Some Helicopter Physics for you to engineer kerbal solutions to!
1)gyroscopic precession https://blog.aopa.org/aopa/2012/01/29/gyroscopic-precession/ on my stock heli's I just turn my joystick 90 degrees to realign the bump from the reaction wheels with the resultant tilt of the rotor disk
2)advancing blade drag: as you move forward faster the speed increases the effective airspeed on the advancing blade side of the rotor disk. the drag from this makes it harder for the motor to turn the rotor the faster you go. Around 40-50 m/s the motor usually becomes overcome by drag and the rotor RPM drops. This is bad! You really really want to maintain a constant rotor RPM.
3)retreating blade stall: as you move forward faster the speed decreases the effective airspeed on the retreating blade side of the rotor disk. You will need to trim the roll of the helicopter to remain level. Critical retreating blade stall is usually about 40-50m/s in KSP.
So about 40-50m/s the motor is working super hard to keep up RPM. Then the retreating blade is losing lift. When the RPM begins to drop from advancing blade drag, the retreating blade really stalls and stability is lost.
hope that is some help