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

WWW.SURVIVALEBOOKS.COMFM 1-506Figure 4-23 schematically describes the fuel controlsystem. Fuel control can be divided on a functional basisinto two sections: the flow control section, consisting ofthe valving which meters engine fuel flow, and the computersection. The computer section includes the elementswhich schedule position of the flow control sectionmetering valves as a function of the control input signals.The computer section also signals the closure of thecompressor bleed valve.This computer operates primarily on four engineparameters gas producer speed (Ng), power turbinespeed (Np), compressor discharge total pressure (Pt3),and compressor inlet total temperature (Tt2) for a givenpower Iever position and power turbine speed-settinglever position. The ratio of engine fuel flow to compressordischarge total pressure is the manipulated variableused to control engine power output. This variable isbiased by Tt2 during acceleration and Ng topping.The position of the engine fuel flow metering valveis established by a multiplication of two positions representingcompressor discharge pressure (P3) and fuel flow(Wf). The (Wf) engine fuel flow is made proportional tothe metering valve position by maintaining a constantdifferential pressure across the valve flow area, whichvaries linearly with valve position. This constant pressurein maintained by the pressure regulating valve. This valveis positioned as necessaryto maintain a 40-psi differentialacross the metering area. To accomplish this, all pumpoutput not required to operate servos and supply engineflow requirements is bypassed back to the pump inlet.The P3 servo (motor) is positioned by directingcompressor discharge (P3) to a bellows, which positionsthe servo directly proportional to P3 air pressure. Theservo is connected to the P3 ramp, which moves proportionalto servo inputs. The ramp works directly againstthe ratio servo and in turn positions the metering valve.The ratio servo maybe positioned by anyone of fourinputs: the Np governing linkage, the Ng governinglinkage, acceleration limiting linkage, or the maximumflow limiter. The inputs, which schedule the lowest fuelflow, determine the position of the metering valve. Theratio servo and the Ng servo provide an input to thecompressor bleed air signal mechanism.The Np governor linkage input to the ratio servo iscontrolled by the Np servo piston whose position isestablished by a flyball-operated pilot valve. It functionsas a droop-type governor, limiting fuel flow proportionalto speed over its operating range, which is set by thepower turbine speed setting lever position. The Npgoverning linkage input to the ratio servo is biased by Ngspeed such that Np governing droop line position increasesin speed as Ng speed decreases.The Ng governing linkage input to the ratio servo iscontrolled by the Ng servo piston whose position is establishedin the same manner as the Np servo. It functionsas a droop-type governor, limiting the ratio servo inverselyproportional to Ng speed over its operating range.Its operating range (ground idle to maximum speed) isestablished by the power and shutoff lever position. Thislinkage is also biased by T2 through the speed resetlinkage.The acceleration limiting linkage input to the ratioservo is controlled by a contour on the 3D cam. The 3Dcam is positioned longitudinally by the Ng servo pistonand rotated by the T2 servo piston. This provides aunique cam positioned for each set of NG speeds and T2conditions. The acceleration finding cam contourschedules the maximum allowable fuel of low P3 ratio ateach speed and temperature setting. The 3D cam alsopositions the speed reset linkage through the speed resetcontour and provides an input to the bleed air signalmechanism.The position of the T2 servo is established by theliquid-filled bellows which positions the servo directlyproportional to the compressor inlet temperature.The maximum flow limiter input to the ratio servolimits the engine shaft torque by limiting fuel flow. Thisis done by positioning the ratio servo as necessary toreduce fuel flow at a safe value.The control computer section also provides apneumatic signal to the bleed air acutator which is usedto open or close the bled air during appropriate phasesof engine operation.4-23