GB2240858A - Controller for reheated turbofan - Google Patents

Abstract

The controller has closed loop channels 21, 24, 27 controlling main fuel feed, nozzle area and reheat fuel food. Main fuel channel 21, in response to a thrust demand at 22, correlates fan pressure ratio to thrust creating a demand value 32 of fan pressure ratio which is compared at 33 with a feedback signal 34 to provide at output 23 a main fuel actuator command. Nozzle area channel 24 creates a demand value 42 of a signal which relates to bypass duct Mach number and which is compared at 43 with a feedback signal 44 to provide at output 26 a nozzle area command. Reheat fuel channel 27 creates a demand valve 52 of PS7/PS1 (PS7=mean static pressure at the nozzle area; PS1 = static pressure at the fan inlet) which is compared at 53 with a feedback signal 54 to provide at output 29 a reheat fuel demand. <IMAGE>

Description

ENGINE CONTROLLER FOR REHEATED TURBOFANS This invention relates to an engine controller for reheated turbofan engines which controls main fuel feed, reheat fuel feed and propelling nozzle area in a coordinated manner in response to imposed thrust commands and in accordance with a commanded or predetermined fan working line/surge margin characteristic.

It is difficult to secure a direct measurement of turbine engine thrust save in test cell operation where reaction forces can be measured by a load cell, but it is obviously desirable to control these engines so as to produce required thrust levels in a safe and predictable way with as much accuracy as is possible. It is important that the engine controller does not control the engine in a way which produces an over provision of thrust in order to guarantee the attainment of a specified thrust, for each increment of thrust raises component temperatures and consumes engine life.In order to control aero turbofan engines to thrust some parameter of engine performance which correlates more or less directly to thrust is used as a control variable and a thrust command is translated to a corresponding value of the control variable to drive the engine under open loop or dosed loop control.

Some current controllers for civil turbofan engines use the overall pressure ratio of the engine (measured in terms of total pressure) as the control variable related to engine thrust and such controllers are satisfactory for unreheated engines with fixed nozzle areas. Reheated engines with variable nozzles require a more comprehensive control system to account for variations in fan working line and also operation under reheat.

Other parameters have been used as control variable in reheated turbofan engines for control of the dry component of thrust produced by the core of the engine. Such parameters as turbine blade temperatue, and the spool speeds of the low pressure compressor (fan) or high pressure compressor have been used as control variable. These parameters are not affected by operation of the engine in reheat providing the core exit pressure and core/bypass flows are not disturbed. Control of reheat boost is achieved through manipulation of the reheat fuel and nozzle area actuators on an open loop basis according to pre-determined schedules with the two controls cross-coupled so that the output of whichever actuator has the slower response generates a reference level for the other actuator control.

However the control variables and the reheat control approach mentioned above, whilst being generally related to dry thrust, do not equate to thrust with sufficient accuracy to avoid over provision of thrust due to heat soakage effects when rapid changes of thrust are required. Moreover these parameters are affected by normal in-service degradation of the engine to a degree which might require over provision of thrust (with increased consumption of hot end life) in an engine at the start of its life to guarantee attainement of specified thrusts when in subsequent service.

The present invention is an engine controller for reheated turobfan engines which uses a different control variable more closely related to dry thrust. It offers the ability to control engine performance more closely to imposed dry thrust commands and the ability to control the reheat thrust to meet overall thrust commands without overloading the core engine. Moreover it provides means by which fan working line/surge margin may be varied in accordance with any desired control strategy.

An explanation of the terms used herein to describe and define the claimed invention is given below together with a description of special purpose pressure instrumentation used to obtain the necessary measurements. The explanation and description refers to Figure 1 of the drawings which comprises a sectional view of a typical reheated turbofan engine. No novelty or invention is claimed for any of the pressure instrumentation for some or all of it might be found in present day research engines if not in current service engines.

The illustrated engine has an inlet 1 leading to its low pressure compressor 4 - hereafter termed the "fan". Within this inlet 1 there are static pressure tappings 2 and there might also be total pressure probes 3. Both these are mounted at a station clear of the face of the fan say at one quarter of inlet diameter forward of the fan. All pressure and other measurements made at this station are indicated with the suffix "1". Downstream of the fan 4 there lies bypass duct 5 and at the entry to this duct there is a second measurement station.

Here are situated duct Mach number rakes designated 6 and each of these comprises two tubes with closed innermost ends. The forward tube of each rake is designated 7 and is perforated by a series of forward facing holes and the rearward tube of each rake is designated 8 and this is perforated by a series of rearward facing holes.

Measurements made at this station are suffixed "2". Tubes 7 yield a total pressure measurement designated PT2 and tubes 8 yield a static pressure measurement designated PS2. The remaining pressure measurement station is at the rear of the engine just before the entry to propelling nozzle 9. Measurements made at this station are suffixed "7" in accordance with established nomenclature and here there are static pressure tappings 10.

In order to yield pressure measurements suitable for control purposes each individual tapping or probe at a given station is connected to a common manifold which interfaces with a single pressure diaphragm to yield a pneumatically averaged reading. This minimises the effects of local variations in conditions. It has been found that the total pressure measurement made at station 1 is especially susceptable to the influence of swirl in the inlet flow. Moreover it is desirable to avoid placing intrusive instrumentation such as probes 3 ahead of rotating machines for obvious reasons of safety. To avoid both these problems the total pressure at station I may be estimated rather than measured and this estimate is derived by reference to engine calibration data and other pressure measurements.

Tabulation and Definition of Terms PO is environmental (static) pressure at aircraft altitude.

MO is the aircraft Mach number.

-P1 is the mean pressure of station 1 being either static pressure (PS1) or total pressure (PT1).

P2 is the mean pressure at station 2 being either static pressure (PS2) or total pressure (PT2).

Fan pressure ratio is the ratio P2/P1, being either PT2/PT1 or PS2/PS1.

FANSPR designates fan pressure ratio in terms of static pressure being the ratio PS2/PS1.

DPUP designates the ratio (PT2-PS2)/PT2 as measured by the duct Mach number rakes and this is a unique function of duct Mach number.

EFANPR designates the estimated value of fan pressure ratio in terms of total pressure (PT2/PT1) obtained by measurement of Pm, PS2 and PS1 and calibration.

PS7 is the mean static pressure measured at station 7.

NH designates the spool speed of the high pressure compressor expressed as a percentage of design value.

NL designates the spool speed of the fan expressed as a percentage of design value.

o is the ratio between aircraft environmental temperature in degrees absolute and the reference temperature (usually 288.16"K).

TBT designates turbine blade temperature as measured by a pyrometer observing the rotor blades.

Surge line is the notional or measured boundary between stable and unstable engine operation on a plot of compressor pressure ratio against air mass flow.

Surge margin is the margin in compressor pressure ratio (%) between an attained or specified condition of stable engine operation and the surge line for a given air mass flow.

Working line is a predetermined relationship between compressor pressure ratio and air mass flow to which the engine is controlled on spool up or spool down.

AN designates nozzle area as measured or as estimated by reference to a calibration plot and to measured values of DPUP and FANSPR ANdry designates the notional value of nozzle area which would produce equivalent core conditions in the dry mode to a value of AN for an engine operating in reheat, this being estimated from the dry engine calibration.

NDGTF designates a non dimensional gross thrust function which is defined by the following expression NDGTF = gross thrust/(AN x PSI) + PO/PSI In this specification all references to surge line and surge margin etc relate to the operation of the fan section of the compressor. For a given engine type a calibration can be obtained relating surge margin, DPUP and FANSPR from engine tests having a knowledge of the surge line. Subsequent assessment of surge margin is made using the calibration plot and measurements of DPUP and FANSPR.

The present invention is a controller for a reheated turbofan engine giving simultaneous control of main fuel, nozzle area and reheat fuel in response to a thrust request, which comprises the following: an input port or ports at which control demands are delivered there being one such port at which a thrust demand is delivered; respective input ports for control reference signals these reference signals including at least the values of P1, P2, PS7, DPUP, NL and o or derived parameters; respective output ports at which actuator demand signals for the main fuel feed actuator, the nozzle area actuator, and the reheat fuel feed actuator are provided;; a main fuel control channel including a respective signal generator which generates a P2/P1 demand signal corresponding to the delivered thrust request by reference to stored calibration data and to control reference signals, a summing junction at which the P2/P1 demand signal and a P2/P1 feedback signal are compared to produce an error signal and a gain scheduled signal compensator which acts upon the error signal to produce the respective actuator demand signal by reference to stored data documenting the P2/P1 open loop dynamic response of the engine to manipulation of the main fuel feed actuator demand signal;; a nozzle area control channel including a respective signal generator which generates a DPUP demand signal corresponding to a delivered surge margin request or to a predetermined fan working line by reference to stored calibration data and to a control reference signal or signals, a summing junction at which the DPUP demand signal and a DPUP feedback signal are compared to produce an error signal and a gain scheduled signal compensator which acts upon the error signal to produce the respective actuator demand signal, by reference to stored data documenting the DPUP open loop dynamic response of the engine to manipulation of the nozzle actuator demand signal; and a reheat fuel control channel including a respective signal generator which generates a PS7/PS1 demand signal corresponding to a required reheat boost by reference to stored calibration data and to control reference signals, a summing junction at which the PS7/PS1 demand signal and a PS7/PS1 feedback signal are compared to produce the respective actuator demand signal, and a gain scheduled signal compensator which acts upon the error signal to produce the respective actuator demand signal by reference to stored data documenting the open loop dynamic response of the engine to manipulation of the reheat fuel actuator demand signal.

The signal generator of the main fuel control channel can embody various degrees of complexity. In all cases an input demand is converted to a related P2/PI demand signal and to do this without ambiguity some knowledge of or assumption on the fan working line is required. The variation lies in the different ways in which the conversion can be made for a given working line or lines and in the degree in which stored or control reference signals are required to support the conversion.

In a basic form of the controller the signal generator of the main fuel control channel incorporates a single look-up table relating engine design values of P2/P1 to pilots control settings nominally related to thrust at a given flight condition, say the sea level static condition, and for a design fan working line. Such a controller makes no attempt to provide an accurate regulation of the engine to a specific thrust for a given input command across the range of flight conditions nor does it provide any correction for an off-design working line. Providing the nozzle area control channel is effective in holding the engine to the design fan working line this basic form of controller will be adequate to provide a predictable response to control inputs at any point across the range of flight conditions.With proper scheduling of the proportional and integral gains in the signal compensator of the main fuel control channel, the basic controller will also enable the engine to respond promptly and without overshoot in thrust to slam changes in the demanded thrust.

Control of acceleration in the initial response to a slam acceleration demand is achieved by use of spool speed/overspeed and acceleration limiters in the conventional way to avoid damaging accelerations.

In more sophisticated versions of the controller the signal generator of the main fuel control channel can produce a P2/P1 demand which relates closely to the anticipated P2/P1 value of the engine at the demand level of thrust for measured flight conditions and achieved for working line. Hence accurate control to thrust (which can be either gross thrust or net thrust) is possible. The way in which this more precise form of control is achieved is described later. The P2/P1 ratio can be either PS2/PS1 or PT2/PT1 but, if the latter, it is preferred that PT1 is estimated rather than measured directly in order to avoid intrusive instrumentation in front of the rotating components of the engine and in order to avoid sensitivity to flow distortion or swirl etc in the inlet airstream.

The nozzle area control channel can be arranged to hold the engine to a single fan working line or alternatively can be arranged to implement a fan working line variation through the range of engine and flight conditions so as to optimise the balance between thrust and surge margin differently under different conditions to take account of manoeuvre requirements etc. The former system is the easiest to implement for only one look-up table is required to be held by each respective signal generator of the main fuel control channel and nozzle area control channel. The control errors introduced by this approach are minimal because changes in fan working line have a relatively small effect on the relationship between thrust and fan pressure ratio.

One of the merits of the invention is that the control of the fan pressure ratio and DPUP can be performed with de-coupled channels manipulating respectively the main fuel feed and nozzle area.

Research engine tests and simulation results have indicated that such control provides good control of thrust and a good thrust response to slam accelerations. The virtue of such a decentralised control is in the avoidance of control equipment complexity and in the greater certainty concerning failure modes. The decentralised controller as described above ignores the effects of main fuel manipluation on DPUP and of nozzle manipulation upon fan pressure ratio.

It is likely that it will not be possible to ignore the cross coupling of the manipluation and the responses in the main fuel control channel and nozzle area control channel for all engine installations without some loss of control effectiveness. A modified version of the controller uses centralised multivariable control techniques providing cross-coupled control to take account of all links between control inputs and engine responses.

This modified form of the controller comprises the following: an input port or ports at which control demands are delivered there being one such port at which a thrust demand is delivered; respective input ports for control reference signals these reference signals including at least the values of P1, P2, PS7, DPUP, NL and 0, or derived parameters; respective output parts at which actuator demand signals for the main fuel feed actuator, the nozzle area actuator, and the reheat fuel feed actuator are provided; a first signal generator which generates a P2/P1 demand signal corresponding to the delivered thrust request by reference to stored calibration data and to control reference signals;; a first summing junction, connected to the output of the first signal generator, at which the P2/P1 demand signal and a P2/PI feedback signal are compared to produce an error signal; a second signal generator which generates a DPUP demand signal corresponding to a delivered surge margin request or to a predetermined fan working line by reference to stored calibration data and to a control reference signal or signals; a second summing junction, connected to the output of the second signal generator, at which the DPUP demand signal and a DPUP feedback signal are compared to produce an error signal;; a dual input-dual output multivariable compensator in which all inputs affect all outputs, having respective input ports at which the P2/P1 error signal and the DPUP error signal are delivered and respective output ports at which are provided an actutator demand signal for the main fuel feed actuator and an actuator demand signal for the nozzle actuator by reference to stored data correlating the open loop dynamic response of the engine in terms of fan pressure ratio and DPUP to manipulation of main fuel feed and nozzle area; and a reheat fuel control channel including a respective signal generator which generates a PS7/PS1 demand signal corresponding to a required reheat boost by reference to stored calibration data, and to control reference signals, a summing junction at which the PS7/PS1 demand signal and a PS7/PS1 feedback signal are compared to produce an actuator demand signal fed to the reheat fuel actuator, and a gain scheduled signal compensator which acts upon the error signal to produce the respective actuator demand signal by reference to stored data documenting the open loop dynamic response of the engine to manipulation of the reheat fuel actuator demand signal.

The calibration data held within the multivariable controller is obtained by calibration in an engine/installation type test using well established control engineering techniques.

The engine controller might include a facility permitting variation of the fan working line across the operating regime, through manipulation of the nozzle actuator demand, to provide an optimised balance between thrust and surge margin according to the needs of the moment. To provide this facility the signal generator which creates the DPUP demand signal has an input port at which surge margin or working line requests are delivered and that signal generator has stored calibration data relating DPUP to NL/4e for various working lines across the range of intended variation. The signal generator which produces the P2/PI demand signal is provided with a control reference signal of surge margin or fan working line and has a store of data relating P2/PI to thrust for various fan working lines across the range of variation also.This facility for variation of fan working line can be incorporated in both the de-coupled and cross-coupled versions of the controller.

The invention is described below by way of example with reference to Figures 2 to 14 which are as follows: Figure 2 is a schematic circuit diagram for one form of the controller; Figure 3 to 9 are various calibration plots for the contoller; Figures 10 illustrates an input interface unit for the controller.

Figures II and 12 are plots of engine response data for a slam increase in thrust demand for the controller.

Figure 13 is a table of engine control data; and Figure 14 is a schematic circuit diagram for a second form of the controller.

The engine controller which is computer-based is designated 20 in Figure 2. In a non-flying research version of the controller all the elements shown within the claim dotted boundary on Figure 2 can be constructed within a single microcomputer, such as a Texas Instruments 990 microcomputer suitably programmed. The controller 20 has three separate, independent channels each of which controls a respective manipulated variable in response to a respective command. A first channel, designated 21, accepts dry thrust commands at an input port 22 and controls the main fuel feed by an output provided at an output port 23. This dry thrust command is derived from the pilot's thrust lever. A second channel of the controller, designated 24 accepts surge margin commands at an input port 25 and controls the engine nozzle acutators by a signal provided at an output port 26. The third channel, designated 27, accepts requests at an input port 28 and controls the reheat fuel supply thrust accordingly by a signal provided at an output port 29.

The functional units within each channel and the interconnection of these are similar in each channel and typical of many control engineering systems employing dosed loop techniques for single input-single output control. Each channel incorporates a signal generator within which a demand value of the reference input is created, a summing point at which an error signal is created by subtracting a feedback signal from the demand signal and a signal compensator which translates the error signal to a corresponding actuator demand through a gain scheduled proportional plus integral action. The gain values ascribed to the proportional and integral actions are derived by conventional methods using data obtained from open loop response tests measuring the dynamic response of the engine (as reflected in the feedback signal) to changes in the actuator demands.

Channel 21 controls the main fuel feed to the engine by manipulation of a fuel feed actuator demand signal provided at output port 23. In a simple version of the controller, without provision for variation of fan working line in response to imposed commands, signal generator 30 incorporates a single look-up table correllating pressure ratio P2/PI against thrust or pilots control setting for the fan working line maintained by the controller and for a single flight envelope condition eg the sea level static condition. In the more comprehensive version described here the signal generator has access to a family of calibration lines, each for a separate fan working line and has the means to interpolate between plotted lines.Moreover providing the thrust coordinate on the plot is scaled as the NDGTF rather than gross thrust or net thrust then a single engine calibration obtained by a sea level static calibration will hold good to a satisfactory degree across the flight envelope. Such a calibration plot is provided at Figure 3. Some correction of NDGTF for secondary flight envelope effects (eg variation in Reynold's number) would be required to secure ultimate accuracy and should such accuracy be required then altitude engine tests would be needed to provide the necessary additional data. This refinement is not incorporated in the version described here.The conversion of gross thrust input command into NDGTF is made within the signal generator 30 using the NDGTF equation stated previously and reference inputs of PO, PSI and AN. the calculated value of NDGTF and a reference input of surge margin fix a corresponding value of pressure ratio P2/PI which is the required demand signal provided at output 32.

The calibration plot shown in Figure 3 relates NDGTF to pressure ratio P2/PI measured as static pressures (ie PS2/PS1). In the alternative a similar plot could be provided using the total pressure version of the same pressure ratio. To avoid the intrusive instrumentation required for the direct measurement of PTI and the sensitivity of the measured value of PT1 to inlet swirl a value of PT1 can be estimated from bypass duct rake measurements of PTI and PT2 together with a measured value of PS1 by reference to the equation: EFANPR = PT2/PSI (1+Y) where Y is a function of NLX and DPUP for which values are obtained by calibration.

The PS2/PSI demand signal produced at output 32 of the signal generator 30 in the controller described is passed to a summing point 33 where it is compared with a feedback value of PS2/PS1 indicated at 34. Difference signal 35 is directed to signal compensator 36. The gain values of the proportional and integral actions of signal compensator 36 are predetermined by engine calibration but are not constant across the range of engine operation because the dynamic response of the engine to actuator manipulation varies from point to point. In order to fix the current response characterstic of the engine reference inputs 37 are provided to signal compensator 36 these comprising NH, 0, PS1 and PO.

An output signal from compensator 36 is provided at 38 and directed to a 'lowest wins' gate 39. Here the level of the compensator output is compared with other competing signals generated by structural and acceleration limiters and the lowest signal is used to provide the actuator demand signal for the main fuel pump at channel output port 23. Upper and lower limits as well as rate of change limits can be applied at various points along channel 21 to protect engine structure and combustion stability etc as is commonplace in aero engine control practice.

The signal generator of channel 24 is designated 40. This has access to a calibration plot of the form shown in Figure 4 for a single fan working line or alternatively to a like calibration covering several fan working lines if a fan working line variation is to be provided by the controller. In the former case the surge margin command input port 25 is not used and the signal generator 40 creates a DPUP demand signal according to its one calibration plot by reference to a reference input at port 41 of NL/O. In the alternative case an input command at port 25 causes the signal generator to generate a DPUP demand signal by seeking the requisite working line (by interpolation between the given lines) then using the reference input b to fix the operating point on that line.The DPUP demand signal is provided at output 42 and at summing point 43 it is compared with a feedback signal of the same, designated 44, to provide an error signal 45 which is delivered to signal compensator 46. This has reference inputs at ports 47 to fix the current dynamic characteristics of the engine and hence the values of the proportional and integral transfer coefficients. An output signal which establishes the nozzle actuator demand is provided at port 26.

As mentioned previously the remaining channel of the controller 20, channel 27, is responsible for manipulating the engine's reheat fuel supply. Reheat thrust boost is conventionally used in aero turbofans to satisfy short term thrust demands which cannot be achieved in the engine concerned when operating in the dry mode without incurring undue consumption of engine life or causing immediate structural damage. In this description of channel 27 it is assumed that the engine is driven to the maximum dry thrust limit before engaging reheat However reheated operation cannot be sustained below a certain value of reheat boost and engagement of reheat will cause a step change in thrust unless the core engine is simultaneously spooled down to a corresponding degree. When continuity of thrust is important this is achieved by incorporation of a suitable command structure within the controller but is not described here.

The input port 28 of channel 27 leads to a signal generator 50 having calibration plots of the form shown in Figure 5 and 6 and additionally (if required) that shown in Figure 7. Reference inputs 51 are provided to the signal generator 50. A command is delivered to input port 28 in the form of a gross thrust signal at a level requiring reheat (XG reheat) as is also a reference input of maximum dry gross thrust (XG maxdry).Providing the engine is held to the same working line by means of channel 24 when switched to reheat, then it is a reasonable assumption that the value of NDGTF will be held sensibly constant and from the definition of NDGTF it follows that: XG maxdry/(AN maxdry x PSI) + PO/PS1 = XG reheat/(AN reheat x PS1) + PO/PS1 Hence thrust and nozzle areas are related as follows: XG reheat/XG maxdry = AN reheat/AN maxdry; and a value of the nozzle area ratio can be calculated by the signal generator 50.

This nozzle area ratio value together with reference inputs of PS2/PS1 and surge margin enables a corresponding value of the ratio PS7/PSl reheat to PS7/PS1 dry to be obtained from Figure 5 by the signal generator 50. At the same time, the dry engine calibration in Figure 6 together with the above mentioned reference inputs lead to a dry engine value of PS7/PSI. This can enable the signal generator 50 to utilize these two to derive directly a demand value of PS7/PS1 reheat.

However the assumption that NDGTF is held constant in the change from dry operation to reheat, by maintenance of working line, is not completely accurate because of the thermal changes incurred by imposition of reheat To compensate for this factor it is arranged that a further refinement of the value the ratio PS7/PS1 reheat to PS7/PS1 dry is made by the signal generator 50 before utilizing this to determine the output demand signal. The first estimate of this reheat/dry ratio of pressures together with reference inputs of PS2/PS1 and surge margin are used by the signal generator 50 in conjunction with the engine calibration shown at Figure 7 to derive a corresponding ratio of NDGTF reheat/NDGTF dry. A value of NDGTF dry is obtained from the plot shown at Figure 3 and this leads to a value of NDGTF reheat.The NDGTF equation enables a corresponding value of AN reheat to be obtained from the NDGTF reheat input value and reference values of XG reheat, PS1 and PO. This is combined with a value of AN maxdry to provide a second iteration of the nozzle area ratio AN reheat/AN maxdry and hence a second iteration of the ratio PS7/PS1 reheat PS7/PS1 dry by a second reference to the calibration plot of Figure 5. At this stage of iteration this ratio of pressures is sufficiently refined to be used by the signal generator 50, in conjunction with the value of PS7/PS1 dry obtained from Figure 6, to create a demand value of the signal PS7/PS1 which is provided as an output from the controller.

This demand value of PS7/PSI is indicated at 52 and at summing point 53 it is compared with a feedback signal 54 of PS7/PSI to create an error signal 55. This error signal is directed to signal compensator 56 having gain scheduled proportional plus integral action and having reference inputs 57 to fix the instant dynamic conditions of the engine and hence vary the values of the proportional and integral gains. An output signal which establishes the reheat fuel actuator signal is provided at port 29. To ensure that no spurious reheat fuel actuator signal is provided when reheat is not required the output of channel 27 is suppressed at other times. A switch 58 is shown to represent this feature of the controller.

In addition to the elements of the three control channels described above, the controller 20 has several additional elements for manipulation of measured signals to provide appropriate reference inputs to the control channels and to provide also an input signal strategy to initiate operation when so required of the reheat control channel 27. These additional elements are described below.

A processor 60 is provided to convert measured DPUP values at input 61 to a corresponding surge margin value by reference to a calibration as shown in Figure 8 and to a reference input of NL/40 designated 62. An output is provided at 63 and this is utilized as a reference input at signal generators 30, 40 and 50, and also to a further processor designated 64. Processor 64 holds a calibration as shown in Figure 9 and an input value of PS2/PS1 provided at input 65 is converted by processor 64 to a corresponding value of AN maxdry by reference to this plot and to a reference input 66 of surge margin.

The AN maxdry output is provided at 67 and this is utilized as a reference input by signal generator 50. Output 67 is also directed to a processor 68 which has reference inputs 69 of NDGTF maxdry, PO and PSI. The NDGTF maxdry input is provided by signal generator 30 as an auxiliary output 70. Processor 68 produces a value of XG maxdry corresponding to its inputs by means of the NDGTF equation stated previously. This value is provided as an output at 71. To ensure appropriate ordering of input commands to the controller 20 a separate functional unit of the form shown in Figure 10 is interposed between the aircraft controls and the notional input parts of the controller.

A gross thrust command is delivered as an input at port 80. This is conveyed as an input 82 to a lowest wins gate 81 with the other input (83) to this gate being the XG maxdry value provided as output 71 of processor 68. An output 84 is provided (this of course being the lower of the two inputs 82 and 83). This output is provided directly to the input port 22 of controller 20 and thus ensures that channel 21 does not drive the PS2/PS1 demand beyond the maximum dry thrust rating of the engine. Output 84 is also provided as an input 85 to a comparator 86. The comparator has another input 87 which is the XG maxdry value produced at the output 71 of processor 68. Comparator 86 produces a logic output when the level of the gross thrust command at port 80 exceeds the XG maxdry value, which causes that gross thrust input command to be routed to input port 28, by operation of a logic switch 88, for control of the reheat fuel channel 27. The generation of an output at the comparator also causes operation of switch 58 which removes the output suppression from channel 27.

Figures 11 and 12 document the slam thrust increase response in an engine controlled using the controller described above. The flat response of the thrust, as measured by an engine bed load cell, to the imposed change is dear as is the correlation between the ratio PS2/PS1 and the measured thrust. These figures relate to a sea level static test performed on a research engine operating without reheat Other parameters sometimes used for engine control purposes such as TBT, NH and NL/40 show a less dean response to the change in demanded thrust and a longer time to reach a stabilized value. The implications of this are that an engine controller controlling to thrust by use of any one of these parameters as the thrust related variable would not produce such a flat thrust response.

Figure 13 provides a table of engine test data for an engine controlled using the controller described above operating across the range from zero to full reheat. In this instance the signal generator 30 of channel 21 was calibrated in terms of the EFANPR version of the fan pressure ratio, and the channel used a feedback signal of the same to create the appropriate error signal. The dose equivalence between the measured value of the controlled parameter EFANPR and the measured value of the uncontrolled parameter PS2/PS1 indicates that either would be satisfactory for control purposes in the steady state at least. The engine static pressure ratio PS7/PS1 decreases as a function of reheat fuel providing the working line is held.

Figure 14 is a partial view of a modified version of the controller 20 which incorporates a single dual-input dual-output multivariable compensator designated 90 in place of the separate compensators 36 and 46 described previously. The inputs to and outputs from this compensator 90 are the same as described previously and are identified by the same designation. In this version of the controller 20 both the PS2/PS1 error signal at 35 and the DPUP error signal at 45 affect both the main fuel actuator signal at 38 and the nozzle area actuator signal at output port 26. The gains for each proportional and integral action of the multivariable compensator 90 are determined as before by isolating one at a time each control variable and the response of the engine to that control. Obviously those gains relating the fuel actuation signal to the P2/PI error and relating the nozzle actuation signal to the DPUP error will be more significant than the gains for the cross-coupling actions of compensator 90.

Claims (6)

1. A controller for a reheated turbofan engine giving simultaneous control of main fuel, nozzle area and reheat fuel in response to a thrust request, which comprises the following: an input port or ports at which control demands are delivered there being one such port at which a thrust demand is delivered; respective input ports for control reference signals these reference signals including at least the values of P1, P2, PS7, DPUP, NL and e or derived parameters; respective output ports at which actuator demand signals for the main fuel feed actuator, the nozzle area actuator, and the reheat fuel feed actuator are provided;; a main fuel control channel including a respective signal generator which generates a P2/Pl demand signal corresponding to the delivered thrust request by reference to stored calibration data and to control reference signals, a summing junction at which the P2/P1 demand signal and a P2/P1 feedback signal are compared to produce an error signal and a gain scheduled signal compensator which acts upon the error signal to produce the respective actuator demand signal by reference to stored data documenting the P2/P1 open loop response of the engine to manipulation of the main fuel feed actuator demand signal;; a nozzle area control channel including a respective signal generator which generates a DPUP demand signal corresponding to a delivered surge margin request or to a predetermined fan working line by reference to stored calibration data and to a control reference signal or signals, a summing junction at which the DPUP demand signal and a DPUP feedback signal are compared to produce an error signal and a gain scheduled signal compensator which acts upon the error signal to produce the respective actuator demand signal, by reference to stored data documenting the DPUP open loop dynamic response of the engine to manipulation of the nozzle actuator demand signal; and a reheat control channel including a respective signal generator which generates a PS7/PSl demand signal corresponding to a required reheat boost by reference to stored calibration data and to control reference signals, a summing junction at which the PS7/PS1 demand signal and a PS7/PS1 feedback signal are compared to produce the respective actuator demand signal, and a gain scheduled signal compensator which acts upon the error signal to produce the respective actutator demand signal by reference to stored data documenting the dynamic response of the engine to manipulation of the reheat fuel actuator demand signal.

2. A controller for a reheated turbofan engine giving simultaneous control of main fuel, nozzle area and reheat fuel in response to a thrust request, which comprises the following; an input port or ports at which control demands are delivered there being one such port at which a thrust demand is delivered; respective input ports for control reference signals these reference signals including at least the values of P1, P2, PS7, DPUP, NL and 0, or derived parameters; respective output ports at which actuator demand signals for the main fuel feed actuator,the nozzle area actuator, and the reheat fuel feed actuator are provided; a first signal generator which generates a P2/PI demand signal corresponding to the delivered thrust request by reference to stored calibation data and to control reference signals;; a first summing junction, connected to the output of the first signal generator, at which the P2/P1 demand signal and a P2/P1 feedback signal are compared to produce an error signal; a second signal generator which generates a DPUP demand signal corresponding to a delivered surge margin request or to a predetermined fan working line by reference to stored calibration data and to a control reference signal or signals; a second summing junction, connected to the output of the second signal generator, at which the DPUP demand signal and a DPUP feedback signal are compared to produce an error signal;; a dual input-dual output multivariable compensator in which all inputs affect all outputs, having respective input ports at which the P2/Pl error signal and the DPUP error signal are delivered and respective output ports at which are provided an actuator demand signal for the main fuel feed actuator and an actuator demand signal for the nozzle actuator by reference to stored data correlating the open loop dynamic response of the engine in terms of fan pressure ratio and DPUP to manipulation of main fuel feed and nozzle area; and a reheat fuel control channel including a respective signal generator which generates a PS7/PS1 demand signal corresponding to a required reheat boost by reference to stored calibration data and to control reference signals, a summing junction at which the PS7/PS1 demand signal and a PS7/PS1 feedback signal are compared to produce an actuator demand signal fed to the reheat fuel actuator and a gain scheduled signal compensator which acts upon the error signal to produce the respective actutator demand signal, by reference to stored data documenting the dyanmic response of the engine to manipulation of the reheat fuel actuator demand signal.

3. A controller as claimed in claim 1 or claim 2 in which the signal generator of the main fuel control channel holds calibration data correlating a non dimensionalised derivative of thrust to P2/P1 and surge margin or working line such that data obtained from a sea level static test of the engine to be controlled is adequate for control purposes at other flight conditions.

4; A controller as claimed in any one of the preceding claims comprising a processor holding calibration data correlating nozzle area to surge margin and P2/Pl and operative to provide an estimate of nozzle area against inputs of surge margin and P2/Pl for control reference purposes elsewhere in the controller.

5. A controller as claimed in any one of the preceding claims comprising a processor holding calibration data correlating surge margin to DPUP and P2/P1 and operative to provide an estimate of surge margin against inputs of DPUP and P2/P1 for control reference purposes elsewhere in the controller.

6. A controller as claimed in claim 1 or claim 2 substantially as hereinbefore described with reference to the drawings.