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

WWW.SURVIVALEBOOKS.COMFM 1-506metering systems and various sensing mechanisms.Electronic fuel controls contain thermocouples,amplifiers, relays, electrical servo system switches, andsolenoids. The discussion of fuel-control theory will addressmainly the hydromechanical type. The simplestcontrol is a plain metering valve to regulate fuel flow tothe engine. This type of control could be installed on anengine used for thrust or gas generation.Fuel controls may sense some or all of the followingengine operating variables:Pilots demands.Compressor inlet temperature.Compressor discharge pressure.Burner pressure.Compressor inlet pressure.RPM.Turbine temperature.Some refinements might include a —Pump to pressurize the fuel.Shutoff valve to stop fuel flow.Relief valve to protect the control when theshutoff valve is closed.Minimum fuel control adjustment to preventcomplete stoppage of fuel by the metering valve.Turboprop and Turboshaft Engine Fuel ControlsLike fuel controls for turbojet and turbofan engines,the fuel control for a turboprop or a turboshaft enginereceives a signal from the pilot for a given level of power.The control then takes certain variables into consideration.It adjusts the engine fuel flow to provide the desiredpower without exceeding the RPM and turbine inlettemperature limitations of the engine. But the turbopropor turboshaft engine control system has an additional jobto do that is not shared by its turbojet and turbofancounterparts. It must control the speed of the propelleror the free turbine, and it usually governs the pitch angleof the propeller blades.Many turboprop and turboshaft engines in productiontoday are the free turbine type. Engines of this kindact principally as gas generators to furnish high-velocitygases that drive a freely rotating turbine mounted in theexhaust gas stream. The free turbine turns the propellerthrough a system of reduction gears. If the engine is aturboshaft model, the free turbine rotates (also throughreduction gears) a helicopter rotor or powers a machine.The General Electric T-700 and the Lycoming T-55-L­712 fuel control system follow.T-700 CONTROL SYSTEMThe engine control system incorporates all controlunits necessary for complete control of the engine. Thesystem provides for the more common functions of fuelhandling, computation, compressor bleed and VG control,power modulation for rotor speed control, and overspeedprotection. The system also incorporates controlfeatures for torque matching of multiple engine installationsand overtemperature protection.The T-700 control system was designed for simpleoperation requiring a low level of pilot attention. Thesystem performs many of the controlling functionsformerly performed by the pilot. This has been done byproviding —Isochronous power turbine and helicopter rotor(N p/N r) governing.Automatic load sharing.Automatic limiting of power turbine inlettemperature.Rapid engine transient response through collectivecompensation.Automatic startingContingency power.Basic system operation is governed through the interactionof the electrical and hydromechanical controlunits. In general, the HMU provides for gas generatorcontrol in the areas of acceleration limiting, stall andflameout protection, gas generator speed limiting rapidresponse to power demands, and VG actuation. TheECU trims the HMU to satisfy the requirements of theload to maintain rotor speed, regulate load sharing, andlimit engine power turbine inlet temperature.Component Arrangement and InterconnectionMetering of fuel to the engine and basic enginecontrol computations are performed in thehydromechanical control unit (Figure 4-18). The electricaland hydromechanical control units compute the fuelquantity to satisfy power requirements of the engine. Thefuel and control system contains the following components:Hydromechanical control unit which includes a9947 RPM (at 100 percent of Ng) high-pressurevane pump package and a closed-loop VGservo-acutator. The basic unit meters the requiredfuel to the engine as a function of N gT 2,P 3, an electrical trimming signal from the4-17