Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 14. Engine Synchroscope (Eclipse)

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 XIV - ENGINE SYNCHROSCOPE (ECLIPSE)

61. Purpose and use.-The use of this instrument is the same as the engine synchronism indicator as given in paragraph 56.

62. Description.-a. The engine synchroscope consists of a. sensitive voltmeter mechanism contained within a standard 1 7/8-inch bakelite case as shown in figure 23. The case is provided with a two-piece aluminum shield to prevent static disturbance in radio and compass equipment in the airplane. The scale is an arc of approximately 180° without graduations of any kind. The pointer is covered with luminous material, provided with a zero adjuster, and mounted on a shaft which rides in jewel bearings, thereby reducing friction to a minimum and making it extremely sensitive.

b. A condenser unit built into a metal shield is provided to smooth out the pointer movement and prevent excessive oscillation due to slight variations in frequencies. It is attached to the rear section of the instrument case shield. Three pairs of resistors are provided with each instrument having resistance values of 35,000 ohms, 50,000 ohms, and 75,000 ohms. They are cartridge type and uniform in over-all dimensions. The three sizes are furnished to meet the variation in voltage of different types of engine magnetos. A No. 18 solid copper wire is silver-soldered into each end of the resistor for attachment into the magneto circuit.

(1) Indicator.                                           (2) Resistors.
FIGURE 23.-Synchroscope.

c. Two single pole, single throw switches are used to turn the synchroscope off and on.

63. Operation.-The synchroscope and its resistors are connected between the primary circuits of two magnetos; that is, one magneto on each engine. Connections are made at the magneto switches as shown in figure 24. When connected to the two magneto switches, two alternating voltages whose frequencies depend on the engine rpm are applied to the instrument. The sum of two alternating voltages of different frequencies produces "beats" or "nodes" in voltage which occur at regular intervals depending on the difference in rpm of the two engines. Each "beat" in frequency produces one complete swing of the synchroscope pointer. At exact synchronism the two voltages add to produce a steady alternating voltage of constant magnitude having no "beats." The pointer therefore stops at a steady indication. Due to variation in magneto construction the resistance necessary to produce the correct deflection will range from 70,000 to 150,000 ohms.

64. Installation.-a. The general points on installation of instruments given in section III are applicable to this instrument.

FIGURE 24 -Diagram of a two-engine synchroscope installation.

b. The instrument, resistors, and switches are connected as shown in figure 24, using not less than No. 16 gage cable, care being taken that the synchroscope is connected to only one magneto lead from each engine. The synchroscope control switches are located as close to the magneto switches as possible. The insulation of the leads between the switches and the instrument must not be broken at any point as a ground in any part of the synchroscope circuit will cause one or both magnetos to cut out.

c. In making an initial installation of the synchroscope the two 50,000-ohm resistors are used first. One engine is operated at 1,200 rpm and the other at 1,500 rpm. Under these conditions the synchroscope needle should have a deflection of approximately one-half the scale. If the deflection is less than this, the two 35,000-ohm resistors are then used. If the pointer deflection is more than one-half full scale, the 75,000-ohm resistors are used. After the correct size of resistor has been determined on the initial trial, the same size is used for replacement unless otherwise directed. It is essential that each pair of resistors on any installation be of the same size.

65. Maintenance.-The general points on maintenance of instruments given in section II are applicable to this instrument. When the synchroscope is not in operation, the pointer should stand at the left end of the scale. A small screw is located at the bottom of the instrument in the cover glass for adjustment purposes when it becomes necessary.