Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 19. Self Synchronous Instruments

TM 1-413, TECHNICAL MANUAL,  AIRCRAFT INSTRUMENTS, Prepared under direction of the Chief of the Air Corps, WAR DEPARTMENT, WASHINGTON February 2, 1942. (This manual supersedes TM 1-413, November 7, 1940)

SECTION XIX - SELF-SYNCHRONOUS INSTRUMENTS

86. Purpose and use.-a. Self-synchronous instruments provide a means for measuring engine and airplane functions and transmitting the measurements electrically to a centrally located instrument panel in the pilot's or flight engineer's station in the airplane. Electrical transmission from the various remote points of measurement eliminates the use of rigid and semirigid connections such as tubing, shafts, capillary line, etc. On large multiple engine airplanes, this flexibility of connection is absolutely essential and can be accomplished satisfactorily only by use of electrical transmission systems. The instruments are used in pairs, one unit serving as a transmitter and the other as an indicator. By means of a selector switch it is possible to use one indicator with several transmitters. In this way indications from several engines or tanks may be had with greatly reduced space on the instrument board.

b. Self-synchronous instruments are used to measure and indicate the following airplane engine functions:

(1) Fuel pressure.

(2) Oil pressure.

(3) Manifold pressure.

(4) Engine rpm.

(5) Fuel level.

(6) Fuel flow.

(7) Oil temperature.

(8) Tail gear position.

(9) Landing gear position,

(10) Flap position.

(11) Bomb door position.

87. Description.-a. The self-synchronous system consists of the indicator located on the instrument panel, the transmitter located with the mechanical measuring device at the point of measurement, and the leads which connect these two units. Both types of indicators are equipped with a ring light, a small 3-volt lamp removable from the front of the instrument being the source of illumination.

(1) Front view.                                                           (2) Rear view.

(2) Rotor.                                                                           (4) Stator.
FIGURE 40.-Single pointer self-synchronous indicator.

b. The indicators are of two general types which differ only in construction, the principle of operation being the same. These are the single pointer indicator which is used with one transmitter and the dual pointer indicator which is used with two transmitters, each transmitter Operating one of the two indicator pointers.

(1) The single pointer indicator (fig. 40) consists of a motor having a suitable dial and a pointer fastened directly to the motor shaft. It is free to rotate in either direction, there being no stops or springs of any kind. The motor is a motor only in the sense that its shape and general construction are in many ways similar to the common small power producing motor. It does not, however, spin nor produce power. The two fundamental parts of each motor are the stator and the rotor. The stator is clamped between the two end castings of the motor. Fifteen slots on the inner side of the stator carry a three-phase winding, the three leads of which are connected to the terminals marked "1," "2," and "3" on the motor case. Within the stator, the rotor is free to revolve without stops and is carried in small deep groove ball bearings. The rotor is of the two-pole type and is carried on small precision ball bearings. It has on it a field winding which is energized by 32-volt alternating current from an outside source. One end of the winding is grounded to the rotor shaft and thence through the bearings and an auxiliary silver brush under the dial to the case of the instrument. The other end of the winding is connected to a silver pin protruding from the back end of the rotor shaft and insulated from it.. A flat leaf brush located under the bakelite terminal plate makes contact with the pin and connects with the outside terminal marked "A." The ground terminal marked "G" screws directly into the case. These two terminals are connected to the 32-volt power supply. The indicators used for the different functions are alike except for the dial with which they are fitted. The dial for each indicator is graduated over the same range as the mechanical instrument of like kind discussed in various sections.

(2) On the dual pointer indicator (fig. 41), both pointers rotate about the same center and are read on the same dial, each hand having a number to identify the engine with which it is associated. The indicator consists of two separate motors mounted in tandem. Both motors are similar in construction to the motor of the single pointer indicator, each having a stator and a rotor. The rotor shaft of the front motor is hollow- and an extension shaft from the back motor passes through it and carries a pointer on its outer end. The extension shaft is carried in jewel bearings and is connected to the rear motor by means of pin and fork coupling so that the two motors can be separated for inspection and repair. The pointer of the forward motor is fixed directly to the end of its rotor shaft. The rotor of each motor is energized by the 32-volt a-c power supply, connection to the rotors being made by means of brushes. One end of each rotor winding is grounded to the shaft by a small spring loaded silver brush on the forward end of each motor. Connection is then made to the terminal "G" on the back of the indicator. The ungrounded end of each rotor winding connects with its respective "A" terminal on the back of the indicator through a brush. The brush of the back motor is located under the terminal plate and contacts a silver pin projecting from the end of the shaft and insulated from it. The small cylindrical silver brush of the front motor is on the side of the case and is held by a small coil spring in contact with a silver slip ring on the rotor shaft.

FIGURE 41.-Dual pointer, self-synchronous indicator.

c. The transmitter (fig. 42) consists of a motor similar to that of the single pointer indicator coupled directly to a mechanism having a diaphragm, Bourdon tube, centrifugal flyweights, or other actuating element, depending on its function. The electrical elements of both the transmitter and indicator are similar except for the cases in which they are mounted. As with the indicator, the stator windings of the transmitter are connected to the terminals marked "1",  "2", and "3" and the rotor winding to the terminals "A" and "G" on the rear of the motor case. The motor of the transmitter may be said to occupy the place of the pointer on a conventional gage mechanism. The diaphragm, Bourdon tube or flyweights, as the case may be, causes the mechanism to rotate and assume a certain position to indicate a given pressure, rpm, or other condition. Thus, instead of rotating a pointer as on a conventional instrument, the transmitter motor is rotated.

FIGURE 42.-Self-synchronous fuel pressure transmitter.

88. Operation.-a. The alternating current supply for the operation of the self-synchronous instruments is generally furnished by an engine-driven alternator or an electrically driven dynamotor. The current supply control switches are located at a convenient point. in the cockpit. When the current is applied, all indicator pointers move from zero to a position on the scale that corresponds with the pressure or position element in the transmitter. Operation of the mechanical element in the transmitter is the same as for any conventional type of instrument of like kind described in other sections.

b. A wiring diagram of the self-synchronous instrument with indicator and transmitter is shown in figure 43. The three windings of the stators of both indicator and transmitter are respectively connected in parallel. Likewise the field windings of the rotors of both units are connected in parallel across the 32-volt a-c supply. When this a-c supply is turned on, the current in the rotors of both units will set up identical magnetic fields at right angles to the flow of the current in the windings as shown by the arrows in the wiring diagram. Since the rotor of the transmitter is fixed in a definite position by the mechanism of that unit, definite induced voltages will be set up in each of the three stator windings of the transmitter proportional to the number of lines of the magnetic field cutting each winding. For any given position of the rotor these three voltage values are fixed and correspond only to that position. Since the three stator windings of both the indicator and transmitter are connected in parallel, these same induced voltages will be present across the respective stator windings of the indicator and hence the indicator rotor, which is free to revolve, will assume the identical position of the transmitter rotor. No further movement of the indicator rotor takes place until the volt-age values in the stator windings are changed by a movement of the transmitter rotor. In this way the indicator, which would otherwise be shown by a pointer mounted on the conventional gage mechanism, is transferred from the transmitter mounted at the place of measurement to the indicator mounted on the instrument panel.

FIGURE 43.-Self-synchronous operation diagram.

89. Installation.-a. Typical installations of the self-synchronous system on a four-engine airplane are given in figure 44, showing the indicators, transmitters, and connecting leads. In figure 44 the single pointer indicators are used, one set being employed for each engine. In figure 44 (1), the dual pointer indicators are used, requiring only one set for the right engines and one for the left. In addition, the use of selector switches allows four of the six engine functions in each case to be measured on one indicator.

b. General information on installation of instruments is given in section III. The indicators are mounted on shockproof instrument panel in the conventional manner. Transmitters are mounted on a vibration absorbing mount as near to the source of indication to be transmitted as possible, usually in the engine nacelle. Installing re-placements requires particular attention in insuring proper wire continuity, including the proper cleaning and the secure fastening of all connections. For details on any particular installation reference is made to Air Corps Technical Orders for the particular airplane involved.

90. Maintenance. - See section II.

FIGURE 44.-Installation diagram of self-synchronous instruments