Patent Publication Number: US-2022213841-A1

Title: Method and device for estimating a dead zone of a turbomachine discharge valve

Description:
The invention relates to a method and a device for estimating a closing dead zone of a turbomachine discharge valve. 
     One field of application relates to aircraft turbomachine, such as for example aircraft turbojets. 
     Document US2007/240677 describes a method for controlling a flap valve for gas recirculation, in which a dead band is taken into account. 
     Discharge valves (VBV for “variable bleed valve”) allow discharging a portion of the primary flow into the secondary flow. 
     The invention applies in particular to discharge valves each comprising a gate actuated in pivoting on a hinge by an actuator, to open and close an orifice provided on a wall of a casing which delimits in part an outer wall of a stream of primary flow of the turbomachine. The gates are generally installed on an inner shroud of a structural casing of the turbomachine, which can for example be an intermediate casing which can in particular constitute a load path for the suspension of the turbomachine from a structure of an aircraft such as a pylon. The inner shroud of the structural casing has a wall which faces, inside, the stream of the primary flow. The opening of the gates allows discharging a portion of the flow rate of the primary flow to a stream of the secondary flow of the turbomachine, while passing through an inter-stream compartment which separates the stream of the primary flow from the stream of the secondary flow. Each gate is equipped with a seal gasket, which is compressed against the orifice so that the latter can be closed with a good seal. 
     There exists however an angular movement range of the gate, called the closing dead zone, during which the gasket remains compressed during the opening of the gate and therefore does not let the primary flow pass into the secondary flow. In other words, as long as the position of the gate remains in the dead zone range, the gate remains sealed even though it is not completely closed. 
     It is desired to estimate what the dead zone is on each turbomachine, because it differs on each turbomachine due in particular to deviations in assembly and manufacture. 
     One possibility is to carry out tests for characterizing the dead zone of each turbomachine, by determining the permeability of its discharge valves, i.e. the angle starting at which the gates are sealed. These tests consist of sweeping different angles of the gate at a fixed speed. These tests are time-consuming and must be carried out for each operating point where it is desired to know the dead zone. 
     The invention aims to obtain a method and a device for estimating the dead zone, which do not necessitate the performance of a specific test on each turbomachine. 
     To this end, a first subject matter of the invention is a method for estimating a closing dead zone of a first gate of a first discharge valve mounted on a first casing of a turbomachine to be investigated, which can operate in flight to propel an aircraft, the first casing delimiting in its inside a primary gas flow of the turbomachine, the first gate being designed to be able to be moved angularly by a first actuator depending on a first control signal of the first actuator between one and the other of:
         a first opening position of a first orifice of the first casing configured to discharge by this first orifice a portion of the primary flow into a secondary gas flow located outside the first casing,   and a first closing position of the first orifice configured to prevent the primary flow from passing through the first orifice,       

     the first gate being provided with a first gas seal gasket, able to be compressed against a first edge of the first orifice in the first closing position to seal the closure of the first orifice, 
     characterized in that 
     for the turbomachine to be investigated, operating in flight, the first actual position of the first actuator is measured when the first control signal is equal to a first set point closing signal of the first gate, 
     based on the first actual position, a measured droop angle of the first gate with respect to the first casing is determined, 
     for the turbomachine to be investigated, operating in flight, a measured closing dead zone of the first gate is determined, in which the first gasket remains compressed and thus seals the closure of the first orifice, by the fact that the measured closing dead zone is equal to a reference dead zone, which was predetermined during operation on the ground on a reference turbomachine which can operate in flight and on the ground, to which has been added the measured droop angle and from which has been subtracted a reference droop angle which was predetermined on the reference turbomachine operating in flight to propel an aircraft. 
     The invention thus defines a dead zone estimator based on static measurements which can be obtained “as a passenger” of other tests and therefore do not necessitate a specific test. 
     According to one embodiment of the invention, which can be applied to the estimating method and/or to the estimating device, the reference turbomachine has a second gate of a second discharge valve mounted on a second casing of the reference turbomachine, the second casing delimiting in its inside a primary gas flow of the reference turbomachine, the second gate being designed to be able to be moved angularly by a second actuator depending on a second control signal of the second actuator between one and the other of:
         a second opening position of a second orifice of the second casing to discharge through this second orifice a portion of the primary flow into a secondary gas flow of the reference turbomachine located outside the second casing,   and a second closing position of the second orifice to prevent the primary flow from passing through the second orifice,       

     the second gate being provided with a second gas seal gasket, able to be compressed against a second edge of the second orifice in the second closing position to seal the closure of the second orifice. 
     According to one embodiment of the invention, for the reference turbomachine operating on the ground, the reference dead zone is determined, which is a second angular closing range of the second gate, during which, between the second closing position and the second opening position, the second gasket remains compressed and thus seals the closure of the second orifice. 
     According to one embodiment of the invention, for the turbomachine to be investigated, operating in flight, a first rotation speed of a first high-pressure compressor of the turbomachine to be investigated, and measured operating parameters comprising at least one quantity selected from among a first atmospheric pressure and a first altitude are determined, 
     based on a model of the reference turbomachine giving a second gate angle of the reference turbomachine with respect to the second casing of the reference turbomachine, depending on a second rotation speed of the second high-pressure compressor of the reference turbomachine and on a second atmospheric pressure of the reference turbomachine or on a second operating altitude of the reference turbomachine, the reference droop angle is determined, which is the second gate angle of the reference turbomachine which corresponds according to the model to the measured operating parameters, 
     the second gate angle corresponding to an angle of the second gate with respect to the second casing, which has been determined based on a second actual position of the second actuator, having itself been measured for the reference turbomachine operating in flight, when the second control signal is equal to a second set point closing signal of the second gate. 
     A second subject matter of the invention is a device for estimating a closing dead zone of a first gate of a first discharge valve mounted on a first casing of a turbomachine to be investigated which can operate in flight to propel an aircraft, the first casing delimiting in its inside the primary gas flow of the turbomachine, the first gate being designed to be able to be moved angularly by a first actuator depending on a first control signal of the first actuator between one and the other of:
         a first opening position of a first orifice of the first casing configured to discharge through this first orifice a portion of the primary flow into a secondary gas flow located outside the first casing,   and a first closing position of the first orifice configured to prevent the primary flow from passing through the first orifice,       

     the first gate being provided with a first gas seal gasket, able to be compressed against a first edge of the first orifice in the first closing position to seal the closure of the first orifice, 
     characterized in that the estimation device comprises 
     a measurement means for measuring, for the turbomachine to be investigated operating in flight, the first actual position of the first actuator when the first control signal is equal to a first set point closing signal of the first gate, 
     a calculator configured to:
         determine, based on the first actual position, a measured droop angle of the first gate with respect to the first casing,   determine, for the turbomachine to be investigated operating in flight, a measured closing dead zone of the first gate, in which the first gasket remains compressed and thus seals the closure of the first orifice, by the fact that the measured closing dead zone is equal to a reference dead zone, which was predetermined during operation on the ground on a reference turbomachine which can operate in flight and on the ground, to which has been added the measured droop angle and from which has been subtracted a reference droop angle which was predetermined on the reference turbomachine operating in flight for propelling an aircraft.       

     According to one embodiment of the invention, the estimation device comprises a measurement means for determining, for the reference turbomachine operating on the ground, the reference dead zone, which is a second angular closing range of the second gate, during which, between the second closing position and the second opening position, the second gasket remains compressed and thus seals the closure of the second orifice. 
     According to one embodiment of the invention, the estimation device comprises on-board measurement sensors of the first turbomachine to determine, for the turbomachine to be investigated operating in flight, a first rotation speed of a first high-pressure compressor of the turbomachine, and measured operating parameters comprising at least one quantity selected from among a first atmospheric pressure and a first altitude, 
     the calculator being configured to determine, from a model of the reference turbomachine giving a second gate angle of the reference turbomachine with respect to the second casing of the reference turbomachine depending on a second rotation speed of the second high-pressure compressor of the reference turbomachine and from a second atmospheric pressure of the reference turbomachine or from a second operating altitude of the reference turbomachine, the reference droop angle, which is the second gate angle of the reference turbomachine which corresponds according to the model to the measured operating parameters, 
     a measurement means for measuring, for the reference turbomachine operating in flight, a second actual position of the second actuator, when the second control signal is equal to a second set point closing signal of the second gate, 
     the calculator being configured to determine the second gate angle corresponding to the angle of the second gate with respect to the second casing, based on the second actual position of the second actuator which was measured. 
    
    
     
       The invention will be better understood upon reading the description that follows, given solely by way of a non-limiting example with reference to the appended drawings, in which: 
         FIG. 1  shows schematically in longitudinal section an example of a turbomachine to which the estimation device and method according to the invention can apply, 
         FIG. 2  shows schematically in longitudinal section an enlarged view in the closing position of a gate of an intermediate casing of the turbomachine of  FIG. 1 , to which the estimation device and method according to the invention can apply, 
         FIG. 3  shows schematically in longitudinal section an enlarged view in the closing position of a gate of an intermediate casing of the turbomachine of  FIG. 1 , to which the estimation device and method according to the invention can apply, 
         FIG. 4  shows schematically in longitudinal section an enlarged view in the closing position of a gate of an intermediate casing of the turbomachine of  FIG. 1 , to which the estimation device and method according to the invention can apply, 
         FIG. 5  shows schematically in longitudinal section an enlarged view in the closing position of a gate of an intermediate casing of the turbomachine of  FIG. 1 , to which the estimation device and method according to the invention can apply, 
         FIG. 6  shows schematically in longitudinal section an enlarged view in the opening position of a gate of an intermediate casing of the turbomachine of  FIG. 1 , to which the estimation device and method according to the invention can apply, 
         FIG. 7  shows schematically forces appearing on the gate of the intermediate casing of  FIG. 6 , 
         FIG. 8  shows schematically in longitudinal section an enlarged view in the closing position of a gate of an intermediate casing of the turbomachine of  FIG. 1 , to which the estimation device and method according to the invention can apply, 
         FIG. 9  shows schematically forces appearing on the gate of the intermediate casing of  FIG. 8 , 
         FIG. 10  is a diagram showing respectively forces which can appear during control of the gate of the turbomachine according to  FIGS. 1 to 5 , 
         FIG. 11  is a diagram showing respectively gate angles which can appear during control of the gate of the turbomachine according to  FIGS. 1 to 5 , 
         FIG. 12  is a diagram showing pressure differences which can appear during control of the gate of the turbomachine according to  FIGS. 1 to 5 , 
         FIG. 13  shows schematically in longitudinal section an enlarged view in the closing position of a gate of an intermediate casing of the turbomachine of  FIG. 1 , to which the estimation device and method according to the invention can apply, 
         FIG. 14  shows schematically forces appearing on the gate of the intermediate casing of  FIG. 13 , 
         FIG. 15  shows schematically a mapping of a reference dead zone, which can be used by the estimation device and method according to the invention, 
         FIGS. 16 and 17  show schematically the composition of the dead zone of the first and second turbomachines, 
         FIG. 18  is a modular synopsis of an example of a dead zone estimator implementing the estimation device and method according to the invention, 
         FIG. 19  is an example of a flowchart of the estimation method according to the invention. 
     
    
    
     An example of a double flow turbomachine is shown in  FIGS. 1 and 2 . The turbomachine  10 ,  1 ,  2  is intended to be installed on an aircraft, not shown, to propel it in the air. The turbomachine  10  or gas turbine engine assembly has a longitudinal axis  24 . The direction extending from the interior to the exterior is the radial direction starting from the longitudinal axis  24 . The turbomachine  10  is for example of the two-bodies type. The turbomachine  10  comprises a fan assembly  28  and an assembly  13  for generating combustion gases and their use, also called a “gas generator.” The gas generator  13  comprises, from upstream to downstream in the flow direction of the gases, a low-pressure compressor  12 , a high-pressure compressor  14 , a combustion chamber  16 , a high-pressure turbine  18  and a low-pressure turbine  20  which define a primary gas flow  22 . The fan assembly  28  comprises an array of fan blades extending radially outward from a rotor disk  25 . The turbomachine  10  has an intake side  29  and an exhaust side  30 . The turbomachine  10  also comprises a set of casings which define an inter-stream compartment  36  also called the “core zone”, located between a primary flow stream and a secondary flow stream of the double-flow turbomachine. An upstream portion of the inter-stream compartment  36  is formed by a structural casing  26  connected by arms  34  to the internal wall  201  of the duct  30  through the bypass channel  39  of the secondary flow  32 . This structural casing  26  is for example an intermediate casing, and will be so designated in the description of the exemplary embodiment that follows, but other types of structural casings are possible, for example an inter-compressor casing interposed between the low-pressure compressor  12  and the high-pressure compressor  14 . 
     In operation, the air flows through the fan assembly  12  and a first portion  22  (primary flow  22 ) of the air flow is routed through the high-pressure compressor  14 , in which the air flow is compressed and sent to the combustion chamber  16 . The hot combustion products (not shown in the figures) originating in the combustion chamber  16  are used to drive the turbines  18  and  20 , and the low-pressure turbine  20  is connected to the fan assembly  28  to produce the major part of the thrust of the turbomachine  10 . The turbomachine  10  also comprises a bypass channel  39  which is used to pass a second portion  32  (secondary flow  32 ) of the air flow vented from the fan assembly  28  around the central gas turbine engine  13 . More precisely, the bypass channel  39  extends between an internal wall  201  of a fan duct  30  or nacelle  30  and an external envelope  202  of the inter-stream compartment  36 , also called IFD for “inner fan duct.” 
     In  FIGS. 2 to 5 , the intermediate casing  26  comprises an internal wall  38  delimiting on the inside the flow space of the primary flow  22 , an upstream flange  40  and a downstream flange  42  connected to the exterior surface of the internal wall  38 , and an external wall  44  connecting on the outside the upstream flange  40  and the downstream flange  42 . The arms  34  are fastened to the outer ends of the upstream flange  40  and of the downstream flange  42 . The intermediate casing  26  is equipped with a discharge valve  48 , or with several discharge valves  48 . 
     A first turbomachine  1  is considered, called the turbomachine to be investigated, similar to the turbomachine  10  as described above, for which a closing dead zone ZM RECH  will be determined. The following reference symbols, ending with  1 , designate the portions and variables of this first turbomachine  1 , introduced by the adjective “first,” of which those mentioned above in  FIGS. 1 to 5  are recalled. 
     Also considered is a second turbomachine  2 , called the reference turbomachine, similar to the turbomachine  10  as described above, for which the reference dead zone ZM REF  has been determined. The following reference symbols, ending in  2 , designate the portions and variable of this second turbomachine  2 , introduced by the adjective “second,” of which those mentioned above in  FIGS. 1 to 5  are recalled. Of course, what is described for the first turbomachine  1  below is equally valid for the second turbomachine  2 . Of course, the reference turbomachine  2  or second turbomachine  2  can be different from that described below and above. 
     In the first turbomachine  1 , each first discharge valve VBV 1 ,  48  comprises a first gate  50 , P 1  mounted on the first intermediate casing  26 , C 1 . The first casing  26 , C 1  delimits on the inside the first primary gas flow  22 , FP 1 . The first gate  50 , P 1  is mounted pivoting on a hinge  51  and is connected to a first actuator V 1  designed to be able to angularly move the first gate  50 , P 1  depending on a first control signal SC 1  of the first actuator V 1 . The first internal wall  38 ,  381  of the first intermediate casing  26 , C 1  comprises a first orifice O 1  with a shape corresponding to the first gate  50 , P 1 . A certain number N of first gates  50 , P 1  can be provided on the first intermediate casing  26 , C 1 , actuated by a certain number M of first actuators, with for example M&lt;N. For example, the first actuator(s) V 1  actuate one or some first gate(s)  50 , P 1 , said to be driving, and all the first gates  50 , P 1  are connected via a ring which allows having uniform actuation of the first gates  50 , P 1 . The need for discharging the first low-pressure compressor  12 , CBP 1  is manifested in protection laws. These laws being a representation of an actual discharge flow rate depending on the configuration and the condition of the engine, they must take into account the effective cross section of the first gate and therefore of the dead zone which affects this flow rate. 
     The first actuator V 1  comprises a first actuator body CV 1  fixed with respect to the first intermediate casing  26 , C 1  and a first shaft T 1  movable in translation with respect to the first actuator body CV 1 . At least one first connection member BI 1 , comprising for example a connecting rod BI 1  and articulation devices, is mounted between the first shaft T 1  and the first gate P 1  to move angularly the first gate P 1  by movement in translation of the first shaft T 1  with respect to the first actuator body CV 1 . The first actuator V 1  can be a hydraulic actuator, the movement in translation of the movable shaft T 1  of which is controlled by the difference in oil pressure on either side of a piston sliding in the body CV 1 . 
     The first gate  50 , P 1  comprises, in  FIGS. 6 and 7 , a first opening position PO 1  of the first orifice O 1 , where the first gasket J 1  is at a distance from the first edge B 1 , to discharge through this first orifice O 1  a portion of the first primary flow FP 1 ,  22  into the first secondary gas flow FS 1 ,  32  located outside the first intermediate casing  26 , C 1 . 
     The first gate  50 , P 1  comprises, in  FIGS. 8 and 9 , a first closing position PF 1  of the first orifice O 1  so as not to allow the first primary flow FP 1 ,  22  to pass through the first orifice O 1  into the first secondary flow FS 1 ,  32 . The first actuator V 1  allows moving the first gate  50 , P 1  from the first opening position PO 1  to the first closing position PF 1  in a closing direction, and from the first closing position PF 1  to the first opening position PO 1  in an opening direction. 
     The first gate  50 , P 1  is provided with a first gas seal gasket J 1 , able to be compressed against a first edge B 1  of the first orifice O 1  in the first closing position FP 1  so as not to let the first primary flow FP 1  pass through the first orifice O 1  into the first secondary flow FS 1 . 
     In  FIGS. 3, 4, 5, 6, 8 and 13 , each first gate  50 , P 1  comprises a first gate casting F 1  against which is fastened the first gasket J 1 . 
     In  FIGS. 6 and 7 , when the first gate  50 , P 1  is in the first opening position PO 1 , the force F VBV  in the first actuator V 1  balances the aerodynamic force F Aero . 
     In  FIGS. 8 and 9 , when the first gate  50 , P 1  is in the first closing position PF 1 , the first gasket J 1  is compressed against the first edge B 1  of the first intermediate casing  26 , C 1  so as to ensure the sealing of the first gate  50 , P 1  and to provide a smoother primary stream in the flow space of the primary flow  22 , i.e. that the first gate  50 , P 1  is aligned with the first internal wall  38 ,  381 . The force F VBV  in the first actuator V 1  then balances the aerodynamic force F Aero , which depends on the operating point and the forces F joint  compressing the gasket, which depend on the angular position of the first gate  50 , P 1  and therefore on the depression of the first gate  50 , P 1  in the primary stream. 
     In the first closing position PF 1 , the first gate  50 , P 1  can occupy, with respect to the first casing  26 , C 1 , C 2  several different angles located in a certain angular range, called the measured closing dead zone ZM RECH , which is due to the flexibility of the first gasket J 1  and during which the first gasket J 1  does not allow the first primary flow FP 1  to pass through the first orifice O 1  into the first secondary flow FS 1  (effective cross section is zero in this case for the first gate  50 , P 1 ). For example, in  FIGS. 3 and 5 , the first gate  50 , P 1  can occupy in the first closing position PF 1  a certain angle ANG 1  with respect to the first casing  26 , C 1 , C 2 , which makes the first gasket J 1  assume a certain compression thickness EP 1   comp  against the first edge B 1 , where the first gasket J 1  does not allow the first primary flow FP 1  to pass through the first orifice O 1  into the first secondary flow FS 1 . In  FIGS. 4 and 5 , the first gate  50 , P 1  can occupy in the first closing position PF 1  another angle ANG 2  more depressed into the primary stream than with the angle 
     ANG 1 , which causes the first gasket J 1  to assume a compression thickness EP 2   comp  against the first edge B 1 , which is smaller than the compression thickness EP 1   comp  and where the first gasket J 1  does not allow the first primary flow FP 1  to pass through the first orifice O 1  into the first secondary flow FS 1 . The angular range of the first gate  50 , P 1  corresponding to the difference between the compression thickness EP 1   comp  and the compression thickness EP 2   comp  is comprised in the measured closing dead zone ZM RECH . 
     According to the invention, the measured closing dead zone ZM RECH  is calculated based on the reference dead zone ZM REF  which was determined on the reference turbomachine  2  operating on the ground, by correcting it by the difference between the droop Stat MES  (or statism or offset) measured on the first gate P 1  of the first turbomachine  1  operating in flight and the reference droop angle Stat REF  which was predetermined on the second turbomachine  2  operating in flight. Reference can be made below to  FIGS. 16 to 18 . 
     According to the invention, during a measurement step E 1 , the first actual position PRV 1  of the first actuator V 1  is measured for the first turbomachine  1  operating in flight when the first control signal SC 1  is equal to a first set point closing signal SCPF 1  of the first gate P 1 , for example by a position sensor of the first actuator V 1  or by other measurement means. For example, the first actual position PRV 1  in translation of the first shaft T 1  of the first actuator V 1  is measured with respect to the first actuator body CV 1 . 
     During a determination step E 2  subsequent to the measurement step E 1 , a first angle Stat MES  of the first gate P 1  with respect to the first casing C 1 , called the measured droop angle Stat MES , is determined based on the first actual position PRV 1 , for example by a calculator CAL or by other measurement means. 
     During a determination step E 3  subsequent to the determination step E 2 , the first closing angular range ZM RECH  of the first gate P 1 , called the measured closing dead zone ZM RECH , which corresponds to the fact that the first gasket J 1  is compressed so as not to allow the first primary flux FP 1  to pass through the first orifice O 1  into the first secondary flow FS 1  and thus seals the closure of the first gate P 1 , is determined for the first turbomachine  1  operating in flight. The measured closing dead zone ZM RECH  of the first turbomachine  1  is equal to the reference dead zone ZM REF , which was predetermined on the second turbomachine  2 , called the reference turbomachine, operating on the ground, to which has been added the measured droop angle Stat MES  and from which has been subtracted a reference droop angle Stat REF  that was predetermined on the second turbomachine  2  operating in flight, according to the following equation: 
         ZM   RECH   =ZM   REF +Stat MES −Stat REF  
 
     Thus, thanks to the invention, tests for measuring the dead zone in operation of each first turbomachine  1  in flight is avoided, this measured closing dead zone ZM RECH  of the first turbomachine  1  depending on the operating point of the first turbomachine  1  and on the altitude. 
     The droop of the discharge valve VBV 1 ,  48  is a phenomenon encountered on many motors, primarily at altitude. It is manifested by an incapacity to close entirely the first gate  50 , P 1  where, although the first control signal SC 1  is equal to a first closing set point signal SCPF 1  of the first gate P 1 , the measured droop angle Stat MES  is not at its set point closing position at 0° but at a different value which could reach 1.5°, as shown by way of an example by the 3 periods ST 1 , ST 2  and ST 3  of droop in  FIGS. 10, 11 and 12 . Droop is a continuous phenomenon and can assume different values between 0° and 3° for example. 
     In  FIG. 11 , the measured droop angle Stat MES  in degrees in the ordinate varies according to the curve C 10 , while the first control signal SC 1  in degrees on the ordinate varies according to the curve C 20 , as a function of time t in seconds (s) in the abscissa.  FIG. 10  shows, in a manner corresponding to  FIG. 11 , the force F VBV  in the first actuator V 1  in Newtons (N) in the ordinate and the aerodynamic force F Aero  in Newtons (N) in the ordinate, as a function of time t in seconds (s) in the abscissa.  FIG. 12  shows, in a manner corresponding to  FIG. 11 , the pressure difference ΔP servo  at the terminals of the first servo-valve  48 , VBV 1  and the pressure difference ΔP V  at the terminals of the first actuator V 1  in bar in the ordinate, as a function of time in seconds (s) in the abscissa. During these droop periods ST 1 , ST 2  and ST 3 , the force F VBV  becomes less than the aerodynamic force F Aero  and the pressure difference ΔP V  at the terminals of the first actuator V 1  becomes equal to the pressure difference ΔP servo  at the terminals of the first servovalve  48 , VBV 1 , which indicates that the entire hydraulic power of the first actuator V 1  is consumed and does not allow complete closing of the first gate  50 , P 1 . 
     This phenomenon is explained by the fact that the compression of the first gasket J 1  requires a greater and greater force as the first gate  50 , P 1  is depressed farther into the primary stream in the first closing position PF 1 . Thus, when the hydraulic power available is no longer sufficient (the effect of altitude in particular), the necessary force for the complete closure of the first gate  50 , P 1  cannot be supplied, as shown in  FIGS. 13 and 14 , and the first gasket J 1  is not completely compressed, while not letting pass, in the first closing position PF 1  the first primary flow FP 1 ,  22  through the first orifice O 1  into the first secondary flow FS 1 ,  32 , even when the force F VBV  in the first actuator V 1  is equal to the maximum force F maxVBV . The primary stream is then not smooth, i.e. in the first closing position PF 1  the first gate  50 , P 1  is not aligned with the first internal wall  38 ,  381  and is slightly lifted by the first primary flow  22 , FP 1  toward the first secondary flow  32 , FS 1 . 
     According to one embodiment, for the second turbomachine  2  serving as a reference, during a determination step E 4  prior to the determination step E 3  and for example prior to the measurement step E 1 , the reference dead zone ZM REF , which is a second closing angular range ZM REF  of its second gate  50 , P 2  during which, between its second closing position PF 2  and its second opening position PO 2  its second gasket J 2  is compressed so as not to allow its second primary flow FP 2  to pass through its second orifice O 2  into its second secondary flow FS 2 , is determined for the second turbomachine  2  operating on the ground, by measurement means, for example on a test bench on the ground. Of course, the reference dead zone ZM REF  can be determined in another manner than according to this embodiment. 
     According to one embodiment, during a determination step E 5  prior to the determination step E 3 , a first rotation speed N 2 _ 1  of the first high-pressure compressor  14 , CHP 1  of the first turbomachine  1 , and a first atmospheric pressure Patm 1  or a first altitude Alt 1 , called measured operating parameters N 2 _ 1 , Patm 1  or Alt 1 , are determined for the first turbomachine  1  operating in flight, for example by measurement sensors carried on board the first turbomachine  1  and/or on the aircraft comprising it. 
     According to one embodiment, the calculator CAL comprises a model MOD of the second turbomachine operating in flight, giving the second gate angle Stat REF  of the second turbomachine  2  with respect to its second intermediate casing  26 , C 2 , as a function of a second rotation speed N 2 _ 2  of its second high-pressure compressor  14 , CHP 2  and of a second atmospheric pressure Patm 2  of the second turbomachine  2  or of a second operating altitude Alt 2  of the second turbomachine  2 . 
     This is illustrated by way of an example in  FIG. 15 , where the second angle Stat REF  is in degrees in vertical ordinate, the second rotation speed N 2 _ 2  is on a first horizontal axis in revolutions per minute (rpm) in the abscissa and the second altitude Alt 2  is on a second horizontal axis in feet (ft) times 10 4  in the abscissa, perpendicular to the first horizontal axis.  FIG. 17  illustrates the situation of the second turbomachine  2 , where in the ordinate the reference dead zone ZM REF  is equal to the sum of the reference droop angle Stat REF =f(Alt 2 , N 2 _ 2 ) of the model MOD and of a second angular gap EA 2   joint  corresponding to the compression of the second gasket J 2 . 
       FIG. 16  illustrates the situation of the first turbomachine  1 , where in the ordinate the measured closing dead zone ZM RECH  is equal to the sum of the measured droop angle Stat MES =f(Alt 1 , N 2 _ 1 ) determined based on the model MOD and on a second angular gap EA 1   joint  corresponding to the compression of the first gasket J 1 . 
     Of course, in the model MOD, the variable “second operating altitude  2  of the second turbomachine  2 ” can be transformed into the variable “second operating pressure Patm 2  of the second turbomachine  2 ” and conversely, given that there exists a bijective relation between them, as is known to a person skilled in the art. 
     According to one embodiment, during a determination step E 6  prior to the determination step E 3  and subsequent to the determination step E 5 , the reference droop angle Stat REF , which is the second gate angle Stat REF  of the second gate  50 , P 2  of the second turbomachine  2 , which corresponds according to the model MOD to the measured operating parameters N 2 _ 1 , Patm 1  or Alt 1 , i.e. for N 2 _ 2 =N 2 _ 1  and Patm 2 =Patm 1  or Alt 2 =Alt 1 , is determined by the calculator CAL, based on the model MOD of the second turbomachine  2  operating in flight. 
     According to one embodiment, the second gate angle Stat REF  of the model MOD corresponds to an angle Stat REF  of the second gate  50 , P 2  with respect to the second casing  26 , C 2  and has been determined for example by a calculator or by other measurement means during a determination step E 7  prior to the determination step E 6  and for example prior to the measurement step E 1 , based on a second actual position PRV 2  of the second actuator V 2 , which for its part was measured for the second turbomachine  2  operating in flight, for example on a test bench in flight by a position sensor of the second actuator V 2  or by other measurement means, when its second control signal SC 2  is equal to a second closing set point signal SCPF 2  of the second gate P 2 . 
     The calculator CAL and the measurement means or sensors described above can be part of an estimation device or estimator, implementing the estimation method described above. The calculator CAL operates automatically and can be embodied by a processor or a computer or a server, which are provided with computer processing programs for carrying out the processing described below and permanent memories for recording there the data and the processing carried out. In  FIG. 18 , an example of an estimator  300  of this type comprises a first input  301  for receiving the measured droop angle Stat MES , a second input  302  for receiving the first altitude Alt 1 , a third input  302  for receiving the first rotation speed N 2 _ 1  and an output  304  for supplying the measured closing dead zone ZM RECH . The inputs  301 ,  302  and  303  can be in the form of a data introduction interface. The output  304  can be in the form of a data output interface or a display screen. The calculator CAL for the second turbomachine  2  can be separated from the calculator CAL for the first turbomachine  1 . 
     Of course, the embodiments, features, possibilities and examples above can be combined with one another or be selected independently of one another.