FM 1-506 Fundamentals of Aircraft Power Plants ... - Survival Books

WWW.SURVIVALEBOOKS.COMFM 1-506supply and indicator, are airframe-supplied components.They may be replaced anytime without recalibration.The remaining three (power output shaft, head assembly,and junction box) are only replaced as a precalibrated set.When torque is imposed on the engine power outputshaft, tension and compression stresses change the magneticreluctance of the shaft. This system measures thechange of magnetic reluctance because of the torsionallyinduced forces (Figure 9-5).The output shaft has welded to it a ferromagneticsleeve which serves as a transformer core in the headassembly. A nonrotating transformer, the head assemblycontains the primary coils which receive alternating currentrequired to power the transformer from the powersupply. It also contains six sets of secondary coils orientedat 90 o to each and 45 o to the shaft axis. They pick upinduced current and register the differential in current onthe dial face of the indicator. In operation, the headassembly surrounds the ferromagnetic sleeve on the shaftwhich acts as the transformer core.The junction box acts as the system conditioner andincludes key calibration controls. The potentiometersstandardize the signal level for indicator display andbalance out effects of magnetic nonhomogeneities. Thebox also rectifies the secondary voltages from the headassembly.The cockpit-mounted indicator reads the engineappliedtorque in percent. A network located within theindicator computes the algebraic difference between thetwo secondary-induced voltages coming from the junctionbox.The power supply is an airframe-mounted unitwhich converts the airframe 28VDC signal to a constant70VAC 2K Hz signal for system operation.The General Electric T-700/701 torque-sensing systemis a reference shaft that is pinned to the front end ofthe drive shaft and extends to the aft end. It is free torotate relative to the drive shaft. The relative rotation isdue to output torque. The resultant phase angle betweenthe drive shaft teeth and reference teeth is electricallysensed by a pickup sensing the two teeth on the drive shaftplus two reference teeth. The electrical signal is conditionedin the electrical control unit, which provides a DCvoltage proportional to torque (Figure 9-5). An intermediatepower (1690 SHP), the output torque is 410pound-feet, and the twist of the shaft is 7.4°.EXHAUST GAS TEMPERATURE INDICATINGSYSTEMTurbine engines may be instrumented for exhaustgas temperature indication at locations before, between,or behind the turbine stages. Exhaust gas temperature isan engine operating limit and is used to monitor themechanical integrity of the turbines. It also checks engineoperating conditions. Actually, the temperature at theturbine inlet is the important consideration. This is themost critical of all engine variables. However, it is impracticalto measure turbine inlet temperature in mostengines. Consequently, thermocouples are inserted atthe turbine discharge. This provides a relative indicationof temperature at the inlet. The temperature at this pointis much lower than that at the inlet. It enables the pilotto maintain surveillance over engine internal operationconditions.Several thermocouples are usually used. They arespaced at intervals around the perimeter of the engineexhaust duct near the turbine exit. The exhaust gastemperature indicator in the aircraft shows the averagetemperature measured by the individual thermocouples.The readings of several thermocouples are usually obtainedindividually during ground engine maintenance bya selective switch. The spread between the lowest andthe highest thermocouple reading is useful in maintenance.It serves to indicate the presence of hot or coldspots in the engine exhaust pattern which may indicate aproblem inside the engine. The importance of exhaustgas temperature cannot be overemphasized. Two systemsthat are in use on Army helicopters follow.Thermoelectrical Systems (T-55-L-11E/T-55-L-712)The chromel-alumel, thermoelectric (MGT)measuring system is independent of all other engineelectrical wiring (Figure 9-6). The engine componentsare five dual-probe segments; each probe is connectedexternally so each segment can be operationally checkedindependently and replaced individually. The 10 thermocoupleprobes protrude into the gas stream at thepower turbine entrance (station 45). The probes reactto variations in temperature by developing a proportionalelectromotive force across the chromel-alumel junction.This potential difference results in meter deflectionsof the cockpit indicator. The cockpit indicator iscalibrated to read temperature in degrees centigrade.In addition, this system incorporates two buss-barslocated on the aft fire shield. This allows for connectionof the five MGT segments to the single MGT harness.9-4