problem with parent/child rotation

I have a nested object (helicopter) that looks like this: EmptyObject -> helicopter (base model) -> main rotor -> tail rotor

Trying to ignore parent rotation by rotor (to always have the same rotating speed) i came up with following code in script on EmptyObject:

 private void FixedUpdate() 
 {
     //W,S for moving forward; A,D for rotating; Q,E for strafe
     horizontalInput = Input.GetAxis("Horizontal");
     strafeInput = Input.GetAxis("StrafeAxis");
     verticalInput = Input.GetAxis("Vertical");

     Vector3 direction = new Vector3(strafeInput, 0, verticalInput);
     playerRb.AddRelativeForce(direction * force);

     //apply rotation to model while moving
     model.transform.localEulerAngles = new Vector3(verticalInput * 15, 0, -strafeInput * 15);

     //compensate for parent rotation
     Quaternion tempRotation = mainRotor.rotation;
     transform.Rotate(0f, turnSpeed * horizontalInput, 0f);
     mainRotor.rotation = tempRotation;
 }

And this leads to a bug when player rotates while the model have a tilt (from moving forward or to the side) - main rotor rotation breaks:

I can't understand why this code leads to rotor having nonzero values in X and Z angles in local rotation.