Understanding Gyroscopic Flight Instruments

A gyroscope flight instrument is a disk, or wheel, mounted on an instrument that is designed to measure angular velocity by utilizing the principle of gyroscopic inertia. Once the wheel has been accelerated, its inertia keeps the disc stable about its axis of rotation. When the instrument is level in flight, a deviation in flight path will move the gyroscopic wheel in its gimbal mount. This movement is then translated to a needle, or card, on the instruments face. Pilots use a gyroscopic attitude indicator, a directional gyroscope, and turn indicators for navigation purposes.

A gyroscopic attitude indicator displays the rotation about the longitudinal axis to indicate the degree of bank, and about the lateral axis to indicate pitch. It uses the rigidity characteristic of the gyroscope to function. When the attitude indicator is in operation, gyroscopic rigidity maintains the horizon bar parallel to the natural horizon. When the aircraft changes pitch or bank, the miniature aircraft on display in the instrument moves with it.

The attitude indicator functions as a vacuum pneumatic system. Air enters through the filter of the gyroscope where passages usher it towards the rear pivot and inner gimbal ring. The air then flows into the housing, where it is directed to the rotor vanes through two openings located on opposite sides of the rotor. It then makes its way through four equally spaced ports in the lower part of the rotor housing and exits through a venturi tube. The chamber that houses the ports is the signaling device that returns the spin axis to its vertical alignment when friction displaces the rotor from the horizontal axis. The increase in air volume through the opening port exerts a force on the rotor housing, mobilizing the gyro. This causes the pendulous vanes to return to a balanced condition.

The directional gyroscope is also vacuum driven and is similar in appearance to a  compass. It is used in determining the angles of yaw and rotation of an aircraft, as well provide course indications. When the gyroscope is spinning it remains rigid in space, creating a stable head reference. The heading indicator that is used has no direction-seeking qualities; thus, it must be calibrated alongside a magnetic compass. This process should take place before flight or in level, unaccelerated flight, when magnetic compass indications are steady. The directional gyro functions with air that is sucked through the rear of the instrument case. Air pressure passes through the filtering system, then through an air bearing, and finally reaches the vertical gimbal ring.

Turn indicators operate on the principle of precession. This is the characteristic of a gyroscope that causes an applied force to produce movement— typically at a point that is 90 degrees from the point of contact in the direction of rotation. A turn indicator consists of a small gyro which can be spun by air or by an electric motor. The gyro is mounted on a single gimbal, with its spin axis parallel to the lateral axis of the aircraft. The axis of the gimbal is parallel with the longitudinal axis.

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