Take a ruler and place one finger either side, in line with each other. When you push, the two forces cancel each other out absolutely, and the ruler stays still.
Now move your fingers apart on the ruler. When you push, the ruler will turn.
That's what happens when the main rotor hub (one force) is not in the same plane as the tail rotor hub (the counter force).
The ruler spins round completely because the force you exert is massive compared to any counter force. In the helicopter, the rolling couple caused by the difference in height between the main and tail rotor hub causes a rotation of the helicopter which is resisted by gravity acting on the mass of the helicopter. The rolling couple moves the mass of the helicopter away from verticle but once it matches the gravitational pull on the mass of the helicopter harmony is achieved, and the helicopter simply flies one-skid-low.
Which skid low depends on whether the main rotor spins clockwise or counter-clockwise.
Simples!
Last edited by John R81; 31st May 2012 at 21:18.