METHOD AND APPARATUS FOR ESTIMATING AN AIRSPEED OF A ROTORCRAFT BY ANALYZING ITS ROTOR

Abstract

A method and apparatus for estimating an airspeed of a rotorcraft by analyzing its rotor. The rotorcraft includes a fuselage and a main rotor that is equipped with a plurality of blades and that rotates about an axis of a hub of the rotor, and in which the free end or “tip” of each blade describes a path in the vicinity of a tip-path plane. The method makes it possible to determine said airspeed of the rotorcraft in a frame of reference united with the tip-path plane by solving a model of the rotor that puts a pitch angle of at least one blade relative to the tip-path plane into relation with the airspeed of the rotorcraft and with an auxiliary speed. The auxiliary speed may be an induced velocity of the air flowing through the rotor or else an axial airspeed at the upstream infinity of the rotorcraft.

Claims

1. A method of estimating an airspeed of a rotorcraft, the rotorcraft including a fuselage and at least one rotor that rotates about an axis of a mast of the rotor, the rotor being provided with the mast, with a hub and with a plurality of blades, each blade having a connected first end and a free second end each blade being mounted to pivot at least about a flapping axis and about a pitch axis, the free second end of a blade describing a path in the vicinity of a mean tip-path plane while the blade is rotating, a longitudinal axis X of the tip-path plane extending in a direction going from the tail of the rotorcraft to the nose of the rotorcraft, and a lateral axis Y of the tip-path plane extending in a direction going from left to right perpendicularly to the longitudinal axis X, wherein the method comprises the following steps: estimating a pitch angle of at least one blade relative to the tip-path plane; determining an auxiliary speed of the rotorcraft, the auxiliary speed being equal: either to an induced velocity of the air flowing through the rotor; or to an axial airspeed at the upstream infinity of the rotorcraft; and determining the airspeed of the rotorcraft in a frame of reference united with the tip-path plane by solving a model of the rotor, the model taking the form of equations putting the pitch angle of the blade into relation with the airspeed of the rotorcraft and with the auxiliary speed.

2. The method according to claim 1, wherein the pitch angle can be broken down into a collective pitch of the blade as well as into a longitudinal cyclic pitch and lateral cyclic pitch of the blade.

3. The method according to claim 1, wherein the step of determining the auxiliary speed equal to the axial airspeed at the upstream infinity includes the following steps: estimating a barometric altitude of the rotorcraft; and determining the axial airspeed at the upstream infinity that is equal to a time derivative of the barometric altitude of the rotorcraft.

4. The method according to claim 1, wherein the step of determining the auxiliary speed equal to the induced velocity includes the following steps: estimating the lift of the rotor; and computing the induced velocity as a function of the lift, of an area swept by the blades of the rotor, and of a forward speed of the rotorcraft.

5. The method according to claim 4, wherein the step of estimating the lift of the rotor includes the following sub-steps: estimating the mass of the rotorcraft; measuring a normal component of a specific force that is being applied to the fuselage of the rotorcraft; estimating an apparent weight of the rotorcraft as a function of the mass and of the specific force that is being applied to the fuselage of the rotorcraft; and computing the lift as a function of the apparent weight.

6. The method according to claim 4, wherein the step of estimating the lift of the rotor includes the following sub-steps: estimating a conicity of the rotor; estimating the speed of rotation of the rotor; and computing the lift as a function of the conicity, of a second moment of area of each blade about its flapping axis, of the number of blades and of the speed of rotation of the rotor.

7. The method according to claim 1, wherein the model of the rotor includes an analytical set of equations of the flight mechanics.

8. The method according to claim 7, wherein the set of equations includes equations expressing the pitch angle as a function of the airspeed of the rotorcraft and of the auxiliary speed.

9. The method according to claim 8, wherein the set of equations is solved by the Newton-Raphson method.

10. The method according to claim 1, wherein the airspeed of the rotorcraft can be broken down into a longitudinal projection and into a lateral projection on the tip-path plane.

11. The method according to claim 1, wherein when the auxiliary speed is the induced velocity, the airspeed of the rotorcraft can be broken down into a longitudinal projection and into a lateral projection on the tip-path plane, and into a projection that is normal to the tip-path plane.

12. The method according to claim 1, wherein the method includes a step of transferring the airspeed of the rotorcraft from the frame of reference united with the tip-path plane to a frame of reference united with the fuselage of the rotorcraft.

13. The method according to claim 1, wherein the step of estimating the pitch angle includes the following steps: measuring a specific force that is being applied to the fuselage of the rotorcraft in a frame of reference united with the fuselage of the rotorcraft; estimating at least one angle of inclination of the tip-path plane relative to a frame of reference united with the fuselage on the basis of the specific force that is being applied to the fuselage of the rotorcraft; measuring a pitch angle in a frame of reference united with the fuselage, the pitch angle including a collective component and a cyclic component; and estimating the pitch angle of the blade as a function of the pitch angle and of the angle(s) of inclination of the tip-path plane.

14. The method according to claim 13, wherein a longitudinal cyclic component of the angle of inclination of the tip-path plane is also estimated on the basis of an estimation of the aerodynamic drag of the fuselage.

15. The method according to claim 13, wherein a lateral cyclic component of the angle of inclination of the tip-path plane is estimated on the basis of the specific force that is being applied to the fuselage of the rotorcraft and of an estimation of lateral thrust from antitorque apparatus of the rotorcraft.

16. Apparatus for estimating an airspeed of a rotorcraft, the apparatus including at least one computer and a plurality of sensors; wherein at least one of the sensors measures a pitch angle of a blade of a rotor of the rotorcraft and the apparatus is configured to implement the method according to claim 1.

17. Apparatus according to claim 16, wherein at least one of the sensors measures a pitch angle of the blade (14) in the tip-path plane or else a pitch angle in a frame of reference united with the fuselage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0110] The invention and its advantages appear in greater detail from the following description of examples given by way of illustration with reference to the accompanying figures, in which:

[0111] FIGS. 1 and 2 show rotorcraft equipped with apparatus for estimating an airspeed of a rotorcraft;

[0112] FIG. 3 shows a representation of the rotor disk and of the components of the airspeed; and

[0113] FIG. 4 shows a block diagram relating the components of the airspeed to the pitches of the blades.

DETAILED DESCRIPTION OF THE INVENTION

[0114] Elements present in more than one of the figures are given the same references in each of them.

[0115] In a manner common to FIGS. 1 and 2, a rotorcraft 10 includes a main rotor 11 positioned above a fuselage 16 of the rotorcraft 10 and an antitorque tail rotor 18 positioned at the rear end of a tail boom 19.

[0116] A fuselage frame of reference or “body frame of reference” (X.sub.F, Y.sub.F, Z.sub.F) is attached to this rotorcraft 10, and more particularly to its center of gravity. A longitudinal axis X.sub.F of the rotorcraft 10 extends in a direction going from the tail of the rotorcraft 10 to the nose of the rotorcraft 10. A normal axis Z.sub.F extends in a direction going downwards perpendicularly to the longitudinal axis X.sub.F, and a lateral axis Y.sub.F extends in a direction going from left to right perpendicularly to the longitudinal axis X.sub.F and to the normal axis Z.sub.F.

[0117] The main rotor 11 includes a mast 12, a hub 13, and blades 14. Each blade 14 has a first end 141 that is connected to the hub 13 and a second end or tip 142 that is free. The mast 12 is united with the hub 13, and drives the hub 13 and the blades 14 in rotation about an axis A1 of the mast 12. Each blade 14 is also mounted to pivot about its pitch axis, as well as about its flapping axis and about its drag axis (these axes not being shown).

[0118] Therefore, while the main rotor 11 is rotating about the axis A1, the hub 13 moves in a hub plane HP that is perpendicular to an axis A1 of the mast 12 while the free second end 142 of each blade 14 describes a substantially plane path, in the vicinity of a mean plane known as the “tip-path plane” TPP. The path of the free second end 142 of each blade 14 is situated on either side of the tip-path plane TPP and the projection of that path in the tip-path plane TPP is substantially circular.

[0119] Another body frame of reference (X.sub.R, Y.sub.R, Z.sub.R) may also be associated with the fuselage 16 of the rotorcraft 10 and more particularly be united with the hub plane HP. This frame of reference (X.sub.R, Y.sub.R, Z.sub.R) does not rotate relative to the fuselage 16, a longitudinal axis X.sub.R being formed by a projection on the hub plane HP of the longitudinal axis X.sub.F of the body frame of reference (X.sub.F, Y.sub.F, Z.sub.F), a lateral axis Y.sub.R being formed by a projection on the hub plane HP of the lateral axis Y.sub.F, and a normal axis Z.sub.R extending in a direction going downwards perpendicularly to the hub plane HP.

[0120] A frame of reference (X, Y, Z) may also be united with the tip-path plane TPP. This frame of reference (X, Y, Z) does not rotate relative to the fuselage 16 of the rotorcraft 10, a longitudinal axis X being formed by a projection on the tip-path plane TPP of the longitudinal axis X.sub.F of the body frame of reference (X.sub.F, Y.sub.F, Z.sub.F), a lateral axis Y being formed by a projection on the tip-path plane TPP of the lateral axis Y.sub.F, and a normal axis Z extending in a direction going downwards perpendicularly to the tip-path plane TPP.

[0121] In addition, the rotorcraft 10 may include apparatus of the AHRS type 6 that, in particular, delivers specific forces that are being applied to the fuselage 16 of the rotorcraft 10, e.g. along the axes X.sub.F, Y.sub.F, Z.sub.F as well as apparatus 1 for estimating an airspeed. The apparatus 1 for estimating an airspeed is based on using the cyclic pitches of the blades 14, relative to the tip-path plane TPP, and makes it possible, in particular to estimate the longitudinal component v.sub.l, the lateral component v.sub.t and, where applicable, the axial component v.sub.a of the airspeed of the rotorcraft 10.

[0122] The apparatus 1 for estimating an airspeed 1 includes a computer 2 provided with a memory 3 and makes it possible to perform a method of estimating an airspeed of a rotorcraft 10 in order to determine estimations of the components of an airspeed of the rotorcraft 10.

[0123] The memory 3 of the computer 2 stores at least one algorithm for performing said method as well as at least one model of the rotor coming from the flight mechanics of the rotorcraft 10, which model defines an equilibrium for the rotor disk and takes the form of equations relating the pitch angle θ.sub.TPP of at least one blade 14 to the airspeed of the rotorcraft 10 and to an auxiliary speed, e.g. the induced velocity of the air flowing through the rotor 11 or else an axial airspeed at the upstream infinity of the rotorcraft 10.

[0124] The model of the rotor may include a set of analytical equations of the flight mechanics that, in particular, relate the longitudinal cyclic pitch θ.sub.C and lateral cyclic pitch θ.sub.S of the blades 14 to the longitudinal component v.sub.l, lateral component v.sub.t and, where applicable, axial component v.sub.a of the airspeed of the rotorcraft 10 and to the auxiliary speed.

[0125] The method of estimating an airspeed of a rotorcraft 10 includes firstly a step of estimating a pitch angle θ.sub.TPP of at least one blade 14 relative to the tip-path plane TPP, this step being performed by the apparatus 1 for estimating an airspeed.

[0126] In FIG. 1, the apparatus 1 for estimating an airspeed also includes a barometric sensor 8 and a first sensor 4. The first sensor 4 may be arranged on a blade 14 or else in a blade 14, and it measures a first angular parameter that is characteristic of the movement of the blade 14. In this way, the first sensor 4 makes it possible to estimate a pitch angle θ.sub.TPP of the blade 14 relative to the tip-path plane TPP. This pitch angle θ.sub.TPP can be broken down into a collective pitch θ.sub.0, a longitudinal cyclic pitch θ.sub.C/TPP and a lateral cyclic pitch θ.sub.S/TPP of the blade 14.

[0127] In FIG. 2, the apparatus 1 for estimating an airspeed includes a computer 2 provided with a memory 3, as well as a barometric sensor 8, a second sensor 5, and a third sensor 7. The second sensor 5 may, for example, be arranged on a non-rotary portion of a swashplate 17 that controls the variations in the pitches of the blades 14 as shown in FIG. 2, or else at control apparatus for controlling the variation in the pitches of the blades 14. The second sensor 5 thus directly or indirectly measures a pitch angle θ.sub.HP relative to the hub plane HP. The pitch angle θ.sub.HP includes a collective component θ.sub.0 and a cyclic component that can be broken down into a longitudinal cyclic component θ.sub.C/HP and a lateral cyclic component θ.sub.S/HP.

[0128] FIG. 3 diagrammatically shows the rotor 11 as well as the tip-path plane TPP and the hub plane HP. The axis A1 of the mast 12 of the rotor 11 is inclined by a few degrees, typically in the range 3° to 4°, relative to the normal axis Z.sub.F. Therefore, the hub plane HP is inclined relative to a horizontal plane formed by the longitudinal axis X.sub.F and by the lateral axis Y.sub.F. In addition, during a flight of the rotorcraft 10, the tip-path plane TPP is generally inclined relative to the hub plane HP, except for the particular situation of a flight with zero airspeed, namely a hovering flight in which the rotorcraft 10 is stationary relative to the mass of air.

[0129] Since the tip-path plane TPP and the hub plane HP are not generally parallel, the longitudinal cyclic pitch θ.sub.C and the lateral cyclic pitch θ.sub.S of each blade 14 of the rotor 11 differ depending on whether they are measured relative to the tip-path plane TPP or relative to the hub plane HP. The relationships for going between these measurements of cyclic pitch can then be written as follows:

θ.sub.C/TPP=θ.sub.c/HP+β.sub.s and  [Math 6]

θ.sub.S/TPP=θ.sub.S/HP−β.sub.c,  [Math 7]

where β.sub.c and β.sub.s, are the longitudinal cyclic component and the lateral cyclic component of the angle of inclination of the tip-path plane TPP relative to the hub plane HP respectively about the longitudinal axis X.sub.F and about the lateral axis Y.sub.F.

[0130] The computer 2 of said apparatus 1 for estimating an airspeed can estimate the pitch angle θ.sub.TPP of the blade 14 as a function of the pitch angle θ.sub.HP and of at least one angle of inclination of the tip-path plane TPP relative to the hub plane HP, said at least one angle of inclination itself being estimated on the basis of a specific force that is being applied to the fuselage 16 of the rotorcraft 10 and that is measured by the apparatus 6 of the AHRS type. Said at least one angle of inclination may include the longitudinal cyclic component β.sub.C and the lateral cyclic component β.sub.S, thereby making it possible to estimate, in particular, a longitudinal cyclic pitch θ.sub.C/TPP and a lateral cyclic pitch β.sub.S/TPP of the blade 14 relative to the tip-path plane TPP.

[0131] Therefore, regardless of its embodiment, the apparatus 1 for estimating an airspeed makes it possible to estimate a pitch angle θ.sub.TPP of the blade 14 relative to the tip-path plane TPP, it being possible for this angle θ.sub.TPP to be broken down into a collective pitch θ.sub.0, a longitudinal cyclic pitch θ.sub.C/TPP and a lateral cyclic pitch θ.sub.S/TPP of the blade 14.

[0132] In addition, the third sensor 7 measures instants at which a first mark 115 attached to the rotor 11 goes past a second mark 165 attached to the fuselage 16 and facing said first mark 115 as it goes past. Said third sensor 7 thus makes it possible to determine firstly the azimuth angle ψ of the blade 14 about the axis A1 of the mast 12, as well as the speed of rotation Ω of the blade 14. For example, the third sensor 7 may be a Hall effect sensor positioned on the fuselage 16 at the second mark 165, a magnet then being positioned at the first mark 115 attached to the rotor 11.

[0133] Then, the method of estimating an airspeed of a rotorcraft 10 may include a step of determining an auxiliary speed of said rotorcraft 10, said auxiliary speed being an induced velocity of the air flowing through the rotor 11 or else an axial airspeed at the upstream infinity of the rotorcraft 10.

[0134] When the auxiliary speed is the axial airspeed at the upstream infinity of the rotorcraft 10, determining the auxiliary speed may include a step of estimating a barometric altitude of the rotorcraft 10, e.g. by means of the barometric sensor 8, and a step of determining the upstream infinity airspeed v.sub.a equal to a time derivative of the barometric altitude of the rotorcraft 10, this step being performed, for example, by the computer 2.

[0135] When the auxiliary speed is the induced velocity of the air flowing through the rotor 11, determining the auxiliary speed may include a step of estimating the lift of the rotor 11 of the rotorcraft 10 and a step of computing the induced velocity as a function of said lift and of an area swept by the blades 14 of the rotor 11.

[0136] The step of estimating the lift of the rotor 11 may be performed in different ways. For example, since the lift of the rotor 11 of the rotorcraft 10 mainly opposes the apparent weight of the rotorcraft 10, the step of estimating the lift may include a step of estimating the mass of the rotorcraft 10, a step of measuring a normal component of the specific force that is being applied to the fuselage 16 of the rotorcraft 10, a step of estimating an apparent weight of the rotorcraft 10 as a function of its mass and of said specific force, and a step of computing the lift as a function of said apparent weight.

[0137] For example, the specific force that is being applied to the fuselage 16 of the rotorcraft 10 may be formed by a vector thereby defining the direction and the modulus of said specific force in a body frame of reference and may be measured by means of the apparatus 6 of the AHRS type.

[0138] Other terms, such as an estimation of the vertical drag of the fuselage 16 of the rotorcraft 10 may also be taken into account in computing the lift.

[0139] In another example, the step of estimating the lift may include a step of estimating a conicity of the rotor 11, a step of estimating the speed of rotation of the rotor 11, and a step of computing the lift as a function of the conicity, of a second moment of area of each blade 14 about its flapping axis, of the number of blades, and of the speed of rotation of the rotor 11.

[0140] For example, the conicity of the rotor 11 may be estimated on the basis of an angle sensor integrated in the flapping hinge of a blade 14.

[0141] The speed of rotation of the rotor 11 may be information delivered by the avionics of the rotorcraft 10 or else be determined by the third sensor 7 as mentioned above.

[0142] Then, with the lift of the rotor 11 being known, the induced velocity can be computed, by the computer 2, as a function of the lift of the rotor 11, of an area swept by the blades 14 of the rotor 11, and of the forward speed of the rotorcraft 10, e.g. by using Froude's momentum theory.

[0143] Finally, the method of estimating an airspeed of a rotorcraft 10 includes a step of determining said airspeed of the rotorcraft 10 in the frame of reference (X, Y, Z) united with the tip-path plane TPP by solving the model of the rotor 11. For example, the model of the rotor 11 may include the following set of analytical equations [Math 8]:

[00003]$\begin{array}{cc}{\theta }_{\mathrm{.75}}=\frac{\left(1+\frac{3}{2}{\mu }^2\right)\left(\frac{6C_T}{\text{?}}+\frac{3}{8}{\mu }^2{\theta }_{\mathrm{tw}}\right)+\frac{3}{2}{\lambda }_{\mathrm{TPP}}\left(1-\frac{1}{2}{\mu }^2\right)}{1-{\mu }^2+\frac{9}{4}{\mu }^4}{\theta }_{1s}=-{\beta }_{1c}-\frac{\frac{8}{3}\mu \left(\frac{6C_T}{\text{?}}+\frac{3}{8}{\mu }^2{\theta }_{\mathrm{tw}}\right)+2{\mathrm{\mu \lambda }}_{\mathrm{TPP}}\left(1-\frac{3}{2}{\mu }^2\right)}{1-{\mu }^2+\frac{9}{4}{\mu }^4}{\beta }_0=\frac{\gamma /8}{1-{\mu }^2+\frac{9}{4}{\mu }^4}\left[\left(1-\frac{19}{18}{\mu }^2+\frac{3}{2}{\mu }^4\right)\frac{6C_T}{\text{?}}+\left(\frac{1}{20}+\frac{29}{120}{\mu }^2-\frac{1}{5}{\mu }^4+\frac{3}{8}{\mu }^6\right){\theta }_{\mathrm{tw}}+\left(\frac{1}{6}-\frac{7}{12}{\mu }^2+\frac{1}{4}{\mu }^4\right){\lambda }_{\mathrm{TPP}}\right]{\theta }_{1c}={\beta }_{1s}+\frac{\frac{4}{3}{\mathrm{\mu \beta }}_0}{1+\frac{1}{2}{\mu }^2}\text{?}\text{indicates text missing or illegible when filed}& \end{array}$

[0144] The radial component v.sub.r may be defined by a sideslip angle β relative to the longitudinal axis X.sub.F of the body frame of reference (X.sub.F, Y.sub.F, Z.sub.F) and relative to the longitudinal axis X of the frame of reference (X, Y, Z) attached to the tip-path plane TPP. This radial component v.sub.r may be broken down into the longitudinal component v.sub.l and the lateral component v.sub.t of the airspeed of the rotorcraft 10 respectively along the longitudinal axis X and along the lateral axis Y of this frame of reference (X, Y, Z) by the following relationships:

v.sub.r=√{square root over (v.sub.l.sup.2+v.sub.t.sup.2)} and  [Math 9]

[Math 10]

[00004]$\beta ={\tan }^{-1}\left(\frac{{\nu }_t}{v_l}\right).$

[0145] For example, the set of three equations of this model may be solved by the Newton-Raphson model by the computer 2, thereby making it possible to estimate the longitudinal component v.sub.l, the lateral component v.sub.t and, where appliable, the axial component v.sub.a of the airspeed of the rotorcraft 10.

[0146] The block diagram 21 shown in FIG. 4 shows the links between the components of the airspeed and the pitches of the blades 14 of the rotorcraft 10 by applying this set of equations. The matrix M.sub.β is a matrix for transfer between the frame of reference (X, Y, Z) attached to the tip-path plane TPP and the body frame of reference (X.sub.R, Y.sub.R, Z.sub.R) attached to the hub plane HP.

[0147] Naturally, the present invention may be subjected to numerous variations as to its implementation. Although several implementations and embodiments are described above, it should readily be understood that it is not conceivable to identify exhaustively all possible implementations and embodiments. It is naturally possible to envisage replacing any of the means described by equivalent means without going beyond the ambit of the present invention.