Portion of aircraft with reduced wave drag
The innovative wing-fuselage junction fairing design with a convex-concave configuration significantly reduces wave drag, improving aircraft performance and fuel efficiency by minimizing shock waves at transonic speeds.
Patent Information
- Application Number
- FR2021014031
- Authority / Receiving Office
- FR · FR
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2021-12-20
- Publication Date
- 2025-01-17
- Estimated Expiration
- 2038-02-14
AI Technical Summary
Subsonic aircraft experience increased drag due to shock waves forming at the wing-fuselage junction when operating near the speed of sound, which is not adequately addressed by existing junction fairings.
The wing-fuselage junction fairing is designed with a horizontal profile that includes a convex front section, a concave intermediate section, and a convex rear section, along with a specific configuration of convex and concave regions in the vertical profile to minimize shock wave formation.
This design reduces wave drag by up to 3% when operating at Mach numbers greater than MMO-0.025, enhancing aircraft performance and fuel efficiency.
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Abstract
Description
Title of the invention: Aircraft portion with reduced wave drag
[0001] The present invention relates to an aircraft portion, of the type comprising a fuselage oriented in a longitudinal direction and having a median longitudinal plane, a wing formed of at least one pair of wings arranged on either side of the fuselage in a transverse direction orthogonal to the median longitudinal plane, and a wing-fuselage junction fairing at the interface between the wing and the fuselage, said junction fairing having an external surface and being delimited in a vertical direction orthogonal to the longitudinal and transverse directions by an envelope surface such that, at each point of said envelope surface, there passes a transverse generatrix contained in said envelope surface and flush with the junction fairing, the projection of said envelope surface in the median longitudinal plane drawing a lower profile of the junction fairing,and the upper surface of each wing being in contact with said external surface of the junction fairing along a junction line whose orthogonal projection in a projection plane orthogonal to the median longitudinal plane draws a horizontal profile of said junction line.
[0002] Subsonic aircraft with low wings are known comprising a wing-fuselage junction fairing at the interface between their wing and their fuselage, said fairing covering the root by which the wing is connected to the fuselage. This junction fairing has an external surface whose shape is designed to promote the flow of air streams and thus avoid aerodynamic disturbances at the root. This fairing is generally also used to house certain systems of the aircraft such as pneumatic and / or hydraulic lines, as well as the landing gear.
[0003] The shape of the external surface of these junction fairings is usually designed so as to minimize the aerodynamic interactions of this fairing with the wing. This gives junction fairings of which:
[0004] - the external surface is in contact with the upper surface of each wing of the aircraft along a junction line whose horizontal profile is slightly convex, almost straight, and - the vertical profile of the lower surface of the junction fairing is also slightly convex, almost straight.
[0005] These known aircraft are not, however, entirely satisfactory. It is indeed common for these subsonic aircraft to be adapted to operate at speeds sufficiently close to the speed of sound so that the airflow around the wings of these aircraft enters a transonic regime, creating shock waves on the upper surface or even, in in some cases, on the lower surface, which increase the aircraft's drag by generating what is commonly called "wave drag".
[0006] An objective of the invention is to use the wing-fuselage junction fairing to reduce the wave drag of the aircraft.
[0007] To this end, the invention relates to an aircraft portion of the aforementioned type, in which the horizontal profile of at least one of the junction lines and / or the lower profile of the junction fairing successively presents, in the longitudinal direction, a convex front section, a concave intermediate section, and a convex rear section.
[0008] According to particular embodiments of the invention, the aircraft portion also has one or more of the following characteristics, taken in isolation or in any technically possible combination(s):
[0009] - the wing has a leading edge and a trailing edge, the intersection between the edge leading edge and the outer surface of the junction fairing defining a front reference point, the intersection between the trailing edge and the longitudinal plane passing through said front reference point defining a rear reference point, and the distance between the front and rear reference points constituting a reference length, and the front section of the horizontal profile comprises a front convexity apex, the intermediate section of the horizontal profile comprises an intermediate concavity bottom, and the rear section of the horizontal profile comprises a rear convexity apex, the tangent to the horizontal profile at each of said convexity apexes and concavity bottom being parallel to the longitudinal direction, the intermediate concavity bottom being set back towards the median longitudinal plane relative to each of the front and rear convexity apexes,
[0010] - the intermediate concavity bottom has a first primary longitudinal gap relative to the front convexity apex, said first primary longitudinal deviation being between 15 and 45% of the reference length, preferably between 25 and 35% of said reference length,
[0011] - the intermediate concavity bottom has a first secondary longitudinal gap relative to the rear convexity apex, said first secondary longitudinal deviation being between 35 and 65% of the reference length, preferably between 45 and 60% of said reference length,
[0012] - the intermediate concavity bottom has a primary transverse gap rela tively at the apex of the front convexity, said primary transverse deviation being between 0.5 and 2.5% of the reference length, preferably between 1.0 and 2.0% of the reference length,
[0013] - the intermediate concavity bottom has a secondary transverse gap rela tively at the apex of rear convexity, said secondary transverse gap being between 2.0 and 4.5% of the reference length, preferably between 2.5 and 4.0% of the reference length,
[0014] - the front convexity vertex is at a first primary longitudinal distance from the front reference point between 5 and 20% of the reference length,
[0015] - the intermediate concavity bottom is at a first longitudinal distance secondary to the front reference point between 35 and 50% of the reference length,
[0016] - the rear convexity vertex is at a first tertiary longitudinal distance from the front reference point between 85 and 100% of the reference length,
[0017] - the front convexity vertex is at a primary transverse distance from the point of front reference between 0.0 and 1.0% of the reference length,
[0018] - the intermediate concavity bottom is at a secondary transverse distance from the front reference point between 0.5 and 2.0% of the reference length,
[0019] - the rear convexity vertex is at a tertiary transverse distance from the point of front reference between 1.5 and 3.0% of the reference length, preferably between 2.0 and 2.5% of the reference length,
[0020] - each point of the junction line has a unique orthogonal projection, specific to said point, on the segment connecting the front and rear reference points,
[0021] - the junction line is at least class C2
[0022] - the front section of the lower profile comprises a front curvature apex and the lower profile has, at the interface between the intermediate region and the rear region, an inflection point, the tangent to the lower profile at the level of said front curvature apex and inflection point being parallel to the longitudinal direction, the inflection point being set back towards the fuselage relative to the front curvature apex,
[0023] - the inflection point has a second primary longitudinal deviation relatively at the apex of the front curvature, said second primary longitudinal deviation being between 40 and 55% of the reference length, preferably between 42 and 51% of said reference length;
[0024] - the inflection point has a vertical deviation relative to the vertex of curvature before, said vertical deviation being between 1.0 and 2.5% of the reference length, preferably between 1.4 and 2.2% of said reference length;
[0025] - the front curvature apex is at a second primary longitudinal distance from the front reference point between 35 and 45% of the reference length, preferably between 37 and 42% of said reference length; and
[0026] - the inflection point is at a second secondary longitudinal distance from the point reference length before between 80 and 90% of the reference length, preferably between 83 and 88% of said reference length.
[0027] Other characteristics and advantages of the invention will appear on reading the description which follows, given solely by way of example and made with reference to the attached drawings, in which:
[0028] - [Fig. 1] is a perspective view of an aircraft including a portion of aircraft according to the invention, - [Fig.2] is a top view of part of the aircraft of [Fig.l], and - [Fig.3] is a side view of part of the aircraft of [Fig.l], taken following a section plane marked III in [Fig.l].
[0029] The aircraft 10 of [Fig.l] is a low-wing aircraft. It comprises, in known manner, a fuselage 12, a wing 14, and a wing-fuselage junction fairing 18 at the interface between the wing 14 and the fuselage 12. The aircraft 10 also comprises a tailplane 20 and a powerplant 22, the latter here being formed of three turbojets 24.
[0030] In the following, the terms of reference are understood with reference to the usual orthogonal reference system for aircraft, which is shown in the Figures, and in which we distinguish:
[0031] - a longitudinal direction X oriented from the front to the rear of the aircraft, - a transverse direction Y oriented from the left to the right of the aircraft, said transverse direction Y forming with the longitudinal direction X a horizontal plane (X, Y), and - a vertical direction Z oriented from bottom to top, said vertical direction Z forming: • with the longitudinal direction X a longitudinal plane (X, Z), and • with the transverse direction Y a transverse plane (Y, Z).
[0032] The fuselage 12 is elongated in the longitudinal direction X. It is in particular cylindrical and is centered on a longitudinal axis (not shown) forming the axis of the fuselage 12.
[0033] The fuselage 12 has a median longitudinal plane M extending along the longitudinal X and vertical Z directions, said plane M forming in particular a plane of symmetry of the fuselage 12. This median longitudinal plane M also forms a median longitudinal plane of the aircraft 10, which is advantageously generally symmetrical with respect to this plane M.
[0034] The wing 14 is formed of a pair of wings 26 arranged on either side of the fuselage 12 in the transverse direction Y. These wings 26 are in particular arranged symmetrically with respect to each other relative to the median longitudinal plane M. Optionally, the wing 14 also comprises at least one other pair of wings (not shown).
[0035] Each wing 26 is mounted on the fuselage 12 at a root (not shown). This root is located in a lower half of the fuselage 12, in particular in a lower quarter of the fuselage 12.
[0036] Each wing 26 has a leading edge 30 and a trailing edge 32.
[0037] The leading edge 30 is formed by the front end line of the wing 26. In in other words, the leading edge 30 is constituted by a line each point of which constitutes a front end of the longitudinal section of the wing 26 passing through this point.
[0038] The trailing edge 32 is constituted by the rear end line of the wing 26. In other words, the trailing edge 32 is constituted by a line each point of which constitutes a rear end of the longitudinal section of the wing 26 passing through this point.
[0039] Each wing 26 also has a lower surface 34 and an upper surface 36. The lower surface 34 is formed by the outer surface of the wing 26 extending from the leading edge 30 to the trailing edge 32, below the wing 26. The upper surface 36 is formed by the outer surface of the wing 26 extending from the leading edge 30 to the trailing edge 32, above the wing 26.
[0040] The wing-fuselage junction fairing 18 comprises two lateral portions 40 each covering the root of a respective wing 26, and a central portion 42 connecting the lateral portions 34 to each other while extending under the fuselage 12.
[0041] The wing-fuselage junction fairing 18 has an external surface 44 oriented away from the fuselage 12. This external surface 44 extends continuously over the lateral portions 40 and over the central portion 42. It is at least class C2.
[0042] With reference to [Fig. 2], for each wing 26, the leading edge 30 of said wing 26 has an intersection with the external surface 44 of the junction fairing 18; this intersection defines a front reference point PL. Furthermore, the trailing edge 32 of the wing 26 has an intersection with the longitudinal plane passing through the first reference point PI; this intersection defines a rear reference point P2. The segment connecting the points PI and P2 constitutes a chord of the wing 26 to the root, and the distance from PI to P2 constitutes a chord length L ([Fig. 3]) of the wing 26 to the root. This chord length L is hereinafter referred to as the “reference length”.
[0043] Still with reference to [Fig.2], the extrados 36 of each wing 26 is in contact with the external surface 44 of the junction fairing 18 along a junction line 46. This junction may be sharp or defined by a fillet having a minimum radius of curvature at the level of said junction line 46. This junction line 46 is at least class C2.
[0044] Each point of the junction line 46 has a unique orthogonal projection, specific to said point, on the segment connecting the front and rear reference points PI, P2. There is thus a bijective application connecting the points of the junction line 46 to the segment connecting the front and rear reference points PI, P2.
[0045] The orthogonal projection of the junction line 46 in a horizontal projection plane draws a horizontal profile 48 of said junction line 46 successively presenting, in the longitudinal direction X, a front section 50, an intermediate section 52, and a rear section 54, each of said sections 50, 52, 54 being continuous.
[0046] These sections 50, 52, 54 are joined to each other; in other words, the horizontal profile 48 does not include a section interposed between two of said sections 50, 52, 54. Furthermore, the front section 50 defines a front end 56 of the horizontal profile 48, merged with the first reference point PI, and the rear section 52 defines a rear end 58 of the horizontal profile 48. Thus, the horizontal profile 48 is constituted by the sections 50, 52, 54.
[0047] According to the invention, the front section 50 is convex, the intermediate section 52 is concave, and the rear section 54 is convex. It will be noted that the terms “convex” and “concave” are understood here and hereinafter with respect to an observation from outside the aircraft 10; thus, a section described as “convex” extends opposite the median longitudinal plane M relative to the segment connecting the longitudinal ends of said section, and a section described as “concave” extends between the median longitudinal plane M and the segment connecting the longitudinal ends of said section. Furthermore, each of the sections 50, 52, 54 is devoid of any inflection point, except for the junction points between said sections 50, 52, 54.
[0048] To facilitate reading of the drawings, the concave and convex aspects of the sections 50, 52, 54 have been accentuated in [Fig.2].
[0049] The front section 50 comprises a front convexity vertex Al such that the tangent to the horizontal profile 48 at said vertex Al is parallel to the longitudinal direction X. Similarly, the intermediate section 52 comprises an intermediate concavity bottom A2 such that the tangent to the horizontal profile 48 at said bottom A2 is parallel to the longitudinal direction X, and the rear section 54 comprises a rear convexity vertex A3 such that the tangent to the horizontal profile 48 at said vertex A3 is parallel to the longitudinal direction X. The intermediate concavity bottom A2 is set back towards the median longitudinal plane M relative to each of the front and rear convexity vertices Al, A3, i.e. the intermediate concavity bottom A2 is closer to the median longitudinal plane M than each of the front and rear convexity vertices Al, A3.
[0050] In particular, the front convexity apex A1 is at a first primary longitudinal distance dxn from the front reference point PI of between 5 and 20% of the reference length L, the intermediate concavity bottom A2 is at a first secondary longitudinal distance dx[2 from the front reference point PI of between 35 and 50% of the reference length L, and the rear convexity apex A3 is at a first tertiary longitudinal distance dxn from the front reference point PI of between 85 and 100% of the reference length L.
[0051] Furthermore, the intermediate concavity bottom A2 has a first primary longitudinal deviation exn relative to the front convexity apex Al advantageously between 15 and 45% of the reference length, for example between 25 and 35% of the reference length L, and a first secondary longitudinal deviation exn relative to the rear convexity apex A3 advantageously between 35 and 65% of the reference length L, for example between 45 and 60% of the reference length L.
[0052] In addition, the intermediate concavity bottom A2 has a primary transverse deviation ey1 relative to the front convexity apex Al preferably comprised between 0.5 and 2.5% of the reference length L, for example comprised between 1.0 and 2.0% of the reference length L, and a secondary transverse deviation ey2 relative to the rear convexity apex A3 preferably comprised between 2.0 and 4.5% of the reference length L, for example comprised between 2.5 and 4.0% of the reference length L.
[0053] For this purpose, the front convexity apex A1 is typically placed at a primary transverse distance dyi from the front reference point PI of between 0.0 and 1.0% of the reference length L, projecting away from the median longitudinal plane M relative to said point PI, the intermediate concavity bottom A2 at a secondary transverse distance dy2 from the front reference point PI of between 0.5 and 2.0% of the reference length L, set back towards the median longitudinal plane M relative to said point PI, and the rear convexity apex A3 at a tertiary transverse distance dy3 from the front reference point PI of between 1.5 and 3.0% of the reference length L, projecting away from the median longitudinal plane M relative to said point PI.
[0054] Advantageously, the ratio between the deviations eyi and exn is between 0.035 and 0.060.
[0055] Advantageously again, the ratio between the deviations ey2 and exi2 is between 0.055 and 0.065.
[0056] According to a particularly preferred embodiment of the invention, the ratio (e>9<»n) / (exJ2X^ is between 1.15 and 1.45.
[0057] It has been found that this particular shape of the horizontal profile 48 of the joining lines 46 makes it possible to significantly reduce the force of the shock waves at the upper surface of the wing and, thus, the wave drag of the aircraft. In fact, thanks to this particular shape alone, a gain of 1 to 2% in the overall drag of the aircraft 10 is observed when it operates at a Mach number greater than or equal to MMO-0.05, MMO being the maximum Mach authorized in operation of the aircraft 10.
[0058] With reference to [Fig. 3], the junction fairing 18 is delimited in the vertical direction Z by an envelope surface 60 such that, at each point of said surface envelope 60, passes a transverse generatrix contained in said envelope surface 60 and flush with the junction fairing 18. The projection of this envelope surface 60 in the median longitudinal plane M draws a lower profile 62 of the junction fairing 18.
[0059] The lower profile 62 successively has, along the longitudinal direction X, a front region 64, an intermediate region 66, and a rear region 68, each of said regions 64, 66, 68 being continuous, the front region 64 having a front end B0 which is substantially vertically aligned with the front reference point PI and the rear region 68 having a rear end B3 which is substantially vertically aligned with the rear reference point P2.
[0060] These regions 64, 66, 68 are joined to each other; in other words, the lower profile 62 does not include a section inserted between two of said regions 64, 66, 68.
[0061] The portion of the lower profile 62 formed by the joining of the regions 64, 66, 68 has a longitudinal extension equal to the longitudinal extension of the chord of the wings 26 at the root.
[0062] In the example shown, the front region 64 is convex, the intermediate region 66 is concave, and the rear region 68 is convex. It will be noted that the terms “convex” and “concave” are always understood here with respect to an observation from outside the aircraft 10; thus, a region described as “convex” extends opposite the fuselage 12 relative to the segment connecting the longitudinal ends of said region, and a region described as “concave” extends between the fuselage 12 and the segment connecting the longitudinal ends of said region. Furthermore, each of the regions 64, 66, 68 is devoid of an inflection point, except for the junction points between said regions 64, 66, 68.
[0063] To facilitate reading of the drawings, the concave and convex aspects of the regions 64, 66, 68 have been accentuated in [Fig.3].
[0064] The front region 64 comprises a front curvature apex B1 such that the tangent to the lower profile 62 at said apex B1 is parallel to the longitudinal direction X. Furthermore, the lower profile 62 has, at the interface between the intermediate region 66 and the rear region 68, an inflection point B2 such that the tangent to the lower profile 62 at said inflection point B2 is substantially parallel to the longitudinal direction X. The inflection point B2 is set back towards the fuselage 12 relative to the front curvature apex B1, that is to say that the inflection point B2 is closer to the fuselage 12 than the front curvature apex B1.
[0065] In particular, the front curvature apex B1 is at a second primary longitudinal distance dx2i from the front reference point PI of between 35 and 45% of the reference length L, in particular of between 37 and 42% of said length L, and the inflection point B2 is at a second secondary longitudinal distance dx22 from the front reference point PI of between 80 and 90% of the reference length L, in particular of between 83 and 88% of said length L.
[0066] Furthermore, the inflection point B2 has a second primary longitudinal deviation ex2i relative to the front curvature apex B1 advantageously between 40 and 55% of the reference length L, for example between 42 and 51% of the reference length L.
[0067] In addition, the inflection point B2 has a vertical deviation ez relative to the front curvature peak B1 preferably between 1.0 and 2.5% of the reference length L, for example between 1.4 and 2.2% of the reference length L.
[0068] Advantageously, the ratio between the deviations ez and ex2[ is between 0.030 and 0.045, for example between 0.035 and 0.042.
[0069] It has been found that this particular shape of the vertical profile of the lower surface of the junction fairing 18 makes it possible to reduce the force of the shock waves at the lower surface of the wing and, thus, the wave drag of the aircraft. It is indeed observed that this alone allows a gain of approximately 1% of the overall drag of the aircraft 10 when it is operating at a Mach number greater than or equal to MMO-0.025, MMO being the maximum Mach authorized in operation of the aircraft 10. When it is combined with the particular shape of the horizontal profile of the junction line 46 described above, it is thus a gain of nearly 3% of the overall drag of the aircraft 10 that it is possible to achieve.
[0070] Thanks to the invention described above, the wave drag of the aircraft 10 is reduced, which allows a significant increase in the performance of the aircraft, in particular in terms of fuel consumption and range.
Claims
Claims
1. - Aircraft portion, comprising a fuselage (12) oriented along a longitudinal direction (X) and having a median longitudinal plane (M), a wing (14) formed of at least one pair of wings (26) arranged on either side of the fuselage (12) in a transverse direction (Y) orthogonal to the median longitudinal plane (M), and a wing-fuselage junction fairing (18) at the interface between the wing (14) and the fuselage (12), said junction fairing (18) having an external surface (44) and being delimited in a vertical direction (Z) orthogonal to the longitudinal and transverse directions (X, Y) by an envelope surface (60), the projection of said envelope surface (60) in the median longitudinal plane (M) drawing a lower profile (62) of the junction fairing (18),and the extrados (36) of each wing (26) being in contact with said external surface (44) of the junction fairing (18) along a junction line (46) whose orthogonal projection in a projection plane orthogonal to the median longitudinal plane (M) draws a horizontal profile (48) of said junction line (46), characterized in that the horizontal profile (48) of at least one of the junction lines (46) successively presents, in the longitudinal direction (X), a convex front section (50), a concave intermediate section (52), and a convex rear section (54), each wing (26) presenting a leading edge (30) and a trailing edge (32), the intersection between the leading edge (30) and the external surface (44) of the junction fairing (18) defining a front reference point (PI), the intersection between the trailing edge (32) and the longitudinal plane passing through said front reference point (PI) defining a rear reference point (P2),and the distance between the front and rear reference points (PI, P2) constituting a reference length (L), the convex front section (50) of the horizontal profile (48) of at least one of the junction lines (46) defining a front end (56) of the horizontal profile (48), merged with the first reference point (PI).,
2. - Aircraft portion according to claim 1, in which the section front (50) of the horizontal profile (48) comprises a front convexity vertex (Al), the intermediate section (52) of the horizontal profile (48) comprises an intermediate concavity bottom (A2), and the rear section (54) of the horizontal profile (48) comprises a rear convexity vertex (A3), the tangent to the horizontal profile (48) at each of said vertices of convexity and concavity bottom (Al, A2, A3) being parallel to the longitudinal direction (X), the intermediate concavity bottom (A2) being set back towards the median longitudinal plane (M) relative to each of the front and rear convexity vertices (Al, A3).
3. - Aircraft portion according to claim 2, in which the intermediate concavity bottom (A2) has a first primary longitudinal deviation (exn) relative to the front convexity apex (Al), said first primary longitudinal deviation (exn) being between 15 and 45% of the reference length (L), preferably between 25 and 35% of said reference length (L).
4. - Aircraft portion according to claim 2 or 3, in which the intermediate concavity bottom (A2) has a first secondary longitudinal deviation (exi2) relative to the rear convexity apex (A3), said first secondary longitudinal deviation (exi2) being between 35 and 65% of the reference length (L), preferably between 45 and 60% of said reference length (L).
5. - Aircraft portion according to any one of claims 2 to 4, in which the intermediate concavity bottom (A2) has a primary transverse deviation (eyi) relative to the front convexity apex (Al), said primary transverse deviation (eyO being between 0.5 and 2.5% of the reference length (L), preferably between 1.0 and 2.0% of the reference length (L).
6. - Aircraft portion according to any one of claims 2 to 5, in which the intermediate concavity bottom (A2) has a secondary transverse deviation (ey2) relative to the rear convexity apex (A3), said secondary transverse deviation (ey2) being between 2.0 and 4.5% of the reference length (L), preferably between 2.5 and 4.0% of the reference length (L).
7. - Aircraft portion according to claims 5 and 6, wherein said primary transverse deviation (eyj) is between 2.0 and 2.5% of the reference length (L), said secondary transverse deviation (ey2) being between 2.0 and 2.5% of the reference length (L).
8. - Aircraft portion according to any one of claims 2 to 7, in which the front convexity apex (Al) is at a first primary longitudinal distance (dxn) from the front reference point (PI) of between 5 and 20% of the reference length (L).
9. - Aircraft portion according to any one of claims 2 to 8, in the intermediate concavity bottom (A2) is at a first distance secondary longitudinal (dxi2) of the front reference point (P2) between 35 and 50% of the reference length (L).
10. - Aircraft portion according to any one of claims 2 to 9, in which the rear convexity apex (A3) is at a first tertiary longitudinal distance (dxn) from the front reference point (PI) of between 85 and 100% of the reference length (L).
11. - Aircraft portion according to any one of claims 2 to 10, in which the forward convexity apex (Al) is at a primary transverse distance (dyj from the forward reference point (PI) of between 0.0 and 1.0% of the reference length (L).
12. - Aircraft portion according to any one of claims 2 to 11, in which the intermediate concavity bottom (A2) is at a secondary transverse distance (dy2) from the front reference point (PI) of between 0.5 and 2.0% of the reference length (L).
13. - Aircraft portion according to any one of claims 2 to 12, wherein the rear convexity apex (A3) is at a tertiary transverse distance (dy3) from the front reference point (PI) of between 1.5 and 3.0% of the reference length (L), preferably of between 2.0 and 2.5% of the reference length (L).
14. - Aircraft portion according to any one of claims 2 to 13, wherein each point of the joining line (46) has a unique orthogonal projection, specific to said point, on the segment connecting the front and rear reference points (PI, P2).
15. - Aircraft portion according to any one of the preceding claims, in which the junction line (46) is at least of class C 2