Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 13. Engine Synchronism Indicator (Weston)

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 XIII - ENGINE SYNCHRONISM INDICATOR (WESTON)

56. Purpose and use. The synchronism indicator and the engine synchroscope (sec. XIV) are designed for use on multiple-engine airplanes for synchronism of two engines, eliminating the necessity of accomplishing this by sound or by readings on two tachometer indicators. The indication on these synchronizing instruments allows the pilot to adjust the throttles and mixture controls so that both engines and propellers are operating at the same speed. This minimizes the vibration and dissonant throbbing otherwise present.

57. Description.-a. The engine synchronism indicator (fig. 20) consists of a high resistance millivoltmeter which is adjusted to measure the unbalanced voltage generated by two tachometer generators running at slightly different speeds. The indicator scale has the zero mark in the center and a range in each direction of 50 rpm.

b. The voltmeter mechanism is placed in a 2 3/4-inch molded bakelite case. The instrument case is provided with a shield to reduce magnetic interference with compasses and radio equipment. Two conventional stud type binding posts protrude through the rear of the case for connection of the indicator to the tachometer circuits. These posts are marked + (positive) and — (negative). The instrument is provided with the standard 3-volt lamp, the receptacle being molded integral with the instrument case.

FIGURE 20.- Synchronism indicator

58. Operation.  a. The synchronism indicator is connected into the circuit with the tachometer indicators and generators as shown in figure 21. Its indication and use are selective with that of tachometer indicators, the control gage switch having three positions, "off," "tachometers on," and "synchronizer on." For conventional operation the switch is set for the "tachometers on" position so that the engine speeds will show on each individual tachometer indicator. When the desired altitude has been attained and the airplane "trimmed," the switch is then set for the "synchronizer on" position.

b. Due to the difference in generated voltage of the two tachometer generators, the engine synchronism indicator shows the difference in speed of the two engines in revolutions per minute. A pointer movement in a clockwise direction indicates that the right engine is running at the greater speed and a counterclockwise rotation of the pointer indicates that the speed of the left engine is the greater. The throttles are adjusted until both engines are running at the same speed and the pointer remains at zero. Accuracy is within 2 rpm. Due to the high torque imposed by the resistance, the sensitivity decreases rapidly at the outer end of the scale and the indicator will not be damaged by a tachometer generator speed difference up to 3,500 rpm.

FIGURE 21.-Wiring diagram for installation of generator voltmeter tachometers and synchronism indicators.

c. After the synchronization procedure the switch position is optional. Simultaneous indication of the tachometers and indicators is not possible. It is necessary to select either one or the other and no damage will result by continuous or intermittent operation on either of the selective positions.

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

b. In addition to these, there are some specific points on installation which are applicable to this engine synchronism indicator.

(1) Before mounting the indicator, the two tachometer generators with which it is to be connected are adjusted to give identical output at the same rpm. The generators are attached to a tachometer test stand and wired to the synchronism indicator as shown in figure 22. The tachometer stand is run at approximately 1,800 rpm. and the reading of the synchronism indicator observed. If it is off center, the adjusting screw cap is removed from one of the tachometer generators and the adjusting screw turned until the synchronism indicator reads zero. If the indicator is off center more than 10 rpm, half the adjustment is made on each generator. The adjusting screw caps are then replaced and safetied.

(2) If a tachometer stand is not available, the above check may be made on one of the engines of the airplane by connecting one of the tachometer generators to each of the tachometer outlets and running the engine at approximately 1,800 rpm. The error in the synchronism indicator at any other speed must be within 2 rpm.

FIGURE 22.-Wiring diagram for connecting two tachometer generators with a synchronism indicator when making bench tests and shunt adjustments.

(3) By means of the zero corrector located at the bottom of the front cover glass, the pointer is set, using a small screw driver, to a point on the scale corresponding to the 0 reading. After this operation it will not be necessary to reset the pointer unless the zero corrector is moved accidentally. After the indicator is mounted on the instrument panel it is connected to the two tachometer circuits as shown in figure 21, using No. 18 gage shielded low voltage airplane power and lighting cable. These connections must be clean and tight, otherwise resistance will be introduced and the readings incorrect.

60. Maintenance.-The general points on maintenance of instruments given in section II are applicable to this instrument. When the generators and indicators are removed for periodic adjustment, the same procedure is used as that preceding the installation.