Two-wing VTOL aircraft

Abstract

Two-wing VTOL aircraft A vertical takeoff and landing aircraft comprises a fuselage (3). The fuselage (3) has a front portion, shaped as a nose (5), and a rear portion, shaped as a tail (7), mutually opposite. The aircraft further comprises a first wing (11) and a second wing (13), attached to the fuselage (3) between the nose (5) and the tail (7). The second wing (13) is arranged behind the first wing (11). The first wing (11) is attached to the fuselage (3) at a first height, and extends, at least in part, generally in a first plane. The second wing (13) is attached to the fuselage (3) at a second height, greater than the first height, and extends, at least over a portion connected to the fuselage (3), generally in a second plane. The aircraft further comprises a first fin (17), arranged at the end of the first wing (11).The first fin (17) comprises a projecting portion (19) projecting from the first plane, opposite the second plane. Fig.2.

Description

Title of invention: Two-wing VTOL aircraft

[0001] The invention relates to the field of vertical take-off and landing aircraft, also referred to as VTOL in the art (from the English equivalent “Vertical Take-Off and Landing”). More particularly, the invention relates to a VTOL aircraft having at least two wings.

[0002] A VTOL aircraft generally comprises a propulsion system having a plurality of rotors, each rotor being configured to rotate about an axis of rotation relative to a respective stator. The rotors, when rotated, are capable of jointly producing an essentially vertical movement of the aircraft, in particular for the takeoff and landing phases thereof. A VTOL aircraft can take off from and land on a reduced ground infrastructure. This makes its use particularly suitable in highly constrained environments, such as cities for example, and makes it possible to reduce the footprint required for its operation.

[0003] A first configuration of VTOL aircraft is known, in which the rotation of the rotors alone provides the lift of the aircraft, not only in the vertical flight phases, but also in the forward flight phases. This is the case, for example, of the configuration of the aircraft known under the name "Volocity", from the company Volocopter. The aircraft is then generally without wings. However, the aircraft of this first configuration have a fairly low forward flight speed, a low energy autonomy and significant noise pollution.

[0004] This is why a second configuration is generally preferred, in which the aircraft has a fuselage provided with at least one wing. This wing produces most of the lift of the aircraft in forward flight, while in vertical flight, the lift remains mainly generated by the rotors.

[0005] An aircraft having this second configuration may comprise several wings, so as to improve its lift in forward flight. A category of VTOL aircraft is thus known comprising two main wings, arranged on the fuselage in a non-superimposed manner so as to form a front wing and a rear wing. This wing configuration is commonly referred to in the art as "tandem wings". A VTOL aircraft with tandem wings may further comprise a tailplane having a third wing, of reduced span.

[0006] In the following, we are interested in VTOL type aircraft with tandem wings for which the front wing and the rear wing are attached to the fuselage at different heights, the rear wing being higher than the front wing. An aircraft of this type generally has good lift in forward flight. However, when this When an aircraft has an angle of incidence exceeding a certain value, typically 15°, it is known that this lift deteriorates.

[0007] Indeed, at a high angle of incidence, turbulence is likely to appear in the wake of the front wing and affect the airflow over the rear wing. This turbulence includes, for example, vortices detaching from the leading edge, the trailing edge and the tips of the front wing. Such turbulence has the effect of causing an increase in the angle of incidence of the rear wing, this increase being able to reach several degrees when the cores of the vortices detaching from the tips of the front wing are positioned at the level of the rear wing. A phenomenon of stalling of the rear wing is then observed, that is to say that the lift of this wing decreases, which leads, more generally, to a reduction in the lift of the entire aircraft. In addition, this stall is likely to cause sudden pitching movements, and therefore to harm the longitudinal stability of the aircraft.

[0008] More particularly, the vortices detaching from the front wing have the effect of increasing the incidence on the external surface of the rear wing (a phenomenon known in the art under the English term "upwash") while reducing the incidence on the internal surface of the latter (a phenomenon known in the art under the English term "downwash"). This effect generates additional forces on the structure of the rear wing. To remedy this, the structure of the rear wing is generally reinforced, which implies an increase in its mass.

[0009] For non-VTOL tandem wing aircraft, i.e. aircraft with a front wing and rear wing without rotors, these drawbacks are generally avoided by providing a suitable front wing pitch angle. The pitch angle corresponds to the angle formed by the front wing relative to the fuselage. This angle is determined so that the front wing always stalls before the rear wing (an effect known in the art as the "nose-down effect"), thus preventing the phenomena described above from being observed. A similar solution is found on non-VTOL "canard" type aircraft, i.e. aircraft with a small wing at the front and a large wing at the rear, as well as on "three-surface" type aircraft (such as the model known as the "Piaggio PI80 Avanti", from the company Piaggio Aero).

[0010] This solution is however not transposable to a tandem wing aircraft of the VTOL type, whose front wing profile may have the natural characteristics of stalling well beyond the stall incidences of the rear wing, typically 15°.

[0011] The state of the art includes VTOL aircraft models with tandem wings whose construction arrangements aim to reduce interactions between the front wing and the rear wing which could impair lift in forward flight.

[0012] For example, a first model, known under the name "Lilium Jet", from the company Lilium, comprises wings whose ends are curved on the upper surface, that is to say upwards, so as to form fins. These fins are commonly called "winglets" in the art. The winglets thus formed reduce the appearance of vortices, in particular at the ends of the front wing. However, the phenomenon of increased incidence described above, which results in a stall of the rear wing, continues to occur beyond a certain angle of incidence value, typically 15°.

[0013] A second model, known as "Plana", from the company Plana Aero, includes counter-rotating rotors at the ends of its wings. These rotors, by rotating, oppose the rotation of the vortices at the end of the wing, so as to prevent an increase in the incidence of the rear wing. However, such a solution is not satisfactory because it considerably increases the weight, size and structural complexity of the aircraft.

[0014] On a third model, known as "Odys Aero", from the company Odys Aviation, the ends of the front and rear wings are connected together so as to form a ring wing, also called a "box wing" in the art. This ring wing shape tends to eliminate the appearance of vortices at the ends of the front wing. However, such a solution is not satisfactory because it imposes strong constraints on the design of the structure of the aircraft, in particular with regard to the size and orientation of its wings.

[0015] In this context, the Applicant sought to improve the situation.

[0016] A vertical takeoff and landing aircraft is proposed, comprising a fuselage. The fuselage has a front portion, shaped like a nose, and a rear portion, shaped like a tail, which are mutually opposed. The aircraft further comprises a first wing and a second wing, attached to the fuselage between the nose and the tail. The second wing is arranged behind the first wing. The first wing is attached to the fuselage at a first height, and extends, at least in part, generally in a first plane. The second wing is attached to the fuselage at a second height, greater than the first height, and extends, at least over a portion connected to the fuselage, generally in a second plane. The aircraft further comprises a first fin, arranged at the end of the first wing. The first fin comprises a projecting portion projecting from the first plane, opposite the second plane.

[0017] The proposed aircraft has at the end of its first wing, or front wing, a particular fin shape. This shape has the effect of reducing the influence of the wake of the front wing on the airflow around the second wing, or rear wing. In particular, the level of the core of the vortex forming at the end of the front wing is lowered, which limits its effect on the rear wing and therefore the appearance of a phenomenon of increased incidence of this wing.

[0018] The proposed fin shape is particularly advantageous because it is capable of improving the lateral stability of the aircraft, by forming a drift surface for this aircraft.

[0019] Optional features of the invention, complementary or substitutable, are set out below: - the projecting portion of the first fin comprises an end portion, generally extending in an inclination plane, this inclination plane forming with the first plane an angle of between 60° and 90°; - the projecting portion of the first fin has, according to a profile, a length of between 50 and 100% of an average aerodynamic chord value of the first wing; - the second wing comprises a portion of sail, partly arranged behind the projecting portion of the first fin; - the wing portion generally extends into the second plane; - the aircraft further comprises a second fin, arranged at the end of the second wing, this second fin having a projecting portion projecting from the second plane generally in the same direction as the projecting portion of the first fin; - the projecting portion of the second fin comprises an end portion, generally extending in an inclination plane, this inclination plane forming with the second plane an angle of between 60° and 90°; - the projecting portion of the second fin has, according to a profile, a length of between 50 and 100% of an average aerodynamic chord value of the second wing; - the aircraft further comprises a through duct, capable of housing a rotor, this through duct being arranged close to the projecting portion of the first fin; - the aircraft further comprises a tail unit, arranged on the tail, the tail unit being shaped, at least in part, into a third wing, which generally extends in a plane, substantially parallel to the first plane.

[0020] Other characteristics and advantages of the invention will appear more clearly on reading the following description, taken from examples given for illustrative and non-limiting purposes, taken from the drawings in which: - [Fig.l] represents a left part of an aircraft according to the invention, in front view; - [Fig.2] represents a part of the aircraft of [Fig.l], in isometric perspective; - [Fig.3] represents a left part of the front wing of the aircraft of [Fig.l], in front view; - [Fig.4] represents the left part of the front wing of the aircraft of [Fig.l], seen from above; - [Fig.5] represents the left part of the front wing of the aircraft of [Fig.l], in side view; - [Fig.6] represents a pressure field on the leading edge of the rear wing, for a state-of-the-art aircraft in the forward flight phase; - [Fig.7] represents a pressure field on the leading edge of the rear wing, for an aircraft according to the invention in the forward flight phase; - [Fig.8] represents an evolution of the lift coefficient of the rear wing as a function of the angle of incidence, for an aircraft of the state of the art and for an aircraft according to the invention; - [Fig.9] represents an evolution of the pitching moment coefficient as a function of the angle of incidence, for an aircraft of the state of the art and for an aircraft according to the invention; - [Fig. 10] represents an evolution of the roll moment coefficient as a function of the sideslip angle, at a given angle of incidence, for several embodiments of an aircraft according to the invention; - [Fig. 11] represents an average speed field of an air flow on the left part of the front wing, for an aircraft according to the invention in vertical flight phase; - [Fig. 12] represents an average speed field and streamlines of an air flow on the left part of the front wing, for an aircraft according to the invention in vertical flight phase.

[0021] The drawings and the description below contain, for the most part, elements of a certain character. They may therefore not only serve to better understand the present invention, but also contribute to its definition, if necessary.

[0022] Reference is made to Figures 1 and 2.

[0023] These figures represent a vertical takeoff and landing aircraft, or VTOL aircraft, according to one embodiment of the invention. This aircraft is designated by the reference numeral 1 in the remainder of the description.

[0024] The aircraft 1 comprises a fuselage 3, which generally extends along a first axis, called the longitudinal axis of the aircraft 1. The aircraft 1 is further characterized by a second axis, or lateral axis, perpendicular to its longitudinal axis. This lateral axis intersects the longitudinal axis at a point corresponding to the center of gravity of the aircraft 1. The aircraft 1 is further characterized by a third axis, perpendicular to its longitudinal axis and to its lateral axis. This third axis passes through the center of gravity of the aircraft 1. Here, this third axis generally extends vertically.

[0025] The aircraft 1 is generally symmetrical, along a plane comprising its longitudinal axis and its third axis. This plane is called the longitudinal plane or plane of symmetry of the aircraft 1 in the remainder of the description. The transverse plane of the aircraft 1 is called the plane comprising the longitudinal axis and the lateral axis of the aircraft 1. Here, this transverse plane generally extends horizontally.

[0026] The angle alpha, or angle of incidence of the aircraft 1, is called the angle formed between the longitudinal axis of the aircraft 1 and a projection of the relative wind in the plane of symmetry of the aircraft 1.

[0027] The axis resulting from a rotation of the longitudinal axis of the aircraft 1 in the plane of symmetry is called axis XI, this rotation having as its center the center of gravity of the aircraft 1 and as its angle value the angle of incidence of the aircraft 1. The angle beta, or sideslip angle of the aircraft 1, is called the angle formed between this axis XI and the direction of the relative wind.

[0028] The fuselage 3 has a front part, shaped like a nose 5, and a rear part, shaped like a tail 7, mutually opposite.

[0029] The aircraft 1 comprises a first wing, or front wing 11, attached to the fuselage 3 between the nose 5 and the tail 7. The front wing 11 has a generally elongated shape.

[0030] The front wing 11 generally extends parallel to the lateral axis of the aircraft 1. The front wing 11 extends mainly along one axis, called the longitudinal axis of the front wing 11. Here, the front wing 11 is arranged close to the nose 5.

[0031] The aircraft 1 further comprises a second wing, or rear wing 13, attached to the fuselage 3 between the nose 5 and the tail 7. The rear wing 13 is attached to the fuselage 3 behind the front wing 11. The front wing 11 and the rear wing 13 are offset along the longitudinal axis of the aircraft 1. The front wing 11 and the rear wing 13 are attached to the fuselage 3 in a non-superimposed manner. The aircraft 1 has a so-called tandem wing configuration.

[0032] The front wing 11 and the rear wing 13 produce most of the lift of the aircraft 1 in the forward flight phase.

[0033] The rear wing 13 has a generally elongated shape. The rear wing 13 generally extends parallel to the lateral axis of the aircraft 1. The rear wing 13 extends mainly along one axis, called the longitudinal axis of the rear wing 13. The longitudinal axis of the rear wing 13 and the longitudinal axis of the front wing 11 are parallel to each other. Here, the rear wing 13 is arranged close to the tail 7.

[0034] The front wing 11 and the rear wing 13 are attached to the fuselage 3 at different heights with reference to the third axis of the aircraft 1. The front wing 11 forms a lower wing for the aircraft 1, while the rear wing 13 forms an upper wing for it.

[0035] The forward dihedral angle is the angle formed between the front wing 11 and the transverse plane of the aircraft 1. Here, the forward dihedral angle is substantially equal to 0°. Here, the front wing 11 extends generally parallel to the transverse plane of the aircraft 1. The front wing 11 generally extends in a first plane, or front wing plane. The front wing plane comprises the longitudinal axis of the front wing 11. The rear wing 13 is arranged above the front wing plane.

[0036] The angle formed between the rear wing 13 and the transverse plane of the aircraft 1 is called the rear dihedral angle. Here, the rear dihedral angle is substantially equal to 0°. Here, the rear wing 13 extends generally parallel to the transverse plane of the aircraft 1. The rear wing 13 generally extends in a second plane, or rear wing plane. The rear wing plane is distinct from the front wing plane. Here, the front wing plane and the rear wing plane are parallel to each other. Here, the rear wing plane extends above the front wing plane. The front wing 11 is arranged below the rear wing plane.

[0037] The span of a wing is the length of this wing along the lateral axis of the aircraft 1. Here, the span of the front wing 11 corresponds to its length along its longitudinal axis. Here, the span of the rear wing 13 corresponds to its length along its longitudinal axis. The rear wing 13 has a span substantially equal to or greater than the span of the front wing 11. Here, the rear wing 13 has a span greater than the span of the front wing 11.

[0038] The aircraft 1 may further comprise, as here, a tail unit 9, arranged on the tail 7 of the fuselage 3. Here, the tail unit 9 has a general T-shaped shape. The tail unit 9 comprises a fin portion 91, which extends from the fuselage 3, here in the third direction of the aircraft 1. The tail unit 9 further comprises a wing portion 93, which extends at one end of the fin portion 91 opposite the fuselage 3. The tail unit 9 may further comprise, as here, a nacelle 95, suitable for covering a horizontal rear engine of the aircraft 1.

[0039] The wing portion 93 has a generally elongated shape. The wing portion 93 generally extends parallel to the lateral axis of the aircraft 1. The wing portion 93 extends mainly along one axis, called the longitudinal axis of the wing portion 93.

[0040] The tailplane dihedral angle is the angle formed between the wing portion 93 and the transverse plane of the aircraft 1. Here, the tailplane dihedral angle is substantially equal to 0°. Here, the wing portion 93 extends generally parallel to the transverse plane of the aircraft 1. The wing portion 93 generally extends in a third plane, or tailplane wing plane. The tailplane wing plane comprises the longitudinal axis of the wing portion 93. The tailplane wing plane is distinct from the front wing plane and the rear wing plane. Here, the tailplane wing plane is parallel to the front wing plane and the rear wing plane. Here, the empennage wing plane extends above the front wing plane and the rear wing plane.

[0041] The aircraft 1 further comprises, at one end of its front wing 11, a first fin, or front winglet 17. Here, the aircraft 1 comprises a pair of front winglets 17, each arranged at one of the ends of its front wing 11. The front winglets 17 each have a projecting portion 19, which projects from the front wing plane opposite the rear wing plane. The projecting portions 19 of the front winglets 17 generally extend downwards. Here, the projecting portions 19 of the front winglets 17 project from the front wing plane.

[0042] The aircraft 1 may further comprise, as here, a second fin, or rear winglet 21, at one end of its rear wing 13. Here, the aircraft 1 comprises a pair of rear winglets 21, each arranged at one of the ends of its rear wing 13. The rear winglets 21 each have a projecting portion 23, which projects from the rear wing 13 generally in the same direction as the projecting portions 19 of the front winglets 17. Here, the projecting portions 23 of the rear winglets 21 project from the rear wing plane. The projecting portions 23 of the rear winglets 21 generally extend downwards. Here, the projecting portions 23 of the rear winglets 21 generally extend towards the front wing plane.

[0043] The aircraft 1 further comprises a plurality of rotors 15, here eight rotors 15, distributed over its front wing 11 and its rear wing 13. The rotors 15 are configured so as to rotate about axes substantially parallel to the third axis of the aircraft 1. The rotors 15 produce most of the lift of the aircraft 1 in the vertical flight phase. The rotors 15 are arranged so as to jointly produce an essentially vertical movement of the aircraft 1. Here, four of the rotors 15 are arranged on the front wing 11 while the other four rotors 15 are arranged on the rear wing 13.

[0044] Reference is made to Figures 1 to 5.

[0045] The front wing 11 is described below. Figures 3 to 5 represent the left part of this wing.

[0046] The front wing 11 has a fairing, which comprises a front part, shaped as a leading edge 35, and a rear part, shaped as a trailing edge 37, mutually opposite. Here, the leading edge 35 and the trailing edge 37 extend generally parallel to each other.

[0047] The forward sweep angle is the angle formed between the leading edge 35 of the front wing 11 and the lateral axis of the aircraft 1. Here, the forward sweep angle is slightly positive, for example of the order of 5°.

[0048] The fairing of the front wing 11 further comprises an upper surface, shaped as an extrados 31, and a lower surface, shaped as an intrados 33, mutually opposed. The extrados 31 and the intrados 33 each connect the leading edge 35 to the trailing edge 37.

[0049] Here, the fairing of the front wing 11 has four through ducts 25, each connecting the lower surface 33 to the upper surface 31. Each of these through ducts 25 is arranged so as to house one of the rotors 15, at least in part. These through ducts 25 are arranged in pairs on either side of the fuselage 3.

[0050] Here, each of the front winglets 17 is arranged close to one of the through ducts 25 of the front wing 11.

[0051] The front winglet 17 of the left part of the front wing 11 is described below.

[0052] The projecting portion 19 of the front winglet 17 has an outer surface 20 and an inner surface 16, which are mutually opposite. The inner surface 16 is oriented towards the fuselage 3. The outer surface 20 connects to the upper surface 31 of the left part of the front wing 11. The inner surface 16 connects to the lower surface 33 of the left part of the front wing 11.

[0053] Here, the front winglet 17 and the left part of the front wing 11 are formed as a single piece. Here, the projecting portion 19 of the front winglet 17 corresponds to a generally curved portion of fairing at the end of the front wing 11. This portion of fairing is curved towards the lower surface 33 of the fairing of the front wing 11. As a replacement, the front winglet 17 could be made as a separate piece, shaped so as to be attached to the front wing 11.

[0054] The projecting portion 19 of the front winglet 17 has an end portion 18, which generally extends along a plane. This plane is called the inclination plane of the front winglet 17 in the remainder of the description. The inclination plane of the front winglet 17 forms with the front wing plane a gamma angle, or angle of curvature, for the front winglet 17 (shown in [Fig. 3]). Here, this angle of curvature corresponds to the angle at which the front wing tip fairing 11 is curved to form the projecting portion 19 of the front winglet 17. Here, the angle of curvature of the front winglet 17 is substantially 90°. Here, the slope plane of the front winglet 17 extends substantially parallel to the third axis of the aircraft 1. Alternatively, the curvature angle of the front winglet 17 could be between 60° and 90°. These curvature angle values ​​ensure that the vortex detaching from the front wingtip moves sufficiently away from the rear wing.The higher the curvature angle value, the greater the distance.

[0055] Furthermore, the shape of the front winglet 17 is characterized by two other angles, the phi angle (shown in [Fig.4]) and the psi angle (shown in [Fig.5]).

[0056] The end portion 18 has a front edge 181. The angle phi corresponds to the angle formed between a projection of this front edge 181 in the front wing plane and a projection of the leading edge 35 in the front wing plane. The angle phi can be between 30° and 120°. Here, the angle phi is approximately 75°.

[0057] The axis X2 is called the axis connecting, on the one hand, the most distal point of the front edge 181 of the end portion 18 and, on the other hand, the connection point between the leading edge 35 of the front wing 11 and the fuselage 3. The angle psi corresponds to the angle formed between the longitudinal axis of the aircraft 1 and a projection of the axis X2 in the plane of symmetry of the aircraft 1. The angle psi can be between 0° and 90°. Here, the angle psi is substantially 25°.

[0058] The values ​​given for the angles gamma, phi and psi make it possible to ensure a good distribution of the lift on the lower surface 33 of the front wing 11. These values ​​make it possible to avoid the appearance of excessively strong aerodynamic gradients, likely to generate a separation of the air layer around the front wing 11, and therefore to generate turbulence and drag.

[0059] The aerodynamic mean chord, or MMC, is the chord of a rectangular wing equivalent in area, lift and aerodynamic moments to a real wing.

[0060] The transverse plane of the front wing 11 is a plane orthogonal to the front wing plane and comprising the longitudinal axis of this wing. The front wing 11 has, according to a section in this transverse plane, a shape profile. The projecting portion 19 of the front winglet 17 may have, on this profile, a length of between 50 and 100% of the aerodynamic mean chord value of the front wing 11. Here, on this profile, the projecting portion 19 has a length of the order of 1 m. As a replacement, this length could be between 0.6 m and 1.7 m.

[0061] This conformation of the front winglet 17 has the effect that the vortex detaching at the end of the front wing 11 moves as far away as possible from the rear wing 13, so that the latter only stalls at very high angle of incidence values. Furthermore, this conformation has the effect that the reduction in lift is progressive on the projecting portion 19, until it is cancelled out on the end part 18. This conformation also has the effect that the vortex detaches from the end part 18 at a single point.

[0062] These characteristics are found on the front winglet of the right part of the front wing 11, in particular in a symmetrical manner with respect to the plane of symmetry of the aircraft 1.

[0063] The rear wing 13 is described below.

[0064] The rear wing 13 has a fairing, which comprises a front part, shaped as a leading edge 45, and a rear part, shaped as a trailing edge 47, mutually opposite.

[0065] The angle formed between the leading edge 45 of the rear wing 13 and the lateral axis of the aircraft 1 is called the rear sweep angle. Here, the rear wing 13 is straight, that is to say that the rear sweep angle is substantially equal to 0°.

[0066] The fairing of the rear wing 13 further comprises an upper surface, shaped as an extrados 41, and a lower surface, shaped as a intrados 43, mutually opposite. The extrados 41 and the intrados 43 each connect the leading edge 45 to the trailing edge 47.

[0067] Here, the fairing of the rear wing 13 has four through ducts 25, each connecting the lower surface 43 to the upper surface 4L. Each of these through ducts 25 is arranged so as to house one of the rotors 15, at least in part. These through ducts 25 are arranged in pairs on either side of the fuselage 3, behind the through ducts 25 arranged on the front wing 11.

[0068] Here, the fairing of the rear wing 13 has a pair of wing portions 27, without a through duct. Each wing portion 27 is arranged between one of the through ducts 25 and one of the rear winglets 21. These wing portions 27 make it possible to improve the lift of the aircraft 1 in forward flight. The wing portions 27 are partly arranged behind the front winglets 17.

[0069] Here, the leading edge 45 extends generally parallel to the longitudinal axis of the rear wing 13. Here, the trailing edge 47 extends generally parallel to the leading edge 45 behind the through ducts 25 and on the wing portions 27. Here, the trailing edge 47 is closer to the leading edge 45 on the wing portions 27 than behind the through ducts 25.

[0070] The rear winglet 21 of the left part of the rear wing 13 is described below.

[0071] The projecting portion 23 of the rear winglet 21 has an outer surface 24 and an inner surface 26, which are mutually opposite. The inner surface 26 is oriented towards the fuselage 3. The outer surface 24 connects to the upper surface 41 of the left part of the rear wing 13. The inner surface 26 connects to the lower surface 43 of the left part of the rear wing 13.

[0072] Here, the rear winglet 21 and the left part of the rear wing 13 are formed as a single piece. Here, the projecting portion 23 of the rear winglet 21 corresponds to a generally curved portion of fairing at the end of the rear wing 13. This portion of fairing is curved towards the lower surface 43 of the fairing of the rear wing 13. As a replacement, the rear winglet 21 could be made as a separate piece, shaped so as to be attached to the rear wing 13.

[0073] In a manner similar to that described for the front winglet 17, the projecting portion 23 of the rear winglet 21 has an end portion 22, which generally extends along a plane. This plane is called the inclination plane of the rear winglet 21 in the remainder of the description. The inclination plane of the rear winglet 21 forms with the rear wing plane a gamma angle, or angle of curvature, for the rear winglet 21. Here, this angle of curvature corresponds to the angle at which the rear wing tip fairing 13 is curved to form the projecting portion 23 of the rear winglet 21. Here, the angle of curvature of the rear winglet 21 is substantially 90°. Here, the plane of inclination of the rear winglet 21 extends substantially parallel to the third axis of the aircraft 1. Alternatively, the angle of curvature of the front winglet 17 could be between 60° and 90°.

[0074] The transverse plane of the rear wing 13 is a plane orthogonal to the rear wing plane and comprising the longitudinal axis of this wing. The rear wing 13 has, according to a section in this transverse plane, a shape profile. The projecting portion 23 of the rear winglet 21 may have, on this profile, a length of between 50 and 100% of the aerodynamic mean chord value of the rear wing 13. Here, on this profile, the projecting portion 23 has a length of the order of 1.3 m. Alternatively, this length could be between 0.8 m and 1.7 m. Here, on this profile, the projecting portion 23 has a length greater than that of the projecting portion 19 of the front winglet 17 on the section in the transverse plane of the front wing 11.

[0075] This conformation of the rear winglet 21 has the effect of ensuring good lateral stability of the aircraft 1 while allowing effective roll control.

[0076] These characteristics are found on the rear winglet of the right part of the rear wing 13, in particular in a symmetrical manner with respect to the plane of symmetry of the aircraft 1.

[0077] Reference is made to Figures 6 and 7.

[0078] [Fig. 6] represents the left part of a VTOL aircraft with tandem wings according to the state of the art. More particularly, this figure represents a pressure field on the leading edge of a left part of the rear wing of this aircraft, in the forward flight phase. Elements similar to those described in relation to Figures 1 to 5 bear the same reference numbers, increased by one hundred.

[0079] The aircraft 101 is essentially distinguished from the aircraft described in relation to FIGS. 1 and 2 in that its front wing 111 is devoid of a front winglet, and in that its rear wing 113 is devoid of a rear winglet. Furthermore, its rear wing 113 is distinguished from the rear wing described in relation to FIGS. 1 and 2 in that it has a positive rear dihedral angle on its wing portions 127. For example, this rear dihedral angle is of the order of 5°.

[0080] [Fig.6] represents a pressure field 150 on the leading edge 145 of the rear wing 113, for an angle of incidence of 15°. The pressure field 150 corresponds to a set of total pressure coefficient values. The total pressure coefficient, or CpO coefficient, includes the dynamic pressure, due to the speed of the aircraft 1, and the static pressure. The CpO coefficient varies from 0 to 1. A A CpO coefficient of 1 is interpreted as characterizing a flow with the same energy as a free flow. A CpO coefficient of 0 is interpreted as characterizing a flow whose energy is dissipated, which contributes to an increase in drag. In Figures 6 and 7 as filed (in color), a CpO coefficient of value 0 is shown in red, while a CpO coefficient of value 1 is shown in white.

[0081] The distribution of the CpO coefficient values ​​makes it possible to visualize the variations in energy of the flow which will impact the leading edge 145 of the rear wing 113, when this rear wing 113 is in the wake of the front wing 111 in the forward flight phase.

[0082] [Fig.6] makes it possible to visualize the influence of the turbulence appearing in the wake of the front wing 111 and affecting the air flow over the rear wing 113 at a high angle of incidence, i.e. greater than 12°. These turbulences comprise vortices detaching from the leading edge 135, the trailing edge and the ends 118 of the front wing 111. The variations observed in the pressure field 150 are characteristic of the influence of these vortices on the airflow around the rear wing 113. In particular, the pressure field 150 has a low intensity zone 151, which corresponds to the core of the vortex forming at the left end 118 of the front wing 111. This low intensity zone 151 is located at a height close to the leading edge 145 of the rear wing 113, on the lower surface 143 side thereof.The pressure field 150 further has four medium intensity zones 152, which extend essentially between the front wing plane and the rear wing plane. One of these medium intensity zones 152 extends above the upper surface 141 of the rear wing 113.

[0083] The pressure distributions illustrated on the pressure field 150 have the effect of causing an increase in the incidence of the rear wing 113, which can reach several degrees. The lift of the rear wing 113 decreases. The rear wing 113 is stalled. The aircraft 101 experiences both a loss of lift and a loss of longitudinal stability, due to a nose-up pitching movement.

[0084] [Fig. 7] represents the left part of a tandem-wing VTOL aircraft according to the invention. More particularly, this figure represents a pressure field on the leading edge of a left part of the rear wing of this aircraft, in the forward flight phase. Elements similar to those described in relation to Figures 1 to 5 bear the same reference numbers.

[0085] The aircraft 1 is essentially distinguished from the aircraft described in relation to Figures 1 to 5 by the shape of its front winglets 17. Here, the front winglets 17 have a curvature angle of substantially 60°. In addition, its rear wing 13 is distinguished from the rear wing described in relation to Figures 1 and 2 in that it has an angle positive rear dihedral on its wing portions 27. For example, this rear dihedral angle is of the order of 5°.

[0086] Similarly to [Fig.6], [Fig.7] represents a pressure field 50 on the leading edge 45 of the rear wing 13, for an angle of incidence of 15°. The pressure field 50 makes it possible to visualize the distribution of the coefficient CpO on the leading edge 45 of the rear wing 13, when this rear wing 13 is in the wake of the front wing 11 in the forward flight phase.

[0087] The pressure field 50 has a low intensity zone 51, corresponding to the core of the vortex detaching from the left end of the front wing 11. Here, this vortex detaches from the end portion 18 of the left front winglet 17. This low intensity zone 51 is at a height close to that of the front wing plane. The pressure field 50 also has two medium intensity zones 52, which extend between the front wing plane and the rear wing plane, at a distance from the leading edge 45 of the rear wing 13. None of these medium intensity zones 52 extends above the upper surface 41 of the rear wing 13.

[0088] Thanks to the invention, at equal angle of incidence, the cores of the vortices generated in the wake of the front wing 11, and materialized by the low intensity zone 51 and the medium intensity zones 52 of the pressure field 50, are generally lowered. The rear wing 13 is no longer arranged directly in the wake of the front wing 11. In particular, the cores of the vortices detaching from the ends of the front wing 11 are located at a distance from the rear wing 13. The influence of the wake of the front wing 11 on the airflow around the rear wing 13 is reduced. No increase in the incidence or stall of the rear wing 13 is observed.

[0089] More generally, the effects of the turbulence of the front wing wake on the airflow around the rear wing are delayed: these effects only appear for very high angle of incidence values, typically above 15°. Such angle of incidence values ​​are, in practice, generally not observed during forward flight phases.

[0090] Reference is made to [Fig.8].

[0091] This figure illustrates the effects of the invention on the evolution of the lift coefficient of the rear wing, as a function of the angle of incidence. A graph Gl, in dashed line, represents this evolution for an aircraft of the state of the art, that is to say without winglets according to the invention. For example, this aircraft of the state of the art is analogous to that described in relation to [Fig.6]. A graph G2, in solid line, represents the evolution of the lift coefficient for an aircraft according to the invention. For example, this aircraft according to the invention is analogous to that described in relation to [Fig.7].

[0092] Graph G1 shows that, between a value AO and a value Al of angle of incidence, the lift coefficient of the rear wing increases regularly, from a value CZO to a value CZ1. Beyond that, between the value Al and a value A2 of angle of incidence, this lift coefficient decreases, until reaching a value CZ2. This decrease corresponds to a stall of the rear wing, as described, for example, in relation to [Fig.6].

[0093] Graph G2 shows that, between 0° and the angle of incidence value A0, the lift coefficient of the rear wing increases, from the value CZ3 to the value CZO. Between the value A0 and the angle of incidence value Al, the lift coefficient of the rear wing continues to increase regularly, from the value CZO to a value CZ4, significantly lower than CZ1. Beyond that, between the value Al and the angle of incidence value A2, this lift coefficient continues to increase, until it reaches a value CZ5, higher than CZ1.

[0094] The value A0 is between 0 and 8°. Here, A0 is of the order of 3°. The value Al is between 8 and 16°. Here, Al is of the order of 12°. The value A2 is between 11° and 19°. Here, A2 is of the order of 15°. The values ​​CZO, CZ1, CZ2, CZ3, CZ4 and CZ5 are here equal to 0.2539, 0.6358, 0.6101, 0.1182, 0.6158 and 0.6492, respectively.

[0095] Thanks to the invention, in particular due to its effects described in relation to Figures 6 and 7, the lift coefficient of the rear wing continues to increase regularly, even for high angle of incidence values. For an angle of incidence between 0° and the value A2, no stall of the rear wing is observed.

[0096] Reference is made to [Fig.9].

[0097] This figure illustrates the effects of the invention on the pitching moment coefficient, as a function of the angle of incidence. A graph G3, in dashed line, represents this evolution for the aircraft of the state of the art of [Fig.8]. A graph G4, in solid line, represents this evolution for the aircraft according to the invention of [Fig.8].

[0098] Graph G3 shows that, between the angle of incidence value A0 and the angle of incidence value Al, the pitching moment coefficient decreases regularly, from a value CM0 to a value CM1. Beyond this, between the angle of incidence value Al and the angle of incidence value A2, the pitching moment coefficient increases, until it reaches a value CM2. This increase corresponds to longitudinal instability and the appearance of a nose-up pitching phenomenon. This means that, when the aircraft 101 is flying in the range of angle of incidence values ​​between Al and A2, in the event of a disturbance, for example in the event of a gust increasing the angle of incidence, the response of the aircraft 101 is to increase the angle of incidence even further. As a result, a so-called "unstable" phenomenon is observed, which leads to a stall of the aircraft 101.

[0099] Graph G4 shows that, between 0° and the angle of incidence value AO, the pitching moment coefficient decreases, from a value CM3 to the value CMO. Between the value AO and the angle of incidence value Al, this pitching moment coefficient continues to decrease regularly, until reaching a value CM4, greater than CM1. Beyond that, between the value Al and the angle of incidence value A2, the pitching moment coefficient continues to decrease slightly, until reaching a value CM5, less than CM2.

[0100] The values ​​CMO, CM1, CM2, CM3, CM4 and CM5 are here equal to -0.202, -0.505, -0.485, -0.094, -0.490 and -0.516, respectively.

[0101] Thanks to the invention, no sudden increase in the pitching moment coefficient is observed beyond the angle of incidence value A1. For an angle of incidence between 0° and the value A2, no longitudinal instability is observed.

[0102] Reference is made to [Fig.10].

[0103] This figure illustrates an evolution of the roll moment coefficient as a function of the sideslip angle, for an angle of incidence equal to 8°. This evolution is represented on graphs G5, G6, G7 and G8, each graph being associated with a different embodiment of the invention.

[0104] Graph G5 is associated with a first embodiment, represented in the window referenced 100. This first mode essentially corresponds to the embodiment described in relation to FIGS. 1 to 5. Graph G6 is associated with a second embodiment, represented in the window referenced 200. This second mode is distinguished from the first mode essentially in that it has a positive tailplane dihedral angle, for example of the order of 5°. Graph G7 is associated with a third embodiment, represented in the window referenced 300. This third mode is distinguished from the second mode essentially in that its rear winglets have an upwardly projecting portion. Graph G8 is associated with a fourth embodiment, represented in the window referenced 400.This fourth mode differs from the third mode essentially in that its rear wing has a positive rear dihedral angle on its wing portions, for example of the order of 5°.

[0105] For the four embodiments, the roll moment coefficient tends to decrease regularly until reaching a zero value, for a sideslip angle varying from a value B2 (negative) to a zero value.

[0106] For the fourth embodiment, graph G8 shows that the roll moment coefficient decreases regularly from a value CL4 to a zero value, for a sideslip angle varying from the value B2 to a zero value.

[0107] For the third embodiment, graph G7 shows that the roll moment coefficient decreases regularly by a value CL3, lower than the value CL4, to a zero value, for a sideslip angle varying from a value B1 (negative) to the zero value. In comparison with the fourth embodiment, the arrangement of the rear wing in a single rear wing plane (corresponding to a zero rear dihedral angle) makes it possible to reduce the rolling moment coefficient, at an equal sideslip angle. The roll controllability of the aircraft is thus improved, while having good lateral stability.

[0108] For the second embodiment, graph G6 shows that the roll moment coefficient decreases regularly from a value CL2, lower than the value CL3, to a zero value, for a sideslip angle varying from the value B1 to the zero value. In comparison with the third embodiment, the downward orientation of the projecting portions of the rear winglets makes it possible to reduce the roll moment coefficient, at an equal sideslip angle. The roll controllability of the aircraft is thus improved, while having good lateral stability.

[0109] For the first embodiment, graph G5 shows that the roll moment coefficient decreases regularly from a value CL1, lower than the value CL2, to a zero value, for a sideslip angle varying from the value B1 to the zero value. In comparison with the second embodiment, the arrangement of the wing portion of the empennage in a single empennage wing plane (corresponding to a zero empennage dihedral angle) makes it possible to reduce the roll moment coefficient, at an equal sideslip angle. The roll controllability of the aircraft is thus improved, while having good lateral stability.

[0110] Here, the value B1 is equal to -15°. The value B2 is equal to -10°. The values ​​CL1, CL2, CL3 and CL4 are here equal to 0.014, 0.019, 0.04 and 0.043, respectively.

[0111] Reference is made to Figures 11 and 12.

[0112] These figures illustrate effects of the invention on an air flow around the front wing in the vertical flight phase.

[0113] These figures represent the left part of the front wing of an aircraft according to an embodiment of the invention. In this embodiment, each of the winglets of the front wing is arranged near a through duct housing a rotor. This embodiment is, for example, similar to that described in relation to Figures 1 to 5. In the remainder of the description, elements similar to those described in relation to Figures 1 to 5 bear the same reference numbers. The characteristics of the air flow around the right part of the front wing 11 are deduced by symmetry with respect to the plane of symmetry of the aircraft 1.

[0114] During the vertical flight phase, the rotors 15 are rotating in the through ducts 25, so as to jointly generate the lift of the aircraft 1.

[0115] [Fig. 11] corresponds to a section along the transverse plane of the front wing 11. This figure represents a field of average speed of the air flow around this wing. This field displays several ranges, corresponding to respective average speed values, in meters per second.

[0116] [Fig. 12] corresponds to a front view of the left part of the front wing 11. This figure shows streamlines of the air flow around this wing. This figure also represents a field of average speed of the air flow around the front wing 11.

[0117] The air flow around the front wing 11 has maximum average speed values ​​at the outlet of the rotors 15. Here, these maximum values ​​are between 40 and 85 m / s. By their rotation, the rotors 15 generate a suction effect. This suction effect causes air to be drawn onto the extrados 31 of the front wing 11.

[0118] The suction effect causes air to be drawn in from the end of the front wing 11, here from the end portion 18 of the projecting portion 19 of the front winglet 17. The air flow undergoes an acceleration on this projecting portion 19, in particular on the outer surface 20 thereof. This acceleration has the effect that the outer surface 20 of the projecting portion 19 functions as an extrados surface for the front wing 11, while the inner surface 16 functions as a lower surface. In particular, we observe the appearance of a new force, materialized in box 80 by the arrow 82. This force is normal to the outer surface 20 of the projecting portion 19 of the front winglet 17. This force has a lift component, generally vertical, which is added to the lift generated by the rotation of the rotors 15. The aircraft 1 thus has improved lift in the vertical flight phase.

[0119] The invention is not limited to the embodiments described above, but encompasses all variants conceivable by those skilled in the art.

Claims

Claims

1. Vertical take-off and landing aircraft, comprising a fuselage (3), having a front part, shaped as a nose (5), and a rear part, shaped as a tail (7), mutually opposite, a first wing (11) and a second wing (13), attached to the fuselage (3) between the nose (5) and the tail (7), the second wing (13) being arranged behind the first wing (11), the first wing (11) being attached to the fuselage (3) at a first height, and extending, at least in part, generally in a first plane, the second wing (13) being attached to the fuselage (3) at a second height, greater than the first height, and extending, at least over a part connected to the fuselage (3), generally in a second plane, a first fin (17), arranged at the end of the first wing (11), characterized in that the first fin (17) comprises a projecting portion (19) projecting from the foreground, opposite the second plane.

2. Aircraft according to claim 1, in which the projecting portion (19) of the first fin (17) comprises an end portion (18), extending generally in a plane of inclination, this plane of inclination forming with the first plane an angle of between 60° and 90°.

3. Aircraft according to one of claims 1 and 2, in which the projecting portion (19) of the first fin (17) has, according to a profile, a length of between 50 and 100% of an average aerodynamic chord value of the first wing (11).

4. Aircraft according to one of the preceding claims, in which the second wing (13) comprises a wing portion (27), partly arranged behind the projecting portion (19) of the first fin (17).

5. An aircraft according to claim 4, wherein the wing portion (27) extends generally in the second plane.

6. Aircraft according to one of the preceding claims, further comprising a second fin (21), arranged at the end of the second wing (13), this second fin (21) having a projecting portion (23) projecting from the second plane generally in the same direction as the projecting portion (19) of the first fin (17).

7. Aircraft according to claim 6, in which the projecting portion (23) of the second fin (21) comprises an end portion (22), extending generally in a plane of inclination, this plane of inclination forming with the second plane an angle of between 60° and 90°.

8. Aircraft according to one of claims 6 and 7, in which the projecting portion (23) of the second fin (21) has, according to a profile, a length of between 50 and 100% of an average aerodynamic chord value of the second wing (13).

9. Aircraft according to one of the preceding claims, further comprising a through duct (25), capable of housing a rotor (15), this through duct (25) being arranged close to the projecting portion (19) of the first fin (17).

10. Aircraft according to one of the preceding claims, further comprising a tail unit (9), arranged on the tail (7), the tail unit (9) being shaped, at least in part, into a third wing (93), which extends generally in a plane, substantially parallel to the first plane.

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