At the risk of getting embroiled once again......
Ground effect is an aerodynamic effect caused by the placement of an obstruction underneath the downward flow from the rotor.
Aerodynamically saying, the rotational airflow remains constant, the induced airflow is slightly decreased by the added restriction placed in the induced flow, thus reducing the induced flow. The relative airflow, being taken from the top of the induced airflow vector is therefore also reduced slightly in angle. As the angle of attack is taken from a constant held pitch angle the angle between the chord/pitch angle and the relative airflow also increases slightly giving greater lift.
Whether or not a small localised pressure increase exists under the helo? yes probably but it has no/or possibly very little contributory effect to 'ground effect'.
This can probably best be seen when over a rig. The small landing pad offers the same ground effect with a small plan area therefore reducing the possibility of the ground effect 'bubble'.
Sigh..... Sit back and wait for flak.
P.s. The ROD is the increased flow from the rotor over the pitot staic system resulting in a small downward deflection caused by position error. Have a look at the beanie on the S-61N to see what mr Sikorsky tried to do about it!!!!
