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Aviation History
1955
1955 - 1728.PDF
FLIGHT, 2 December 1955 847 PNEUMATIC CONTROL —for Turbojets and Ramjets: a New System by Solar THE use of pneumatic pressure ratios as control parametersfor turbojet and ramjet engine cycles was described in alecture by Mr. Wendell E. Reed before the Society of Auto- motive Engineers in Los Angeles recently. With the titleA New Approach to Turbojet and Ramjet Engine Control, the paper described the development by the Solar Aircraft Company(of which Mr. Reed is project engineer) of a simplified computing element known as the Microjet control. In his introduction Mr. Reed pointed out that, in the Mach 2.5aircraft of 1960 which were on the drawing-boards today, the pilot would have to endure a temperature of 450 deg F, were themachine not equipped with an elaborate refrigeration system. At three times the speed of sound the lowest air temperature any-where in the aircraft was 650 deg F. The engine and associated control components, however, must be capable of operating atmuch higher temperatures—between 800 and 1,000 deg F. By contrast, many of the available electronic componentsrequired for control systems were limited to operating tempera- tures of less than 250 deg F. Even highly developed electro-mechanical components, such as electric motors and solenoids, were marginal about 500 deg F. At this temperature, the bestknown hydraulic fluids began to decompose, and the most advanced types of plastic materials used for seals became unsatis-factory at more than 600 deg F. To overcome the disadvantages of electronic and hydraulicequipment at high temperatures, the use of pneumatics was a possible solution. Turbojet and ramjet engines were basicallyair-pressure producing machines and small amounts of leakage presented no problem and certainly no fire hazard. Thus, thedesign of seals was less critical and not limited to the use of resilient materials which possessed a temperature limitation.Another advantage of pneumatics was that the need for storage tanks, pumps and power supplies was eliminated and, so longas the engine was operating, the source of fluid in the pneumatic system was assured. The substitution of compressed air in place of electric poweror hydraulic fluid as the operating medium for a control system, the lecturer continued, did not, however, permit the full realiza-tion of the potential advantages of an all-pneumatic control system. Not only the muscles, or actuating elements, but alsothe brains, or computing elements, of the system must undergo considerable face-lifting in exploiting the conversion to pneu-matics. It was conventional practice, for example, in controlling the variable-area jet nozzle of a turbojet, to utilize turbine-inlettemperature or turbine-discharge temperature as the primary control parameter. The selection of this parameter demanded theuse of electronics in the system. The thermocouples which were used to measure the temperature created a weak electrical signal,which must be amplified electronically before it became usable. Although non-electric methods of temperature measure-ment were available, these were eliminated from practical con- sideration on the basis of response time. The benefits of pneu-matics could best be realized if pneumatic control parameters were used in place of temperature parameters. It was significantthat the engine-matching characteristics could be controlled using three engine pressure ratios. If the r.p.m. and two pressureratios were known, the actual turbine-inlet temperature could be controlled. It was possible, merely by sensing pressure-ratioparameters, to control the variable-area jet nozzle, afterburner fuel flow, engine fuel, compressor air bleed, variable inlet guidevanes, and variable inlet geometry, in order to maintain the engine variables within the prescribed mechanical and thermodynamicrequirements of a high-performance engine. Microjet control in part section. OUTPUT,Y The operating principleof the Microjet pneumatic computing control wasshown in the illustration (Fig. 1). The primarypurpose of the device was to provide a mechanical-displacement output signal which indicated the mag-nitude of the ratio between the two pressures. Onepressure was applied to the upper side of a flexiblediaphragm, and the other pressure was used togenerate a controlled ref- erence pressure which wasapplied to the lower sides of the diaphragm. This reference pressure was generated by bleed air, generally from the com-pressor discharge, through a series of two orifices, the area of one of which was controlled by a tapered portion of a needleattached to the diaphragm. In operation, the flexible diaphragm, which was substantiallyfree to move in response to pressure differences, operated in such a manner as to maintain the reference pressures exactly equal tothe pressure applied above the diaphragm. An increase in the upper pressure resulted in a downward movement of the dia-phragm which caused a corresponding decrease in the area of the second orifice. This raised the reference pressure and retardedthe movement of the diaphragm untii equilibrium was re-estab- lished and the diaphragm displaced to a new position at whichthe two pressures were equal. This position was directly related to the new value of the ratio between the two appliedpressures. Comparing the Microjet pressure-ratio control with the con-ventional pressure-ratio sensing device, the main advantages of the former arose from the latter's use of precision bellows tomeasure differential pressures. The requirements of the con- ventional system were for precision-evacuated bellows, precisiontemperature-compensated reference springs, complicated and precision dividing linkage, delicate instrument-type constructionand skilled servicing; the output function was non-linear unless compensated by additional cams. The Microjet-type control,however, was relatively simple and more versatile. It used a single non-linear diaphragm, the size of which was not critical;it was easily calibrated and adjusted; there was no friction or wearing parts; construction was simple yet rugged; the contouredneedle provided any type of output function; and the device was easily serviced by any trained technician.A typical application of a Microjet pressure-ratio control was the variable-area jet nozzle control shown in the diagram (Fig. 2).Early turbojets had been designed for use with a fixed jet nozzle, resulting in their operating essentially at a fixed turbine pressureratio. Later it became necessary to provide a variable-area jet nozzle to compensate for the increased volume of exhaust gasresulting from afterburner combustion, and to maintain "balanced cycle" control, i.e. to maintain the constant turbine-pressure ratiofor which the engine and control system had been designed. It was for this purpose, in conjunction with an afterburnerdevelopment project, that the Microjet principle had been initially conceived at the Solar Company—after consideration ofvirtually every other possible means of obtaining the required response and reliability. In 1949, a Microjet control was first applied to a Westinghouse J35-WE-34 engine and flight-tested in a Chance Vought XF7U aircraft.The system (Fig. 2) consisted of a Microjet pressure-ratio sensingelement, the output needle of which controlled an integrating servo valveand actuator combination to move the jet nozzle. The control sensedtwo engine pressures, the turbine discharge pressure applied to theupper side of the diaphragm Fig. 1. The operating principle:— 7P, NEEDLE CONTOUR FUNCTION
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