PURPOSE: To accurately detect the airspeed and wind-direction/wind-velocity of an extremely low speed fuselage with a light-weight economical device for the purpose of automatic control, navigation, etc., of a helicopter.

CONSTITUTION: The inclination of a thrust vector (line perpendicular to the rotating plane of a rotor) of a main rotor is determined only by a component of airspeed in the direction of the nose of aeroplane and is independent from the position of the center of gravity of the fuselage. Similar relationship is established between the lateral inclination of the thrust vector of the main rotor and the lateral component of the airspeed. The airspeed component in the direction of the thrust vector of the main rotor is characteristically detected from the back and forth/left and right inclination of the thrust vector.