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

WWW.SURVIVALEBOOKS.COMFM 1-506Compares actual N pto a reference N pto computea speed error input signal for use in electricalcontrol computation.Compares actual torque of the engine to torqueof the other engine for load sharing. If theengine torque (Q 1) is less than that of the otherengine (Q 2), supplies a torque error signal to thecomputer. No error signal appears if Q 1isgreater then Q 2. This arrangement (frequentlyreffered to as a match-the-maxixmun principle)prevents the load-sharing circuit from causinga good engine to reduce power to match apower-limited engine. The torque error signalis added to the N speed reference signal andpcan increase up to 3 percent over the selectedN .pCompares measured engine TGT to a fixedreferenceTGT. When (and only when)temperature is above the reference, develops asignal to reduce fuel flow to correct the overtemperature.Compares the combined speed error and loadsharingsignal to the overtemperature signal inthe selector circuit of the ECU. Selects thesignal calling for the lowest N gand passes it tothe HMU to reduce gas generator power asrequired.Sends an electrical output from the selectordrives to a torque motor in the HMU. Thetorque motor can shift the load demandscheduled as if the PAS were moved in areduced power demand direction. The torquemotor can retard Ng no further than ground idleand can only allow power to increase to the PASsetting. The ECU may be overridden at anytime by advancing PAS beyond the normal(120°) flight position to the 130° stop. PAS maythen be retarded to trim the engine manually.Note that this has no effect on the Np overspeedsystem, which remains fully operational.Takeoff and ClimbBefore takeoff, the PAS lever is advanced fromground idle to 120°, which is the normal flight position.This allows the rotor head to accelerate to 100 percentNp/Nr with the collective still in flat pitch. Ng will rise asthe PAS is advanecd but will cut back after the Np governingspeed of 100 percent is reached. (Np governing canbe selected at 95-101 percent, but 100 percent is consideredstandard.)To take off, collective pitch is increased, increasingthe torque load on the power turbine. Simultaneously,the LDS rotates, calling for an increase in gas generatorspeed to keep Np/Nr from falling below 100 percent. TheECUs of each engine perform a fine trim of fuel flow tomatch torques and trim Np/Nr to 100 percent.As Ng inereases, the HMU schedules the bleed valveclosed and the variable stators open to increase airflowthrough the combustor and turbine.If collective pitch is increased to a very high angle,TGT may approach the temperature limiting value.When this occurs, the ECU prevents any further increasein fuel to the engine. If torque load is increased further,Np/Nr will droop below 100 percent because power turbinegoverning must be sacrificed to protect the engineagainst overtemperature.Cruise and DescentAt the end of the climb segment, less rotor lift isrequired and collective pitch is recked. The LDSrotates to reduce fuel flow and Ng; the variable stators willclose slightly to optimize part-power fuel consumptionand preserve stall margin.Again, ECUs trim fuel flow. Upon entering a descent,the same sequence of events reduces Ng to thepoint that the bleed valve maybe opened. If the collectivepitch is fully lowered, autorotation (power-off descent) isreached, and the torque drops to zero; overrunningclutches in the airframe transmission preclude a negativetorque situation. Once the engines are uncoupled fromthe rotor, Nr is free to accelerate above 100 percent; andboth engines will continue to govern Np at 100 percent.Nr and Np rotor speeds are held in reserve topick up the rotor load when collective is again increased.This condition (100 percent Np zero torque) is known asflight idle.T-55-L-llE/T55-L-712 FUEL CONTROL SYSTEMThe fuel control for the Lycoming T55-L-712 freepower turbine engine is a hydromechanical type designatedby Hamilton Standard as the JFC 31-22. It consistsof the following main units:Single-element fuel pump.Gas producer speed governor.Power turbine speed governor.Acceleration and deceleration control.Air bleed signal mechanism.Maximum flow limiter.Fuel shutoff valve.Temperature compensator.4-22