Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 12. Chronometric Tachometers

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 XII - GENERATOR VOLTMETER TACHOMETERS

51. Purpose and use.-Generator voltmeter tachometers are designed to measure the speed of the engine crankshaft in revolutions per minute. Since the operation of this instrument is independent of the distance between the engine and the instrument panel, it is a desirable tachometer for use on multiple engine airplanes or on two-place airplanes where complete sets of engine instruments are installed in both cockpits. Attempts to use shaft-driven tachometers with a special two-way adapter on the latter type installations have not proved satisfactory because the abruptness of the bends in the shafts causes wear and friction resulting in a quick deterioration of the shaft. The generator voltmeter tachometer is used on any installation, single or multiple engine, where there may be interference of fuel tanks, anummition boxes, etc.; the distance from the cockpit to the engine is excessive; or multiple sharp turns would be required in a flexible shaft drive. In most cases it is given preference over the chronometric type when installation difficulties of this kind are encountered.

52. Description.-a. The generator voltmeter tachometer (fig. 18) is an electrical instrument composed of three principal units; the indicator, which is mounted on the instrument panel; the generator, which is attached to the tachometer drive of the engine; and the connecting leads which are of insulated low-voltage cable.

b. The mechanism contained within the indicator is a permanent magnet moving coil voltmeter which is directly connected to the pointer. This type of mechanism permits only 270° rotation of the pointer, and since the desired scale range is 3,500 rpm, the smallest scale units are 100 rpm. The case is of bakelite and is provided with a shield to prevent interference with the operation of the magnetic compass and radio. As with all other types of airplane tachometers the indicator mechanism is adjusted for a crankshaft speed ratio of 2 :1.

c. The magnetic generator is a small compact unit designed so that it may be attached easily to the tachometer drive on all engines. The armature shaft is mounted in ball bearings and provided with a tang for insertion into the drive unit on the engine. The brush gear and commutator are of conventional type and construction and are provided with suitable terminals for connecting the generator electrically with the indicator. Direction of the rotation of the generator is optional and will be governed by the particular type of engine to which it is attached. Referring to figure 18, if the direction of rotation is clockwise, the right-hand terminal will be positive ( + ), and if the direction of rotation is counterclockwise, the left-hand terminal will be positive ( + ). The leads are made of standard No. 18 aircraft power and lighting cable. This cable is copper wire, tin-coated. and covered with varnished cambric insulation. Lengths will vary to suit the requirements of any particular installation.

(1)Generator.                                                           (2) Indicator.
FIGURE 18.-Generator voltmeter tachometer

53. Operation.-The tachometer generator turns one-half crank-shaft speed. and being connected electrically to the voltmeter indicator which is calibrated accordingly, an instantaneous reading on the indicator can be obtained. As the engine speed increases or decreases. the resulting reading on the indicator varies because the actual voltage output of the generator is directly proportional to the engine speed. Due to the perfect balancing of the coil and the fact that it rotates in jewel bearings, the pointer on the indicator does not oscillate to any great extent and a steady and accurate reading is possible at all times.

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

FIGURE 19.- Wiring diagram of general or voltmeter tachometer installation.

b. The following specific points will be observed when installing generator voltmeter tachometers :

(1) When mounting the generator unit the threads on the tachometer drive outlet on the engine and the union coupling nut of the generator are cleaned and a drop of oil added. The generator is placed against the tachometer drive outlet on the engine and the union coupling nut tightened. It may be mounted with the terminal studs in any direction convenient for installation. The indicator is connected with the generator using suitable lengths of No. 18 cable, and no attempt should be made to substitute any other gage or kind of cable. If the indicator reads backward when the engine is started, reverse the connections of the generator. The direction of rotation of the generator armature is optional ; however, if its rotation is reversed, its polarity is changed and the leads to the voltmeter indicator must be changed accordingly.

(2) This tachometer may be used with a single or double indicator installation for a single engine. In either case when making the connections with the indicator, locate the numbers and signs on the terminals and make the connections as shown in figure 19.

55. Maintenance.-a. The general points on maintenance of instruments given in section III are applicable to this instrument.

b. Several specific points of service maintenance and inspection of generator voltmeter tachometers must be considered. The generator and indicator units of this tachometer are sealed units and any repair or adjustments necessary to the internal mechanisms are major repairs. No lubrication on either unit is required in the field. Zero adjustment of the pointer on the indicator is provided by means of a small screw on the face of the indicator. This adjustment is permissible and may be accomplished when calibrating the indicator. Connection of the leads at all terminals must make clean contacts and should be safetied with lock washers to prevent looseness caused by engine vibration. The leads must be of the specified gage and long enough to prevent tautness at any point. Insulation of the leads must be unbroken at all points and the leads anchored to structural members of the airplane at proper intervals.