THE bombs had been dropped, visibility was good and the Axis was three ships and one fighter plane less. Then the fog and clouds over the Mediterranean between Crete and Bengasi thickened. The triumphant Fortress was suddenly lost in the overcast.
The radio operator strained to pick up the signal of a friendly station or of any known station on which a bearing might be taken. Only noise or garbled Axis communications came through.
The pilot lifted the Fortress higher for a look at the stars but the soup seemed to extend to the top of the sky. There was no visibility at the plane's ceiling. Off its course, with no means of navigating to its home base, a forced landing seemed inevitable. That meant an even chance of landing in enemy territory.
As the Fortress began to descend through the black masses of cloud and fog, the radio operator exclaimed with excitement, "Hold it! Base is coming through."
No one else spoke but those near enough fixed their eyes upon the dial of the automatic radio compass as the needle swung slowly through an arc, halted, pulsed erratically with a crash of static, and then settled down to a steady position.
The Fortress swung about until the plane was aligned with the needle. Blind, through fog and clouds, it rode the radio wave home.
Safe at their base, with bomb bays empty, the crew jumped to the ground and their completed mission seemed far away. But under the streamlined nose was the tear-drop shaped housing which protects the loop that is the heart of the Bendix automatic radio compass. Without its guiding radio compass, the Fortress would never have returned to fly again against the enemy's strategic targets.
The radio compass is to the airman what the magnetic compass has been for centuries to the mariner. But it is really more. The mariner's compass gave him only the cardinal directions. The automatic radio compass gives the airman the direction of his home base and points the way there.
The discovery of the compass properties of the magnetic needle is an event lost in the maze of history distorted by fiction, but since about the 12th century, far-reaching sea voyages have been made through navigating by the magnetic compass.
Diaz in 1486 navigated around the Cape of Good Hope with the aid of a magnetic compass. Columbus used a single needle-shaped magnet, supporting a paper compass rose mounted on a steel point. Long distance marine navigation was made possible by the magnetic compass more than by any other single instrument outside of ships themselves.
Adaptation of the compass to air flight was an obvious step, but something more was needed in a plane off course and lost in fog with neither Earth nor stars visible. Knowing merely the direction of north or south is of little help in such circumstances.
Radio compass navigation by no means replaces the regular dead reckoning and celestial navigation adapted from the mariner for the airman. But in circumstances where the conventional methods are insufficient, which happen frequently under modern flying conditions, the automatic radio compass is irreplaceable.
To understand where the usefulness of the radio compass supplements that of the magnetic compass, let us refer to the diagrams.
Figure 1 is a diagram of the earth, on which three airplanes are located. In each airplane is a modern magnetic compass provided with a dial and rotating pointer for indicating the plane's heading, that is, the position of the pointer indicates the angle between true north and the direction of the plane's longitudinal axis. Ordinarily, the compass needle points towards the Magnetic North Pole, whose direction from an observer is in most cases displaced a number of degrees from the direction of the Geographic North Pole.
In the modern compass it is possible to correct the position of the pointer to account for the angle at the point of observation between magnetic north and true north. It is also possible to correct the pointer position for local magnetic influences in the plane. The corrected position of the pointer then designates the true heading of the plane. For purposes of simplification, therefore, in Figures 1, 2, 4 and 7 the compass needle is represented as indicating true north. Referring to Figure 1, all three planes are headed true north, although they have three different destinations unless perhaps they are on a polar flight.
Now consider a small portion of the earth as shown in Figure 2. Here is a navigation chart on which the location of La Guardia Field is indicated. Again, three planes are shown flying in the same direction. But notice that only Plane 2 will reach La Guardia Field. In each plane the magnetic needle points north while the longitudinal axis makes an angle 70° with the meridian.
The magnetic compass tells the pilot the angle between his line of direction and the meridian, but it does not tell him where he is going. It indicates a direction but not a destination. For when the magnetic compass discloses the plane's direction it merely discloses one of an infinite number of parallel directions. Unless the pilot can see the earth so that he knows where he is at the moment, he cannot tell where he is going.
The radio compass, on the other hand, is both direction and destination finder. This is how it works: Suppose the air is still but visibility is poor and there is an overcast sky so that the pilot does not know his position. His plane, however, is equipped with a Bendix automatic radio compass. Note the radio station at La Guardia Field shown in Figure 3.
The pilot tunes in his radio compass on that station. Then inside the "bomb that is never dropped", the little loop turns and faces the radio station. On the control panel, visible to the pilot, there is a little dial with a pointer that turns with the loop and stops when the loop stops. The pilot turns his plane until the pointer rests on zero. Then he knows that his plane is pointed toward the radio station which has become a pole not the faraway North Pole, but a pole at his place of destination. His radio compass has picked a direction not one of an infinite number of parallel directions, but the particular direction to his desired destination. It is, in truth, an automatic direction finder and this designation, "ADF," will be used throughout this article. By so heading the plane that the ADF pointer stays on zero, the pilot "homes" into La Guardia Field.
When the pilot of Plane 1 (Figure 3) tunes in on the same station, his pointer will indicate to the right of zero because the radio station is located to the right of the longitudinal axis of his plane. So he turns his plane to the right until his ADF pointer rests on zero. Then he, too, can home into La Guardia Field. In this simple manner our fighting planes home into their bases with the aid of the ADF after completing their missions.
The station at La Guardia is an aircraft radio range station. Is it necessary to tune in on this station? Not at all. Near this field there are dozens of broadcast stations. The ADF can be tuned in on any of these stations and the plane home in on the one selected. In combat zones there are no established airways with radio ranges. But wherever our air forces are based, fixed or mobile radio transmitters are set up for the express purpose of bringing our aircraft back.
You may be wondering what happens when there is a strong wind blowing. Figure 4 shows a plane heading east with a northeast wind to combat. The dashed lines indicate the easterly direction of the plane, but the wind keeps blowing it toward the south. Even though the plane sets its course on a direct line toward La Guardia Field, it would pass far to the south. The fact that the magnetic compass discloses the plane's line of direction correctly would not help the pilot find the airfield.
What happens when ADF is on the job? Starting in position 1 (Figure 5) when the plane happens to be headed toward the east on a line through the radio station, the ADF pointer rests on zero. In a short while the plane is blown far enough south so that the pointer swings left, as in Position 2. The pilot then turns into the wind until the pointer comes back to zero, and continues to steer the plane so that the ADF pointer stays right on zero. Finally, the plane approaches its destination as in Position 5, heading into the wind.
In case there is objection to taking this curved path, the pilot can fly the straight path shown in Figure 6. To do this, the plane must be turned into the wind until the radio station is on the right of the plane's heading. By a little maneuvering it is possible to keep a constant angle between the plane's heading and the path of the radio wave. Suppose, for example, this angle happens to be 10° to the right. Then the ADF pointer rests 10° to the right of the zero mark. If held there the plane will home in on a straight line.
Homing is much more valuable than these simple examples indicate. There are times in domestic flight when the pilot receives instructions to land at an alternate airport because his scheduled landing field is closed on account of weather conditions. He may not know his position accurately nor can he listen for a radio range signal. He must immediately pick a path to the unscheduled airport. Again his ADF comes to the rescue. The pilot tunes in a station near the alternate field and guides the plane to it. In combat zones, all landing fields are alternates. After battle a plane's position is rarely known and, in the absence of this information, landings at fields other than the base airport are frequently made.
For long distance military and commercial transport there is even greater need for ADF than in homing. It is true that all methods of navigation are used to determine the plane's position on such flights. Whenever weather conditions permit the sextant is used to sight the stars or planets. But when an overcast prevents the use of the sextant it is a lifesaver to have an ADF and a good magnetic compass. How do these devices work for position finding?
First it is necessary to assume an imaginary straight line passing between the plane and a radio station when the plane is not headed toward the station. In Figure 7, the plane is headed 70° east of north, so the magnetic compass tells us. The ADF pointer reads 40°, indicating that the radio station is 40° to the right of the plane's heading. Therefore, the line through the radio station and the plane makes an angle of 110° to the right of north. This line, passing through the plane and a known point on the earth, is now drawn on a navigation chart with a Weems or Warner Aircraft Plotter. The operation is repeated, using another radio station, and the second line drawn on the chart. By extending these lines, the location of the plane is found at the point of intersection. For the sake of accuracy a third line is used, in the hope all three will intersect at one point.
To make the application specific, note in Figure 8 the three radio stations, La Guardia Radio, Washington Radio and Bellefonte Radio. In quick succession, at predetermined intervals, the pilot tunes in on each of the three stations, then, by using the imaginary line, determines the directions from the stations to the unknown position of the plane at the time the station were tuned in. Each line is then plotted on the chart. By extending these lines, the intersection is found to form a small triangle. The plane is in the center of the triangle. The whole procedure is completed in three minutes and the plane is known to be over Columbia, PA.
The advantage in speed with which a fix is obtained cannot be overestimated in air navigation. By celestial navigation a fix can rarely be found in less than eight minutes and at the speeds which planes are now making even five minutes is a lot of time. It would be misleading, however, to make unjustifiable claims for ADF. There are times when a celestial fix is more accurate than a radio fix. Navigators, however, have told the writer of occasions when, in spite of the accuracy of the celestial fix, they put more faith in their ADF. Then there are occasions when a celestial fix can be obtained although an ADF fix cannot. And the reverse is also true, as already observed.
Navigation of airplanes naturally attracts our interest but not to the exclusion of ships of the sea. It is true, ships at sea can weather storms or heave to, drift off course without too much danger, "run out of gas" without crashing. Still, they too must know their position. They can afford to rely extensively on celestial navigation for they have more time for getting fixes and delay does not involve such great risks as it does for their sisters of the air. Nevertheless, on the Great Lakes, along the coast, on certain inland water-ways at night and under overcast weather conditions, they, too, look to the radio direction finder for guidance. For many years, ocean ships have used manually-operated radio direction finders, requiring skilled radio men. Now however, they are looking to ADF.
There is real drama, however, when radio and the heavens combine to avert impending disaster and bring about a happy ending. Such an incident occurred when a large war cargo plane, approaching its tiny island destination in the North Atlantic, developed battery trouble. No electrical equipment could be used. To add to the desperate circumstances, the navigator could not find a break in the sky. The crew waited patiently, hoping for sunrise before the gas tanks ran dry. At last the navigator was able to take a sight on the sun and from it to plot a line of position. The radio operator, by heroic or miraculous efforts, got enough life out of the batteries to obtain a line of position through the radio station on the island. Where the two lines crossed was the plane's position they were a number of miles past the island. Again, radio compass guided the crew to safety.
This article was originally published in the September, 1944, issue of Flying magazine, vol 35, no 3, pp 58-59, 106, 110.
The PDF of this article includes the figures above, a photo of a B-29 seen from 4 o'clock high with an inset showing a cutaway of the "football", and a photo of the radio compass dial.
Photo credits to Bendix, AAF.