Method of optimizing sections of a tail boom for a rotary wing aircraft

Abstract

A method of optimizing sections of a tail boom for a rotary wing aircraft, and also to a tail boom including such sections. The method comprises the step of creating a database characterizing standard sections for a tail boom that give precedence to minimizing a negative lift and/or to increasing a lateral force generated by the air stream from the main rotor of the aircraft flowing over the tail boom, a step of establishing looked-for aerodynamic and structural characteristics for said tail boom, and a step of defining the sections of the tail boom as a function of the standard sections and of the looked-for aerodynamic and structural characteristics. The tail boom as defined in this way optimizes the reduction in the negative lift and/or the increase in the lateral force generated by the air stream from the main rotor.

Claims

1. A method for producing a tail boom for a rotary wing aircraft having a main rotor and a fuselage, the tail boom to extend from the fuselage in a longitudinal direction, the tail boom to be constituted by successive sections with each section being defined in a plane perpendicular to the longitudinal direction by a chord between a leading edge and a trailing edge of the section and by a maximum thickness measured perpendicularly to the chord, the method comprising: an optimization process for defining the sections of the tail boom, the optimization process including the steps of: a step of performing studies, digital simulations, and/or tests for determining standard sections for the tail boom; a step of creating a database characterizing the standard sections for the tail boom, the database having four types of standard sections for the tail boom, a first type of standard sections being symmetric and maximizing a lateral force of the tail boom, a second type of standard sections being asymmetric and maximizing the lateral force of the tail boom, a third type of standard sections being symmetric and minimizing a negative lift of the tail boom, and a fourth type of standard sections being asymmetric and minimizing the negative lift of the tail boom; each standard section being defined by extreme characteristic points A0, A1, A2, A3, and by intermediate characteristic points A1, A2, A11, A12, A21, A22, A31, A32, the characteristic points being defined in a secondary reference frame attached to each standard section and defined by the directions of the chord and of the maximum thickness of the standard section, one unit along the abscissa axis or along the ordinate axis being equal to the chord, the extreme characteristic points A0, A1, A2, A3, belonging to an outline of the standard section, A0 being situated at the trailing edge and having coordinates (1,0), A2 being situated at the leading edge and having coordinates (0,0), the extreme characteristic points A1 and A3 forming the maximum thickness and having respective coordinates t/(2.c) and t/(2.c), the intermediate characteristic points A01, A02, A11, A12, A21, A22, A31, A32 being construction points for constructing the outline of the standard section serving to define a direction of the outline of the section at each extreme characteristic point A0, A1, A2, A3; a step of establishing looked-for aerodynamic and structural characteristics of the tail boom in terms of lateral force and negative lift generated by an air stream from the main rotor flowing over the tail boom; a step of defining the sections of the tail boom as a function of the standard sections and of the looked-for aerodynamic and structural characteristics for the tail boom in order to give precedence to minimizing the negative lift and/or to maximizing the lateral force of the tail boom; and the method further comprising providing the tail boom having the sections as defined per the optimization process.

2. The method according to claim 1, wherein each standard section being further defined as a function of the ratio of the maximum thickness of the standard section divided by the chord of the standard section.

3. The method according to claim 2, wherein for each standard section of the first type, the maximum thickness of the standard section is arranged downstream from the middle of the chord of the standard section in the flow direction of the air stream from the main rotor; for each standard section of the second type, the maximum thickness of the standard section is arranged downstream from the middle of the chord of the standard section and in the proximity of the middle of the chord of the standard section; and for each standard section of the third type and for each standard section of the fourth type, the maximum thickness of the standard section is arranged upstream from the middle of the chord of the standard section.

4. The method according to claim 1, wherein the database comprises the following first table defining ranges for the characteristic points for standard sections of the first type: TABLE-US-00013 t/c value uA1, uA3 vA01 uA02 uA11 vA12 vA21 uA22 uA31 vA32 50% low 0.55 0.6 0.1 0.6 0.08 0.08 0.6 0.1 0.6 high 0.95 0.95 0.25 0.95 0.25 0.25 0.95 0.25 0.95 60% low 0.55 0.6 0.1 0.6 0.08 0.08 0.6 0.1 0.6 high 0.95 0.95 0.25 0.95 0.25 0.25 0.95 0.25 0.95 70% low 0.55 0.6 0.08 0.6 0.08 0.08 0.6 0.08 0.6 high 0.95 0.95 0.4 0.95 0.25 0.25 0.95 0.4 0.95 80% low 0.55 0.6 0.1 0.6 0.08 0.08 0.6 0.1 0.6 high 0.95 0.95 0.25 0.95 0.25 0.25 0.95 0.25 0.95 90%-130% low 0.55 0.6 0.1 0.6 0.08 0.08 0.6 0.1 0.6 high 0.99 0.95 0.25 0.95 0.25 0.25 0.95 0.25 0.95 the database includes the following second table defining ranges for the characteristic points for standard sections of the second type for the main rotor rotating in the counterclockwise direction when seen from above: TABLE-US-00014 t/c value uA1, uA3 vA01 uA02 uA11 vA12 vA21 uA22 uA31 vA32 50% low 0.45 0.08 0.6 0.08 0.6 0.6 0.08 0.25 0.1 high 0.75 0.25 0.95 0.25 0.95 0.95 0.25 0.45 0.3 60% low 0.45 0.08 0.6 0.08 0.6 0.6 0.08 0.6 0.1 high 0.75 0.25 0.95 0.25 0.95 0.95 0.25 0.95 0.3 70% low 0.45 0.08 0.6 0.08 0.6 0.6 0.08 0.6 0.1 high 0.75 0.25 0.95 0.25 0.95 0.95 0.25 0.95 0.3 80% low 0.45 0.08 0.6 0.08 0.6 0.6 0.08 0.6 0.1 high 0.75 0.25 0.95 0.25 0.95 0.95 0.25 0.95 0.3 90%-130% low 0.5 0.1 0.6 0.08 0.6 0.6 0.08 0.6 0.1 high 0.99 0.6 0.95 0.25 0.95 0.95 0.25 0.95 0.3 the database includes the following third table defining ranges for the characteristic points for standard sections of the third type: TABLE-US-00015 t/c value uA1, uA3 vA01 uA02 uA11 vA12 vA21 uA22 uA31 vA32 50% low 0.2 0.5 0.1 0.2 0.1 0.1 0.2 0.1 0.5 high 0.5 0.9 0.4 0.4 0.4 0.4 0.4 0.4 0.9 60% low 0.2 0.5 0.1 0.3 0.1 0.1 0.3 0.1 0.5 high 0.5 0.9 0.4 0.7 0.4 0.4 0.7 0.4 0.9 70% low 0.2 0.5 0.1 0.3 0.1 0.1 0.3 0.1 0.5 high 0.5 0.9 0.4 0.7 0.4 0.4 0.7 0.4 0.9 80% low 0.2 0.5 0.1 0.4 0.1 0.1 0.4 0.1 0.5 high 0.5 0.9 0.4 0.7 0.4 0.4 0.7 0.4 0.9 90%-130% low 0.2 0.5 0.1 0.7 0.1 0.1 0.7 0.1 0.5 high 0.5 0.9 0.4 0.95 0.4 0.4 0.95 0.4 0.9 and the database includes the following fourth table defining ranges for the characteristic points for standard sections of the fourth type for the main rotor rotating in the counterclockwise direction when seen from above: TABLE-US-00016 Value uA1, uA3 vA01 uA02 uA11 vA12 vA21 uA22 uA31 vA32 Low 0.2 0.1 0.55 0.1 0.65 0.65 0.1 0.65 0.1 High 0.5 0.3 0.85 0.4 0.9 0.9 0.25 0.9 0.3.

5. The method according to claim 4, wherein the ordinate coordinates of the intermediate characteristic points A02, A11 are equal to the ordinate coordinate of the extreme characteristic point A1, the ordinate coordinates of the intermediate characteristic points A22, A31, are equal to the ordinate coordinate of the extreme characteristic point A3, the abscissa coordinates of the intermediate characteristic points A01, A32 are equal to the abscissa coordinate of the extreme characteristic point A0 and the abscissa coordinates of the intermediate characteristic points A12, A21 are equal to the abscissa coordinate of the extreme characteristic point A2.

6. The method according to claim 1, wherein each section of the tail boom is formed by 4.sup.th-order Bezier curves connecting the extreme characteristic points A0, A1, A2, A3, A0 together in pairs and constructed using the intermediate characteristic points A01, A02, A11, A12, A21, A22, A31, A32.

7. A tail boom for a rotary wing aircraft having a main rotor and a fuselage, the tail boom comprising: a plurality of successive sections extending in a longitudinal direction, the sections including a lead section connectable to the fuselage to connect the tail boom to the fuselage, each section being defined in a plane perpendicular to the longitudinal direction by a chord between a leading edge and a trailing edge of the section and by a maximum thickness measured perpendicularly to the chord; and wherein the sections of the tail boom are defined using an optimization process including the following steps: a step of performing studies, digital simulations, and/or tests for determining standard sections for the tail boom; a step of creating a database characterizing the standard sections for the tail boom, the database having four types of standard sections for the tail boom, a first type of standard sections being symmetric and maximizing a lateral force of the tail boom, a second type of standard sections being asymmetric and maximizing the lateral force of the tail boom, a third type of standard sections being symmetric and minimizing a negative lift of the tail boom, and a fourth type of standard sections being asymmetric and minimizing the negative lift of the tail boom; each standard section being defined by extreme characteristic points A0, A1, A2, A3 and by intermediate characteristic points A01, A02, A11, A12, A21, A22, A31, A32, the characteristic points being defined in a secondary reference frame attached to each standard section and defined by the directions of the chord of the standard section and of the maximum thickness of the standard section, one unit along the abscissa axis or along the ordinate axis being equal to the chord, the extreme characteristic points A0, A1, A2, A3 belonging to an outline of the standard section, A0 being situated at the trailing edge and having coordinates (1,0), A2 being situated at the leading edge and having coordinates (0,0), the extreme characteristic points A1 and A3 forming the maximum thickness and having respective coordinates t/(2c) and t/(2c), the intermediate characteristic points A01, A02, A11, A12, A21, A22, A31, A32 being construction points for constructing the outline of the standard section serving to define a direction of the outline of the section at each extreme characteristic point A0, A1, A2, A3; a step of establishing looked-for aerodynamic and structural characteristics of the tail boom in terms of lateral force and negative lift generated by an air stream from the main rotor flowing over the tail boom; and a step of defining the sections of the tail boom as a function of the standard sections and of the looked-for aerodynamic and structural characteristics of the tail boom in order to give precedence to minimizing the negative lift and/or to maximizing the lateral force of the tail boom.

8. The tail boom of claim 7 wherein: each standard section being further defined as a function of the ratio of the maximum thickness of the standard section divided by the chord of the standard section.

9. The tail boom of claim 8 wherein: for each standard section of the first type, the maximum thickness of the standard section is arranged downstream from the middle of the chord of the standard section in the flow direction of the air stream from the main rotor; for each standard section of the second type, the maximum thickness of the standard section is arranged downstream from the middle of the chord of the standard section and in the proximity of the middle of the chord of the standard section; and for each standard section of the third type and for each standard section of the fourth type, the maximum thickness of the standard section is arranged upstream from the middle of the chord of the standard section.

10. The tail boom of claim 7 wherein: the database includes the following first table defining ranges for the characteristic points for standard sections of the first type: TABLE-US-00017 t/c value uA1, uA3 vA01 uA02 uA11 vA12 vA21 uA22 uA31 vA32 50% low 0.55 0.6 0.1 0.6 0.08 0.08 0.6 0.1 0.6 high 0.95 0.95 0.25 0.95 0.25 0.25 0.95 0.25 0.95 60% low 0.55 0.6 0.1 0.6 0.08 0.08 0.6 0.1 0.6 high 0.95 0.95 0.25 0.95 0.25 0.25 0.95 0.25 0.95 70% low 0.55 0.6 0.08 0.6 0.08 0.08 0.6 0.08 0.6 high 0.95 0.95 0.4 0.95 0.25 0.25 0.95 0.4 0.95 80% low 0.55 0.6 0.1 0.6 0.08 0.08 0.6 0.1 0.6 high 0.95 0.95 0.25 0.5 0.25 0.25 0.95 0.25 0.95 90%-130% low 0.55 0.6 0.1 0.6 0.08 0.08 0.6 0.1 0.6 high 0.99 0.95 0.25 0.95 0.25 0.25 0.95 0.25 0.95 the database includes the following second table defining ranges for the characteristic points for standard sections of the second type for the main rotor rotating in the counterclockwise direction when seen from above: TABLE-US-00018 t/c value uA1, uA3 vA01 uA02 uA11 vA12 vA21 uA22 uA31 vA32 50% low 0.45 0.08 0.6 0.08 0.6 0.6 0.08 0.25 0.1 high 0.75 0.25 0.95 0.25 0.95 0.95 0.25 0.45 0.3 60% low 0.45 0.08 0.6 0.08 0.6 0.6 0.08 0.6 0.1 high 0.75 0.25 0.95 0.25 0.95 0.95 0.25 0.95 0.3 70% low 0.45 0.08 0.6 0.08 0.6 0.6 0.08 0.6 0.1 high 0.75 0.25 0.95 0.25 0.95 0.95 0.25 0.95 0.3 80% low 0.45 0.08 0.6 0.08 0.6 0.6 0.08 0.6 0.1 high 0.75 0.25 0.95 0.25 0.95 0.95 0.25 0.95 0.3 90%-130% low 0.5 0.1 0.6 0.08 0.6 0.6 0.08 0.6 0.1 high 0.99 0.6 0.95 0.25 0.95 0.95 0.25 0.95 0.3 the database includes the following third table defining ranges for the characteristic points for standard sections of the third type: TABLE-US-00019 t/c value uA1, uA3 vA01 uA02 uA11 vA12 vA21 uA22 uA31 vA32 50% low 0.2 0.5 0.1 0.2 0.1 0.1 0.2 0.1 0.5 high 0.5 0.9 0.4 0.4 0.4 0.4 0.4 0.4 0.9 60% low 0.2 0.5 0.1 0.3 0.1 0.1 0.3 0.1 0.5 high 0.5 0.9 0.4 0.7 0.4 0.4 0.7 0.4 0.9 70% low 0.2 0.5 0.1 0.3 0.1 0.1 0.3 0.1 0.5 high 0.5 0.9 0.4 0.7 0.4 0.4 0.7 0.4 0.9 80% low 0.2 0.5 0.1 0.4 0.1 0.1 0.4 0.1 0.5 high 0.5 0.9 0.4 0.7 0.4 0.4 0.7 0.4 0.9 90%-130% low 0.2 0.5 0.1 0.7 0.1 0.1 0.7 0.1 0.5 high 0.5 0.9 0.4 0.95 0.4 0.4 0.95 0.4 0.9 and the database includes the following fourth table defining ranges for the characteristic points for standard sections of the fourth type for the main rotor rotating in the counterclockwise direction when seen from above: TABLE-US-00020 uA1, value uA3 vA01 uA02 uA11 vA12 vA21 uA22 uA31 vA32 low 0.2 0.1 0.55 0.1 0.65 0.65 0.1 0.65 0.1 high 0.5 0.3 0.85 0.4 0.9 0.9 0.25 0.9 0.3.

11. The tail boom of claim 10 wherein: the ordinate coordinates of the intermediate characteristic points A02, A11 are equal to the ordinate coordinate of the extreme characteristic point A1, the ordinate coordinates of the intermediate characteristic points A22, A31 are equal to the ordinate coordinate of the extreme characteristic point A3, the abscissa coordinates of the intermediate characteristic points A01, A32 are equal to the abscissa coordinate of the extreme characteristic point A0, and the abscissa coordinates of the intermediate characteristic points A12, A21 are equal to the abscissa coordinate of the extreme characteristic point A2.

12. The tail boom of claim 7 wherein: each section of the tail boom is formed by 4.sup.th-order Bezier curves connecting the extreme characteristic points A0, A1, A2, A3, A0 together in pairs and constructed using the intermediate characteristic points A01, A02, A11, A12, A21, A22, A31, A32.

13. A rotary wing aircraft comprising: a fuselage and a main rotor having a plurality of blades; a tail boom extending from the fuselage in a longitudinal direction, the tail boom being constituted by successive sections, each section being defined in a plane perpendicular to the longitudinal direction by a chord between a leading edge and a trailing edge of the section and by a maximum thickness measured perpendicularly to the chord; and wherein the sections of the tail boom are defined using an optimization process including the following steps: a step of performing studies, digital simulations, and/or tests for determining standard sections for the tail boom; a step of creating a database characterizing the standard sections for the tail boom, the database having four types of standard sections for the tail boom, a first type of standard sections being symmetric and maximizing a lateral force of the tail boom, a second type of standard sections being asymmetric and maximizing the lateral force of the tail boom, a third type of standard sections being symmetric and minimizing a negative lift of the tail boom, and a fourth type of standard sections being asymmetric and minimizing the negative lift of the tail boom; each standard section being defined by extreme characteristic points A0, A1, A2, A3 and by intermediate characteristic points A01, A02, A11, A12, A21, A22, A31, A32, the characteristic points being defined in a secondary reference frame attached to each standard section and defined by the directions of the chord of the standard section and of the maximum thickness of the standard section, one unit along the abscissa axis or along the ordinate axis being equal to the chord, the extreme characteristic points A0, A1, A2, A3 belonging to an outline of the standard section, A0 being situated at the trailing edge and having coordinates (1,0), A2 being situated at the leading edge and having coordinates (0,0), the extreme characteristic points A1 and A3 forming the maximum thickness and having respective coordinates t/(2c) and t/(2c), the intermediate characteristic points A01, A02, A11, A12, A21, A22, A31, A32 being construction points for constructing the outline of the standard section serving to define a direction of the outline of the section at each extreme characteristic point A0, A1, A2, A3; a step of establishing looked-for aerodynamic and structural characteristics of the tail boom in terms of lateral force and negative lift generated by an air stream from the main rotor flowing over the tail boom; and a step of defining the sections of the tail boom as a function of the standard sections and of the looked-for aerodynamic and structural characteristics of the tail boom in order to give precedence to minimizing the negative lift and/or to maximizing the lateral force of the tail boom.

14. The rotary wing aircraft of claim 13 wherein: each standard section being further defined as a function of the ratio of the maximum thickness of the standard section divided by the chord of the standard section.

15. The rotary wing aircraft of claim 14 wherein: for each standard section of the first type, the maximum thickness of the standard section is arranged downstream from the middle of the chord of the standard section in the flow direction of the air stream from the main rotor; for each standard section of the second type, the maximum thickness of the standard section is arranged downstream from the middle of the chord of the standard section and in the proximity of the middle of the chord of the standard section; and for each standard section of the third type and for each standard section of the fourth type, the maximum thickness of the standard section is arranged upstream from the middle of the chord of the standard section.

16. The rotary wing aircraft of claim 13 wherein: the database includes the following first table defining ranges for the characteristic points for standard sections of the first type: TABLE-US-00021 t/c value uA1, uA3 vA01 uA02 uA11 vA12 vA21 uA22 uA31 vA32 50% low 0.55 0.6 0.1 0.6 0.08 0.08 0.6 0.1 0.6 high 0.95 0.95 0.25 0.95 0.25 0.25 0.95 0.25 0.95 60% low 0.55 0.6 0.1 0.6 0.08 0.08 0.6 0.1 0.6 high 0.95 0.95 0.25 0.95 0.25 0.25 0.95 0.25 0.95 70% low 0.55 0.6 0.08 0.6 0.08 0.08 0.6 0.08 0.6 high 0.95 0.95 0.4 0.95 0.25 0.25 0.95 0.4 0.95 80% low 0.55 0.6 0.1 0.6 0.08 0.08 0.6 0.1 0.6 high 0.95 0.95 0.25 0.95 0.25 0.25 0.95 0.25 0.95 90%-130% low 0.55 0.6 0.1 0.6 0.08 0.08 0.6 0.1 0.6 high 0.99 0.95 0.25 0.95 0.25 0.25 0.95 0.25 0.95 the database includes the following second table defining ranges for the characteristic points for standard sections of the second type for the main rotor rotating in the counterclockwise direction when seen from above: TABLE-US-00022 t/c value uA1, uA3 vA01 uA02 uA11 vA12 vA21 uA22 uA31 vA32 50% low 0.45 0.08 0.6 0.08 0.6 0.6 0.08 0.25 0.1 high 0.75 0.25 0.95 0.25 0.95 0.95 0.25 0.45 0.3 60% low 0.45 0.08 0.6 0.08 0.6 0.6 0.08 0.6 0.1 high 0.75 0.25 0.95 0.25 0.95 0.95 0.25 0.95 0.3 70% low 0.45 0.08 0.6 0.08 0.6 0.6 0.08 0.6 0.1 high 0.75 0.25 0.95 0.25 0.95 0.95 0.25 0.95 0.3 80% low 0.45 0.08 0.6 0.08 0.6 0.6 0.08 0.6 0.1 high 0.75 0.25 0.95 0.25 0.95 0.95 0.25 0.95 0.3 90%-130% low 0.5 0.1 0.6 0.08 0.6 0.6 0.08 0.6 0.1 high 0.99 0.6 0.95 0.25 0.95 0.95 0.25 0.95 0.3 the database includes the following third table defining ranges for the characteristic points for standard sections of the third type: TABLE-US-00023 t/c value uA1, uA3 vA01 uA02 uA11 vA12 vA21 uA22 uA31 vA32 50% low 0.2 0.5 0.1 0.2 0.1 0.1 0.2 0.1 0.5 high 0.5 0.9 0.4 0.4 0.4 0.4 0.4 0.4 0.9 60% low 0.2 0.5 0.1 0.3 0.1 0.1 0.3 0.1 0.5 high 0.5 0.9 0.4 0.7 0.4 0.4 0.7 0.4 0.9 70% low 0.2 0.5 0.1 0.3 0.1 0.1 0.3 0.1 0.5 high 0.5 0.9 0.4 0.7 0.4 0.4 0.7 0.4 0.9 80% low 0.2 0.5 0.1 0.4 0.1 0.1 0.4 0.1 0.5 high 0.5 0.9 0.4 0.7 0.4 0.4 0.7 0.4 0.9 90%-130% low 0.2 0.5 0.1 0.7 0.1 0.1 0.7 0.1 0.5 high 0.5 0.9 0.4 0.95 0.4 0.4 0.95 0.4 0.9 and the database includes the following fourth table defining ranges for the characteristic points for standard sections of the fourth type for the main rotor rotating in the counterclockwise direction when seen from above: TABLE-US-00024 uA1, value uA3 vA01 uA02 uA11 vA12 vA21 uA22 uA31 vA32 low 0.2 0.1 0.55 0.1 0.65 0.65 0.1 0.65 0.1 high 0.5 0.3 0.85 0.4 0.9 0.9 0.25 0.9 0.3.

17. The rotary wing aircraft of claim 16 wherein: the ordinate coordinates of the intermediate characteristic points A02, A11 are equal to the ordinate coordinate of the extreme characteristic point A1, the ordinate coordinates of the intermediate characteristic points A22, A31 are equal to the ordinate coordinate of the extreme characteristic point A3, the abscissa coordinates of the intermediate characteristic points A01, A32 are equal to the abscissa coordinate of the extreme characteristic point A0, and the abscissa coordinates of the intermediate characteristic points A12, A21 are equal to the abscissa coordinate of the extreme characteristic point A2.

18. The rotary wing aircraft of claim 13 wherein: each section of the tail boom is formed by 4.sup.th-order Bezier curves connecting the extreme characteristic points A0, A1, A2, A3, A0 together in pairs and constructed using the intermediate characteristic points A01, A02, A11, A12, A21, A22, A31, A32.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIGS. 1 and 2 are two plan views of a rotary wing aircraft;

(3) FIG. 3 is a view showing the flow of the air stream around the tail boom;

(4) FIG. 4 is a summary diagram showing a method of optimizing tail boom sections; and

(5) FIG. 5 shows an example of a tail boom section.

(6) Elements present in more than one of the figures are given the same references in each of them.

DETAILED DESCRIPTION OF THE INVENTION

(7) In FIGS. 1 and 2, a rotary wing aircraft 1 is shown as seen from above. The aircraft 1 comprises a fuselage 2, a tail boom 10, a main rotor 3, and an anti-torque auxiliary rotor 4.

(8) A main rectangular (X,Y,Z) reference frame is associated with the aircraft 1. The longitudinal direction X extends from the front of the aircraft 1 towards the rear of the aircraft 1, and the transverse direction Y extends from left to right perpendicularly to the longitudinal direction X. The direction in elevation Z then extends upwards and is perpendicular to the longitudinal and transverse directions X and Y.

(9) The longitudinal direction X is the roll axis of the aircraft 1, the transverse direction Y is the pitching axis, and the elevation direction Z is the yaw axis.

(10) The main rotor 3 is positioned above the fuselage 2 and has an axis of rotation that is substantially vertical, i.e. substantially parallel to the direction in elevation Z. The main rotor 3 is provided with four main blades 31 driven in rotation by a power plant of the aircraft 1, and it serves to provide the aircraft 1 with propulsion and also with lift.

(11) The tail boom 10 is connected to the fuselage 2 and extends in the longitudinal direction X from the fuselage 2 and towards the rear of the aircraft 1.

(12) The auxiliary rotor 4 is positioned at the free end of the tail boom 10 and its axis of rotation is substantially horizontal, parallel to the transverse direction Y. The auxiliary rotor 4 is also driven in rotation by the power plant of the aircraft 1.

(13) In FIG. 1, the main rotor 3 rotates clockwise and a rotor torque C.sub.R acting counterclockwise is then applied to the fuselage 2 about the axis of rotation of the main rotor 3. In order to balance the rotor torque C.sub.R, the auxiliary rotor 4 generates a transverse force F.sub.acp directed to the left of the fuselage 2, thereby creating a main torque opposing the rotor torque C.sub.R.

(14) Furthermore, the tail boom 10 is swept by the air stream generated by the main rotor 3 rotating clockwise, and consequently generates a lateral aerodynamic force F.sub.L directed to the left of the fuselage 2, thereby creating a torque that adds to the main torque.

(15) In FIG. 2, the main rotor 3 rotates counterclockwise and a rotor torque C.sub.R oriented in the clockwise direction is then applied to the fuselage 2 about the axis of rotation of the main rotor 3. In order to balance this rotor torque C.sub.R, the auxiliary rotor 4 generates a transverse force F.sub.acp directed to the right of the fuselage 2, thereby creating a main torque opposing the rotor torque C.sub.R.

(16) In addition, the tail boom 10 is swept by the air stream generated by the main rotor 3 rotating in the counterclockwise direction, and consequently it generates a lateral aerodynamic force F.sub.L directed to the right of the fuselage 2, thereby creating a torque that is additional to the main torque.

(17) FIG. 3 is a view showing the flow of the air stream around the tail boom 10. In FIG. 3, a section of the tail boom 10 is seen from the rear of the aircraft 1 in a plane perpendicular to the longitudinal direction X of the aircraft 1. This air stream is generated by a main rotor 3 rotating in the clockwise direction, as shown in FIG. 1, during hovering flight or flight at a slow speed of advance of the aircraft 1. This air stream sweeps the tail boom 10 from a leading edge 11 situated facing the main rotor 3 towards a trailing edge 12.

(18) It can be seen that the air stream does not sweep the tail boom 10 in a direction that is purely vertical, but rather in a direction that is inclined from right to left as a result of the effect of the clockwise rotation of the main rotor 3. Consequently, the air streams sweeping the right side and the left side of the tail boom 10 are not identical.

(19) The difference between these air streams, and the shape of the section of the tail boom 10 contribute to the lateral aerodynamic force F.sub.L appearing, which force is a horizontal force in the transverse direction Y. Furthermore, a downwardly-directed vertical aerodynamic force, referred to as negative lift F.sub.D, also appears at the trailing edge of the tail boom 10.

(20) Advantageously, and as shown in FIGS. 1 and 2, the lateral force F.sub.L creates torque that adds to the main torque generated by the auxiliary rotor 4 and contributes to balancing the rotor torque C.sub.R. Consequently, the need of the auxiliary rotor 4 for mechanical power is reduced, thereby enabling the power plant of the aircraft 1 to deliver additional mechanical power for driving rotation of the main rotor 3.

(21) Nevertheless, depending on the shapes of the sections of the tail boom 10 and on the orientation of the air stream, the lateral force F.sub.L might create a torque opposing the main torque generated by the auxiliary rotor 4. Consequently, the auxiliary rotor 4 has an increased need for mechanical power, thereby penalizing the performance of the aircraft 1.

(22) In contrast, the negative lift F.sub.D, which is always directed downwards, always opposes the lift generated by the main rotor 3. This negative lift F.sub.D is thus always penalizing for the performance of the aircraft 1.

(23) FIG. 5 shows an example of a section of a tail boom 10 corresponding to a standard type of section that is symmetric and optimized for minimizing negative lift F.sub.D. This section is defined by a method of optimizing sections of a tail boom, which method is summarized as shown in FIG. 4.

(24) In FIG. 5, a secondary (U,V) reference frame is associated with the leading edge 11 of the section of the tail boom and is situated in a plane perpendicular to the longitudinal direction X. The abscissa axis U is defined by the direction of the chord c of the section of the tail boom 10 and extends from the leading edge 11 to the trailing edge 12 parallel to the direction in elevation Z. The ordinate axis V is defined by the direction of the maximum thickness t of the section of the tail boom 10 and is parallel to the transverse direction Y. One unit along the abscissa axis U and along the ordinate axis V is equal to the chord c.

(25) The method of optimizing sections of a tail boom comprises a step 101 of creating a database characterizing standard sections of a tail boom. The database has been established as a result of studies and numerical simulations, and has subsequently been confirmed by testing.

(26) This database comprises four types of standard sections for the tail boom 10, a first type of standard sections that are symmetric and a second type of standard sections that are asymmetric, both types maximizing the lateral force F.sub.L, together with a third type of standard sections that are symmetric and a fourth type of standard sections that are asymmetric, both minimizing the negative lift F.sub.D.

(27) Each standard section is defined by extreme characteristic points A0, A1, A2, A3 and by intermediate characteristic points A01, A02, A11, A12, A21, A22, A31, A32 defined in the secondary (U,V) reference frame. The extreme characteristic points A0, A1, A2, A3 are situated on the outline of the standard section, whereas the intermediate characteristic points A01, A02, A11, A12, A21, A22, A31, A32 are points for constructing the outline. The outline of each section of the tail boom 10 is defined by four 4.sup.th-order Bezier curves respectively connecting together pairs of extreme characteristic points A0, A1, A2, A3, as shown in FIG. 5.

(28) Thus, the intermediate characteristic points A01, A02, A11, A12, A21, A22, A31, A32 are construction points for each Bezier curve, defining firstly the shape of the outline and secondly the tangency of the outline at each extreme characteristic point A0, A1, A2, A3. For example, the shape of the section is more or less elongated at each extreme characteristic point A0, A1, A2, A3 depending on whether each intermediate characteristic point is further away from or closer to the extreme characteristic point A0, A1, A2, A3 to which it is attached. The outline of each section of the tail boom 10 may also be formed by connecting together the extreme characteristic points by other types of polynomial curve.

(29) The database comprises the following tables defining ranges firstly for the abscissa coordinates of the extreme characteristic points A1, A3 and secondly for the characteristic values of the abscissa coordinates of the intermediate characteristic points A02, A11, A22, A31 having the same ordinate coordinates as the intermediate characteristic points A01, A11, A12, A32 for each type of standard section.

(30) The abscissa coordinates of the extreme characteristic points A1, A3 are defined directly in the secondary (U,V) reference frame. The characteristic values of the intermediate characteristic points are defined relative to the distance between the two extreme characteristic points respectively on either sides of each intermediate characteristic point.

(31) These ranges, and consequently these standard sections, are a function of the ratio t/c of the maximum thickness t of a section of a tail boom 10 divided by the chord c of the section. This ratio t/c characterizes the relative thickness of this section of the tail boom and generally varies along a tail boom 10 in the longitudinal direction X of the aircraft 1.

(32) The first table below relates to standard sections of the first type that are symmetric and that maximize the lateral force F.sub.L of the tail boom:

(33) TABLE-US-00009 t/c value uA1, uA3 vA01 uA02 uA11 vA12 vA21 uA22 uA31 vA32 50% low 0.55 0.6 0.1 0.6 0.08 0.08 0.6 0.1 0.6 high 0.95 0.95 0.25 0.95 0.25 0.25 0.95 0.25 0.95 60% low 0.55 0.6 0.1 0.6 0.08 0.08 0.6 0.1 0.6 high 0.95 0.95 0.25 0.95 0.25 0.25 0.95 0.25 0.95 70% low 0.55 0.6 0.08 0.6 0.08 0.08 0.6 0.08 0.6 high 0.95 0.95 0.4 0.95 0.25 0.25 0.95 0.4 0.95 80% low 0.55 0.6 0.1 0.6 0.08 0.08 0.6 0.1 0.6 high 0.95 0.95 0.25 0.95 0.25 0.25 0.95 0.25 0.95 90%-130% low 0.55 0.6 0.1 0.6 0.08 0.08 0.6 0.1 0.6 high 0.99 0.95 0.25 0.95 0.25 0.25 0.95 0.25 0.95

(34) The second table below relates to standard sections of the second type that are asymmetric and that maximize the lateral force F.sub.L of the tail boom:

(35) TABLE-US-00010 t/c value uA1, uA3 vA01 uA02 uA11 vA12 vA21 uA22 uA31 vA32 50% low 0.45 0.08 0.6 0.08 0.6 0.6 0.08 0.25 0.1 high 0.75 0.25 0.95 0.25 0.95 0.95 0.25 0.45 0.3 60% low 0.45 0.08 0.6 0.08 0.6 0.6 0.08 0.6 0.1 high 0.75 0.25 0.95 0.25 0.95 0.95 0.25 0.95 0.3 70% low 0.45 0.08 0.6 0.08 0.6 0.6 0.08 0.6 0.1 high 0.75 0.25 0.95 0.25 0.95 0.95 0.25 0.95 0.3 80% low 0.45 0.08 0.6 0.08 0.6 0.6 0.08 0.6 0.1 high 0.75 0.25 0.95 0.25 0.95 0.95 0.25 0.95 0.3 90%-130% low 0.5 0.1 0.6 0.08 0.6 0.6 0.08 0.6 0.1 high 0.99 0.6 0.95 0.25 0.95 0.95 0.25 0.95 0.3

(36) The third table below relates to standard sections of the third type that are symmetric and that minimize the negative lift F.sub.D of the tail boom, with an example of such a section being shown in FIG. 5:

(37) TABLE-US-00011 t/c value uA1, uA3 vA01 uA02 uA11 vA12 vA21 uA22 uA31 vA32 50% low 0.2 0.5 0.1 0.2 0.1 0.1 0.2 0.1 0.5 high 0.5 0.9 0.4 0.4 0.4 0.4 0.4 0.4 0.9 60% low 0.2 0.5 0.1 0.3 0.1 0.1 0.3 0.1 0.5 high 0.5 0.9 0.4 0.7 0.4 0.4 0.7 0.4 0.9 70% low 0.2 0.5 0.1 0.3 0.1 0.1 0.3 0.1 0.5 high 0.5 0.9 0.4 0.7 0.4 0.4 0.7 0.4 0.9 80% low 0.2 0.5 0.1 0.4 0.1 0.1 0.4 0.1 0.5 high 0.5 0.9 0.4 0.7 0.4 0.4 0.7 0.4 0.9 90%-130% low 0.2 0.5 0.1 0.7 0.1 0.1 0.7 0.1 0.5 high 0.5 0.9 0.4 0.95 0.4 0.4 0.95 0.4 0.9

(38) Finally, the fourth table below relates to standard sections of the fourth type that are asymmetric, and that minimize the negative lift F.sub.D of the tail boom:

(39) TABLE-US-00012 uA1, value uA3 vA01 uA02 uA11 vA12 vA21 uA22 uA31 vA32 low 0.2 0.1 0.55 0.1 0.65 0.65 0.1 0.65 0.1 high 0.5 0.3 0.85 0.4 0.9 0.9 0.25 0.9 0.3

(40) These values for this fourth type of standard sections are independent of the ratio t/c.

(41) Furthermore, the symmetric standard sections of the first and third types are independent of the direction of rotation of the main rotor 3. In contrast, the asymmetric standard sections of the second and fourth types differ depending on the direction of rotation of the main rotor 3. Thus, an asymmetric standard section defined for a main rotor 3 rotating counterclockwise in a plan view needs to be made symmetric relative to the abscissa axis U in order to define an equivalent asymmetric section for a main rotor 3 rotating in the clockwise direction.

(42) These tables of the database provide values for a main rotor 3 rotating clockwise in FIG. 1.

(43) The abscissa and ordinate coordinates not supplied in the tables are defined by the designer and may also be imposed by manufacturing or tangency constraints.

(44) In particular, as shown in FIG. 5, a first extreme characteristic point A0 is situated at the trailing edge 12 with coordinates (1,0). A second extreme characteristic point A2 is situated at the leading edge 11 with coordinates (0,0). Furthermore, the extreme characteristic points A1 and A3 forming the maximum thickness t of the standard section have respective coordinates t/(2.c) and t/(2.c). These extreme characteristic points A1 and A3 have the same abscissa coordinates.

(45) Furthermore, for the tail boom section shown in FIG. 5, the abscissa and ordinate coordinates that are not supplied by these tables are imposed by tangency constraints at each extreme characteristic point A0, A1, A2, A3 such that the standard section is tangent continuous. The ordinate coordinates of the intermediate characteristic points A02, A11 are equal to the ordinate coordinates of the extreme characteristic point A1 with which they are associated, whereas the ordinate coordinates of the intermediate characteristic points A22, A31 are equal to the ordinate coordinate of the extreme characteristic point A3 with which they are attached. Likewise, the abscissa coordinates of the intermediate characteristic points A01, A32 are equal to the abscissa coordinates of the extreme characteristic point A0, and the abscissa coordinates of the intermediate characteristic points A12, A21 are equal to the abscissa coordinate of the extreme characteristic point A2.

(46) Furthermore, in FIG. 5, it can be seen that for symmetric standard sections of the third type, the maximum thickness t is arranged upstream from the middle of the chord c in the flow direction of the air stream. Likewise, for asymmetric standard sections of the fourth type, the maximum thickness t is likewise arranged upstream from the middle of the chord c in the flow direction of the air stream.

(47) In contrast, for symmetric standard sections of the first type and for asymmetric standard sections of the second type, the maximum thickness t of the section of the tail boom 10 is arranged downstream from the middle of the chord c in the flow direction of the air stream from the main rotor 3. Furthermore, for asymmetric standard sections of the second type, the maximum thickness t of the section is arranged in the proximity of the middle of the chord c.

(48) Consequently, the method of optimizing the sections of a tail boom comprises a step 102 of establishing the aerodynamic and structural characteristics looked for in the tail boom 10 that define in particular the needs of the aircraft 1 in terms of lateral force F.sub.L and of negative lift F.sub.D as generated by the air stream from the main rotor 3 flowing over the tail boom 10, together with the general dimensions of the tail boom 10 and its relative thickness for certain sections.

(49) These looked-for aerodynamic characteristics of the tail boom 10 are defined mainly by the types of mission that the aircraft is to perform. Thus, if the aircraft 1 is to perform a large number of vertical climbing flights, the looked-for aerodynamic characteristics of the tail boom 10 are specifically minimizing the negative lift F.sub.D. In most other cases, the looked-for aerodynamic characteristics are for maximizing the lateral force F.sub.L.

(50) Furthermore, the structural characteristics of the tail boom 10 define dimensional ranges and mechanical strength structural ranges for the tail boom 10 respectively characterizing firstly connecting the tail boom 10 with the architecture of the aircraft 1, and secondly stiffness and mechanical strength of the tail boom 10.

(51) Finally, the method of optimizing sections for a tail boom includes a step 103 of defining sections of the tail boom 10. Sections are defined as a function of the standard sections in the database, of the looked-for aerodynamic and structural characteristics of the tail boom 10, and generally of the ratio t/c of each section.

(52) In addition, selecting between a tail boom 10 having sections that are symmetric or else asymmetric may depend on criteria that are structural. Such a tail boom having symmetric sections is simple to construct and serves to limit its weight, while presenting behavior that is symmetric relative to the air flow from the main rotor 3.

(53) In contrast, a tail boom 10 having sections that are asymmetric generally presents structural complexity and the aircraft 1 has overall behavior that is non-linear as a function of this asymmetry. Nevertheless, the aerodynamic improvements obtained as a result of standard sections from the database of the method of the invention and in particular as a result of an increased in the lateral force F.sub.L can make it possible to compensate for such structural complexity and its accompanying weight.

(54) Naturally, the present invention may be subjected to numerous variations as to its implementation. Although several embodiments are described, it can readily be understood that it is not conceivable to identify exhaustively all possible 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.