Probably because you're at a high speed? I'm almost certain that the rudder is only supposed to be effective at lower speeds.

Yeah , the tail rotor won't make you turn on high speed ... that's normal ... reduce speed or move your mouse left or right then try to raise your nose ... that's how you turn at high speed... like a plane in most common games ...

Without knowing it I think what you are experiencing is the inability to "turn" the helicopter with the pedals at high speed. Remember that the helicopters, especially the Ka-50, will only be able to turn around its own vertical axis with the rudder pedals at stationary to low speeds.

This is somewhat how a real helicopter should behave. I say somehow as the current flightmodels are not "flightsim" precise models, but do capture some of the real life implications of flying helicopters.

As guys above me said. This is how real helicopters behave and its designed same way in game.

Thank you for your feedback.