battery again makes it want to turn left and I need to trim in in another direction.

In other words it seems like battery voltage variation through the flight is affecting rudder. I thought this heli has HH gyro