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Aviation History
1964
1964 - 2325.PDF
332 FLIGHT International, 27 August (96 Powerplant Control for the TSR.2 OPERATION OF THE ULTRA SYSTEM THE greater the complexity of high-speed propulsion systems,the greater is the need for simplicity in their methods ofoperation. This may well have been the guiding principle in the development of the BAC TSR.2 aircraft, described in our April 9 issue. It has certainly been applied to the electrical throttle control system which will enable the TSR.2 pilot to handle the enormous power at his command with the minimum of thought and effort. Automation applied to throttle control is well-established as a feature of high-performance aircraft. That it is so is largely due to the efforts of Ultra Electronics Ltd—the first company to develop a system of this type for use in a subsonic engine—who have now produced the equipment to control the Bristol Siddeley Olympus engines of the supersonic TSR.2. It was in the immediate post-war years that Ultra began to consider the great advantages of electrical control systems for aircraft engines. They established such controls as a worthwhile proposition with the Bristol Britannia. A similar system was specified for the Short Belfast, followed by a major development programme for the comprehensive system for the TSR.2. A similar requirement exists for the Anglo-French Concord. Electrical controls provide facilities—including top-temperature limiting and acceleration control—which cannot be achieved with adequate performance by other means. A linear response and an accuracy of better than 0.2 per cent of full-throttle travel is main- tained independently of widely varying load-torques, aircraft- structure flexing, distance between cockpit and engines, differential thermal expansion of throttle linkage, ambient temperature, altitude, vibration, acceleration and power-supply variation. Electrical governors control engine speed within +0.25 per cent under all these conditions. Temperature limiting allows operation of the engine to within 2£°C of its top-temperature limit. Acceler- ation control allows the pilot to obtain maximum thrust in minimum time, electrical control alone providing the complex scheduling when required. Both the overhaul period and total engine life are increased when the engine is protected from excessive temperatures, speeds and acceleration. The main electrical control unit in the Ultra system is mounted well away from the engines in a cooler part of the fuselage. Input signals are fed into a computer for the provision of automatic control of compressor speed, thrust, acceleration, turbine temper- ature and over-pressure. The signals are compared with pre- determined data values, and adjustable feed-back loops are included to match the engine characteristics without oscillation or overshoot. Operation is fully automatic, and the pilot is not required to tak any special action when calling-up engine power changes, or ti counter variation of aircraft speed and ambient conditions o temperature and pressure. The control of speed and temperatur is important because the engine must not accelerate too quicklj neither must it be allowed to overheat. An all-speed governor i employed to control r.p.m. from flight idling to maximum, and i limiter is included to prevent excessive temperature and surge. When the pilot's throttle lever is moved through a given angle, i proportional voltage is produced by means of an electrical pick-ofl the motor of which is connected to the lever. This voltage form one of the computer inputs. Two further inputs come from engine mounted transducers, which measure the 1-p and h-p compresso speeds. These transducers are U-shaped electro-magnetic probes the ends of each of which bridge the teeth of an appropriate gear ii the engine. Frequency of the output waveform is proportional t( the compressor speed in each case. There are two other transducers mounted on the engine. Thi inter-stage thermocouple harness consists of 11 twin thermocouple: buried in the turbine stators, and measures inter-stage turbin< temperature. Intake temperature is measured by a platinum- resistance thermometer. The 11 couples in each group of the twin thermocouple harness are connected in parallel and are vernier spaced over the eight burner sections of the engine to give a mear turbine temperature. Maximum response and accuracy of measure- ment are obtained because of the design of the gas passages. Resistance values of the undivided couples are compensated sc that all junctions have the same impedance measured from tht paralleling terminals. This is known as a ladder-type harness. Both groups are electrically independent, but record the same mean temperature. The output of one group is used for engine control, and the other output indicates a turbine temperature. This means that, if a fault develops in the control group, the flight deck will still have an indication of the temperature available. Two other transducers, mounted remote from the engine, measure intake pressure and compressor delivery pressure. The second is a safety device expected to operate only if the compressor delivery pressure is above datum level. The intake-pressure transducer has a force-balance system, connection to the engine being by pipe. In the Ultra system, there are five main servo control loops. The hydraulic loop—the primary servo loop—consists of two engine- driven feed pumps, with a common gallery. The electrical control amplifier operates a motorized throttle valve, located in the main fuel supply to the burners; and a hydraulic servo, operating on the main stroke of the fuel pumps, maintains constant pressure-drop across the valve. For steady power constant with a dry engine, the main control loop has a hybrid positioner-governor. The positioner system schedules the fuel supply to the main engine, in accordance with the setting of the pilot's lever, from shut-off, through flight idling, to speeds just above idling, where high-speed-spool governing takes over. Take-over is determined from the ambient engine conditions by a "highest wins" circuit. The positioner-governor signal is taken Concluded at foot of page 334 The main control unit, one of the actuators—for reheat—and the pilot's throttle levers
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