Tachometers Electric Tachometers Mechanical Tachometers The tachometer, or tach, is an instrument that indicates the  speed of the crankshaft of a reciprocating engine. It can be  a direct- or remote-indicating instrument, the dial of which  is calibrated to indicate revolutions per minutes (rpm). On  reciprocating engines, the tach is used to monitor engine power  and to ensure the engine is operated within certified limits. Gas turbine engines also have tachometers. They are used  to monitor the speed(s) of the compressor section(s) of  the engine. Turbine engine tachometers are calibrated  in percentage of rpm with 100 percent corresponding to  optimum turbine speed. This allows similar operating  procedures despite the varied actual engine rpm of different  engines.  [Figure 10-52] In addition to the engine tachometer, helicopters use a  tachometer to indicator main rotor shaft rpm. It should also  be noted that many reciprocating-engine tachometers also  have built-in numeric drums that are geared to the rotational  mechanism inside. These are hour meters that keep track of the  time the engine is operated. There are two types of tachometer  system in wide use today: mechanical and electrical. on the flyweights and moves them to an angular position.  This angular position varies with the rpm of the engine. The  amount of movement of the flyweights is transmitted through  the gear mechanism to the pointer. The pointer rotates to  indicate this movement on the tachometer indicator, which  is directly related to the rpm of the engine.  [Figure 10-53] A more common variation of this type of mechanical  tachometer uses a magnetic drag cup to move the pointer in  the indicator. As the drive shaft turns, it rotates a permanent  magnet in a close-tolerance aluminum cup. A shaft attached  to the indicating point is attached to the exterior center of the  cup. As the magnet is rotated by the engine flex drive cable,  its magnetic field cuts through the conductor surrounding it,  creating eddy currents in the aluminum cup. This current flow  creates its own magnetic field, which interacts with the rotating  magnet’s flux field. The result is that the cup tends to rotate,  and with it, the indicating pointer. A calibrated restraining  spring limits the cup’s rotation to the arc of motion of the  pointer across the scale on the instrument face.  [Figure 10-54]  It is not practical to use a mechanical linkage between the  engine and the rpm indicator on aircraft with engines not  mounted in the fuselage just forward of the instrument panel.  Greater accuracy with lower maintenance is achieved through  the use of electric tachometers. A wide variety of electric  tachometer systems can be employed, so manufacturer’s  instructions should be consulted for details of each specific  tachometer system. Mechanical tachometer indicating systems are found on small,  single-engine light aircraft in which a short distance exists  between the engine and the instrument panel. They consist  of an indicator connected to the engine by a flexible drive  shaft. The drive shaft is geared into the engine so that when  the engine turns, so does the shaft. The indicator contains a  flyweight assembly coupled to a gear mechanism that drives  a pointer. As the drive shaft rotates, centrifugal force acts  A popular electric tachometer system makes use of a small AC  generator mounted to a reciprocating engine’s gear case or the  10-30 Figure 10-52.  A tachometer for a reciprocating engine is calibrated in rpm. A tachometer for a turbine engine is calculated in percent of rpm. 5 3 10 25 20 35 30 15 X100 HOURS 10 RPM 0 0 0 0 0 0 10 20 30 40 50 60 70 80 90 100 PERCENT RPM 0 5 1 9 8 7 6 4 5 5