Hi thealx,
Thanks for your answer. The problem is no joystick related at all. Using the keyboard for cyclic control has the same effect, the Helicopter increases forward pitch until it can't keep airborne. I am no Helicopter pilot but I can't believe this is normal, even if the physics are right the flight assist must compensate the effect or no one would want to ride them. With Long Bow there was not this problem, if you left the stick centered the craft keeps a stable position.
I will try your suggestion. Thanks.