RU2829526C2 - Box-like monolithic structure from composite material for aircraft fuselages and wings and method of making said structure - Google Patents

Abstract

FIELD: aviation.

SUBSTANCE: invention relates to aircraft engineering. Method of making a box-like monolithic structure for aircraft fuselages and wings includes a step of using two or more auxiliary tools (15), having a shape that complements the shape of cavities (18), and a composition based on a reinforcing material and a polymer. Auxiliary tools (15) allow direct lamination of their outer walls (16) with prepreg material and are made to achieve a flexible elastic state at a temperature below curing temperature and above 50 °C. During curing, curing pressure is applied both outside the formed structure and inside auxiliary tools (15), walls (16) of which become flexible, providing pressure on material of curable prepreg.

EFFECT: simplified manufacturing and higher quality of surface finishing.

9 cl, 7 dwg

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority from Italian Patent Application No. 102020000003976, filed on February 26, 2020, the entire disclosure of which is incorporated herein by reference.

AREA OF TECHNOLOGY

The present invention relates to a box-shaped monolithic structure made of composite material for aircraft fuselages and wings, in particular for an aircraft horizontal stabilizer, to which direct references will be made in the following description without loss of applicability.

The present invention also relates to a method for manufacturing the above-mentioned box-shaped monolithic structure.

LEVEL OF TECHNOLOGY

As is known, the horizontal stabilizer is a structural element with fixed surfaces, which, as a rule, is an element of the tail section of the aircraft fuselage.

The stabilizer inside has a box-shaped monolithic structure made of composite material and, in essence, contains:

- the top wall;

- a lower wall facing the upper wall and located at a non-zero distance from the latter; and

- a plurality of connecting elements or spars, continuing between the upper and lower walls and limiting them, together with the aforementioned upper and lower walls of the corresponding elongated cavities, each of which continues between the front and rear openings located in planes transverse with respect to the walls themselves and to the spars.

The use of composite material allows us to reduce the overall weight of the aircraft while still producing very strong structures.

The upper and lower walls are approximately parallel, or more precisely, they converge slightly towards the front or rear openings.

The spars are represented by partitions that are essentially perpendicular or, more generally, transverse to the top and bottom walls.

The described structure is manufactured using a known method that includes the following steps, starting with the application of a prepreg of a thermosetting matrix (resin), reinforced with fibers of various natures, such as carbon fibers, aramid fibers, glass fibers, etc.

First of all, each spar in the prepreg coated state is preformed using suitable preforming tools by producing two elongated shaped rods with C-shaped sections formed from the back side and two wings projecting perpendicularly from opposite end edges of the back side; to form the spar, the two shaped rods are joined together along their respective back sides to form opposite concave surfaces.

In particular, each C-shaped rod is laminated onto three flat surfaces of a preforming tool with essentially a parallelepiped-shaped profile; the first surface of the tool is completely covered with prepreg, while the other two surfaces adjacent to the first surface, perpendicular to the latter and mutually parallel, are covered only for tension.

After the required vacuum and under predetermined temperature conditions, the C-shaped bars are joined two by two as described above to produce the required number of side members, which are held in place and at a predetermined distance for the subsequent curing step by suitable rigid auxiliary tools in the form of elongated pins.

Preforming is a process of applying vacuum and temperature to allow the prepreg material to achieve a cured shape without polymerization.

Each auxiliary tool consists essentially of a rigid and solid elongated body in the form of a parallelepiped, limited by flat surfaces, and its cross-section corresponds to the shape of the elongated cavities of the structure being manufactured.

Before placing each side member, during the pre-forming stage with prepreg, between two auxiliary tools, the latter are subjected to straightening, consisting of the following successive stages:

- applying a layer of insulating material, for example in the form of a film, to each auxiliary instrument to facilitate subsequent removal of the auxiliary instrument itself from the corresponding elongated cavity;

- installation of a bag made of tubular film on each auxiliary tool obtained in this way, with provision of an excess amount of the latter at each end of the auxiliary tool itself for the subsequent compaction operation;

- wrapping each auxiliary tool and the outer part of the bag made of tubular film with fabric to ensure ventilation, fixing the folded edges of the fabric with a sealant;

- installation of a separating sleeve film on each auxiliary tool obtained in this way, ensuring in this case an excess amount of the latter at each end of the auxiliary tool itself for the subsequent compaction operation;

- sealing the ends of the bag made of tubular film and separating tubular film with a sealant;

- application of vacuum and holding for shrinkage of the straightening with separating sleeve film of the corresponding auxiliary tool.

At this point, preformed side members, each of which is represented by two C-shaped profile bars joined together along their respective back sides, are placed between auxiliary tools previously subjected to the straightening operation described above; in particular, each preformed side member is arranged so that:

- the back faces of the respective pair of C-shaped bars that make up the spar were positioned between two facing flat surfaces of two adjacent auxiliary tools; and

- the wings of each shaped rod were located on the corresponding flat surfaces of the corresponding auxiliary tool, parallel to each other and, in essence, perpendicular to the flat surface that supports the back side of the shaped rod itself.

The assembly thus formed, represented by pre-formed side members and pre-straightened auxiliary tools, is installed in a mold for forming, which includes a lower plate, an upper plate and two opposite side walls connecting the lower and upper plates.

In particular, each of the two upper and lower plates is laminated with one or more layers of prepreg material, which are intended to form the upper and lower outer skins, which, after the curing stage, represent the upper and lower walls of the manufactured box structure.

More precisely, on the lower plate of the mold for molding, bearing the lower outer covering, pre-formed spars are arranged, held in place by appropriate auxiliary tools; subsequently, the upper plate of the mold for molding, bearing the upper outer covering, is closed by the side walls of the mold itself and the assembly represented by the pre-formed spars and auxiliary tools.

At this point, the release film, ventilation fabric and bag film are placed sequentially throughout the mold; the bag film is sealed with a sealant at the base of the mold.

The open-ended separating sleeve film is placed on each auxiliary tool; the excess portion in length relative to the corresponding auxiliary tool is cut off.

The sleeve bags are unwound at the ends and compacted using the appropriate auxiliary tools.

Thus, the so-called envelope bag is formed by sealing the edge of the film of the outer bag located on the mold and the ends of the sleeve bags of the auxiliary tools, as well as by sealing the ends of the adjacent sleeve bags.

At this point, a vacuum is created inside the envelope bag until the material of the outer bag shrinks towards the outer surface of the mold.

The ends of the sleeve bags are then opened and, while the vacuum continues to form, the sleeve bags themselves are removed from the corresponding auxiliary instruments, minimizing the volume inside the envelope bag.

At this point, the auxiliary tools are removed and the unit formed in this way is placed in an autoclave for the curing procedure to be carried out at predetermined pressure and temperature values (for example, for epoxy resins the curing temperature is about 180°C and the curing pressure is in the range of 6-7 bar).

The described method and the tools used allow the blanks to be correctly positioned and their position to be maintained during the mold closing procedures for forming and manufacturing the envelope bag.

Removing the auxiliary tools before the cure cycle prevents undue deformation of the tools under the required curing pressure and temperature conditions and ensures that pressure is applied evenly to all composite parts.

Instead, sleeve bags allow pressure to be applied evenly to the composite parts they come into contact with.

However, the applicant noted that the described method and the resulting box-shaped structure could be improved. In particular, there is a need in this area to simplify the method in order to increase its efficiency and reduce the cost; in addition, the quality of the surface finish, in particular the surfaces inside the box-shaped structure, requires further improvement.

OBJECTIVE AND ESSENCE OF THE INVENTION

The aim of the present invention is to provide a highly reliable, low-cost method for manufacturing a box-shaped monolithic structure made of composite material for an aircraft fuselage, which allows satisfying at least one of the above requirements associated with methods for manufacturing box-shaped monolithic structures made of composite materials of a known type.

In accordance with the present invention, this objective is achieved in a method for manufacturing a box-shaped monolithic structure from a composite material for an aircraft fuselage, as claimed in paragraph 1 of the formula of the invention.

Another object of the present invention is to manufacture a box-shaped monolithic structure made of composite material for an aircraft fuselage that has a high quality surface finish and satisfies the above requirements.

In accordance with the present invention, this objective is achieved by a box-shaped monolithic structure made of composite material for the fuselage of an aircraft, as claimed in paragraph 9 of the claims.

BRIEF DESCRIPTION OF DRAWINGS

For a better understanding of the present invention, a preferred embodiment of the invention is described below, solely as a non-limiting example, with reference to the accompanying drawings, in which:

- Fig. 1 is a perspective view of the tail section of an aircraft, inside which a box-shaped monolithic structure made of composite material is installed, manufactured in accordance with the principles of the present invention;

- Fig. 2 shows a perspective view, on an enlarged scale, with parts of the box-shaped monolithic structure in Fig. 1 removed for clarity;

- Fig. 3 is a perspective view of the auxiliary tool in successive stages during the lamination procedure of the side member portion of the structure in Figs. 1 and 2;

- Fig. 4 is a detailed perspective view with a reduced scale of a number of auxiliary tools shown in Fig. 3 during the assembly of a plurality of side members of the structure shown in Figs. 1 and 2;

- Fig. 5 is an expanded perspective view of a mold used to produce the structure shown in Fig. 1 and 2 and in which a number of auxiliary tools shown in Fig. 3 are planned to be installed for this purpose; and

- Fig. 6 and 7 are perspective views on an enlarged scale of the mould for forming according to Fig. 5 during successive stages of the method for producing the structure according to Fig. 1 and 2.

DETAILED DESCRIPTION

With reference to Fig. 1 and 2, position 1 generally denotes a box-shaped monolithic structure made of composite material for the fuselage of an aircraft, in particular for a horizontal stabilizer 2 of a fuselage 3 of an aircraft, for example an aircraft 4.

Construction 1 (Fig. 2) essentially contains:

- top wall 5;

- a lower wall 6 facing, along its surface with greater extension, towards the surface with greater extension of the upper wall 5 and located at a non-zero distance from the latter; and

- a plurality of connecting elements or side members 7, continuing between the upper 5 and lower 6 walls and limiting them, a number of elongated cavities 8 on the above-mentioned upper 5 and lower 6 wall, each of which continues between the corresponding front and rear openings 9 (only the front openings are visible in Fig. 2), located in planes transverse with respect to the walls 5, 6 themselves and the side members 7.

The upper 5 and lower 6 walls in the example shown are approximately parallel to each other. In a possible alternative embodiment not shown, the upper 5 and lower 6 walls may also converge slightly in the direction of the front or rear openings 9.

The spars 7 are represented by partitions which are essentially perpendicular or, more generally, transverse in relation to the upper 5 and lower 6 walls and are extended in the longitudinal direction, essentially parallel to the upper 5 and lower 6 walls.

Construction 1 is made of a prepreg with a polymer matrix, for example a thermosetting resin, reinforced with fibers, which can be of different nature, for example carbon fibers and/or aramid fibers and/or glass fibers, etc.

Alternatively, structure 1 may also be made from a matrix prepreg made from a thermoplastic resin reinforced with fibres of the type specified above.

In both cases, the upper 5 and lower 6 walls are made starting from the corresponding outer coverings 5a, 6a (Fig. 5) with the above-mentioned fibre-reinforced prepreg applied to the corresponding flat surfaces, as will be described in more detail below.

With reference to Fig. 3 and 4, each spar 7 is made of two shaped rods 11, extended in the said longitudinal direction, has C-shaped sections and consists of a back side 12 and two end projections 13, protruding in the transverse direction, essentially perpendicular to the direction from the opposite end edges of the rear side 12 itself and located on one side of the spar. In particular, to form the spar 7, two shaped rods 11 are connected together along the corresponding back sides 12 so that they have opposite concavities.

Preferably, the shaped rods 11 in the above-mentioned prepreg material are applied to corresponding auxiliary tools 15, extended in the said longitudinal direction, which are subsequently intended to hold the side members 7 in predetermined positions inside the structure 1 during the manufacturing stage and have a composition based on a reinforcing material and a corresponding polymer to ensure a transition from a rigid state to a flexible elastic state and vice versa under the influence of heating and, accordingly, cooling, i.e. under the influence of temperature exposure.

The polymer constituting the auxiliary tools 15 is preferably a thermosetting or thermoplastic polymer with shape memory of a known type. The polymer can be, for example, an epoxy polymer with shape memory, a polymer based on a cyanate ester with shape memory, a polyurethane polymer with shape memory, a vinyl polymer with shape memory, a polyamide polymer with shape memory, a maleimide polymer with shape memory or combinations thereof, including copolymers.

Due to the properties of the shape memory polymer, the auxiliary tools 15 can regain their original rigid shape despite excessive repeated use and after numerous heating and cooling cycles.

The reinforcing material of the auxiliary tools 15 includes one or more elastic fibers.

In particular, the reinforcing material may also comprise nylon fibers, lycra fibers, polyester fibers, carbon fibers, glass fibers, aramid fibers, boron fibers, basalt fibers, polymer fibers, chopped fibers, meshes, three-dimensional fiber preforms, normal weft fabrics, twill fabrics or other types of fabrics and combinations thereof. A suitable commercial example of nylon fibers is nylon manufactured by Invista (Wichita, Kansas).

The auxiliary tools 15 in some embodiments may comprise two or more different types of reinforcing materials.

As shown in Fig. 3-7, each auxiliary tool 15 has the shape of a parallelepiped, which is extended in the indicated longitudinal direction and has a cavity in the same direction. In particular, each auxiliary tool 15 has a cross-section with an external polygonal profile, complementary to the profile of the corresponding cavity 8, which must be formed in the structure 1. In the example shown, each auxiliary tool 15 is limited by four side walls 16, parallel to two, wherein each is perpendicular to two adjacent walls 16.

The walls 16 are connected to each other by means of beveled edges 17 and limit the longitudinal through cavity 18 of the corresponding auxiliary tool 15.

Each shaped rod 11 is laminated externally on three adjacent walls 16 of the corresponding auxiliary tool 15 in a rigid state after applying a resin-based adhesive substance (known as a "tackifier").

In particular, the wall 16 of the auxiliary tool 15 in the rigid state is completely covered with the prepreg of the corresponding shaped rod 11, and the other two walls 16 adjacent to it are covered only for the given tension.

In this way, each auxiliary tool 15 can support on opposite sides two prepreg-coated shaped rods 11 with a C-shaped profile.

Due to its rigid state, one or more layers of prepreg can be applied to each auxiliary tool 15; in addition, due to their composition, there is no need to straighten the auxiliary tools 15 before laminating the shaped rods 11.

Having placed the shaped rods 11 on the corresponding walls 16 of each auxiliary tool 15, the latter are closed and sealed in an outer bag (well known and not shown), in which a vacuum is then formed in a known manner to obtain a seal of the shaped rods 11 themselves.

At this point, by bringing together or placing in one row in the lateral direction to each other the auxiliary tools 15, carrying on the outside the corresponding compacted shaped rods 11, covered with prepreg (Fig. 4), it is possible to connect the shaped rods 11 themselves in pairs along the corresponding back sides 12 to form the side members 7, which are then held at the desired distance from each other with the help of the auxiliary tools 15.

In particular, each prepreg-coated spar 7 is designed in such a way that:

- the back sides 12 of the corresponding pair of shaped rods 11 that form it were placed between two front walls 16 of two adjacent auxiliary tools 15; and

- the end projections 13 of each shaped rod 11 were located on the corresponding walls 16 of the corresponding auxiliary tool 15, essentially parallel to each other and essentially perpendicular to the wall 16 that supports the back side 12 of the shaped rod 11 itself.

Since the flat walls 16 of each auxiliary tool 15 are connected by beveled edges 17, the shaped rods 11 take on an external shape similar to the walls 16 themselves on which they are laid, as well as the shape of the beveled edge 17 that connects them.

Thus, between each pair of adjacent coplanar end projections 13, a recess 19 is formed, which has an approximately wedge-shaped cross-section. The recesses 19 are filled with corresponding filling spheres 20 of prepreg or threads (Figs. 4 and 5), which have a profile complementary to the profile of the recesses 19 themselves.

The unit formed in this manner, represented by prepreg-coated spars 7, auxiliary tools 15 holding the spars in place, and filler spheres 20, is inserted into a mold 21 for molding, including a lower plate 22, an upper plate 23 and two opposite side plates 24 connecting the lower plate 22 and the upper plate 23 (Fig. 5).

In particular, the mold 21 for molding is hollow inside and, therefore, has at the front and at the back corresponding end openings 25 (only the front one is shown in Fig. 5), limited by the corresponding front 27 and rear 28 surfaces of the frame, defined by the opposite upper edges of the lower, upper and side plates 22, 23, 24.

In more detail, on each of the two lower and upper plates 22, 23 one or more layers (called "layers") of fibre-reinforced prepreg material of the type described above are applied, intended to define the upper 5a and lower 6a outer coverings, which after the curing step will constitute the upper 5 and lower 6 walls of the structure 1 being manufactured.

More specifically, a resin-based adhesive (known as a "tackifier") is applied to each of the lower and upper plates 22, 23 in advance, and then various layers of fiber-reinforced prepreg material are applied. This adhesive is applied to at least parts of the lower and upper plates 22, 23 intended to accommodate the fiber-reinforced prepreg material.

Preferably, after laminating a predetermined number of layers, for example every four layers, the vacuum sealing step is carried out by wrapping the respective bottom 22 or top plate 23 and the prepreg in a sealed bag (well known and not shown), inside which a vacuum is formed in a known manner.

At this point, the pre-formed side members 7 and the filling spheres 20, held in place by the corresponding auxiliary tools 15, are positioned on the lower plate 22 of the mould 21 for forming, which carries the lower outer covering 6a.

Then the upper plate 23 of the mold 21 for molding, bearing the lower outer covering 5a, is closed from the side of the side plates 24 of the mold 21 itself for molding and from the side of the unit represented by pre-formed side members 7, filling spheres 20 and auxiliary tools 15.

In this state (Fig. 6 and 7), the auxiliary tools 15 protrude from the respective front and rear end portions 15a (only the front portions are visible in Fig. 6 and 7) from the front and rear openings 25 of the mold 21 for forming.

A ventilation fabric (well known per se and not shown) and a sleeve bag 30 protruding from the front and back of the auxiliary tool 15 itself are then installed inside each auxiliary tool 15.

On each of the front 27 and rear 28 surfaces of the frame, the axial end 31 of the additional outer sleeve bag 32, suitable for placement during use around the corresponding front 15a or rear end parts 15 of the auxiliary tools 15, is sealed with a sealant.

At this point, the ends of the sleeve bags 30 projecting in front of the corresponding auxiliary tools 15 are sealed with a sealant to the axial end 33 of the corresponding outer sleeve bag 32 opposite the axial end 31; a similar sealing operation is performed between the corresponding outer sleeve bag 32 sealed in the rear part of the mold 21 for molding, and the ends of the sleeve bags 30 projecting behind the corresponding auxiliary tools 15.

Then all ends of adjacent sleeve bags 30 are closed and sealed together.

In this way, so-called envelope bags are formed, the front and back parts, each of which is designated by the reference number 35.

The mold 21 produced in this manner for molding together with the auxiliary tools 15, the upper and lower outer coverings 5a, 6a, the spars 7, the filling spheres 20 and the envelope bags 35 is fed into an autoclave to perform the curing operation under conditions of specified pressure and temperature values (for example, for epoxy resins the curing temperature is about 180 C, and the curing pressure is in the range of 6-7 bar).

During the curing stage, the walls 16 of the auxiliary tools 15 change from a rigid state to a flexible elastic state due to the temperature effect. In fact, the auxiliary tools 15 are designed to achieve a flexible elastic state at a temperature below the curing temperature and above 50 C. The curing pressure acts both outside the structure 1 being formed and inside the auxiliary tools 15, and therefore inside the cavities 8 of the structure 1 itself through the sleeve bags 30, which thus press on the walls 16, which become flexible due to the change in state. As a result of the curing pressure, the walls 16 then press uniformly on the polymerizing prepreg material.

After completion of the curing stage, the auxiliary tools 15 are reheated to transition to a flexible elastic state to ensure their removal from the cavities 8 of the newly formed structure 1.

As a result of the above-described method and, in particular, the use of auxiliary tools 15, which change from a rigid state to a flexible elastic state during the curing operation, it is possible to obtain a structure 1 having, on all surfaces located between the upper wall 5 and the lower wall 6, a surface roughness with an average value less than or equal to 2 microns.

This result is particularly important since it allows obtaining smooth surfaces on the structure 1, providing an accurate and stable stop for connecting devices or other structures of the fuselage 3 of the aircraft 4.

The described method is simpler and more productive than the known ones, since the shaped rods 11 are laminated directly on the auxiliary tools 15, and not on special tools for preliminary molding, and then transferred to the auxiliary tools. In addition, there is no need to remove the auxiliary tools 15 before the curing stage, since these tools ensure that a flexible elastic state is achieved during the curing procedure, thus ensuring uniform pressure on the prepreg material.

It is clear that changes may be made to the method and design 1 described and illustrated here, without, however, going beyond the scope of protection defined by the claims.

Claims (31)

1. A method for manufacturing a box-shaped monolithic structure (1) for fuselages (3a) and wings (3b) of an aircraft (4) using a fiber-reinforced prepreg material with a polymer matrix containing - the top wall (5); - a lower wall (6) facing the said upper wall (5) and located at a distance from the upper wall (5); and - at least one connecting element (7) extending transversely between the said upper (5) and lower (6) walls, connected to them and delimiting the corresponding elongated cavities (8) of the upper (5) and lower (6) walls; the method includes the steps a) obtaining at least two elongated auxiliary tools (15), each of which has a cross-section with an external polygonal profile complementary to the profile of the cavities (8) formed in the said structure (1); b) placing on the outside, on three adjacent walls (16) of each auxiliary tool (15), an elongated shaped rod (11), made of the said prepreg material, having a C-shaped cross-section and having a back side (12) and two end projections (13) protruding in the transverse direction from the said back side (12) and located on the same back side (12); c) connecting the said shaped rods (11) along the corresponding back sides (12) with the placement of the said auxiliary tools (15) in a row so that the shaped rods (11) themselves have opposite concavities; d) applying one or more layers of said prepreg material on the bottom plate (22) of the mould (21) for forming and creating a bottom outer covering (6a), by means of which said bottom wall (6) of said structure (1) is formed; e) inserting the assembly formed by said auxiliary tools (15) and said shaped rods (11) in step c) onto said lower outer covering (6a) supported by said lower wall (6) of said mould (21) for moulding, and between the corresponding side plates (24) limiting the mould (21) itself for moulding; wherein said shaped rods (11) are provided with corresponding end projections (13) coplanar in contact with said lower outer covering (6a); f) applying one or more layers of said pre-impregnated material to the top plate (23) of said mould (21) to form and create an upper outer covering (5a) by means of which said upper wall (5) of said structure (1) is formed; g) placing said upper plate (23) on said side plates (24) of said mould (21) for moulding in a position facing said lower plate (22), so that said upper outer covering (5a) is in contact with the end projections (13) of said shaped rods (11) opposite the end projections (13) in contact with the lower outer covering (6a); h) transferring the mould (21) prepared in this way and filled with the upper and lower outer coatings (5a, 6a) and shaped rods (11) into an autoclave for curing at the specified parameters of temperature and curing pressure; characterized in that - the said auxiliary tools (15) are hollow inside and have a composition based on a reinforcing material and a polymer, suitable for transition from a rigid state to a flexible elastic state and vice versa, upon heating and, accordingly, cooling; - the said auxiliary tools (15) are designed with the possibility of achieving a flexible elastic state at a temperature below the curing temperature and above 50°C; - step b) is carried out by applying said prepreg directly to the outer surface of the walls (16) of said auxiliary tools (15) in a rigid state; and - in step h) the curing pressure is applied both inside the mould (21) for forming and inside the said auxiliary tools (15), the walls (16) of which become flexible due to the transition from a rigid state to a flexible elastic state, are pressed by the curing pressure itself and adhere to the said shaped rods (11) and to the said lower (6a) and upper (5a) outer coverings. 2. The method according to paragraph 1, characterized in that it additionally includes, before step h), the steps i) inserting into each auxiliary tool (15) at least one sleeve bag (30) protruding from the respective opposite front and rear end portions (15a) of the auxiliary tool (15) itself; l) seals at each opposite front and rear end (27, 28) of the mould (21) for moulding the first axial end (31) of a corresponding outer sleeve bag (32) suitable for receiving and applying around the corresponding front or rear end portion (15a) of auxiliary tools (15) protruding from the mould (21) itself for moulding; and m) sealing the adjacent ends of the sleeve bags (30) protruding from the respective auxiliary tools (15) to each other and to the respective second axial ends (33) of the respective outer sleeve bags (32) opposite the first axial ends (31); wherein said curing pressure is applied inside said sleeve bags (30). 3. The method according to claim 1 or 2, characterized in that step b) is preceded by step n) of applying a first resin-based adhesive substance to at least parts of the walls (16) of the auxiliary tools (15) in a rigid state to obtain the said prepreg material. 4. A method according to any one of the preceding claims, characterized in that steps d) and f) are preceded by a step o) of applying a second resin-based adhesive substance to at least part of said lower (22) and upper (23) plates of the mold (21) for molding, and receiving said prepreg material. 5. A method according to any of the preceding claims, characterized in that the polymer of said auxiliary tools (15) is a thermosetting or thermoplastic polymer with shape memory. 6. A method according to any of the preceding claims, characterized in that the reinforcing material of the said auxiliary tools (15) contains one elastic fiber or several elastic fibers. 7. A method according to any of the preceding claims, characterized in that at least two connecting elements (7) are formed at said stage a) at least three said elongated auxiliary tools (15) are provided; and at said step b) on the walls (16) of at least one of said auxiliary tools (15) two said shaped rods (11) are laminated directly on opposite sides. 8. A method according to any of the preceding claims, characterized in that it additionally comprises, after step h), step p), heating the auxiliary tools (15) to bring them into a flexible elastic state and to remove them from the corresponding cavities (8) of the molded structure (1). 9. A box-shaped monolithic structure (1) for fuselages (3a) and wings (3b) of an aircraft (4), manufactured by the method according to any of the preceding claims, characterized in that on all surfaces located between said upper wall (5) and said lower wall (6), the average value of surface roughness is less than or equal to 2 microns.