T

HE 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.

Photo credits to Bendix, AAF.