Military Aviation Radio Equipment

Perhaps one of the most remarkable phases of the war has been the tremendously rapid advance in tactical use and l technical development of the radio by the air forces of the world. Where only a few years ago the total radio equipment on a military plane might consist of a relatively simple receiver and transmitter, for flying radio ranges and communicating with control towers, the great progress made since the war's beginning and the increased needs dictated by military considerations has necessitated the addition of so much electronic equipment to our big planes that some cynics profess to view the close approach of the day when the overzealous radio engineers and designers will seal the bomb bays to obtain additional space for their complex and multitudinous apparatus, thereby converting a bomber into a veritable electronic actuated device.

Already the list of non-secret equipment installed in our giant bomb carriers is formidable, and when it is realized that there are several other sets, whose functions cannot be mentioned, in addition to these, it is readily apparent that the field of aviation radio is a rapidly expanding one, and now is nearing the attainment of the engineers' dreams of having a plane with complete automatic control. It is not too far off to envision the day when a pilot will be able to direct his plane in the manner of a glorified elevator operator, by merely pushing a button marked with the name of his destination, whereupon his plane will take off, fly its course controlled from a master airways control room, and land, every move having been handled automatically by electronic devices. As far back as 1937 the feasibility of this scheme had been proven in part by army engineers, who connected an automatic radio compass to the gyropilot and were able to make a plane fly a course over a route marked by beacons on the ground.

The requirements for an installation in the P-47 fighter, a B-17 or a B-24 are not quite as complex as this, for the functional uses of radio can be divided into three groups: communication, navigation, and detection.

For communication purposes, the following demands must be met: a relatively low power, short range, voice-operated transmitter for talking between planes, or for receiving landing or take off instructions from the local control tower; a high power, long range transmitter, which can use either voice or code transmission, and span the long distances met in intercontinental operations; an interior communications system in the plane which will enable personnel to talk with each other and into which the outputs from the various communication and navigation radios can be channeled for distribution to the proper crew member; and an emergency transmitter, completely independent of the plane for a source of power or antenna structure, which can summon help in the event of a forced landing.

In regard to radio and its use as a navigational aid, the primary object in any navigation problem is to locate one's self on a line of position, if it is desired to fly a course along that line, and on two or more non-coincidental lines of position, if one wishes to get a position point from their intersection. Also a means may be provided for obtaining a definite pinpoint location from a single object or radio transmission, provided a known object is sighted or a highly localized radio transmission which extends over a small area is received.

Lines of position may be found in two ways from a plane. One method fixes the line on the equi-signal strength intersection of two radio beams transmitted at an angle to each other from a radio range station. This is excellent when the plane is "on the beam," but if not, only an approximate area of position can be found, and that after going through a somewhat complicated orientation problem.

Another manner in which position lines may be drawn at any point is by use of the loop antenna, with either aural null or automatic indication. Any radio broadcasting station or radio range station that can be received with sufficient strength may be used, thus affording, in this country, at least, a large choice of position lines and making it relatively easy to fix a location. Positive position indication from a single transmitter uses a beam of concentrated ultra-high frequency energy directed upward from the station, and through which beam the plane will pass if correctly flying the radio range or blind approach system for landings.

These navigational problems can all be solved by use of radio, providing the proper radio facilities are available, both for transmission on the ground and reception in the plane. Moreover, the location of all ground stations must be accurately known, and a radio facilities map of the region being flown over is an indispensable aid to the use of radio navigation equipment.

Bulkiest of all equipment is the big liaison transmitter with extra coils, for wide frequency coverage, stowed away in an adjacent rack. Used in conjunction with this is a sensitive superheterodyne receiver with band switching. Under good conditions distances of several thousand miles may be covered with this equipment. The transmitter and receiver are both located at the radio operator's position and under his direct control.

Remotely controlled from the pilot's and copilot's positions are the low powered command sets. The high-frequency set, with dual receivers and transmitters, also incorporates a third receiver for reception of low-frequency radio ranges. The ultra-high-frequency command set, if used, is located near the other set, with a remote control box at the pilot's fingertips. A small two-tube 75 MHz receiver has its output combined with that of the range receiver to give indication of the plane's position over a fan or Z marker beacon, if flying a domestic airway.

At the navigator's position are the tuning controls for the automatic radio compass, together with the azimuth indicator, whose needle instantly swings to the bearing of any radio station that is tuned in. A remote control and indicator are also provided for the pilot's use; and back near the tail is the one set which the crew never wants to be forced to use: the emergency, hand-cranked sea-rescue transmitter which will float when thrown from the plane.

Intercommunication control boxes at every crew member's position have an individual volume control and a switch which selects the output of the compass, liaison or command receivers, or else the one tube interphone amplifier, which is separate from the radio sets. A typical simultaneous use of the system might find the pilot flying the radio range on one receiver, the copilot talking with an accompanying plane, navigator taking some cross bearings with the radio compass, radio operator working his home base, and a couple of gunners chatting over the interphone about last night's dates, all at the same time. In order that any crew member may cut into all other channels at once to attract everyone's attention, should some occasion demand, a switch position is provided on the interphone box.

These present types will probably continue in use to the end of the war, for they are performing excellently, and large scale conversion at that date would hardly be feasible, but their very multiplicity points to the need for a unified, integrated design for the future super-aircraft of the post war period. The desirability of this will be readily appreciated by anyone who has seen the interior of a bomber, crammed with radio equipment filling half a dozen racks, scattered in various places from nose to tail, each having its own dynamotor for voltage conversion and requiring a great mass of interconnecting cables.

This situation has been brought about by the fact that our original peacetime aircraft radio program started with but a few simple pieces of equipment, and progressive development resulted in the gradual addition over a period of years of all the sets in use today, with the outcome being that they had to be fitted in already designed planes, wherever room could be found. With the increasing electrical demands for radio and auxiliary equipment, such as selsyns or remote indicating devices, we have reached a state where the big plane now has a large number of small generators and dynamotors, each converting the primary 12 or 24 VDC from the engine driven generators to the higher AC or DC voltages required for their particular equipments. This is an extremely wasteful and inefficient method of converting power from one voltage to another, for several reasons: all of these machines require maintenance and inspection at frequent intervals. Brushes wear out, especially at high altitudes, bearings must be greased, and commutators cleaned. Efficiencies, especially in the smaller machines are low, only about 50%, in many cases. A large amount of space and weight is taken up, and very heavy loads are imposed on the engine-driven generators. A great deal of copper is needed in cables to carry the low-voltage primary currents. The solution, and one which is already close to accomplishment, is the installation of a large alternating current generator in the plane, which functions as the modern power house feeding a distribution system with 120 VAC at 400[Hz] to 500[Hz] and filter components is achieved. Transmission set will step the voltage up or down, rectifiers will convert it to DC.

This article was originally published in the December, 1943, issue of Air Tech magazine, vol 3, no 6, pp 38-39, 52, 62.
The original article includes 2 uncaptioned photos and 12 captioned photos.
Photos credited to Bendix Aviation Corp, Radio Corp of America.

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