Patent Description:
Overspeed is a condition in which an engine is allowed or forced to turn beyond its design limit. In a propeller-based aircraft, various scenarios may cause an overspeed. For this reason, overspeed protection systems are provided to avoid the damage that may be caused to the engine by the overspeed event. However, under certain specific circumstances, it may be preferable to avoid triggering the overspeed protection.

<CIT> discloses an in-flight restart system and a method for a free turbine engine.

According to an aspect of the present invention, there is provided a method for operating an aircraft turboprop engine. The method comprises controlling a propeller of the turboprop engine based on a selected one of a reference propeller rotational speed and a minimum propeller blade angle while the turboprop engine is running; detecting an inflight restart of the turboprop engine; and controlling the propeller during the inflight restart in accordance with at least one of a modified reference propeller rotational speed and a modified minimum propeller blade angle to maintain an actual propeller blade angle above an aerodynamic disking angle during the inflight restart.

According to another aspect of the present invention, there is provided a system for operating an aircraft turboprop engine in accordance with claim <NUM>.

<FIG> illustrates a powerplant <NUM> for an aircraft of a type typically provided for use in subsonic flight, comprising an engine <NUM> and a propeller <NUM>. The powerplant <NUM> generally comprises in serial flow communication the propeller <NUM> attached to a shaft <NUM> and through which ambient air is propelled, a compressor section <NUM> for pressurizing the air, a combustor <NUM> in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section <NUM> for extracting energy from the combustion gases. The propeller <NUM> converts rotary motion from the shaft <NUM> of the engine <NUM> to provide propulsive force for the aircraft, also known as thrust. The propeller <NUM> comprises two or more propeller blades <NUM> that are adjustable in angle position. The blade angle may be referred to as a beta angle, an angle of attack or a blade pitch. The engine <NUM> may be a single or multi-spool gas turbine engine, where the turbine section <NUM> is connected to the propeller <NUM> through a reduction gearbox (RGB).

With reference to <FIG>, there is illustrated an example of a system <NUM> for operating the powerplant <NUM>. In the illustrated embodiment, a control system <NUM> receives a power lever request from a power lever <NUM> of the aircraft, controlled by a pilot or other aircraft operator. The power lever request is indicative of a position of the power lever <NUM> and represents a thrust demand. Several power lever positions can be selected, including those for (<NUM>) maximum forward thrust (MAX FWD), which is typically used during takeoff; (<NUM>) flight idle (FLT IDLE), which may be used in flight during approach or during taxiing on the ground; (<NUM>) ground idle (GND IDLE), at which the propeller <NUM> is spinning, but providing very low thrust; (<NUM>) maximum reverse thrust (MAX REV), which is typically used at landing in order to slow the aircraft. Intermediate positions between the abovementioned positions can also be selected.

The control system <NUM> is configured to control the engine <NUM> and the propeller <NUM> based on the power lever request. An engine request is output to an engine actuator <NUM> for adjusting engine fuel flow, and a propeller request is output to a propeller actuator <NUM> for adjusting the blade angle of the propeller <NUM>. The engine actuator <NUM> and/or propeller actuator <NUM> may each be implemented as a torque motor, a stepper motor or any other suitable actuator. The propeller actuator <NUM> controls hydraulic oil pressure to adjust the blade angle based on the propeller request. The engine actuator <NUM> adjusts the fuel flow to the engine <NUM> based on the engine request. The engine request and/or propeller request are determined as a function of the power lever request and one or more inputs that take into account various engine and/or operating conditions. For example, actual engine and propeller parameters such as propeller rotational speed (NP), propeller blade angle (β), and gas generator speed (NG) are used to determine how the fuel flow and blade angle are to be adjusted in order to provide the power lever request. Flight conditions such as aircraft speed (CAS), altitude (ALT), outside ambient temperature (OAT), and the like may be taken into account as well in setting the engine request and/or propeller request, in combination with a corresponding schedule for fuel flow and/or blade angle.

While the control system <NUM> is illustrated as separate from the powerplant <NUM>, this is for illustrative purposes. In addition, control of the propeller <NUM> and engine <NUM> may be effected by separate controllers, such as an electronic engine controller (EEC) and a propeller control unit (PCU) (which may be electronic or hydraulic), or by a single controller that combines the functionalities of the EEC and the PCU.

In normal operation, the propeller <NUM> is controlled using one of two control laws: (<NUM>) based on a reference (or target) NP; or (<NUM>) based on a minimum β. Under particular flight conditions (i.e. OAT, ALT, CAS), the minimum β, which is typically set as a design parameter, may cause a propeller overspeed event that can trigger the feather solenoid overspeed protection system. As a result, sudden thrust variations can be felt by the pilot and the passengers, which can be undesirable. With reference to <FIG>, graph <NUM> illustrates the relationship between aircraft speed (CAS) and an aerodynamic disking angle of the propeller for constant ALT, OAT, and NP. The aerodynamic disking angle is the angle at which the rotational drag of the propeller is at its minimum value for a given set of flight conditions. As shown in graph <NUM>, the aerodynamic disking angle <NUM> varies with CAS. When the blade angle of the propeller is greater than the aerodynamic disking angle, an increase in β results in a decrease of NP at a constant shaft horse power (SHP). When the blade angle of the propeller is smaller than the aerodynamic disking angle, an increase in in β results in an increase of NP at a constant shaft horse power (SHP).

Region <NUM> is bounded by the aerodynamic disking angle <NUM> and by a minimum β schedule <NUM> and represents an operating regime where the minimum β is lower than the aerodynamic disking angle and the propeller is windmilling. At the initiation of an engine inflight procedure, the actual NP is below the reference NP. The β is reduced to increase NP towards the reference NP. However, for a given range of CAS, if the value of β is reduced below the aerodynamic disking angle <NUM>, the behavior of the propeller changes such that increasing β causes an increase in NP. Therefore, when the reference NP is reached and β is increased to maintain the reference NP, an overspeed occurs.

In order to avoid the overspeed event during an inflight restart, the propeller is controlled so as to ensure that β does not fall below the aerodynamic disking angle. A first approach is to use a modified minimum β for inflight restarts, by temporarily setting the minimum β to a value that is greater than or equal to the aerodynamic disking angle. This directly prevents β from being reduced below the aerodynamic disking angle. A second approach is to use a modified reference NP during inflight restarts, by temporarily setting the reference NP to a lower value. This indirectly prevents β from being reduced below the aerodynamic disking angle as the reference NP is reached before reaching the minimum β value. A third approach is to use a combination of a modified minimum β and a modified reference NP by temporarily setting the minimum β to an increased value and temporarily setting the reference NP to a lower value, such that the combination of an increased minimum β and a decreased reference NP will ensure that the actual β remains above the aerodynamic disking angle during the inflight restart.

<FIG> illustrates an example method <NUM> for operating an aircraft turboprop engine to prevent an overspeed event during an inflight restart. At step <NUM>, the propeller of the turboprop engine is controlled based on a selected one of a reference NP and a minimum β. At step <NUM>, when an inflight restart is detected, the propeller is controlled during the inflight restart in accordance with a modified reference NP and/or a modified minimum β to maintain an actual β above the aerodynamic disking angle during the inflight restart.

The modified reference propeller rotational speed and/or modified minimum propeller blade angle are referred to collectively as a modified schedule. It will be understood that the expression "modified schedule" includes embodiments where the reference NP and/or minimum β is modified by providing a separate and dedicated modified schedule, as well as embodiments where the reference NP and/or minimum β is modified through the application of a gain or a bias to normal schedule values. In both cases, the result is that the actual propeller blade angle is maintained above the aerodynamic disking angle during the inflight restart.

In some embodiments, no distinction is made between an inflight restart and a ground start, and the modified schedule is applied whenever an engine start is detected. An example is illustrated in <FIG>, where a switch <NUM> is used to select either a normal schedule <NUM> or a modified schedule <NUM> based on the engine state, which is either a normal state or a start state. The modified schedule may be selected among a plurality of modified schedules based on flight conditions.

In some embodiments, an inflight restart is detected using a combination of engine state and flight conditions. For example, if the engine state is an engine start and the altitude and/or aircraft speed is greater than a threshold, then an inflight restart is detected and the selected schedule corresponds to a modified schedule. In a first example implementation illustrated in <FIG>, the switch <NUM> is designed to make the distinction between the in-flight restart and the normal operation. In an alternative implementation illustrated in <FIG>, the distinction is not made via a switch but is instead applied more generally to the selection of a most suitable schedule given the PLA, the flight conditions, and the engine state, whereby at least one of the schedules is the modified schedule having the reference NP and/or minimum β values that will maintain the actual propeller blade angle above the aerodynamic disking angle during the inflight restart.

Additional parameters may be used to further limit the application of the modified schedule to certain specific circumstances. For example, and as illustrated in <FIG>, the operating region <NUM> to be avoided is only present above certain aircraft speeds and for negative SHP values. Therefore, in some embodiments, detecting the inflight restart may comprise determining that the aircraft is operating within a given range of aircraft speeds and/or that SHP < <NUM>.

The method <NUM> may be implemented in the control system <NUM> with one or more computing device <NUM>, an example of which is illustrated in <FIG>. For simplicity only one computing device <NUM> is shown but the control system <NUM> may include more computing devices <NUM> operable to exchange data. The computing devices <NUM> may be the same or different types of devices. Note that the control system <NUM> can be implemented as part of a full-authority digital engine controls (FADEC) or other similar device, including electronic engine control (EEC), engine control unit (ECU), electronic propeller control, propeller control unit, and the like. Other embodiments may also apply.

The methods and systems for operating an aircraft turboprop engine described herein may be implemented in a high level procedural or object oriented programming or scripting language, or a combination thereof, to communicate with or assist in the operation of a computer system, for example the computing device <NUM>. Alternatively, the methods and systems for operating an aircraft turboprop engine may be implemented in assembly or machine language. The language may be a compiled or interpreted language. Program code for implementing the methods and systems for operating an aircraft turboprop engine may be stored on a storage media or a device, for example a ROM, a magnetic disk, an optical disc, a flash drive, or any other suitable storage media or device. The program code may be readable by a general or special-purpose programmable computer for configuring and operating the computer when the storage media or device is read by the computer to perform the procedures described herein. Embodiments of the methods and systems for operating an aircraft turboprop engine may also be considered to be implemented by way of a non-transitory computer-readable storage medium having a computer program stored thereon. The computer program may comprise computer-readable instructions which cause a computer, or more specifically the processing unit <NUM> of the computing device <NUM>, to operate in a specific and predefined manner to perform the functions described herein, for example those described in the method <NUM>.

Claim 1:
A method (<NUM>) for operating a turboprop engine (<NUM>) for an aircraft, the method (<NUM>) comprising:
controlling (<NUM>) a propeller (<NUM>) of the turboprop engine (<NUM>) based on a selected one of a reference propeller rotational speed and a minimum propeller blade angle while the turboprop engine (<NUM>) is running;
detecting an inflight restart of the turboprop engine (<NUM>); and characterized in that it further comprises
controlling (<NUM>) the propeller (<NUM>) during the inflight restart in accordance with at least one of a modified reference propeller rotational speed and a modified minimum propeller blade angle to maintain an actual propeller blade angle above an aerodynamic disking angle (<NUM>) during the inflight restart.