804,206. Flight training apparatus. CURTISSWRIGHT CORPORATION. Oct. 25, 1955 [Nov. 23, 1954], No. 30490/55. Class 4. Ground training apparatus for aircrew to simulate the speed response of a turbo-jet engine comprises a first electro-mechanical servo system 37 responsive to simulated throttle operation by the pilot for producing an electrical control quantity variable in magnitude at a predetermined maximum rate to represent scheduled fuel supply to the engine, a second electro-mechanical servo system 1 normally responsive to said electrical quantity and operable to represent engine speed, and feedback control means for said engine speed system responsive to operating conditions of the engine for varying the feedback characteristics whereby the response of the engine speed system to both said electrical quantity and said feedback control simulates the speed response characteristics of the engine. The system 1 is conventional, having a summing amplifier 2 fed from a network 3 to operate a motorgenerator M-G which operates the wipers 14 and 15 of cards 12 and 13 which respectively supply voltages to contacts 97 and 17 of amplifier 2 representative of turbine speed and starting conditions. The motor-generator also operates a switch cam 18 and an engine speed indicator 19. Starting operation. The pilot presses a simulator starter button 24 to close a start switch 22 thereby energising a card 13 of servo system 1 through line 26 so that the amplifier input at 17 represents initial starting speed and the M-G set moves wipers 14 and 15 upwardly until slider 15 reaches grounded tap 27, and cam 18 closes switch 30. Before simulated " firing " can take place throttle lever 31 must be moved to an " idle speed position to advance a slider 32 on a " throttle card 33 to feed a voltage by lead 34 to throttle control selector switch 35 and lead 36 to the amplifier 38 of a throttle servo system 37, the motor 39 of which operates the slider 40 of a card 41. The voltage derived at 40, which represents the programmed control of fuel supply to the engine, provides a feedback to amplifier 38, an input voltage, on leads 42 and 43, for an " idle fuel flow " thyratron 44 for controlling idle engine speed and an input voltage to terminal 45 of amplifier 2 of servo 1 through lead 46, an " air plug " card 47, lead 49, a contact of switch 50, the a contact of a " firing relay " switch 51 and lead 52. The " firing " relay 23 is energized when the starting switch 22 is closed and cam switch 55 is closed by movement of throttle lever 31 to " idle " position, when cam switch 30 is closed by operation, of servo 1, and when an instructors " fuel available " switch 58 is closed. While the start switch is on and before the " firing " relay is held by a holding circuit, two feedback voltages Efb(1) and Efb(2) are supplied to the amplifier 2 to simulate sluggish operation by the starter. When the " firing relay " is energized the feedback Efb(2) is cut-out to permit servo 1 to pick up speed in simulation of the actual firing of the engine. Idle fuel flow and idle speed operation. The thyratron 44, which simulates idling operation of the engine, is energized by voltages representing respectively fuel flow at terminal 75, servo throttle position at terminal 76, a constant voltage representing idle fuel flow at terminal 77 and the actual turbine speed at terminal 78. For stable operation the' oppositely phased voltages at terminals 76 and 78 are balanced so that when the positive fuel voltage at 75 is less than the negative idle fuel flow voltage at 77 the thyratron does not fire. When the positive voltage becomes greater than the negative voltage the thyratron fires and the relay 21 is energized. Of the above inputs the voltage at 75 comes from a fuel servo 80 which operates slider contacts 82 and 83 of cards 84 and 85 respectively, the voltage at 83 being fed to terminal 97 of amplifier 2, when relay 21 is de-energized and " firing " relay 23 is energized. As previously stated the input at 76 comes from card 41 of throttle servo 37, and the voltage at 78 is derived from card 12 of servo 1. Throttle control. Movement by the pilot of throttle lever 31 to increase engine speed for normal operation will put the apparatus in the condition where (a) " air shutter " control 20 is " open ", (b) thyratron relay 21 is energized, (c) the start button 24 is " off " and (d) firing relay 23 is at firing position. Under these conditions a voltage from card 41 of system 37 is fed as previously stated to terminal 45 of amplifier 2. An answer voltage of opposite sense from answer card 12 is now fed to terminal 97 of amplifier 2. The throttle servo 37 tends to follow the position of throttle lever 31 but as the pilot may advance the lever too rapidly for direct fuel supply to the engine the response characteristics of servo 37 are such that it tends to follow the throttle lever at a predetermined maximum speed. In actual aircraft the throttle control operates through electric control circuits and the pilot is provided with a throttle control override switch. This is simulated in the present arrangement by a switch 100 which is energized through a throttle control selector switch 35 which the pilot has to operate when an instructor disables the throttle lever control by moving a " throttle fail " switch 105. During this operation, for the period when the override switch is not operating, the servo 37 is locked against drifting by the provision of a relay 102 which operates a switch 103 to short circuit the control winding of servo motor 39 when override switch 100 is in neutral position. A switch 108 is provided to enable an instructor to simulate an interruption in the A.C. power supply and switches 58 and 64 enable him to simulate a loss of engine fuel, and under this latter condition a signal is fed to amplifier 2 from a servo 115 representative of the condition in which the engine rotor is windmilling. An instructor's control dial 128 is provided to operate the wiper of a potentiometer 125 whereby " increase or decrease engine speed " signals may be applied to terminal 126 of amplifier 2 to simulate faulty operation of the engine.