This is difficult to explain without a diagram (and quite difficult to explain with one too!) but here goes. On a rotary wing there are three components to the airflow acting on a blade. 1. Rotational flow, 2. Forward airflow, (1 & 2 should be self evident) 3. Induced flow (the column of air displaced by the rotor disc - similar to fixed wing downwash). The actual flow experienced by a blade is a combination of these three, known as Relative Airflow. The angle of attack experienced depends on the velocity (direction and magnitude)of this relative airflow as well as the pitch angle of the blade. As you will know, lift always occurs upwards at 90 degrees to the relative airflow and drag acts 90 degrees backwards from the top of the lift vector (even on plank wings!). There is however, a difference between fixed and rotary wings at this point. For rotary wing the resultant between the lift and drag vectors (known as the Total Reaction) is resolved vertically into Rotor thrust (up the rotor mast) and horizontally along the "Plane of Rotation"of the rotor system. Now here is the crux of your question...If the horizontal component lies behind the rotor's axis of rotation it acts as rotor drag and needs power to keep the whole show turning i.e. we are in helicopter mode. However, in the case of an autogyro / gyrocopter or a helicopter in autorotation the Induced Airflow is coming from below the rotor disc (yes, the disc appears to be "tilted back" as you say, or for a helicopter it requires a rate of descent to get the same effect). This changes the Relative Airflow so that the Total Reaction moves forward and there is a "positive rotor drag" or autorotative force. For all this to happen the blades are set at a low pitch angle (permanently on simple autogyros, or the collective lever lowered to the bottom of its travel for a helicopter). The principle is similar to a fixed-wing propellor windmilling after engine failure but in this case, due to the engine axis being roughly in line with the direction of flight the effect is seen as a large dragging force rather than a vertical rotor thrust. (There, now you wish you never asked, I'll bet. Cleans off blackboard)
P.S. Mine's a MacAllans
[This message has been edited by Skycop (edited 06 June 1999).]