INSTALLATION OF RADIOS By A. R. Applegarth, Jr. Vice-President—Advanced Scientific Planning National Aeronautical Corporation (Reprint from National Pilots Service Bulletin Distributed by the National Pilots Association) (EDITOR'S NOTE: Modern aircraft radios are well engineered and built to high standards. But reliability cannot be had without proper installation. The following is the first of two installments on proper radio installations by an outstanding specialist in the field, A. R. Applegarth, Jr., VicePresident of NARCO).

protection while the master protectors will only function in the event of a wiring short circuit outside of the radios. This requires that the radio master protector be capable of handling a greater load than the sum of the individual radio loads, and should be the maximum capacity which the master radio wire size will allow. (Fig. 1 illustrates the above points). r,,j ., S.' ...j

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AVE YOU ever stepped into the left seat of an unfamiliar airplane and been completely baffled by H the radio system? Have you ever been in a tight situation

in your own familiar airplane where you have pushed the wrong switch or forgotten to push the right switch, resulting in an embarrassing communications error? If you have not, you are either a superman, fly very simple and rare radio installations, or stay on the ground. Today's light airplanes frequently have radio systems which rival the most elaborate airliners in complexity, and which require a level of operating skill which even the best trained ATR pilot would find challenging. The purpose of these articles is to discuss the factors which should be considered in laying out a radio system particularly from the pilot's viewpoint, so that an aircraft owner can guide his radio installation people toward a system which fits his pilot's human needs as well as his technical requirements. The material will be presented in a pilot's language, but with sufficient technical detail to enable him to provide his radio installer with the definite technical guidance information needed to produce the desired results. POWER SUPPLY CONSIDERATIONS

Radio Master Switch

In more complex installations where several separate radio equipments are used, it is indeed a convenience to the pilot to have one common radio master switch which will remove all power from all radio equipment. Individual switches can be retained (these are usually built into the equipment) to provide the capability to turn off those specific radios which are not in use. For example, this may be important in a ground taxi situation where the engine(s) are not turning fast enough to provide generator power for more than one communication radio. The radio master switch can be installed directly in the branch of power distribution system feeding all radio gear. A solenoid (power relay) can be used to avoid long runs of heavy wire, and to permit a small operating switch to be employed. Your radio installer should provide a fuse or circuit breaker in the feed wire to each individual radio, so as to insure that a failure in one equipment will not result in power loss to another.

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Power distribution system for radio equipment. COMMUNICATIONS

Single Radio System

Those aircraft which need but one radio communication set present a minimum installation design problem. No microphone, speaker, or phones switching is necessary, or desirable. When navigation equipment is included, please refer to the appropriate paragraphs listed below. Multiple Radio System

Multiple radio communications capability usually establishes a need for selecting the particular transmitter and receiver to be used for any given communications. Two basic approaches are possible — one using pilot operated selector switches, and the other pilot operated volume controls. Sometimes a combination of both systems is used. In a system which uses the "volume control method", two or more receivers are permanently coupled to one loud speaker (or head set). Selection of the desired receiver is made by individual volume control manipulation. Flexibility is great — any combination of receiver output at any combination of volume levels can be obtained. Thus several receivers can be simultaneously monitored covering several channels and services such as local tower and FSS, tower and ground control, tower and approach control, center and approach control, etc. The disadvantages of such great flexibility are pilot confusion, caused by multiple voices and by the need to manipulate the volume controls of several radios when changing communications frequencies, and possible mistakes in coordinating volume

Radio Master Circuit Protector

A master radio protective device such as a circuit breaker or fuse in the wire feeding power to all radio equipment should be avoided. Sometimes, because of a long run between the electrical "buss" and the individual equipment protectors, FAA may require radio master protection. If this is the situation, the radio master fuse or circuit breaker should be located where the pilot can reestablish the circuit (by replacing a fuse, or resetting a breaker) unless an automatic reset circuit breaker is installed. In a properly designed power distribution system, the individual radio protectors will provide the principal

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Communications selector system (Continued on page 15) SPORT AVIATION

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CORBEN SUPER ACE . . . (Continued from page 10)

spar between the ribs at both top and bottom. Nail and glue the plywood to these strips. If you have trouble getting the plywood to take the curve of the leading edge of the wing, it may be steamed and wrapped around the wing and held in place with string until dry, after which it is then glued and nailed. The Vi in. spruce strips are also nailed to the rear spar at top and bottom along the aileron opening in the same manner. This will hold the fabric the same height as the ribs. While the first Corben Super-Ace only had a wing spread of 25 ft., we have increased it to 27 ft. The 25 ft. job flew very well and our only reason for increasing its wing is in favor of the fellows who live in high altitudes. As this increase naturally gives a quicker take-off and a slower landing speed, and by increasing the rpm a little on the old Ford motor, we are still able to keep the speeds about the same. Oh, yes. Some letters have come to me in regard to the wing curve, saying the dimensions for the rib layout were not given on the drawings. This was not shown, for by the time it was rescued for publication, it would have been too small and no doubt would have resulted in several mistakes, therefore it will be necessary for you to lay the airfoil out from a Clark "Y" chart. The gas tanks on the Super-Ace are installed one in each wing. They are held in place by three steel straps, two on the bottom and one across the top of the tank. The top strap has a small turnbuckle brazed in it so this strap is adjustable. In making up the bottom straps be careful to make them just the right length so the tank will not protrude below the bottom curve of the wing. The strap nearest the root end attaches to the root end fitting bolt, both at the front and rear. The gas tanks are made of terne plate. This is a leadcoated material that will not corrode nor rust and neither tin nor galvanized steel should be substituted for it. The filler neck is the same as used on the Model "T" Ford radiator. The drain fitting is cast brass and is soldered to the tank. When the tanks are installed, lay strips of tape or felt between the tanks and straps, otherwise the shifting of the tanks might rub a hole in the metal and you would have to uncover part of the wing to make repairs. See that your tanks are well fastened and held tight. I do not think any time will have to be spent on the aileron details for they are very clearly drawn upon ths plans, so we will move on to the tail group drawing. The entire tail group is constructed of steel tubing

INSTALLATIONS OF RADIOS . . . (Continued from page 9)

control manipulations with transmitter selections. This latter problem is more serious than many of us realize, particularly when channels have to be changed quickly while performing other cockpit duties. A receiver selector system which depends solely on volume control operation should be limited to the very simplest installations which

include nothing more than one l1/^ com-nav radio. Audio switching which is the other basic approach,

can offer the pilot instant, fairly obvious manipulations,

but unless carefully planned can challenge the most experienced telephone switchboard operators. In this area some research has been conducted with useful results. In either the volume control or switchboard approaches, the

selection of the desired transmitter is usually made by

and later covered with fabric the same as the wings. You

will have quite a little welding to do, so if you are not a real good welder it will pay you to get in touch with someone who will do the welding for you, for it is very important that all welds are just right. First, cut and fit all of the tubing as detailed, make up the ribs for the elevators and rudder as shown. These are made out of sheet steel and formed on a tinner's "break", the ends are cut out to the size of the tubing to which they are welded. It will be a good idea to lay the tail group out full size on a wood bench or floor and fit all the pieces to this layout. In other words, it is to be a temporary jig, nails are driven in around the tubing to hold it in place, in the same manner you would use when building a model plane. All joints are tack-welded in place, then welded later; care should be taken to keep the different units from warping during the tacking and welding, however, if they do warp they should be straightened and lined up before the cover is put on. You will note from the drawing there are several small tubes welded onto the leading edge and the rear spar of the stabilizer. The front tubes are for attaching the stabilizer to the fuselage while the rear ones are for the brace wires and elevator hinges. A hole is first drilled through the stabilizer tubing and the small tube is then inserted into the hole and welded at each end. It is a good idea to make the small tubes about one quarter of an inch longer than desired as it is much easier to weld the ends. After the welding is done the ends are cut off to the length shown on the drawing. When making up the elevators and rudder it is necessary that the hinges be made up and slipped onto the spar and into their proper places as detailed. This also applies to the hinges, collars and control horn details. When welding the control horns, care should be taken to have them 93 deg. to the line of control surface. You will note that the horn is made up of .125 steel and is welded onto a tube sleeve having a fish-cut mouth at each end. This is then welded to the spar with a fish-mouth weld. All steel used in the construction of the tail group as well as the tubing is S.A.E. 1025 steel and no other kind or grade of steel should be used. The brace tubes shown must be heated to a cherry red before bending or flattening as detailed, the gussets are welded on after the tube is bent. In the next article I will give you the plans for the fuselage and the fuselage fittings, so have plenty of good hack saw blades on hand, for their will be a lot of cutting and fitting to do. (To Be Continued)

means of a selector switch, although separate microphone jacks are possible, and may even be preferable in certain limited applications. The transmitter selector switch can be arranged to simultaneously switch the associated receivers outputs. The loss in flexibility that results is offset by the large gain in reducing the pilot load to the manipulation of a single switch when changing channels, etc., and of the firm knowledge that a given transmitter will

always be paired with its companion receiver with no possibility of cross-radio cockpit confusion. I have used this arrangement for some 3% years in my personal airplane for some 850 flight hours ranging from the simplest VFR

situation to IFR approaches at AGA, DCA, LSX, ORD, etc.,

and have yet to feel the pinch of lost flexibility. (See Fig. 2).

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