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 , 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