Post by blockwood on Jan 7, 2011 22:59:25 GMT -5
I found the following info in ""The American Flight Navigator" by John Dohm, published by Pan American Navigation Service in 1958. If you ever wondered how aircraft could fly in the arctic, this might help.
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The Eclipse-Pioneer “Polar Path” compass is designed especially to replace the conventional magnetic compass and directional gyro in the higher latitudes, although it can be used with equal effectiveness anywhere. It has been notably successful in overcoming the polar directional problem in trans-Arctic airline operations and is now standard equipment in military service as well.
The Polar Path combines both a pendulous flux gate compass and an extremely precise directional gyro with a minimum drift rate of less than one degree per hour. Two other components-a consol control and a “compass coupler” which houses the motors, amplifiers, power transformer, etc. complete the system. The console can be mounted at the navigator’s station, or, in a fighter plane, on the pilot’s instrument panel.
Any one of three different kinds of directional reference can be selected by means of a switch on the control panel: (1) directional gyro alone, (2) directional gyro plus flux gate, (3) flux gate alone.
The directional gyro mode of operation is used in the Polar Regions not only because the magnetic reference is unreliable but because a grid heading (a great circle) is called for. In order to follow the great circle a constant correction must be made for both the gyro's apparent drift (which is a trigonometric function of the latitude) and any known actual drift of the instrument. This the Polar Path does automatically by the setting of a drift correction knob calibrated in latitude. If a rhumb line heading is desired, one additional correction is set into the system.
The second mode of operation-called "slaved directional gyro" combines both gyro and flux gate compass operation to provide an accurate rhumb line heading in the lower and middle latitudes where magnetic reference is reliable. Here the signal from the directional gyro gives the short term reference, and is gradually over ridden by the signal from the flux gate, thus offering the advantages of both gyro and compass systems. The accuracy and stability of this second mode of Polar Path operation are much higher than in systems which rely solely on the earth inductor element for directional reference.
In the third mode, which is provided as a standby or auxiliary operation, only the flux gate reference is used.
The heading information from the Polar Path gyro and flux gate is displayed on standard panel mounted indicators such as the radio magnetic indicator (RMI) or the repeater type. When the gyro is switched in to the aircraft’s autopilot, any departure of the aircraft from the gyro reference sends corrective signals to the autopilot controls and the aircraft is brought back on to the reference heading.
The single most important feature of the Polar Path is its gyro, in which virtually all precession other than that caused by the earth’s rotation is eliminated. This in itself is a valuable contribution to polar flying.
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The Eclipse-Pioneer “Polar Path” compass is designed especially to replace the conventional magnetic compass and directional gyro in the higher latitudes, although it can be used with equal effectiveness anywhere. It has been notably successful in overcoming the polar directional problem in trans-Arctic airline operations and is now standard equipment in military service as well.
The Polar Path combines both a pendulous flux gate compass and an extremely precise directional gyro with a minimum drift rate of less than one degree per hour. Two other components-a consol control and a “compass coupler” which houses the motors, amplifiers, power transformer, etc. complete the system. The console can be mounted at the navigator’s station, or, in a fighter plane, on the pilot’s instrument panel.
Any one of three different kinds of directional reference can be selected by means of a switch on the control panel: (1) directional gyro alone, (2) directional gyro plus flux gate, (3) flux gate alone.
The directional gyro mode of operation is used in the Polar Regions not only because the magnetic reference is unreliable but because a grid heading (a great circle) is called for. In order to follow the great circle a constant correction must be made for both the gyro's apparent drift (which is a trigonometric function of the latitude) and any known actual drift of the instrument. This the Polar Path does automatically by the setting of a drift correction knob calibrated in latitude. If a rhumb line heading is desired, one additional correction is set into the system.
The second mode of operation-called "slaved directional gyro" combines both gyro and flux gate compass operation to provide an accurate rhumb line heading in the lower and middle latitudes where magnetic reference is reliable. Here the signal from the directional gyro gives the short term reference, and is gradually over ridden by the signal from the flux gate, thus offering the advantages of both gyro and compass systems. The accuracy and stability of this second mode of Polar Path operation are much higher than in systems which rely solely on the earth inductor element for directional reference.
In the third mode, which is provided as a standby or auxiliary operation, only the flux gate reference is used.
The heading information from the Polar Path gyro and flux gate is displayed on standard panel mounted indicators such as the radio magnetic indicator (RMI) or the repeater type. When the gyro is switched in to the aircraft’s autopilot, any departure of the aircraft from the gyro reference sends corrective signals to the autopilot controls and the aircraft is brought back on to the reference heading.
The single most important feature of the Polar Path is its gyro, in which virtually all precession other than that caused by the earth’s rotation is eliminated. This in itself is a valuable contribution to polar flying.
