My first spool-up went rather well. The wind pushed over the rotor quite a bit, but I read that until the 3GX is actually airborn, it can’t compensate for anything. So I gave the throttle stick a push and indeed, the wind was compensated away. The controls were very sensitive; too sensitive for my taste; so I landed and increased the expo on the swash to 30% (it was 15%). During the next spoolup, there was a sudden “tick”, accompanied by a kick of the tail. Right after liftoff, the heli was pirouetting like crazy. Luckily I only was ~20cm high, so just dropping the pitch to -2º was enough to get it back on the ground without any damage.
I double-checked the gyro direction, the control direction and the direction of rotation; all seem to be correct. Until I took a closer look at the tail rotor itself:
The tail rotor hub had moved 6mm from the end of the shaft, and the set screw was gone. Moving the hub back into position was hard, it was jammed fairly hard.