SYSTEM FOR DETERMINING THE ANGULAR SETTING OF AN ANNULAR ROW OF STATOR VANES

Abstract

A method determines the angular setting (β_2) of an annular row of stator vanes arranged downstream of a pusher propeller of a propulsion system with a longitudinal axis. The annular row of stator vanes receives an air flow having a velocity (V2) including a longitudinal component (Viz) and a tangential component (V_iθ) associated with the velocity of gyration generated by the pusher propeller. The method includes the steps of establishing a theoretical model of the pusher propeller using a power (P1) and a mechanical speed (N1) associated with said pusher propeller, and flight conditions that include a velocity of the airflow incident on the pusher propeller, the altitude of said propulsion system and ambient temperature. The method further includes the step of determining an angular setting (β_1) of the pusher propeller from the theoretical model.

Description

TECHNICAL FIELD OF THE INVENTION

The present document relates to the determination of the angular setting of an annular row of stator vanes arranged downstream of a pusher propeller in a propulsion system of an aircraft.

STATE OF PRIOR ART

Conventionally, an aircraft, as illustrated in FIG. 1, comprises a propulsion system 1 with a longitudinal axis 2 comprising a pusher propeller 4 formed by an annular row of vanes movable about the longitudinal axis 2. An annular row of stator vanes 6 is arranged downstream of said pusher propeller 4 in order to convert gyration induced by the pusher propeller 4 into an axial advance velocity and thereby increase the thrust generated. Upstream and downstream are defined with respect to the direction of gas circulation within said propulsion system.

It is known to achieve an angular setting of the vanes of the pusher propeller 4 in order to optimise propulsion of the aircraft. While the setting of the stator vanes 6 is also known, however, no details are given regarding the method and calculations required to achieve an optimised angular setting of said annular row of stator vanes 6 as a function of the flight phase. The implementation of control laws for said angular setting is complex and there is no optimised, simple and effective system for angularly setting an annular row of stator vanes 6 arranged downstream of a pusher propeller 4 of an aircraft propulsion system, which propeller is also a variable-pitch propeller.

This document aims to address these drawbacks in a simple, reliable and cost-effective way.

DISCLOSURE OF THE INVENTION

The present document relates to a method for determining the angular setting of an annular row of stator vanes arranged downstream of a pusher propeller of a propulsion system with a longitudinal axis, said annular row of stator vanes receiving an air flow having a velocity V₂ including a longitudinal component V_iz and a tangential component V_iθ associated with the velocity of gyration generated by the pusher propeller, the method comprising the steps:

• • a) establishing a theoretical model of the pusher propeller using a power P₁ and a mechanical speed N₁ associated with said pusher propeller, and flight conditions comprising a velocity of the airflow incident on the pusher propeller, the altitude of said propulsion system and ambient temperature;
  • b) determining an angular setting of said pusher propeller from said theoretical model;
  • c) from said theoretical model of the pusher propeller, defining dimensionless parameters including at least a power coefficient C_p,1, a pull coefficient C_T,1 and an advance ratio J₁ of said pusher propeller defined by the following formulae:

$\begin{array}{c}{C}_{P,1}=\frac{P_1}{\rho ·N_1^3·D_1^5}\\ C_{T,1}=\frac{T_1}{\rho ·N_1^2·D_1^4}\\ J_1=\frac{V_0}{N_1·D_1}\end{array}$

whereρ corresponds to the density of an ambient air,V₀ corresponds to a flight velocity of said propulsion system,N₁ corresponds to said mechanical speed of said pusher propeller,D₁ corresponds to a diameter of said pusher propeller,P₁ corresponds to said power of said pusher propeller,T₁ corresponds to a pull of said pusher propeller;

• • d) calculating the longitudinal component V_iz and the tangential component V_iθ of said velocity V₂ of the airflow incident on said annular row of stator vanes from said dimensionless parameters and deducing an angle φ₁₂ between said velocity of the airflow incident on said annular row of stator vanes and a plane of rotation of said pusher propeller;
  • e) determining an angular setting to be applied to said annular row of stator vanes from said angle, a Mach number associated with the velocity of the airflow incident on the pusher propeller and a database associating with each said angle, different angular settings of said annular row of stator vanes obtained for different Mach numbers.

A database is pre-constructed so as to facilitate real-time angular setting as a function of the position of the pusher propeller. Indeed, from a power and a mechanical speed associated with said pusher propeller, from a velocity of the airflow incident on the pusher propeller, from the altitude of said propulsion system and from ambient temperature, the angular setting of the pusher propeller is obtained and the method makes it possible, subsequently, to determine the optimum angular setting of each of the stator vanes from the database.

The way of determining the angular setting to be applied to said annular row of stator vanes provided herein makes the control of said annular row of stator vanes easier to implement, more robust to changes in flight conditions such as the velocity of the airflow incident on the pusher propeller, the altitude of said propulsion system and ambient temperature. Ambient temperature here refers to the surrounding temperature in which the propulsion system is immersed.

It is possible to obtain an optimum thrust of the propulsion system by virtue of the optimum angular setting of said annular row of stator vanes as a function of the setting of the pusher propeller.

Said axial component V_iz of said velocity V₂ of the airflow incident on the annular row of stator vanes can be calculated from the following formula:

$V_{iz}=\frac{V_0}{2J_1}\left[\sqrt{J_1^2+K_1·C_{T,1}}-J_1\right]$

where K₁ is a constant related to the radial dimension of the pusher propeller.

Said tangential component V_iθ of said velocity V₂ of the airflow incident on the annular row of stator vanes can be calculated from the following formula:

$V_{i\theta }=K_2·\frac{V_0·C_{P,1}}{J_1}·\frac{1}{J_1+\sqrt{J_1^2+K_1·C_{T,1}}}$

where K₁ and K₂ are constants related to the radial dimension of the pusher propeller.

Said angle φ₁₂ can obey the following formula:

${\phi }_{12}=\frac{180}{\pi }{\tan }^{-1}\left(\frac{V_0+V_{iz}}{V_{i\theta }}\right).$

Said angular setting β₂ of said annular row of stator vanes may be communicated by a FADEC system to actuators which control the angular setting β₂ of said annular row of stator vanes. By the way it works, the present method requires less data and is therefore easier to implement in a calculator such as a FADEC (Full Authority Digital Engine Control).

The database may be a table constructed by simulation or testing in which several operating conditions are calculated from all combinations of parameters such as the rotor speed N1, flight Mach number, angular setting β₁ of the pusher propeller and angular setting β₂ of said annular row of stator vanes, giving a table of maximised pull coefficient C′_T,2 as a function of the flight Mach number, angle φ₁₂ and angular setting β₂ of said annular row of stator vanes. The advantage of this database is that it allows very quick access to the information. Indeed, once said database has been created, a simple reading of the optimum value of the angular setting β₂ of said annular row of stator vanes is necessary. The database as it is constituted is lighter and can therefore be integrated into the FADEC.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic view of a pusher propeller system with an annular row of stator vanes.

FIG. 2 is a schematic view of the air flow and more precisely of the velocity of this flow received by a pusher propeller.

FIG. 3 is a schematic view of the air flow and more specifically of the velocity of this flow received by the annular row of stator vanes downstream of the pusher propeller shown in FIG. 3.

FIG. 4 is a diagram representing the method for determining the angular setting of a pusher propeller and the angular setting of the annular row of stator vanes downstream of said pusher propeller.

FIG. 5 is a schematic diagram of the operation of the setting model of the annular row of stator vanes.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 illustrates an air flow at the inlet of a rotating propeller 4 of a propulsion system 1 with a longitudinal axis 2 according to FIG. 1. When the pusher propeller 4 has an angular setting β₁, then it receives air at a flight velocity V₀ oriented perpendicular to the longitudinal axis 2.

In such a configuration, as illustrated in FIG. 3, an annular row of stator vanes 6 located downstream of said pusher propeller 4 then receives an air flow having a velocity V₂ having a longitudinal component comprising said flight velocity V₀ (longitudinal only) and a longitudinal velocity V_iz and a tangential component V_iθ where V_iz represents the longitudinal component and V_iθ the tangential component associated with the velocity of gyration generated by the pusher propeller 6.

The angle φ₁₂ between said velocity of the airflow incident on said annular row of stator vanes 6 and a plane of rotation 8 of said pusher propeller 4 is very important for defining the aerodynamic performance of the annular row of stator vanes 6. In combination with the angular setting β₂ to be applied to said annular row of stator vanes 6, the angle φ₁₂ defines the angle of attack. If the angle of attack is too high, a stall of the annular row of stator vanes 6 is observed. This stall results in a high level of pressure loss and significant gyration, which reduces the propulsive efficiency of the pusher propeller 4.

The angle of attack therefore has to remain within an acceptable range defined according to the aerodynamic robustness of the profile used, known at the time of its design. Furthermore, there is an optimum angle of attack for which the performance of the annular row of stator vanes 6 is maximum. In order to optimise performance of the propulsion system 1, it is therefore necessary to be close to this angle of attack during the entire flight phase. The angular setting β₂ to be applied to said annular row of stator vanes 6 therefore has to be controlled by said angle φ₁₂.

Thus, the present document provides an efficient way to combine the parameters influencing the aerodynamics of said pusher propeller 4 and the annular row of stator vanes 6 in order to determine the angular setting β₂ of the annular row of stator vanes 6 as a function of only two parameters, namely said angle φ₁₂ and a Mach number 10 associated with the velocity of the airflow incident on the pusher propeller 4.

FIG. 3 illustrates a diagram representing the method for determining the angular setting β₁ of a pusher propeller 4 and the angular setting β₂ of the annular row of stator vanes 6 downstream of said pusher propeller 4. This method 12 can be implemented in a calculator of said propulsion system (FADEC—“Full Authority Digital Engine Control”).

Said method 12 comprises a first step of establishing a theoretical model 14 of the pusher propeller 4. For this, a power P₁ and a mechanical speed N₁ associated with said pusher propeller, and flight conditions 16 comprising a velocity V₀ of the airflow incident on the pusher propeller, the altitude of said propulsion system and ambient temperature are used as an input to said theoretical model of the pusher propeller. This theoretical model 14 of the pusher propeller 4 makes it possible to determine an angular setting β₁ of said pusher propeller 4. Said pusher propeller is thus modelled by means of said theoretical model, which is in the form of a table comprising a set of dimensionless coefficients, comprising an advance ratio J₁, a power coefficient C_p,1 and a pull coefficient C_T,1 defined for a plurality of angular settings β₁ of said pusher propeller 4, for a plurality of velocities V₀ of incident airflow. As an output of the theoretical model 14 of the pusher propeller, the dimensionless parameters 17 including at least the power coefficient C_p,1, the pull coefficient C_T,1 and the advance ratio J₁ of said pusher propeller 4 may be used and will be defined by the following formulae:

$\begin{array}{c}{C}_{P,1}=\frac{P_1}{\rho ·N_1^3·D_1^5}\\ C_{T,1}=\frac{T_1}{\rho ·N_1^2·D_1^4}\\ J_1=\frac{V_0}{N_1·D_1}\end{array}$

where ρ corresponds to the density of an ambient air, V₀ corresponds to a flight velocity of said propulsion system, N₁ corresponds to said mechanical speed of said pusher propeller, D₁ corresponds to a diameter of said pusher propeller, P₁ corresponds to said power of said pusher propeller, T₁ corresponds to a pull of said pusher propeller.

These dimensionless parameters 17 associated with the pusher propeller 4 are then communicated to a setting model of the annular row of stator vanes 18, as shown in FIG. 5. Indeed, from said dimensionless parameters 17, the longitudinal velocity V_iz and the tangential component V_iθ are calculated on the basis of Froude's law of conservation of momentum for the longitudinal velocity V_iz and Euler's law for the tangential component V_iθ. These two velocities V_iz and V_iθ respect the following relationship:

$V_{iz}=\frac{V_0}{2J_1}\left[\sqrt{J_1^2+K_1·C_{T,1}}-J_1\right]$

where K₁ is a constant related to the radial dimension of the pusher propeller 4 and:

$V_{i\theta }=K_2·\frac{V_0·C_{P,1}}{J_1}·\frac{1}{J_1+\sqrt{J_1^2+K_1·C_{T,1}}}$

where K₁ and K₂ are constants related to the radial dimension of the pusher propeller 4.

Said angle φ₁₂ is then obtained from said longitudinal velocity V_iz and tangential component V_iθ according to the following relationship:

${\phi }_{12}=\frac{180}{\pi }{\tan }^{-1}\left(\frac{V_0+V_{iz}}{V_{i\theta }}\right).$

A database 20 is constructed beforehand. It allows each said angle φ₁₂ to be associated with different angular settings β₂ of said annular row of stator vanes 6 obtained for different Mach numbers.

To implement said database 20, a pull coefficient C′_T-2 of the straightener is calculated from the following formula:

$C_{T,2}^{\prime }=\frac{T_2}{\rho ·V_2^2·D_2^2}$

where D₂ corresponds to a diameter of said annular row of stator vanes 6, T₂ corresponds to a pull of said annular row of stator vanes 6, and V₂ corresponds to the velocity received by said annular row of stator vanes 6. This pull coefficient is necessary to create the database.

The database is a table which associates with each Mach number and angle φ₁₂ a value of angular setting β₂ of said annular row of stator vanes 6. This database is constructed by simulation or by testing. Several operating conditions associated with all combinations of parameters are calculated. The parameters considered are: the rotor speed N1, flight Mach number, angular setting β₁ of the pusher propeller 4 and angular setting β₂ of said annular row of stator vanes 6. For each of these operating points, the angle φ₁₂ and pull coefficient C′_T,2 are calculated. This gives a table of pull coefficient C′_T,2 as a function of the flight Mach number, angle φ₁₂ and angular setting β₂ of said annular row of stator vanes 6. For each flight Mach number and angle φ₁₂ in this table, the value of angular setting β₂ of said annular row of stator vanes 6 which maximises the coefficient C′_T,2 is chosen. Thus, the control law in the form of the angular setting β₂ of said annular row of stator vanes 6 as a function of Mach number and angle φ₁₂ is obtained.

The creation of a database as described above makes it possible to quickly and simply perform the determination of the optimum value of the angular setting β₂ to be applied to said annular row of stator vanes. This database can be integrated into a memory unit of the FADEC which does not require the addition of complex calculation means.

From this database 20, said angle φ₁₂ and a Mach number 10 associated with the velocity of the airflow incident on the pusher propeller, an angular setting β₂ to be applied to said annular row of stator vanes 6 is determined.

Finally, said angular setting β₂ of said annular row of stator vanes 6 is communicated by a FADEC system to actuators which control the setting of said annular row of stator vanes 6.

The system and logic provided herein make the determination of the angular setting β₂ of the annular row of stator vanes row easier to implement, more robust to changes in flight conditions 16 and easier to store in a FADEC, as said method 12 requires less data. As a result of this determination of the angular setting β₂, the annular row of stator vanes 6 always provides the optimum amount of thrust for the given flight phase.

Claims

1. A method for determining an angular setting (β2) of an annular row of stator vanes arranged downstream of a pusher propeller of a propulsion system with a longitudinal axis, said annular row of stator vanes receiving an airflow having a velocity (V2) including a longitudinal component (Viz) and a tangential component (Viθ) associated with a velocity of gyration generated by the pusher propeller, the method comprising the steps of: a) establishing a theoretical model of the pusher propeller using a power (P1) and a mechanical speed (N1) associated with said pusher propeller, and flight conditions comprising a velocity of the airflow incident on the pusher propeller, an altitude of said propulsion system and ambient temperature;b) determining an angular setting (β1) of said pusher propeller from said theoretical model;c) from said theoretical model of the pusher propeller, defining dimensionless parameters including at least a power coefficient (Cp,1), a pull coefficient (CT,1) and an advance ratio (J1) of said pusher propeller defined by the following formulae:

2. The method according to claim 1, wherein said longitudinal component (Viz) of said velocity (V2) of the airflow incident on the annular row of stator vanes is calculated from the following formula:

3. The method according to claim 1, wherein said tangential component (Viθ) of said velocity (V2) of the airflow incident on the annular row of stator vanes is calculated from the following formula:

4. The method according to claim 1, wherein said angle (φ12) obeys the following formula:

5. The method according to claim 1, wherein said angular setting (β2) of said annular row of stator vanes is communicated by a FADEC system to actuators which control the angular setting (β2) of said annular row of stator vanes.

6. The method according to claim 1, wherein the database is a table constructed by simulation or testing in which several operating conditions are calculated from all combinations of parameters such as the rotor speed N1, flight Mach number, angular setting β1 of the pusher propeller and angular setting β2 of said annular row of stator vanes, giving a table of maximised pull coefficient C′T,2 as a function of the flight Mach number, angle φ12 and angular setting β2 of said annular row of stator vanes.