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Aviation History
1962
1962 - 0979.PDF
FLIGHT International, 21 June 1962 977 Fig. 2 Schematic block diagram of the Smiths electrical control system intake pressure. This form of control is essentially open-loop, and has probably been used only by cautious pilots on first acquaintance with axial turbojets. Incipient-surge Sensors Closer to ideal, perhaps, would be a controller that sensed incipient surge, since this would avoid the problem of non-unique limiting values. Unfortunately, a single parameter with the required characteristics has not been dis covered, although several attempts have been made to isolate compressor "noises" and correlate them with the onset of surge. The most promising controllers of this type so far studied seek to use the change in balance that can occur between two parameters as the surge condition is entered: pressure ratio or turbine temperature may be balanced against acceleration, using a nominal function of speed.* Unfortunately, it is almost impossible to predict exactly the changes that will occur under all surge conditions, and the practical application of such controllers may be delayed. Limiters It is possible to compute for engine parameters, such as acceleration and temperature, limiting values as a function of speed, which correspond to the limiting conditions imposed by the compressor characteristics. A controller can then be designed to limit the chosen parameter in accordance with a generated function of speed, but a difficulty remains in that the limiting values are not unique for all engine conditions. Choice of Method and Parameters Several factors enter into the choice of method and parameter for control, and the most important is that the sign of control signals must be in the correct sense if for any reason surge does occur. Yet another consideration is the need to preserve a margin between steady-running and surge-line values of the controlled parameter. These and other factors are:— (i) If surge is entered, the sign of the control signal should be such that fuel-flow is reduced, (ii) The control equation should be non-dimensional, (iii) The controller should not be sensitive to fuel density, (iv) The margin between steady-running and surge-line values of a controlled parameter should be as wide as possible, (v) The generation of an accurate function should not call for extreme accuracy in the measurement of the generating parameter, (vi) The minimum number of additional parameters should be required to cancel the non-unique characteristics of the surge line. Application of some of these "gauges" to an acceleration con troller with the control relationship:— t N for p > h, i, I —= I reduce fuel flow WTj/ shows that this is non-dimensional; also that there is a 100 per cent margin between steady-running and surge-line values of N. But against these it may be noted that, if the engine surges, N will be reduced and the sense of control will be reversed, thus driving the engine deeper into surge. Also, for a typical engine, a 1 per cent change in -^-produces a 10 per cent change in "1^77^ )• Another form of control employs temperature as the controlled parameter, the control relationship being:— for =? > h,( —— 1 reduce fuel flow r, VvTi/ which is non-dimensional; and, as the engine surges T4 will be increased, which is in the correct sense. On the other hand, the margin between steady-running and surge-line values of T4 may be N typically only 20 per cent. Moreover, a 1 per cent change in —7= vTi produces a 5 per cent change in h2( —=• I. The choice of acceleration control for a particular application is not easy, but the techniques available with electrical controllers can assist materially by offering great flexibility in signal mixing and computing. More particularly is this so early in the development of an engine, since it is difficult to define beforehand all the para meters that will be required to achieve satisfactory control. Thermocouples When temperature is the controlled parameter, the important transducer is the thermocouple. Development has recently been carried out to improve the time response and life of thermocouples for this application. The requirement of long life and quick response are not compatible for a simple couple, since quick response can be obtained only if the mass of the wires is small; but, by careful design to shroud the material more susceptible to erosion in the high-velocity high-temperature gas stream, improve ments have been obtained. Thermocouple response characteristics are a function of gas- stream conditions and design detail, and the response to step changes of flowing gas temperature has been shown by experiment and theory to approximate closely to the form:— -t -t 9 = 1 - (1 - K)eTi - KeT* where K, rj and T2 are functions of (i) gas-stream total and static pressures (ii) design details, the most important of which determine the rate at which heat is taken up by the junction and lost by conduction along the leads. Automatic compensation for the response is simplified if the response is a single exponential. Much of the work to effect improve ments has been directed to this end. Improvement in response time can be achieved by maintaining the gas flow sonic in the region of the element, and in many cases the measures taken to improve response time will also improve the accuracy of temperature measurement, either by a gain in the recovery of total-head tempera ture or by minimizing heat conduction along the wires away from the junction. An Electrical Control System These improvements in thermocouple design, together with the improvement in amplifier size and response obtained by the use of transistors, have made it possible for Smiths Aviation Division to demonstrate an engine acceleration control system for high- performance aircraft using temperature as the controlled para meter. It has been developed to be compatible with some of the features of existing fuel systems, and particular attention was given to ensuring that flexibility existed to permit non-unique character istics of the engine surge line to be accommodated in a particular application. The system that was demonstrated (Fig. 2) included steady-running speed and temperature control. Fuel-flow is controlled by setting the stroke of a reciprocating 'British Patent 10050/57.
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