Inside The Compass

About various airplane compasses, what they
are like, how they work, what their faults
are and how to go about remedying them.

If one were to ask the average veteran bomber pilot which of the maze of instruments that pattern his dash he would select as an absolute minimum for a safe flight back from a bombing objective he would probably select five. These would, in most cases, be the altimeter, air speed indicator, drift sight, turn and bank indicator and compass.

The choice would be a hard one. It would depend somewhat on where he was when the picking started and what kind of a fix he was in as to weather and visibility. If he came out of a dog fight minus a navigating navigator he'd probably need having the compass most of all. With that he could add 180 degrees to his takeoff flight course and back-track on the new compass course. Why 180 degrees? Well, let's start at the beginning.

Fig 1

First, look at the face of the compass (Fig 1). It's an honest face, by the way; compasses tell only tiny lies and accurate records of the few degrees they "vary" or "deviate" from "true" directions is posted on a card underneath the face. Notice that N (North) reads both 0 degrees and 360 degrees. This is because N marks the beginning and end of a circle. (Degrees are always used in circular measure.)

If you headed East toward Germany you'd steer 90; so to backtrack on this course you would steer 90 plus 180, or 270 which you will see, if you look at the compass face again, is West. Your compass is mounted in the plane so that when the plane is headed North, N on the compass is also supposed to point north. But the compass needle can't know this and it doesn't point toward the north pole because it can't. As a matter of fact it doesn't even "point." Actually the needle tip is pulled toward a large mass of what the ancients called lodestone ("lead stone") that is buried 'way up in Baffinland near the North Pole. This ore deposit is an enormous natural magnet and the way we get the compass needle to keep turning toward it, no matter which way we kick the rudder, is to make the needle into a tiny bar-shaped magnet. After that, nature takes its course: "Like poles repel, unlike poles attract." So the North Pole of a magnet, in this case a compass needle, is actually its south pole, "seeking" a north pole; in other words it is the north-seeking pole. And don't forget, it seeks the iron ore deposit called the North Magnetic pole, not the north geographical pole. If the needle points to 354 when the plane is headed N we have a little lie being told, and this error is called variation (sometimes declination). In this case the variation is 6 degrees West. If the needle pointed at, say, 4 degrees, the variation would be 4 degrees East.

There is no guesswork on the part of flyers as to how much or in what direction their compass is going to "vary" at any one place in the United States. The Coast and Geodetic Survey people supply new maps every year showing how much to add or subtract to any course to be plotted. Taking off at New York you'd add 10°, at Dayton you'd be even-Steven but before you pulled into San Francisco you'd be subtracting about 15° from the true course.

The radio, loose tools and partially magnetized metals aboard or part of a plane cause a compass error called deviation. This can be mostly compensated for by carefully adjusting the position of tiny compensating magnets built into the compass for that purpose. Unless these two errors are applied mathematically in figuring out a course to fly, the compass does not fulfill its destiny, that of being the most important navigation instrument aboard a plane. (Yes, you can fly a radiobeacon course along an established airway without even looking at old man Magnetic Compass. You can fly blind-folded with your hands tied, too, but — you don't!)

There are two other reasons why a compass may put you into Omaha instead of Kansas City: faulty functioning of moving parts; vibration. As these reasons lie mostly within the compass let's look at the insides, captioned here as a sectional view of a compass, Fig 2.

Fig 2

In the card type compass the needle, a small number of bundled magnets, carries an indicator card. The card is shaped like a clock face and is graduated and marked in degrees from N, which is 360 and/or 0, clockwise. Notice in Fig 1 that the zeros are omitted, that N, E, S and W take the place of 0, 90, 180 and 270 respectively. The navigator sees the whole face of his instrument, the pilot only sees the bent over or beveled edge and he sees this through a cut-out or window in the instrument panel.

The needle-and-card assembly is pivoted to balance and is floated in a bowl to provide buoyancy and reduce friction. These four things — needle, card, bowl and liquid are the essential parts. Note the refinements shown in Fig. 2.

If additional magnets or equivalent filaments are added to damp down the swinging motion of the needle when ruddering the plane an "aperiodic" type compass is developed.

One of the commonest mechanical troubles is caused by excessive vibration of the plane. Bumpy weather causes erratic swinging; vibration causes moveable compass parts to stick or the pivot bearing to be dislodged.

If a large bubble forms in the liquid it means you are losing the float liquid and the card is apt to settle down and jam as it moves, causing a faulty reading.

The only other error you have to watch out for is that caused by turning and banking the plane, called the "northerly turning error." Most pilots learn how much to allow for this, by actual experience acquired in flight.

Fig 3

Fig 3 pictures the face of an aperiodic magnetic compass which indicates the direction of flight by a single arrow-tipped pointer. Remember, in considering how this differs from the ordinary compass, that the bowl of the latter is mounted in a fixed position and that the needle swings and carries its direction card. A "lubber's line" is marked on the bowl and this points fixedly at the nose (and tail) of the plane. The heading of the plane is the angle between the fore and aft line of the plane (the lubber's line) and the reading on the card.

In the direction indicator the card remains stationary and the single pointer swings to the direction the plane is ruddered into. For convenience the pilot usually sets the double set of pointers on the course he wishes to hold to and then rudders the plane so that the single pointer is bracketed by the pair of stationary pointers. This is a big help on a long, tiresome hop. (Note that this is a direction indicator; actuating it there is a master compass aboard, its reading being transmitted to the indicator electrically by self synchronizing motors.)

This is almost a necessity in certain military planes where a crew man, like a bombardier, is in the middle of so much metal the magnetic compass would tell him nothing but lies. There is another type of flying not well served by the magnetic compass — acrobatics that call for steep banked turns. In this case centrifugal force cants the compass, the needle dips strongly and doesn't follow through accurately or promptly to the new course. A gyro doesn't have this turning error.

The directional gyro, according to its makers, Sperry Gyroscope Company, "is a fixed indicator of direction for steering straight courses and for making precision turns." All large planes carry this instrument. It is essential in 'blind" (instrument) flying. Call it a mechanical compass, if you will, which operates on certain gyroscopic laws or properties of a spring wheel.

Fig 4

Assen Jordanoff, an authoritative writer on airplane instruments, explains it this way — with illustrations. "The cutaway view (Fig 4) shows the mechanism. The normal position of the gyro is shown in Fig A. The gyro wheel is spun by the dynamic pressure of air which is injected into the case through two jets and acts directly upon the gyro blades. The use of two jets instead of one tends to keep the gyro wheel in position. Whenever the wheel leans to one side, as shown in the lower right of the illustration, more pressure is transmitted to the blades of the gyro from the jet on that side, and the wheel is brought back to normal position."

Fig 5

Fig 5 in its case gives a clearer picture of the general arrangement of the mechanism. Note that the spinning gyro is mounted on a gimbal ring, thus permitting the gyro to turn along the vertical as well as the horizontal axis of the ring at the same time that it spins around its own horizontal axis. The air is sucked out by the vacuum pump of the plane, which is connected by a line to the case. Inrushing air enters the air-jet line through a filter located in the lower portion of the case, and, after striking the rotor blade, passes on through the vacuum line.

When Byrd made his polar flights he took along a sun compass in addition to gyro and magnetic types** because any magnetic compass is apt to go haywire when quite close to the magnetic pole and a directional gyro has to be checked against some other type as to direction because of its mechanical behavior.

Commander P V H Weems, one of the Navy's outstanding air navigators, explains that the sun compass works on the same principle as a sun dial. The dial registers apparent time and when the latitude and longitude is known and the compass set up accordingly the shadow indicates the direction.

This article was originally published in the April, 1943, issue of Air Tech magazine, vol 2, no 4, pp 54-55, 62.
Photos credited to Pioneer, Eddy, Jordanoff.
The PDF of this article includes a photo showing the indicators of three types of compasses, installed in the instrument panel of an airliner.