Flux gate compass

By Robert H Carlton

Imagine a world without a compass, a wold of uncertain poles, depending entirely on the sun and the stars for guidance in commerce or even in the everyday of human relations. Imagine such a world, and you are thinking of things as they were centuries before the birth of Christ — by any standards a world of primitive living, of laborious travel with limitations which for generations now have been nonexistent.

Vastly helping to bridge the gap between those tightly circumscribed Old World centuries and the sprawling civilization of today was the compass — the common little magnetized pole-seeking needle which has meant as much to mankind as any other invention ever created.

The arrow that quivered, settled in man's early compasses, was pointing to more than just the direction of north or south. It was pointing the way to progress and expansion and men followed, until today an ocean is spanned, a continent is crossed in a matter of hours, when weeks and months were consumed before. The mosaic of progress would doubtless have been there, but its framework might never have taken the shape of things today had it not been for that little swinging arrow.

Utilization of the compass in the past quarter century in aerial navigation has contributed heavily to its earlier value as exemplified in terrestrial navigation. Today, when men and machines are locked in combat in uncharted skies, its value finds an added, accentuated meaning. Warfare itself, whether in the air or on the ground, is fundamentally a thing of direction and of maps, which are based upon the north-pointing compass needle.

This has always probably been true, for it was in connection with warfare that the compass first made the pages of history. Records show that the earliest mention of such a device was in a battle communique issued in the sixty-fourth year of the reign of a Chinese emperor named Ho-Ang-Ti, about 2634 BCE.

When Ho-Ang-Ti found his troops frustrated in an attack on Tchi-Yeou in the plains of Tchou-Lou by what may have been the earliest smoke screen, he had built a tchi-nan, or compass chariot, consisting of a figure on a magnetized base mounted on a cart. Natural magnetic forces caused the hand of the figure to point south. Having thus oriented himself, Ho-Ang-Ti presumably went on to triumph. From primitive beginnings of this sort, the compass changed little through the years until American ingenuity created the first basically new device of this kind for general use. The gyro compass, based on the gyroscope invented by the French physicist, Foucault, was developed in the middle of the Nineteenth Century.

That original gyro compass, though bulky, slow to assume initial stability, did provide a meridian- seeking shipboard compass, unaffected by rolling, turns or magnetic variations. From it came the aircraft gyro compass, which offers stability during rough air and turns for a brief period but which requires initial setting from the magnetic compass and frequent resetting in flight, especially when the going is rough or erratic in direction.

It thus has been considered a compass aid, rather than a primary navigation instrument.

Seven years ago necessity again proved the mother of invention. W A Reichal, director of engineering at Eclipse-Pioneer Division of the Bendix Aviation Corporation, started working on a new compass.

Today that answer has materialized in the Gyro Flux Gate Compass, a revolutionary type which enables Allied airmen to fly straight to their target under virtually any conditions, return just as unerringly to their bases after showering destruction on enemy objectives.

Charles Marcus, vice president in charge of engineering, calls the new device "as great an advance over the conventional magnetic compass as that compass was over the lodestone." It incorporates a new principle, for the sluggish and frequently wavering needle is replaced by a fixed coil system in which actuating currents combine with energy generated by the earth's magnetic field to turn the compass indicator.

Technically, the compass is called a remote indicating earth inductor system, consisting of a gyro stabilized flux gate transmitter, an amplifier, a master indicator and from one to six Magnesyn remote indicating repeaters. It is designed to provide an accurate indication of the magnetic direction of an aircraft under all possible flight conditions up to an angle of sixty-five degrees with the horizon.

The flux gate, or magnetic azimuth sensitive element of the system (magnetic azimuth is direction with relation to magnetic north) is fully electrical and is maintained at the horizontal by an electrically driven horizon gyro. Because of this arrangement, the compass system's indications are not appreciably affected by the sudden maneuvers in flight. A stable compass proves its worth to airmen who dive, turn their planes at 300-400 mph.

The device will not go off its reading when the plane dives or climbs suddenly, and as Reichel describes it, "will not lag or overshoot during a turn and will not oscillate or 'hunt' back and forth in rough weather."

One of the compass problems arising from use of aircraft equipped with armament, armor plate and laden with bombs has been solved with the remote indicating phase of the system. The magnetic azimuth sensitive element, or transmitter, is placed where interference will be at a minimum — that is, at a point remote from current carrying conductors or causes of local magnetic deviations such as armament, which would impair the accuracy of the standard compass.

Through this arrangement, the indications of the transmitter in its out-of-the-way place are registered on the master indicator. Further, other indicators are linked to the compass through the Magnesyn system, which makes possible remote readings of measurements received from a master source. Pilot, co-pilot navigator and bombardier have their own dials, and compass readings are transmitted to them or to as many as six different points about the aircraft.

In the ordinary compass, deflection must be considered, and for this purpose a correction card is used. The pilot, navigator have to refer to this card, correct the deviation. In combat mistakes are inevitable. In the gyro flux gate system provision has been made for compensating the compass, with the result that fully corrected readings are immediately available, and the possibility of human error in moments of stress is eliminated.

This compensating mechanism takes care of both variation, which is the work of natural forces, varying in different parts of the world, and deviation, which is caused by local magnetic influences like the armament mentioned previously. Variation correction is applied by rotating a knurled knob on the face of the indicator. This knob offsets the dial either East or West until the desired variation is indicated on the lower part of the dial.

Compensation for deviation is applied by adjustment of screws arranged around the periphery of the dial, which control the contour of a cam plate. As all compensation takes place at the main indicator, there is a mechanism for duplicating the corrections at the other dials in the aircraft.

Position of the master indicator pointer is relayed to the repeater indicators by a transmitting Magnesyn unit. This unit is connected through a gear train to the main indicator mechanism. When the automatic correction takes place on the master dial, immediate transmission is recorded on the repeater indicator indicators. To see uncorrected reading, there is a cut-out at the top of the main dial which records original indication.

Those seven years of research in the Bendix division's laboratories have filled the need for fast, undisturbed and accurate azimuth indication in flight, and through the repeater arrangement has placed this information at the fingertips of all key men in the aerial fighting team.

The Gyro Flux Gate Compass is now in production at the Philadelphia Division of the Bendix Aviation, reputedly the world's, largest aircraft instrument plant. This plant's September production of scientific instruments which turn the blazing guns of victory on the Axis, hit the staggering total of 175,000 units — enough to equip more than 100,000 planes with twenty-one instruments each in a year's time.

In order for the instrument to function perfectly, each one of the 120 parts involved must be matched precisely. Even a pin-point of perspiration would ruin the amazing accuracy of the compass. All assembly on the gyro flux gate is done in air conditioned, dust-free rooms. Once assembled, sealed under exacting conditions, the compass can operate in any climate, at any altitude.

One major problem was developing new test equipment capable of measuring the accuracy, stability of the compass. Test equipment had to represent flying speeds of 300-400 mph, extreme temperatures, and violent movement.

More aircraft instruments are shipped from this one plant in six hours than were produced by the entire aviation industry in one month before 1940, and it will ship more instruments this year than were turned out in the world in all the years before 1940.

That the flux gate compass is, at this moment, contributing to the rising crescendo of the all-out United Nations effort to crush our enemies is indicated in the fact that one or more of them have fallen into Axis hands — hence the revelation of its existence.

But possession of such a sample of American progress will give the big and little Hitlers small comfort. Engineers agree that there is no possibility that the enemy can catch up with us, because it will be impossible for him to duplicate the performance of this compass, much less put it into volume production during the war.

This article was originally published in the March, 1944, issue of Air News magazine, vol 6, no 2, pp 39-41.
The PDF of this article is extensively illustrated with photos and diagrams of the elements of the fluxgate compass and of its installation in a B-17.
The original printing was on 9½ by 12¾ paper. The pages in the PDF have been reduced to print on letter-size paper.
Photos credited to Bendix.