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Journal of a Seaplane Cruise Around The World
Part One: The Atlantic Ocean

Notes on the use of pelorus to compare compass heading with sun's azimuth.

Flight to Greenland These calculations in the last notations of the journal were made in order to "swing the compass" while the airplane was in flight. While in Boston, the magnetic compass was carefully compensated and its readings were checked at intervals of 15° around the compass circle. However, the dependability of a compass decreases steadily with continued hours of flying, particularly if the flights pass through latitudes near the magnetic pole. This is due to added magnetization of the entire steel structure of the airplane because of a change of intensity and direction of the earth's magnetic lines of force. The error is particularly great if the flight is continued day after day in the same general direction, as was true on the route from New Haven to Angmagssalik.

It is just as necessary for the flyer to know direction as it is for the mariner, and if the errors of the magnetic compass are changing in so short a period of time as a few days, it is necessary to compare the readings of the compass with some known guide to direction. One of the easiest of these guides is the position of the sun, and this may be determined while the airplane is in flight just as readily as when it is at rest on the ground. The only requirements are a knowledge of Greenwich time within 5 minutes, a knowledge of the position of the airplane within 60 miles, a book of tables for the sun's azimuth (U.S. Hydrographic Office Publication 71), and a perorus. The procedure of determining the ship's heading from the sun's azimuth is greatly facilitated by the use of the gyrosopic compass and a two-way telephone system between the pilot and the observer. The determination can be made in five minutes or less.

The observer finds from the printed tables the bearing of the sun, reckoned from North, for a given local civil time. With this in mind he sets the bearing circle to the true course. He dons smoked glasses, helmet, goggles, and earphone, and opens the hatch to set the bearing plate in its bracket. The sightings arm is set to the sun's azimuth on the bearing circle.

The pilot meanwhile is flying his regular course and when everything is prepared for the determination, shuts down the gyroscopic compass and adjusts its dial to read the true course plus the magnetic variation (magnetic course). As the time of observation approaches the observer looks along the sighting arm and directs the pilot through the microphone to steer right to left until the sun appears in line with the instrument. As this alignment is made he calls "mark" and the pilot sets the gyroscope into action again.

The gyroscopic compass now indicates direction which is accurate within 1° or less, and will retain for a period of 15 or 20 minutes, when the precession of the instrument introduces substantial errors. During this interval the pilot can make frequent references to his magnetic compass and determines what reading it gives when the ship is pointed in the direction which has just been determined. This compass direction may be the same as that which was determined at the last "swinging" of the compass on the ground, but if the magnetization of the ship has changed, it probably will point to some slightly different direction.

Before procession occurs in the gyroscope it is possible also to check other points of the magnetic compass, preferably those in the range of the probable headings of the plane during the next few days of flying.

So long as steel fuselage construction is maintained in aircraft we cannot hope for good operation from magnetic compasses. In any case, ordinary compasses are not reliable for the pilot to steer by, because of the motions imparted to them by the jerks and jars of the airplane. The gyroscopic compass is ideal for steering, but its rate of precession makes its readings valid for periods of no longer than 15 or 20 minutes. The great need in airplane compass development is the perfection of a gyroscopic compass which will retain its setting for periods of 24 hours or longer. Before each flight the gyroscope would be set in rotation and adjusted to indicate true directions, and would then give unerring points of the compass until cessation of flight.

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