My rechargeable Rx pack turned up today, so I thought I'd fit the radio gear. No rudder movement at all. I wiggled the rudder manually, and the servo moved slightly. Response from the Tx stick was very jerky, which made me think that the servo was faulty.
I found an unused Hitec HS55 "feather" servo amongst my old RC plane stuff, which is the right size, but uses a different spline for the arm. I removed the rudder linkage - a push-pull setup - and the reason for the limited movement became apparent. The nuts that secured the metal fittings to the servo arm had been "locked" with superglue, but way too much. The fittings were glued solid to the arm. No rotation at all.
A quick dunk in acetone got everything separated, and I fitted the new Hitec arm, adding washers either side of the arm (and a tiny drop of superglue to lock the nuts),
While I was in there, I pulled the rudder shaft from it's tube. No lubricant, no washer between the top of the tube and the rudder arm.
Not impressed by the build quality of what is supposed to be an "ARTR" kit. I'd be seriously annoyed if I'd bought it at full price.