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Aviation History
1955
1955 - 1729.PDF
848 FLIGHT, 2 December 1955 PNEUMATIC CONTROL . . . and the compressor discharge pressure which was applied to theseries of two orifices to produce an intermediate reference pressure on the lower side of the diaphragm. In operation, adecrease in turbine pressure ratio (for example, such as would be caused by light-off of the afterburner) caused the turbinedischarge pressure to rise, displacing the diaphragm down- ward. As the contoured needle decreased the size of the secondorifice, it raised the reference pressure until the diaphragm reached a new equilibrium position corresponding to the errorin turbine pressure ratio. As the needle moved downward, it operated the pneumatic servo valve which bled compressor-discharge air to one side of the pneumatic actuator, moving the jet nozzle in the open direction. This reduced the turbine-dis-charge pressure, restoring it towards its original value. Con- versely, should the afterburner flame be extinguished, the suddenincrease in turbine pressure ratio would cause the Microjet control to return the jet nozzle to the closed position. With the succeeding generation of turbojet engines, the after-burner had been adapted as an integral part of the engine design. The next logical step was to utilize the variable-jet nozzle toimprove the non-afterburning performance of the engine by maintaining the maximum permissible turbine-blade temperature.This had resulted in a requirement for a constant turbine-inlet temperature control. Since it had proved impractical to utilizethermocouples in the high-temperature zone ahead of the turbine, however, the use of turbine discharge temperature was accepted asa compromise substitution. In order to maintain constant turbine discharge temperatureat constant military r.p.m., it was necessary to increase the turbine pressure ratio by opening the jet nozzle as the compressor inlettemperature rose. In initial attempts to use the Microjet control to maintain constant temperature, the turbine pressure-ratiosetting was biased as a function of compressor inlet temperature by utilizing a temperature-compensating element. This was notconsidered the most desirable solution, because of the slow rate of response of this type of temperature element. The system was then re-evaluated on the basis of substitutingother engine parameters for the compressor-inlet temperature. With the engine operating at constant military r.p.m., the com-pressor ratio varied almost as a linear function of the compressor inlet temperature. Therefore, by establishing a scheduled rela-tionship between turbine pressure ratio and compressor pressure ratio, a constant turbine-discharge-temperature operation couldbe achieved. Repeated flight-tests to altitudes in excess of 5O,OOOft, however,had indicated that still another factor had to be considered. Above 40,000ft, the decreased density of the air had a markedeffect on die efficiency of the engine compressor, as a result of a wide variation in the value of Reynolds number. As compressorefficiency decreased at high altitude, relatively more work was required to turn the compressor. If constant turbine dischargetemperature was to be maintained, this additional work could be obtained only by increasing the turbine pressure ratio by openingthe jet nozzle slightly to hold the temperature constant. In the previous examples, the selection of schedules had beenmade on the basis of close linear approximations. Not all schedules, however, could be reduced to this degree of simplicityand high pressure-ratio engines, in particular, tended to have greater non-linear characteristics. Furthermore, the modernnozzle-area control must provide not only constant temperature at military r.p.m., but ideally must also provide an appropriatenozzle-area for cruise operation and must maintain a pre-selected nozzle position duringengine acceleration. By combining two Microjetelements into a single con- trol, this type of non-linear schedule could be achieved.This modified control consisted simply of twounits combined into one. The primary unit sensedturbine pressure ratio and the secondary unit sensedcompressor pressure ratio. The output needle of thesecondary unit changed the position of a con-toured element in the first orifice of the primaryunit, thus biasing the tur- Fig. 2. Typical application of the Microjet control to a variable-area nozzle. bine pressure-ratio setting as a function of compressor pressureratio. If the taper of the needle of the basic Microjet control were re-versed a unique characteristic was achieved. With this configura- tion, the diaphragm assembly became unstable and snapped toeither stop, depending on pressure-ratio level. Thus, whereas the basic unit normally produced an output signal that was a con-tinuous proportional function of the error, the reference taper intro- duced a discontinuous characteristic pneumatically analogous to theaction of the simple toggle-switch. This type of control was used with two-position jet nozzles where it was desired to establishand maintain the jet nozzle in the full-open position when the afterburner ignited, and to return it to the closed position whenthe afterburner was extinguished. Extracts from the lecturer's detailed description of the Microjetcontrol include: — Use as Net Thrust Indicator. A compound Microjet is used whereinthe primary unit senses turbine-presure ratio and the secondary unit senses compressor-pressure ratio. By appropriate shaping of the con-toured needle, and operating the primary Microjet unit with both orifices choked, the quotient of these two parameters is computedpneumatically such that the output displacement of the primary unit is a function only of the engine pressure-ratio, which is itself a functionof thrust. Airflow Requirement. Standard units generally require less than0.05 lb/sec at sea-level conditions, or less than 0.05 per cent of the engine airflow. Insensitivity to Dirt. Service experience on over 2,000 Microjetcontrols has indicated that the units are relatively free from malfunction caused by dirt or impurities in the air supply . . . The heavier particlesare carried past the compressor discharge opening by the high-velocity airstream and most of the smaller particles are separated by centrifugalaction as a result of the design of the housing. Furthermore, the orifices are large enough so that blocking is virtually impossible. Permissible Operating Temperatures. Military environment qualifica-tion tests have been successfully completed in an ambient temperature of 350 deg F, using compressor discharge air at 925 deg F. Other unitshave been operated successfully at ambient temperatures as high as 600 deg F . . . The operating temperature limit is determined only bythe physical properties of the plastic diaphragm material. Current research into new types of material and new mechanical configurationsfor the diaphragm promise to eliminate altogether the limitations caused by operating temperature. Accuracy. One of the design considerations contributing to the highdegree of accuracy of the control is that large diaphragm areas—of the order of 5 and 6 sq in—are generally selected, so that signal actuatingforces are available even at high-altitude operating conditions. Relatively large displacements of the order of 0.125 to 0.5in are used, so thatinaccuracies due to manufacturing tolerances are minimized. An accu- racy within approximately 0.3 per cent is achieved over a wide range ofaltitude operation. Afterburner Light-Off and Blow-Out Detection. In some of thecurrent production engines equipped with conventional electronic temperature-sensing variable-nozzle controls, the slow response of thethermocouples has created problems associated with the ignition of the afterburner . . . Turbine pressure ratio, on the other hand, changesinstantaneously when the afterburner ignites and, as a result, the Micro- jet control is used in several such applications to initiate rapid openingor closing of the nozzle. Overspeed Control. In another application of the above detector,Microjet control is used for preventing over-speed in a two-spool engine, by reducing engine fuel flow when a sudden change in turbine-pressureratio indicates that afterburner blow-out has occurred. Jet-Nozzle Control. The use of the Microjet principle for maintain-ing constant turbine inlet temperature by controlling the variable jet nozzle has already been successfully demonstrated in ground tests,altitude tank tests, and actual flight-tests. The use of pressure ratio for fully-variable control of the jet nozzle is being given serious con-sideration by many of the major engine manufacturers because of the significant reduction of weight, increase in reliability, and fasterresponse. Turning to the application of the Microjet control to fuelmetering for ramjets and afterburners, the lecturer said that this was currently the subject of an extensive development pro-gramme. The basic fuel-controls in each case were quite similar, the primary purpose being to establish and maintain a basic fuel-schedule that was at least roughly proportional to engine airflow. For the ramjet, this fuel schedule was required mainly for ignition,the schedule being trimmed or over-ridden thereafter by a shock- wave positioning control. The control of fuel flow in the turbojet engine was much moredifficult than in the case of ramjets or afterburners. It was con- ventional practice to schedule engine acceleration fuel as afunction of r.p.m., compressor inlet temperature and compressor discharge pressure. In the interest of reducing compressor-surgemargin and eliminating the effect of variations in fuel characteris- tics, studies had been made by the Solar Company to determinewhether a new concept, utilizing closed-loop control or com- pressor pressure ratio, might provide an answer to the control ofengine acceleration. These studies had indicated many interest- ing possibilities-^-that engine acceleration fuel as well as inletguide-vane position and compressor-bleed airflow could be con- trolled by the use of pressure-ratio parameters.
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