Control system and method for propeller-speed overshoot limitation in a turbopropeller engine

An electronic control system (35) for a turbopropeller (2) having a gas turbine engine (20) and a propeller assembly (3) coupled to the gas turbine engine (20), the control system (35) having: a propeller control stage (35a), implementing a closed loop control for controlling operation of the propeller assembly (3) based on a scheduled propeller speed reference (Nrref) and a propeller speed measure (Nr); a gas turbine control stage (35b), implementing a closed loop control for controlling operation of the gas turbine engine (20) based on a scheduled reference (Ngdotref) and at least a feedback quantity. The control system (35) further envisages an auxiliary control stage (35c), coupling the propeller control stage (35a) and the gas turbine control stage (35b) and determining a limitation of the operation of the gas turbine engine (20), if a propeller speed overshoot is detected, with respect to the propeller speed reference (Nrref).

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority from European patent application no. 18425019.9 filed on 23 Mar. 2018, the entire disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present solution relates to a control system and method for propeller-speed overshoot limitation in a turbopropeller (or turboprop) engine of an aircraft.

BACKGROUND ART

As it is known, a turbopropeller includes a gas turbine engine and a propeller assembly, coupled to the gas turbine engine via a gearbox assembly.

Aircraft solutions are known, in which a single operator-manipulated input device (a power, or throttle, lever) is used by the pilot to input an engine power request; the same input device is also used to determine a propeller pitch angle or setting.

Propeller control is generally based on a closed-loop tracking of propeller speed and/or propeller pitch references, taking into account operating and environmental conditions and the input power request provided via the input device. The output of the control action is generally a driving quantity, e.g. an electric current, that is supplied to a propeller actuation assembly of the turbopropeller engine (e.g. to a servo-valve) to control the pitch angle of the propeller blades, in order to regulate the value of the propeller speed.

In a corresponding manner, gas turbine control is generally based on a feedback tracking of one or more quantities related to engine operation, such as the acceleration (or speed rate of change) of a gas generator, or a measured torque, again taking into account the operating and environmental conditions and the input power request. The output of the control action is generally a driving quantity, e.g. an electric current, that is supplied to a fuel metering unit (FMU) to control the quantity or rate of fuel provided to the gas turbine engine.

During flight, propeller speed overshoot may arise, i.e. a rapid increase of the propeller speed above a set reference speed, which may be caused by a high power increase rate, e.g. due to a maximum power requested by the pilot when the aircraft is flying at high altitude in idle condition and at a certain speed; a propeller speed overshoot may also be caused by a propeller control anomaly.

In these situations, the propeller control action (pitch-based) may not be able to maintain the propeller speed to the reference value, and the propeller may overshoot and reach an overspeed value, leading to a potentially dangerous situation for the aircraft and even to engine-failure.

This situation is schematically depicted inFIG.1, which shows plots vs time of: a propeller speed measure, denoted with Nr; a torque TQ, that increases quickly according to the power increase, starting from time t0; and a measure of a gas generator acceleration, denoted with Ngdot.FIG.1also shows the value of a propeller speed reference Nrref.

As shown in the sameFIG.1, the rapid increase rate of the requested power (and the corresponding increase rate of the gas generator acceleration) causes overshoot of the propeller speed Nr with respect to the propeller speed reference Nrref, reaching a value for example up to 40-50 rpm above the same propeller speed reference Nrref.

The present Applicant has realized that known turbopropeller control solutions may not be able to cope with the above discussed issue concerning propeller speed overshoot, thus not preventing the occurrence of potentially dangerous operating conditions for the aircraft.

DISCLOSURE OF INVENTION

The aim of the present solution is to provide an improved control solution for a turbopropeller engine, allowing to achieve a suitable limitation of propeller-speed overshoot.

According to the present solution, a control system and a control method are therefore provided, as defined in the appended claims.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG.2shows a perspective view of an exemplary aircraft1, provided with a turbopropeller2; only a propeller assembly3of the turbopropeller2is visible in the sameFIG.2.

The aircraft1includes an airframe4defining a cockpit5; a single operator-manipulated input device (a power, or throttle, lever)6and an instrument control panel7having a display8are provided within the cockpit5.

The propeller assembly3comprises a hub9and a plurality of blades10extending outwardly from the hub9. A gas turbine engine of the turbopropeller2(here not shown) generates and transmits power to drive rotation of propeller assembly3, thus generating thrust for the aircraft1.

The aircraft1defines an orthogonal coordinate system, including three orthogonal coordinate axes. In particular, the three orthogonal coordinate axes include a lateral axis L, a longitudinal axis T, and a vertical axis V. During operation, the aircraft10can move along at least one of the lateral axis L, the longitudinal axis T, and the vertical axis V; in particular, forward and reverse operating modes of the aircraft1imply movement in respective, and opposing, directions along the longitudinal axis T.

FIG.3shows the operator-manipulated input device6, used by the pilot of the aircraft1to control engine power; additionally, the operator-manipulated input device6controls a minimum propeller pitch angle or low pitch setting (LPS) based on a position thereof.

In the embodiment depicted, the operator-manipulated input device6defines an axial direction A, and includes a lever11having a handle12. The lever11is movable along the axial direction A between a first position14, corresponding to a TAKEOFF or MAXIMUM POWER setting, and a second position16, corresponding to a MAXIMUM REVERSE setting. Accordingly, moving the lever11towards the first position14increases thrust of the aircraft1in a first direction along the longitudinal axis T, whereas moving the lever11towards the second position16increases thrust of the aircraft1in a second direction along the same longitudinal axis T, opposite to the first direction. In addition, the lever11includes one or more of intermediate third positions15disposed between the first and second positions14,16; in particular, the intermediate third positions15can include an IDLE position.

With the single operator-manipulated input device6, the pilot may set at a same time power (or thrust) and propeller speed requirements. The propeller speed demand or reference Nrrefand power are a function of the lever angular position (or level angle, PLA), according to a predetermined schedule being developed to operate the propeller as close as possible to an optimal operating point, assuming a typical aircraft mission.

As schematically shown inFIG.4, the gas turbine engine of the turbopropeller2, here denoted with20, generally comprises:axial/centrifugal compressors22, coupled to an air intake23;a high-pressure turbine24, so called “gas generator”, coupled to the axial/centrifugal compressors22via a gas generator shaft25; anda low-pressure turbine26, so called “power turbine”, mechanically decoupled from the gas generator shaft25and driven by hot gas expansion.

The propeller assembly3is coupled to the gas turbine engine20via a propeller shaft27and a gearbox28.

More specifically, the gearbox28can include a first gear28aand a second gear28bin mesh with the first gear28a. The first gear28acan be connected to the propeller shaft27, in turn coupled to the hub9of the propeller assembly3, and the second gear28bcan be connected to a power turbine shaft27′, in turn coupled to the low-pressure turbine26. During operation, the gearbox28can step-down a rotational speed of the power turbine shaft27′, so that a rotational speed of the propeller shaft27can be less than the rotational speed of the power turbine shaft27′.

An actuation assembly29is coupled to the propeller assembly3, to control the pitch angle β of the propeller blades10, in order to regulate the value of the propeller speed Nr; as shown in the sameFIG.4, the pitch angle β may be defined as the angle between a chord30extending between leading and trailing edges31,32of each propeller blade10and a direction R about which the propeller blades10are rotatable.

The turbopropeller2is managed by an electronic control system35(shown schematically inFIG.4), that includes an electronic processing unit (e.g. a microprocessor, a microcontroller, or similar processing unit) provided with a non-volatile memory storing suitable software instructions, in order to implement an engine control strategy to meet an input power demand, originated from the operator-manipulated input device6. The electronic control system35may define one or more of a full authority digital engine controller (FADEC), an engine control unit (ECU), or an electronic engine control (EEC); in particular, according to an embodiment of the present solution, the electronic control system35implements both a propeller electronic control (PEC) and a turbine electronic control (TEC).

As shown inFIG.5, the control system35comprises a propeller control stage35a, implementing a closed loop control aimed at controlling the propeller speed Nr based on a scheduled propeller speed reference Nrrefand at least a feedback measure. In particular, the propeller control stage35ais configured to generate a driving quantity IP, for example an electrical current, designed to drive the actuation assembly29of the propeller assembly3to set a controlled pitch angle β of the propeller blades10, and to receive, as a feedback measure, at least a measure of the propeller speed Nr; a measure of the pitch angle β may also be received by the propeller control stage35a, as a further feedback measure.

The control system35further comprises a gas generator (or turbine) control stage35b, implementing a respective closed loop control aimed at controlling the engine power based on a scheduled reference and at least one feedback measure. In particular, the gas generator control stage35bis configured to generate a respective driving quantity IF, for example an electrical current, designed to drive a fuel metering unit to set a controlled amount of fuel (or fuel rate) Wffor the gas turbine engine20, and to receive, as a feedback, at least the gas generator acceleration Ngdot (that may be determined from a sensed gas generator speed Ng); a measure of the engine torque TQ or thrust may also be received by the gas generator control stage35b, as a further feedback measure.

As will be discussed in more detail in the following, according to a particular aspect of the present solution, the electronic control system35further comprises an auxiliary control stage35c, which couples the propeller control stage35aand the gas generator control stage35band is configured to determine a limitation of the operation of the gas turbine engine20, in particular a limitation of the engine power increase (as will be discussed in detail, achieved by limiting a reference of the gas generator acceleration control), if a propeller speed overshoot is detected (i.e. if the propeller speed has a given relation with a propeller speed overshoot threshold).

According to an embodiment, the auxiliary control stage35cis configured to acquire a measure of the propeller speed from the propeller control stage35a, and to determine a limitation to the fuel provided to the gas turbine engine20, based on the relation between the propeller speed and the propeller speed overshoot threshold. In particular, the auxiliary control stage35cis configured to limit a scheduled reference used by the gas generator control stage35b, in order to cause the desired limitation of engine power increase.

A possible embodiment of control system35is now discussed in more details with reference toFIG.6.

The propeller control stage35a, implementing the propeller electronic control (PEC) to adjust propeller blade pitch angle β to control the propeller speed Nr, comprises:a first reference generator36, including a first scheduler configured to receive a lever angle signal PLA indicative of the positioning angle of the operator-manipulated input device6; it determines a value of the reference propeller speed Nrref(corresponding to the positioning angle of the lever6), according to a preset schedule that characterizes turbopropeller engine operation (e.g. provided by the manufacturer and stored in the non-volatile memory of the processing unit of the electronic control system35);a first adder block38, receiving at a first (positive, or summation) input the reference propeller speed Nrrefand at a second (negative, or subtraction) input a measure of the propeller speed Nr, as a feedback, measured by a suitable sensor coupled to the turbopropeller engine2, and providing at the output a propeller speed error ep, as a function of the subtraction between the reference propeller speed Nrrefand the measured propeller speed Nr (in a possible embodiment, the measured propeller speed Nr corresponds to the rotational speed of the power turbine shaft27′); anda first regulator39, receiving at its input the propeller speed error epand a measure of the pitch angle β, measured at the propeller assembly3, and generating at its output, based on a regulation scheme, the first driving quantity IP, for example an electrical current, which is provided to a propeller control unit39′, designed to control actuation of the actuation assembly29moving the pitch angle β of the propeller blades10(according to propeller speed error epin alpha control mode, during forward operation, or directly the pitch angle β in beta control mode, e.g. during reverse operation).

The gas generator control stage35b, implementing the turbine electronic control (TEC) to adjust the fuel rate Wfto control engine power, in turn comprises:a second reference generator40, configured to receive the signal PLA indicative of the input power request and to determine a value of a scheduled gas generator acceleration reference Ngdotref;a second adder block41, receiving at a first (positive, or summation) input a control reference Ngdotref′, being a function of the scheduled gas generator acceleration reference Ngdotref(as discussed in the following) and at a second (negative, or subtraction) input a gas generator acceleration Ngdot, as a feedback (which may be measured by a suitable sensor coupled to the gas turbine engine20or, as in the shown embodiment, be the first order derivative of a measured gas generator speed Ng, which may correspond to the rotational speed of the gas generator shaft25), and providing at the output a gas generator acceleration error eNgdot, as a function of the subtraction between control reference Ngdotref′ and gas generator acceleration Ngdot; anda second regulator42, receiving at its input the gas generator acceleration error eNgdot, and generating at its output, based on a regulation scheme aimed at minimizing the same gas generator acceleration error eNgdot, the second driving quantity IF, for example an electrical current, which is provided to a fuel metering unit42′, designed to control the fuel rate Wfprovided to the gas turbine engine20.

In the discussed embodiment, the auxiliary control stage35cis configured to acquire the measure of the propeller speed Nr and the reference propeller speed Nrreffrom the propeller control stage35a, and to cause a limitation to the engine power if the measured propeller speed Nr overcomes the reference propeller speed Nrrefby a given overshoot value, i.e. the propeller speed overshoot threshold, here denoted with THov.

In particular, the auxiliary control stage35cis configured to determine the control reference Ngdotref′, based on the gas generator acceleration reference Ngdotrefat the output of the second reference generator40and based on the difference between the sum of the reference propeller speed Nrrefand the propeller speed overshoot threshold THov, and the measured propeller speed Nr.

The auxiliary control stage35ctherefore comprises:a third adder block45, receiving at a first (positive, or summation) input the propeller speed overshoot threshold THovand at a second (also positive, or summation) input the reference propeller speed Nrref, and providing at the output the sum between the same propeller speed overshoot threshold THovand the same reference propeller speed Nrref(Nrref+THov);a fourth adder block46, receiving at a first (positive, or summation) input the above sum Nrref+THov, from the output of the third adder block45, and at a second (negative, or subtraction) input the measured propeller speed Nr, and providing at the output the difference between the same sum Nrref+THovand the propeller speed Nr ((Nrref+THov)−Nr);a third regulator47, in the example of the PI (Proportional Integral) type, coupled to the output of the fourth adder block46, so as to receive the above difference (Nrref+THov)−Nr, and configured to generate a regulation output OutFIas a function of the value of the same difference (i.e. as a function of the difference between the reference propeller speed Nrrefplus the propeller speed overshoot threshold THovand the measured propeller speed Nr);a conversion-table block48, receiving the regulation output OutFIfrom the third regulator47and converting the value of the same regulation output OutPI, generally ranging from 0% to 100%, into a regulation value Reg, being a scalar lower than or equal to 1, i.e. comprised between a minimum value (higher than 0) and1, according to a suitable conversion table; anda multiplication block49, receiving at a first multiplication input the scheduled gas generator acceleration reference Ngdotrefand at a second multiplication input the regulation value Reg, and providing at the output the control reference Ngdotref′, being limited as the result of the multiplication between the scheduled gas generator acceleration reference Ngdotrefand the above regulation value Reg, which thus has a direct action on the control reference Ngdotref′.

The minimum value of the regulation value Reg may be, for example, equal to 0.2, thereby determining a maximum reduction of 80% of the scheduled gas generator acceleration reference Ngdotref(and a corresponding reduction of the engine power increase rate). In more general terms, the above minimum value is set as a function of a desired maximum intervention of the auxiliary control stage35cin the control action exerted by the electronic control system35; for example, a minimum value of 0.5 at the output of the conversion-table block48determines a maximum reduction of 50% of the scheduled gas generator acceleration reference Ngdotref(and thus a lower intervention by the auxiliary control stage35cwith respect to the previously considered example). The conversion-table block48in any case determines a correspondence between the regulation output OutPI, generally comprised in the full range 0%-100%, and the desired range for the regulation value Reg, comprised between the desired minimum value and 1 (the latter value corresponding to a 100% value of the same regulation output OutPI).

The present Applicant has realized that it may not be convenient to have a minimum value of the regulation value Reg lower than 0.2 (and correspondingly a maximum reduction of the scheduled gas generator acceleration reference Ngdotrefhigher than 80%), in order to maintain a sufficient acceleration capability for the gas turbine engine20.

During operation, when the propeller speed Nr is in steady state (i.e. the value of the same propeller speed Nr is lower than the sum Nrref+THov), the error at the input of the third regulator47is positive, so that the same regulator will saturate to the 100% value. In this case, the regulation value Reg is equal to 1 and the auxiliary control stage35ctherefore does not affect at all the operation of the electronic control system35, in particular of the gas generator control stage35bimplementing the turbine electronic control unit (TEC).

When the propeller speed Nr instead overcomes the reference propeller speed Nrrefby a value higher than propeller speed overshoot threshold THov(i.e. the value of the same propeller speed Nr is higher than the sum Nrref+THov), the third regulator47starts to operate, decreasing its regulation output OutPIto decrease the acceleration reference so that less fuel will be supplied to the gas turbine engine20, thereby determining a reduction of the power increase rate.

The advantages of the present solution are clear from the previous discussion.

In any case, it is again underlined that the present solution allows to achieve an improved control action, in particular limiting propeller speed overshoot and thus reducing the risk of propeller overspeed and possible engine damages.

In this regard,FIG.7shows a comparison between the overshoot of the propeller speed Nr in a traditional control solution (as previously shown inFIG.1), depicted with a dashed line, and the overshoot of the propeller speed that may be obtained with the present control solution, shown with a continuous line. In particular, the present solution allows to greatly reduce the amount of overshoot and more importantly exerts a control action on the same overshoot, which is instead not controlled in any manner in the traditional solutions. The overshoot of the propeller speed Nr inFIG.7is, in the example, the result of a rapid increase rate of the requested power, as was previously discussed with reference toFIG.1.

Advantageously, the present solution allows to limit the propeller speed overshoot, and at the same time does not entail any limitation by the control action on the scheduled acceleration during normal operating conditions (i.e. when the propeller speed Nr does not overcome the propeller speed reference Nrrefby a value higher than the overshoot threshold THov), thus not impairing normal engine performance.

The present solution indeed limits propeller speed overshoot, maintaining the possibility to have arbitrary fast engine accelerations at altitude conditions and desired aircraft speed; in other words, the present solution allows to achieve a proper propeller speed limitation regardless of the engine acceleration schedule.

Finally, it is clear that modifications and variations can be made to what is described and illustrated herein, without thereby departing from the scope of the present invention as defined in the appended claims.

In particular, it is underlined that different implementations could be envisaged for the propeller control stage35aand/or the gas generator control stage35bin the electronic control system35, without however implying any modification to the discussed solution for limiting propeller speed overshoot. In particular, a further regulator could be envisaged in the propeller control stage35a, e.g. based on the pitch angle β (during “beta mode”, for reverse engine operation), cooperating with the first regulator39; likewise, further regulators could be envisaged in the gas generator control stage35b, e.g. based on a torque or a measure of the gas generator speed, cooperating with the second regulator42.

Moreover, it is underlined that, although generally valid for a fixed-wing aircraft, the present disclosure may further apply to rotary-wing aircraft, tilt-rotor aircraft, or other apparatuses including a pitch-changing propeller assembly and a gas generator coupled to the aircraft.