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Aviation History
1962
1962 - 0978.PDF
976 FLIGHT International, 21 June 1962 Electrical Control of Gas Turbines By D. A. RUSH, CGIA, AMIEE* OVER the years there has been a steady increase in the number of applications of electricity to the control and monitoring of aircraft gas turbines, and of these the "black box" responsible for maximum temperature control is perhaps the most familiar. This particular control consists of three main parts: thermocouples in the jet-pipe, a magnetic amplifier, and a solenoid- or motor-actuated trimmer in the fuel system. With few exceptions there has been no requirement for very rapid response or wide authority from this type of controller; its function has been that of a relatively slow-acting trim to prevent excess temperature occurring when the engine is running steadily near maximum speed. But gas temperature is a prime parameter of the engine, and it may be employed to effect accurate and quite sophisticated control during acceleration. When the engine is accelerated slowly the relationship of gas temperature to speed usually remains reasonably close to the steady- running values. But during a rapid acceleration it may be greatly in excess; in fact, if these high values of excess temperature did not occur rapid acceleration would be impossible. Apart from the problem of limiting this transiently permitted excess temperature to a value determined by, say, a metal thermal shock criterion, there is a further restriction on rapid acceleration. The compressor can be subject to aerodynamic stall if acceleration occurs too quickly, and a condition known as engine surge results. This is a compressor phenomenon not unlike the standing waves in electrical transmission lines, which results in fluctuation in engine airflow. Consequently mechanical damage may occur; and at appropriate points in the fluctuation cycle the engine may be starved of air, which leads to excess temperatures, or be surfeited with air and the flame extinguished. The values of the para meters that define proximity or otherwise to the surge conditions are not necessarily unique. This has made for difficulties in the design of acceleration control systems. In Fig. 1 an engine steady-running line has been superimposed on the compressor characteristics. This line is a characteristic of the complete engine; and there is a reasonable margin between this line and the area in which surge may occur. When the excess fuel necessary for acceleration is burnt in the combustion chambers the p. pressure ratio = is increased, and if sufficient fuel is supplied the ratio could increase to a value sufficient to cause surge. This will happen if the excess fuel is supplied more rapidly than the engine can accelerate to a new value of speed at which the compressor can P2 accommodate the new =• . It follows that a slow opening of the "i * Aviation Division of S. Smith & Sons England) Ltd \The surge line plots any parameter computed for a condition close to the surge. 1 Fig. I Typical compressor characteristics, with a steady- running line superimposed No 3 test cell at the NGTE, showing the Rolls-Royce Avon 208 currently engaged in simulated altitude trials with the new Smiths electrical control system. Later this year the engine is to undergo flight trials in a Sea Vixen throttle is unlikely to surge the engine, while a fast opening mav. The compressor characteristics can be modified by bleeding air from an appropriate stage. This represents one possible medium of control, which is sometimes used to bend the characteristics—but not to effect complete dynamic control during acceleration, which has to be achieved by regulating the fuel flow. Ideally an engine should be accelerated to follow a trajectory as close as possible to the surge line,t since this will give minimum acceleration time. Although not often called for in the normal life of an engine, minimum acceleration time is important for overshoot and collision avoidance. It is, of course, also important for military aircraft. Methods of Control Fuel Schedule Control A method of control much employed in hydraulic controllers schedules fuel flow as a function of pressure ratio, and is probably the most successful method that has had wide application to date. It possesses one or two shortcomings, including sensitivity to non-uniqueness of parameter relationship. Rate of Change of Fuel-flow Control It is theoretically possible to limit the rate of change of fuel-flow to values that would avoid surge, provided that the limiting values are a function of engine speed and
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