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artificial horizon[¦ärd·ə¦fish·əl hə′rīz·ən]
also called gyro horizon or attitude indicator, a gyroscopic device designed to measure the roll and pitch angles of an aircraft. The pitch angle is the angle between the longitudinal axis of the aircraft and the horizontal plane, and the roll angle is the angle of inclination of the longitudinal plane of the craft to the vertical. The main parts of the artificial horizon are a gyroscope with three degrees of freedom which retains the position of its axis fixed in space and a pendulum correction system which eliminates deviations of the gyroscope rotor axis from the true vertical. Whenever a gyroscope rotor axis is out of alignment with the true vertical, the correction system, consisting of a pendulum built into the inner frame of the gyroscope and correction motors, causes precession of the gyro axis (moving it in the plane perpendicular to the direction in which the force is applied) until the axis resumes its preset position. Precession ceases as soon as the gyro axis lines up with the vertical. The most commonly encountered artificial horizon is one in which the front panel of the instrument shows a silhouette of the aircraft remaining fixed relative to the artificial horizon housing, and consequently to the aircraft as a whole. A gyro-stabilized sphere is located behind the silhouette. The bottom half of the sphere is colored blue (sky), and the top half is colored brown (earth). The interface between the two colored hemispheres serves as the artificial horizon bar. Scales are also drawn on the sphere as aids in measuring the pitch angle and the roll angle. By observing the position of the aircraft silhouette on the instrument scale relative to the moving artificial horizon bar, the pilot gains information on the presence and magnitude of those angles. A slip indicator with a zero position indicator is built into the housing of the artificial horizon. Artificial horizons with pneumatically driven gyroscopes and correction devices were used in the past; subsequently, artificial horizons with electrically driven gyroscopes and correction devices came into use. The total error in the readings of an electrically powered artificial horizon in level flight is not greater than 1°, and it is not greater than 2° when a plane is coming out of a bank turn at a 20° roll angle at a speed of 400 km/hr, an accuracy which in practice is sufficient for blind flying.
REFERENCEFridlender, G. O., and M. S. Kozlov. Aviatsionnye giroskopicheskie privbory. Moscow, 1961.
ii. A device, such as a spirit level or a pendulum, that establishes a horizontal reference in a navigation instrument.