Method of stopping a rotorcraft engine in overspeed, and a system and a rotorcraft associated therewith

Abstract

A method of stopping an engine of a rotorcraft in overspeed, the engine comprising a gas generator and a power assembly. When the engine is in operation, a relationship is established giving a limit derivative that varies as a function of the speed of rotation of the power assembly. The speed of rotation, referred to as the current speed, reached by the power assembly is measured and the time derivative of the speed of rotation is determined and referred to as the current derivative. The engine is stopped automatically when the limit derivative corresponding to the current speed as determined by the relationship is less than or equal to the current derivative.

Claims

1. A method of stopping an engine of a rotorcraft when the engine is in overspeed, the engine including a gas generator and a power assembly, the power assembly having a power turbine set in rotation by gas coming from the gas generator and a power shaft constrained to rotate with the power turbine, the power assembly being rotatable about a longitudinal axis at a speed of rotation, the method comprising: prior to a flight of the rotorcraft, establishing a relationship that provides a limit derivative threshold over a range of speeds of rotation, the limit derivative threshold according to the relationship having a non-constant value varying over the range of speeds of rotation as a function of speed of rotation values within the range of speeds of rotation; and during a flight of the rotorcraft: measuring a current speed of rotation of the power assembly; determining a time derivative of the current speed of rotation, wherein the time derivative of the current speed of rotation is referred to as a current derivative; and determining that the current speed of rotation is in the range of speeds of rotation specified by the relationship and that the limit derivative threshold corresponding to the current speed of rotation in application of the relationship is less than or equal to the current derivative and automatically stopping the engine in response to the current speed of rotation being in the range of speeds of rotation specified by the relationship and the limit derivative threshold corresponding to the current speed of rotation in application of the relationship being less than or equal to the current derivative.

2. The method according to claim 1, wherein the gas generator is fed by a fuel metering device, the fuel metering device is set between a minimum limit inducing a minimum fuel flow rate and a maximum limit inducing a maximum fuel flow rate, and wherein automatically stopping the engine is performed by setting the fuel metering device of the engine at the minimum limit.

3. The method according to claim 1, wherein the engine includes a cock on a fuel pipe, and wherein automatically stopping the engine is performed by closing the cock.

4. The method according to claim 1, wherein the engine is fed with fuel by a pump, and the pump is shut down following automatic stopping of the engine.

5. The method according to claim 1, wherein the limit derivative threshold decreases between: an intermediate value reached for an intermediate speed of rotation value; and a minimum value reached for a maximum acceptable speed of rotation value, the maximum acceptable speed of rotation value being greater than the intermediate speed of rotation value, the minimum value of the limit derivative threshold being less than the intermediate value of the limit derivative threshold.

6. The method according to claim 5, wherein the minimum value of the limit derivative threshold is zero.

7. The method according to claim 5, further comprising not automatically stopping the engine when the speed of rotation is less than the intermediate speed of rotation value, whatever the current derivative.

8. The method according to claim 5, further comprising automatically stopping the engine when the speed of rotation is greater than the maximum acceptable speed of rotation value, whatever the current derivative.

9. The method according to claim 5, wherein the limit derivative threshold decreases linearly between the intermediate value and the minimum value.

10. The method according to claim 1, wherein in a phase diagram plotting the speed of rotation along the abscissa axis and the time derivative of the speed of rotation up the ordinate axis, the relationship providing the limit derivative threshold over the range of speeds of rotation takes the form of a curve that is not parallel to the ordinate axis, the curve splitting the plane of the phase diagram into an authorized sector of operation that does not require the engine to be stopped and an unauthorized sector of operation that does require the engine to be stopped, the unauthorized sector being located downstream from the curve in the direction of increasing abscissa values, the engine to be stopped when an operating point corresponding to the current derivative and to the current speed is located in the unauthorized sector.

11. An overspeed safety system for an engine of a rotorcraft, the engine including a gas generator and a power assembly, the power assembly having a power turbine set in rotation by the gas generator and a power shaft constrained to rotate with the power turbine, the power assembly being rotatable about a longitudinal axis at a speed of rotation, the overspeed safety system comprising: a shutdown system for stopping operation of the engine; a processor unit connected to the shutdown system; a speed sensor for measuring the speed of rotation of the power assembly; the processor unit being connected to the speed sensor, the processor unit being configured to store a relationship, established prior to a flight of the rotorcraft, that provides a limit derivative threshold over a range of speeds of rotation, the limit derivative threshold according to the relationship having a non-constant value varying over the range of speeds of rotation as a function of speed of rotation values within the range of speeds of rotation; and during a flight of the rotorcraft: use the speed sensor to measure a current speed of rotation of the power assembly; determine a time derivative of the current speed of rotation, wherein the time derivative of the current speed of rotation is referred to as a current derivative; and determine that the current speed of rotation is in the range of speeds of rotation specified by the relationship and that the limit derivative threshold corresponding to the current speed of rotation in application of the relationship is less than or equal to the current derivative and use the shutdown system to automatically stop the engine in response to the current speed of rotation being in the range of speeds of rotation specified by the relationship and the limit derivative threshold corresponding to the current speed of rotation in application of the relationship being less than or equal to the current derivative.

12. The overspeed safety system according to claim 11, wherein the shutdown system includes a fuel-metering device conveying fuel to the gas generator.

13. The overspeed safety system according to claim 11, wherein the shutdown system includes a pump conveying fuel to the gas generator.

14. The overspeed safety system according to claim 11 further comprising a shield ring surrounding the power turbine, the power turbine further having a plurality of blades, each blade being fastened to a fuse member.

15. The overspeed safety system according to claim 11, wherein the processor unit is a full authority digital engine controller (FADEC) of the engine.

16. A rotorcraft comprising a rotor and at least one engine, each engine of the at least one engine driving a power transmission train connected to the rotor, wherein the rotorcraft further includes at least one overspeed safety system according to claim 11, each overspeed safety system of the at least one overspeed system being connected to an engine of the at least one engine.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The invention and its advantages appear in greater detail in the context of the following description of embodiments given by way of illustration and with reference to the accompanying figures, in which:

(2) FIG. 1 shows a rotorcraft provided with a single engine;

(3) FIG. 2 shows an aircraft provided with two engines; and

(4) FIG. 3 is a phase diagram explaining the invention.

DETAILED DESCRIPTION OF THE INVENTION

(5) FIG. 1 shows an aircraft 1, in particular a rotorcraft. The members of the aircraft that are not directly involved in the invention are not shown in the figure in order to avoid overloading the figure pointlessly.

(6) In particular, this aircraft 1 is a rotorcraft including a lift and/or propulsion rotor 2. This rotor 2 is rotated by a power plant including at least one engine 10 and one power drive train 3. Such a power drive train 3 includes for example a free wheel 56 and a main gear box 55. The main gearbox 55 is provided with a mast driving a hub of the rotor 2.

(7) Consequently, at least one engine 10 is mechanically connected to said power drive train 3.

(8) The engine 10 of the rotorcraft is in particular a turboshaft engine.

(9) Consequently, the engine 10 includes a gas generator 11. Conventionally, the gas 11 generator is provided with at least one compressor 12, a combustion chamber 13, and at least one expansion turbine 14. The expansion turbine 14 is connected rigidly to the compressor 12 by a shaft referred to as a drive shaft 13.

(10) FIG. 1 shows a single compressor 12 and a single expansion turbine 14. Nevertheless, the number of compressor(s), and expansion turbine(s) may be optimized according to requirements, and does not restrict the ambit of the invention at all.

(11) In addition, the compressor 12, the expansion turbine 14 and the drive shaft 13 are suitable for rotating jointly about a longitudinal axis AX of the engine 10. More precisely, the compressor 12, the expansion turbine 14, and the drive shaft 13 are constrained to rotate together about this longitudinal axis.

(12) The gas generator then rotates at a speed N1, said speed N1 corresponding to the speed of the rotary assembly of the gas generator that comprises the compressor 12 together with the expansion turbine 14 and the drive shaft 13.

(13) In addition, the turboshaft engine 10 comprises a power assembly 19 located downstream from the gas generator. The power assembly is set in movement by the gas generated by the gas generator.

(14) The power assembly 19 comprises at least one power turbine 15 located downstream from the gas generator. The term downstream is to be considered relative to the direction of gas flow within the engine 10.

(15) This power turbine may be connected to the gas generator. However, in FIG. 1, the power turbine is a free turbine that is independent of the gas generator.

(16) Consequently, the power turbine 15 is secured to a power shaft 16 that is connected to the power transmission train 3. Conventionally, the power transmission train 3 is fastened to the power shaft by a member (not shown) for accommodating angular and axial misalignments.

(17) FIG. 1 shows a power assembly 19 including a single power turbine 15. Nevertheless, the number of power turbine(s) may be optimized depending on requirements, and does not restrict the ambit of the invention at all.

(18) The gas leaving the gas generator then sets in rotation the power assembly of the turboshaft engine about the longitudinal axis AX at a speed of rotation N2.

(19) In addition, the rotorcraft comprises at least one tank 4 of fuel 6 for feeding the combustion chamber 13 with fuel.

(20) Consequently, a fuel feed line provided with at least one pump 5 and a metering device 7 connects the tank 4 to the combustion chamber 13. The engine may further comprise a cock 100 on an internal fuel pipe.

(21) The rotorcraft is further provided with an overspeed safety system 20 in order to avoid overspeeding of the engine 10.

(22) This overspeed safety system 20 comprises a processor unit 21.

(23) The processor unit may include a logic circuit or equivalent.

(24) In the variant shown in FIG. 1, the processor unit 21 is for example provided with a storage device 23 and a computer 22. By way of example, the computer may include a processor or the equivalent for executing instructions stored in the storage device for applying the method of the invention. This storage device may include a non-volatile memory 24 storing such instructions and a volatile memory 25 storing parameter values, for example.

(25) The processor unit 21 may be an integral part of a system for controlling a turboshaft engine, such as a system known under the acronym ECU for Engine Control Unit or FADEC for Full Authority Digital Engine Control. Consequently, the computer of the processor unit is the computer of the control system, the storage device being the device for storing said control system.

(26) The processor unit 21 is connected by wire and/or wireless connections to a speed sensor 30. The speed sensor 30 is arranged on the power assembly in order to measure the speed of rotation N2 of said power assembly.

(27) Consequently, the speed sensor 30 transmits a signal conveying the speed of rotation N2 to the processor unit.

(28) In addition, the processor unit 21 is connected to a shutdown system 40 suitable for stopping the engine 10. This shutdown system comprises in particular the metering device 7 feeding the engine 10 and/or the cock 100 with fuel.

(29) Under such circumstances, in the method of the invention, a rotorcraft manufacturer establishes a limit operating relationship prior to flight of the rotorcraft.

(30) Thus, during a development stage, the manufacturer determines a relationship giving a limit time derivative for the speed of rotation N2 of the power assembly. This relationship limits the authorized speed range for the engine 10. For at least one possible range of values for the speed of rotation N2 of the power assembly, the relationship makes it possible to determine a limit value referred to as a limit derivative.

(31) The term range should be understood as a set of values for the speed of rotation N2 extending from a lower limit to an upper limit.

(32) This relationship is then stored in the processor unit 21.

(33) The stored relation ship may take the form of one or more mathematical equations, or indeed a database, for example.

(34) In flight, the processor unit 21 determines whether the engine is in an overspeed condition, and stops said engine 10 automatically if necessary.

(35) To this end, the processor unit 21 performs a series of calculation operations continuously, at a predetermined repetition rate.

(36) During each iteration, the processor unit 21 determines the current speed of the power assembly 19 starting from the signal coming from the speed sensor 30.

(37) The processor unit 21 further deduces the time derivative of said current speed, referred to as the current derivative.

(38) Furthermore, when the current speed is located within a range of said relationship, the processor unit implements the stored relationship in order to determine whether the current derivative is greater than or equal to the limit derivative obtained from the current speed.

(39) By way of example, the processor unit inputs the current speed value into the stored relationship, and deduces the limit derivative resulting therefrom.

(40) If the current derivative is greater than the limit derivative, the engine 10 is considered as being in an overspeed situation. The processor unit then actuates the shutdown system for stopping the engine 10.

(41) If necessary, each pump 5 is also shut off.

(42) FIG. 3 illustrates a relationship of the invention by means of a phase diagram 80.

(43) This phase diagram 80 plots the speed of rotation N2 of the power assembly 19 along the abscissa axis. By way of example, this speed of rotation is expressed as a percentage of a nominal speed of rotation. Furthermore, the phase diagram 80 plots the time derivative of said speed of rotation up the ordinate axis.

(44) The relationship providing the speed of rotation limit derivative then takes the form of a curve 100 in this phase diagram 80.

(45) The curve 100 is not parallel to the ordinate axis, unlike a straight line 95 representing a constant speed of rotation. This curve 100 splits the plane of the phase diagram 80 in authorized and unauthorized sectors of operation 81 and 82. The unauthorized sector 82 is located downstream from said curve 100 in the direction of increasing abscissa values. In FIG. 3, the authorized sector 81 is thus located on the left of the curve 100, with the unauthorized sector being located on the right of said curve 100.

(46) Under these conditions, the engine 10 is stopped when an operating point Pt2 corresponding to the current derivative and the current speed is located in the unauthorized sector 82.

(47) Conversely, if the operating point Pt1 is located in the authorized sector 81, the engine is not stopped.

(48) In order to illustrate the advantage of the invention, FIG. 3 plots a first line 91 that shows a tight turn of the rotorcraft.

(49) A second line 92 shows a tight turn performed violently.

(50) If the manufacturer implements a low constant threshold as represented by the first vertical line 95, said constant threshold will give rise to undue stopping of the engine during the violent maneuver as represented by second line 92.

(51) Furthermore, if the manufacturer implements a high constant threshold as represented by a second vertical line 96, stopping of the engine may be late if overspeeding takes place following a stage of flight that induces a speed of rotation N2 that is relatively moderate, e.g. during a turn performed as represented by the first line 91.

(52) The curve 100 illustrating the relationship of the invention aims to avoid undue stopping, while optimizing the stopping of the engine 10 in the event of overspeed.

(53) In the relationship shown in the diagram, the limit derivative decreases linearly along a segment 102 between:

(54) an intermediate value 302 reached for a speed of rotation referred to as an intermediate speed 201, of the order of 110% of the nominal speed for example; and a minimum value 301 reached for a speed of rotation referred to as a maximum acceptable speed 202, of the order of 130% of the nominal speed for example.

(55) By way of example, this minimum value 301 is zero. Furthermore, the intermediate value 302 is of the order of 40% of the nominal speed, for example.

(56) It should be remembered that, the lift rotor is driven in rotation at a nominal speed, said nominal speed inducing a nominal speed of rotation of the power assembly known to the person skilled in the art.

(57) By way of example, the relationship takes the following form where DL represents said limit derivative and N2 represents the speed of rotation:

(58) $\mathrm{DL}=\frac{-325}{130}N2+325$
for N2 lying in the range 110% to 130% of the nominal speed.

(59) However, the relationship may take other forms, possibly non-linear forms.

(60) By way of example, the segment 102 could be curved.

(61) The curve 100 thus further presents the form of a half-line 101 extending parallel to the ordinate axis from the intermediate value, and going away from the abscissa axis.

(62) In addition, the curve 100 takes the form of a half-line 103 extending parallel to the ordinate axis from the minimum value, and going away from the abscissa axis.

(63) With reference to FIG. 1, in addition to an electronic system, the overspeed safety system may include a mechanical system of the blade shedding type.

(64) Consequently, each blade 51 of the power turbine 15 may be fastened to a hub by a fuse member 52. This fuse member is dimensioned so as to break in the event of overspeed.

(65) In the example shown, each blade is fastened to the power shaft by a fuse member.

(66) In addition, the overspeed safety system comprises a shield ring 50 for containing the blades inside the engine 10.

(67) The invention applies to a rotorcraft provided with a single engine 10 in accordance with the embodiment of FIG. 1.

(68) Nevertheless, the invention also applies to a rotorcraft including a plurality of engines 10 in accordance with the embodiment of FIG. 2.

(69) Consequently, at least one engine 10 is provided with an overspeed safety system 20. Preferably, each engine 10 includes this overspeed safety system 20.

(70) Naturally, the present invention may be subjected to numerous variants as to its implementation. Although several implementations are described, it should readily be understood that it is not conceivable to identify exhaustively all possible implementations. It is naturally possible to envisage replacing any of the means described by equivalent means without going beyond the ambit of the present invention.

(71) By way of example, the processor unit may be independent of the FADEC, possibly being arranged in parallel with said FADEC.