a condition of improved performance (lift) due to the interference of the surface with the airflow pattern of the rotor system when a helicopter or other VTOL (vertical take-off and landing) aircraft is operating near the ground, the change in aerodynamic characteristics due to modification of the air flow past the aircraft caused by the presence of the ground, a beneficial gain in lifting power when the helicopter is operating near the surface - caused by the rotor downwash field being altered from its free air state by the presence of the surface