METHOD FOR MANUFACTURING A RIGID PART AND AIRCRAFT CELL MADE WITH THIS METHOD

Abstract

TITLE: METHOD FOR MANUFACTURING A RIGID PART AND AIRCRAFT CELL PRODUCED WITH THIS METHOD The method (20) for manufacturing a rigid part (70) comprises: - a step (26) of closing a mold (50) whose internal wall is covered with a skin (56) made of flexible material fibers impregnated with resin (58), - before the resin hardens, a step (28) of introducing an expanding foam into the mold and a step (29) of expanding the foam, - a step (30, 31) of drying the foam, - before the complete glass transition of the foam during the drying step, a step (31) of baking the part at a resin polymerization temperature and - after the resin hardens, a step (34) of demolding the rigid part (70). (Figure for abstract: Figure 1)

Description

Title of the invention: METHOD FOR MANUFACTURING A RIGID PART AND AIRCRAFT CELL PRODUCED WITH THIS METHOD Technical field of the invention

[0001] The present invention relates to an aircraft cell and a method for manufacturing a rigid part. It applies, in particular, to the field of drones and the manufacture of extremely light and rigid parts, for example drone cells. State of the art

[0002] The manufacture of rigid parts has evolved from parts made of solid material, for example wood or metal, to hollow parts and, finally, to parts made of composite material.

[0003] Composite materials derive their properties from the combination of a reinforcement and a resin. While we can consider that this second ingredient has equal properties in all directions, this is not the case for reinforcements, which are, for the most part, flat. Their properties are linked to the orientations of the threads or fibers that compose them. One of the advantages of composite materials is thus being able to take advantage of these orientations to design the part accordingly. This can be an advantage, but also a disadvantage, as the "thickness" properties can hardly be significant. The design of structures must therefore, ideally, take this anisotropy into account, by adapting the draping of the reinforcements as well as their number to the desired maximum load or displacement.

[0004] For example, it is possible to reinforce parts loaded in one direction by one or more plies of unidirectional reinforcements without adding unnecessary mass by taking a balanced fabric, in which the fibers at 90° to the main stress would not have been stressed at all.

[0005] The most common materials are presented below, very briefly, in terms of their performance in terms of density, tensile strength and tensile modulus. Material compositions can offer the cumulative advantages of low mass, high strength and high rigidity. For the production of composites and sandwiches used in various ways, the following can be mentioned, without being exhaustive, in increasing order of performance and therefore cost:

[0006] Fibers: Glass, Aramid®, Carbon (High Tenacity, Intermediate Modulus),

[0007] Binders (Resins): Polyester, Epoxy family of cooking and temperature ambient at (Low Tg), Epoxy cooking High temperature (High Tg), ...

[0008] The Souls: Foams (PS, PVC, PET, Acrylic, PU, ​​Phenolic), Honeycomb Thermoplastics: (Polypropylene ...), Balsa, Nomex® Honeycomb (Aramid / phenolic resin), Aluminum Honeycomb,

[0009] We distinguish between monolithic composite materials and sandwich materials.

[0010] The former are simply made of fibers (glass, carbon, for example) and resin (Thermo-hard, for example Epoxy, or Thermoplastic, for example Polyester), while the latter are characterized by two skins made of composite material also composed of fibers and resin, and a core (balsa, foam, honeycomb) whose thickness is several orders of magnitude greater or less than the skins. This thickness generates little additional mass, but provides considerable properties outside the tangent plane to the skin.

[0011] In summary, the advantage of the sandwich over the monolithic composite is a gain in mass on parts which work essentially outside this plane. For example, a horizontal sandwich beam embedded subjected to a vertical force from top to bottom at its end sees its upper skin working in tension (mainly stressing the fiber of these plies) while its lower skin works in compression (mainly stressing the resin of these plies). The same monolithic beam of the same total mass has a much more mediocre behavior. Thickness is an important parameter and here we highlight the relative uselessness of the "neutral fiber" (that of the center of the monolithic plies) in this stress.It is shown, for example, that a monolithic tube with a thickness of five mm can certainly withstand the loads and displacements required by its user, but that a sandwich tube of the same thickness will have the same behavior but a much lower mass.

[0012] It is noted that carbon fibers have a low, or even zero, coefficient of expansion. The basic materials are threads, themselves made up of filaments. These threads are then used to form the reinforcements. These are classified into several categories, the most common being fabrics, braids, and multiaxials.

[0013] The fabrics are made on a loom, and can be recognized by the fact that they have a warp and a weft, i.e. two directions of threads. These fabrics are differentiated by - their mass per m2, - their weave, i.e. the way in which these threads are intertwined, and - the type of thread used.

[0014] These three factors influence the mechanical characteristics of the final composite, but also the way in which they will behave, that is to say their ease of being handled, placed in different shapes (called "drapeability").

[0015] The parts made from sandwiches according to methods known in the prior art nevertheless have a rigidity to weight ratio which is not optimal.

[0016] Furthermore, drones are unmanned vehicles whose piloting is automatic or remote controlled, whether they are flying, land-based or amphibious, for civil or to that of the armed or security forces of a State. Depending on the capabilities sought, their mass varies from a few grams to several tons. Their autonomy can reach up to several dozen hours (compared to the typical two hours of autonomy of a fighter). In the aeronautical field, the term "drone" designates an automated aerodyne and the associated implementation system are also called UAV (for "unmanned aerial vehicle") and increasingly often UAS (for "unmanned aircraft System"). Among these drones, one family is made up of multicopters, (also called tricopters, quadcopters, hexacopters or octocopters, etc., depending on the number of propellers). Multicopters have a cell comprising arms, which connect the central body of the drone to peripheral motors equipped with propellers.

[0017] This cell must be as light as possible to maximize the drone's autonomy and maneuverability. As a result, traditional monocoque structures consist of rigid skins held together with frames whose interior is hollow.

[0018] These drones include sensors, in particular geolocation sensors and, often, at least one camera whose image, transmitted to the pilot remotely, allows the pilot to control the functions of the drone, including its configuration, its orientation and the power developed by its motors.

[0019] However, the deformation of the parts of this drone, in particular the arms carrying the motors and the propellers, leads to a delay in the interpretation of the physical position into an electrical signal from the flight controllers relative to the cell. Schematically, the "brain" of the multicopter must receive, in real time, the position information from the arms carrying the motors. If the cell is deformed, this information is misinterpreted: there is then distortion of information between the flight controller and the cell. This distortion is harmful to piloting and can lead to the crash of the multicopter, because the flight controllers no longer know how to interpret a situation, whether aerial or on the ground. Summary of the invention

[0020] The invention aims to reduce this distortion of information between the flight controller and the aircraft cell. It also aims to make the drone cell isotropic and to reduce its capacity to deform under the action of constraints.

[0021] The inventor measured the reduction in vibration resulting from the implementation of the invention, in comparison with the reaction of a flight controller in a hollow structural cell. He was thus able to measure vibration differences of the order of 80%, as explained below.

[0022] The technical objectives of the invention include that the drone cell is more rigid and deforms less, in order to improve the behavior of the machines in flight: - direct gain in wind resistance in flight, - the manufacturing process allows a reduction in vibration compared to a traditional monocoque structure, and - remove the frames, refine the skin, without adding mechanical welding.

[0023] In addition, the core material is preferably fireproof, self-extinguishing, so that a battery fire is not a source of crash (it is recalled that lithium-based batteries are quite unstable and sensitive to temperature). Embodiments of the invention make it possible to insulate the batteries from heat using a core material that traps oxygen.

[0024] The invention also aims to increase the repairability of the cells and to allow the cell to integrate a maximum number of functions, such as structural functions, combined with electromagnetic shielding functions, for example, and / or energy circulation functions by digging trenches in the mold used to manufacture this cell.

[0025] Preferably, any sharp edges are avoided and fillets are applied, for example with a minimum radius of 7 mm, to avoid sewing two surfaces perpendicular to each other.

[0026] Preferably, at least one mold integrates all of the functionalities such as cable passages or fixing inserts.

[0027] Objects of the invention

[0028] According to a first aspect, the present invention relates to a method for manufacturing a rigid part, which comprises: - a step of closing a mold whose inner wall is covered with a skin made of flexible material fibers impregnated with resin, - before the resin hardens, a step of introducing an expanding foam into the mold and a step of expanding the foam, - a foam drying step, - before the complete glass transition of the foam during the drying stage, a stage of baking the part at a resin polymerization temperature and - after the resin has hardened, a step of demolding the rigid part.

[0029] The foam drying step and the resin curing step are thus partially simultaneous. They thus jointly achieve a single phase of joint hardening of flexible materials (the foam and the resin impregnating the skin) of different natures in contact with each other. Thanks to these arrangements, neither the resin nor the foam reaches its glass transition before the other of these constituents has begun its hardening, respectively by polymerization and by drying. The mechanical cohesion of these two constituents is thus optimized. The resin and the skin made of soft material fibers adhere extremely strongly to the foam. The molded part thus produced has very high rigidity, even with a very low skin thickness. In addition, this molded part is particularly light, thanks to the low skin thickness. The stiffness / weight ratio is thus very high. Finally, the molded part has high repairability since it can be cut around a break and a weld on the main part of the part can be made with a part of the same consistency corresponding to the broken or cut parts.

[0030] Furthermore, the inventor has observed that a part thus manufactured has good vibration absorption capabilities in the frequency range generated by the rotation of the motors of a multicopter. A multicopter cell produced according to the method which is the subject of the invention thus makes it possible to produce a drone which has better behavior in flight or on the ground.

[0031] In embodiments, the skin has a thickness less than or equal to 0.4 mm and is made of carbon fibers.

[0032] The inventor has determined that, thanks to the implementation of the present invention, parts having sufficient rigidity, for example to constitute an aircraft or drone cell, could have a carbon fiber skin of such thickness. This very low thickness allows a weight saving of the part thus manufactured.

[0033] In embodiments, the resin is a slow-setting epoxy resin having a working time greater than or equal to 24 hours. Thanks to these provisions, the resin cannot reach its glass transition before the foam has completely dried. The adhesion between the foam and the resin is thus maximized when the resin has polymerized.

[0034] In embodiments, the foam is an epoxy foam with the addition of expanded polystyrene beads.

[0035] These balls, with a diameter preferably less than or equal to 10 mm and, even more preferably, less than or equal to 6 mm, increase the absorption of vibrations by the part manufactured by implementing the invention.

[0036] In embodiments, during the step of placing a skin made of flexible material fibers on the surface of several parts of a mold, the skin is made to protrude from one part of the mold and, during the step of closing the mold, the protrusion of the skin beyond one part of the mold is superimposed with the skin placed on another part of the mold. This superposition ensures the welding of the two skins during the polymerization of the resin.

[0037] In embodiments, the method which is the subject of the invention further comprises a step of placing, in a mold part, at least one mandrel surrounded by a skin made of flexible material fibers covered with resin.

[0038] Such mandrels correspond, after their removal from the demolded part, to create reservations for passages of wires or cables through the part.

[0039] In embodiments, at least one mandrel is made of a material having a high coefficient of thermal expansion, the method further comprising, after the baking step, a step of cooling the mold comprising the molded part to a temperature below 0°C and a step of removing at least one mandrel at this temperature.

[0040] By choosing a material with a high coefficient of thermal expansion, the shrinkage of the mandrels is obtained by the drop in temperature which causes them to retract, while the demolded part, and in particular its foam, does not deform with the drop in temperature.

[0041] In embodiments, at least one mandrel is made of polyoxymethylene (or polyformaldehyde or polyacetal). This material has a high coefficient of thermal expansion.

[0042] According to a second aspect, the present invention relates to a molded part obtained by implementing the method which is the subject of the invention. In particular, the present invention relates to a vehicle cell, and more particularly an aircraft cell, obtained by implementing the method which is the subject of the invention.

[0043] The advantages, aims and particular characteristics of this part, of this vehicle cell and of this aircraft cell being similar to those of the method which is the subject of the invention, they are not recalled here. Brief description of the figures

[0044] Other advantages, aims and particular characteristics of the invention will emerge from the description which follows, given for explanatory and in no way limiting purposes with regard to the appended drawings, in which:

[0045] [Fig.l] represents, in the form of a flowchart, steps of a particular embodiment of the method which is the subject of the invention,

[0046] [Fig.2] represents, in the form of a chronogram, the drying times of a foaming and polymerization of a resin, in the process illustrated in [Fig.l],

[0047] [Fig.3] represents, in schematic top view, a part of an empty mold cor corresponding to a half-cell of a quadcopter,

[0048] [Fig.4] represents, according to a view along a section AA marked in [Fig.3], the part of empty mold illustrated in [Fig.3],

[0049] [Fig.5] represents the part of the mold illustrated in [Fig.4] after the installation of a skin,

[0050] [Fig.6] represents the mold part illustrated in [Fig.5] after impregnation of the skin with resin,

[0051] [Fig.7] represents the mold part illustrated in [Fig.6] after the positioning of a mandrel itself surrounded by a skin impregnated with resin,

[0052] [Fig-8] represents, in schematic top view, the mold part illustrated in [Fig.3] and the positioning of mandrels,

[0053] [Fig.9] represents, according to the sectional view marked in [Fig.3], the closed mold,

[0054] [Fig. 10] represents the mold illustrated in [Fig.9] after introduction of the expanding foam into the mold,

[0055] [Fig. 11] represents the mold illustrated in figures 9 and 10, after the expansion of the foam,

[0056] [Fig. 12] represents the mold illustrated in [Fig. 11], after drying of the foam and cooking of the resin,

[0057] [Fig. 13] represents the demolded part, after extraction from the mandrel, and

[0058] [Fig. 14] shows, in a schematic top view, the cell manufactured after removal of the mandrels.

[0059] Description of the preferred embodiments of the invention

[0060] It should be noted, from now on, that the figures are not to scale.

[0061] In [Fig.l], we observe a method 20 for manufacturing a mechanical part, by example a vehicle cell, in particular an aircraft and, more particularly, a drone or multicopter.

[0062] During a step 21, several mold parts are formed, the inner wall of which corresponds to the outer shape of the part to be manufactured. These mold parts are, for example, made of metal or composite material.

[0063] During a step 22, a release film is applied to the internal surface of each mold part. Preferably, the release film is of the wax type in ten layers with polishing after each layer, which facilitates the sliding of the skin.

[0064] During a step 23, a skin made of flexible material fibers is placed on the inner surface of the parts of a mold. To ensure, in the following steps, overlapping and welding of the skins present on the different parts of the mold, this skin is made to protrude beyond the inner wall of at least one part of the mold (see [Fig.6]). For example, this skin is made of carbon fibers or glass fibers. Depending on the variants, this skin is pre-impregnated with heat-curable resin or remains to be impregnated. Preferably, the skin is made of carbon fibers and has a thickness less than or equal to 0.4 mm and, even more preferably, less than or equal to 0.3 mm. For example, the skin is made of carbon (in accordance with the desired strength), with a weight of 600 g / m2 to manufacture a cell of a drone having a total flight mass of 25 kg. Alternatively, the skin is made of glass fibers.

[0065] In the case where the skin is not pre-impregnated, during a step 24, the skin is impregnated with resin mixed with a polymerizing agent, their system constituting a heat-curable compound, in each part of the mold. For example, a layer of a slow epoxy resin system of the "24 Hours" type, i.e. with a working time greater than or equal to 24 hours. For example, it is a casting epoxy resin, with a slow glass transition.

[0066] During a step 25, which may be simultaneous with step 23, mandrels, themselves covered with a resin-impregnated skin, are positioned in the areas of mold parts corresponding to reservations, or fillets, in the part to be molded. The mandrels are solid cylindrical parts, for example with a circular director, which are positioned along the axes of the arms of the cell being manufactured. Preferably, the mandrels have a high coefficient of thermal expansion. For example, the mandrels are made of POM (Polyoxymethylene), a semi-crystalline thermoplastic from the polytetrafluoroethylene or “PTFE” family. The insertion of the mandrels into the mold is done cold: these mandrels are brought to a temperature of -20°C, which, due to their high coefficient of thermal expansion, reduces their diameters. Then, due to their heating in ambient air to 20°C, their expansion locks them in position.

[0067] During a step 26, the mold is closed by assembling the mold parts, while leaving open evacuation paths for excess foam. During this step, the skin overhangs of one mold part come inside the skin of the other mold part which is assembled to it (see [Fig.9]).

[0068] During a step 27, which can be carried out simultaneously with at least one of steps 22 to 25, an expanding foam is prepared. Preferably, their density is reduced by adding additives. These additives also promote a reduction in the propagation of vibrations while being compatible with the core material. For example, this preparation is carried out with epoxy foam with the addition of 5% of the mass in expanded polystyrene beads and mixing of an agent polymerizing the epoxy foam. These beads have a diameter preferably less than or equal to 10 mm and, even more preferably, less than or equal to 6 mm.

[0069] For example, the 2080 M17(R) and 2080 M25(R) systems have free expansion coefficients of 6.6 and 4.5, allowing final densities of 170 and 250 kg / m3 to be obtained. The slow foaming of these systems allows mixing without a machine. A composite material mold is sufficient because the thrust is low and slow, with a vent being provided in the mold at the top to allow air and excess foam to escape.

[0070] These foaming epoxy resins combine many advantages: - the castings can be demolded and used without post-baking depending on the applications, - they have very good compatibility with pre-impregnated materials and epoxy resins during crosslinking, - they have excellent water resistance, - they have excellent thermal and mechanical resistance, and - they have excellent density homogeneity.

[0071] During a step 28, before the resin hardening step, the expanding foam is introduced into the mold, by gravity or with an injector.

[0072] During a step 29, the expanding foam begins to expand. In the example given above, the expansion is allowed to take place for 20 minutes, then the vents are closed while maintaining grooves (slots along the mold) to evacuate the surplus.

[0073] During a step 30, the expanding foam finishes its expansion, with possible fining towards the outside of the mold via the evacuation paths left open in the mold, and begins its drying. In the example given above, the foam is left to dry for 24 hours at 22°C.

[0074] During a step 31, before the complete glass transition of the foam during the drying step, the part being manufactured is cured at a resin polymerization temperature. In the example given above, the skins are cured when the 60 to 85°C phase of the foam drying phase is entered. The curing is prolonged with a temperature increase at a rate of 0.1°C / min, from 80°C, until reaching 100°C. When this value is reached, the curing continues at 100°C for 24 hours. It is recalled here that the glass transition makes the resins smooth in appearance, which practically causes a bonding incompatibility. This transition would result in a detachment of the core material from the skin.

[0075] When the epoxy resin is mixed with the hardener, the curing process begins instantly. The epoxy group triggers a polymerization reaction between the resin and the hardener, creating crosslinks and the mixed compounds harden.

[0076] For example, for the VTC 401® epoxy resin impregnated carbon system, post-curing is done at 100°C to avoid damaging the foam during the same cure. VTC401 is an epoxy resin system designed to give a low temperature initial cure or an instant high temperature cure with increased stiffness, and a higher service temperature, providing greater flexibility in component manufacturing. It can be supplied on a variety of fabrics in UD (unidirectional) format.

[0077] Main features and benefits - Curing temperature from 65°C to 120°C, - Service temperature up to 135°C after polymerization, - Low CET (Coefficient of Elasticity or Thermal Expansion) - Shelf life at 20°C: 21 days - Shelf life at -18°C: 12 months - Very low VOC (volatile organic compound) content - Curing cycle from 45 min (at 120°C) to 16 hours (65°C) with a post-curing time.

[0078] Prepreg materials require curing in order to polymerize, which is not the case with epoxy foams. These foams are cured after ambient drying for 24 hours. According to the invention, the two materials harden together in the same phase, so that the skin and the core are welded in one go. This process also makes it possible to avoid intermediate sanding phases which would be harmful to the structural skin.

[0079] Pre-impregnated materials are compacted when placed in the mold, whereas foams must be able to expand for a given period of time.

[0080] By implementing the invention, the epoxy foam (core material) is made compatible with the pre-impregnated skin, during their curing. The curing step allows slow polymerization of the core material, but at a sufficiently high temperature to activate the pre-accelerated resins of the prepregs.

[0081] During a step 32, the molded part is cooled to a temperature below 0°C. Due to the high coefficient of expansion of the mandrel material, the mandrels shrink and detach from the skin surrounding them, which is retained by the dried foam. For example, the mold is frozen at -20°C for two hours.

[0082] During a step 33, the mandrels are removed by axial traction.

[0083] During a step 34, the manufactured part is demolded, by shrinkage mold parts, at stabilized room temperature.

[0084] In [Fig.2], on the timing diagram 40, we observe the duration 41 of drying of the foam and the duration 44 of polymerization of the resin. The drying duration 41 extends from the instant 42, which corresponds to the start of step 27, until the instant 43, which is located during the cooking step 31. The polymerization duration 44 extends from the instant 45, which is located during the step 31, when the cooking temperature has reached a predetermined value for triggering the polymerization of the resin, until a moment 46, still during the cooking step 31.

[0085] Because time 43 occurs after time 45, the foam has not yet reached its glass transition. Thus, neither the resin nor the foam reaches its glass transition before the other of these constituents has begun to harden, respectively by polymerization and by drying. The mechanical cohesion of these two constituents, once hardened, is thus optimized. The resin and the skin of flexible material fibers adhere extremely strongly to the foam. The molded part thus produced has very high rigidity, even with a very low skin thickness.

[0086] In [Fig. 3], we observe a part 50 of a mold comprising two identical parts 50 and 50' (see [Fig. 9]) intended to close on each other to form a complete mold. Of course, depending on the external shape of the part to be manufactured, the mold parts may be different and, in particular, may not be symmetrical with respect to a horizontal plane (the plane of [Fig. 3]), nor with respect to a vertical plane (which passes through the axis of symmetry 53, in [Fig. 3]). The mold parts are, for example, made of metal or composite material.

[0087] This mold part 50 has an internal recess, or cavity 51, the wall of which corresponds to the external shape of the corresponding part of the part to be manufactured and a thickness 55. In the case of the quadcopter cell to be manufactured serving as an example, the internal recess 51 corresponds to the body of the quadcopter, in the center, and has extensions 52 corresponding to the four arms of the quadcopter carrying the electric motors setting the propellers in rotation. These extensions 52 have an axis 54. On this axis 54, a cylindrical recess 59 serves as a mandrel support.

[0088] A section AA, perpendicular to the axis 54 of one of the extensions 52 is marked in [Fig. 3]. [Fig. 4] represents this section AA. It shows the elements illustrated in [Fig. 3]. [Fig. 5] represents this section AA after step 23, in the case where the skin is not pre-impregnated with resin. A skin 56 made of flexible material fibers has been applied to the internal wall of the mold part 50. In addition, this skin 56 has projections 57 beyond the internal wall of the mold part 50. As illustrated in [Fig. 5], these projections are preferably inclined towards the internal wall of the mold part 50. This inclination prevents, when closing the mold (step 26 and [Fig. 9]), these projections from coming into contact with the skin placed on the mold part 50' and risking folding one of the skins.

[0089] [Fig.6] represents the section AA after step 23, in the case where the skin 56 is pre-impregnated with resin 58 or after step 24, when the skin 56 has been impregnated with resin 58.

[0090] [Fig.7] represents the section AA after step 25, when a mandrel 60 has been positioned in the support 59, this mandrel 60 also being surrounded by a skin 61 impregnated with resin 62.

[0091] [Fig. 8] represents the positioning of a mandrel 60 in each of the extensions 52, along an axis 54. Optionally, a central reservation is made by adding a central part 64, itself surrounded by a skin impregnated with resin. For example, this reservation will be used for positioning a battery and an electronic module in the quadcopter cell being manufactured. Preferably, the skin of this central part 64 also has projections covering the skin applied to the internal wall of the mold 50. It can be seen, in [Fig. 8], that the mandrels 60 preferentially come into contact with this central part 64 so that electrical cables can connect motors placed at the distal ends of the extensions 52 to the electronic module which will take the place of the central part 64 in the cell of the quadricopter.

[0092] [Fig. 9] represents the section AA after step 26, when the mold has been closed. The mold part 50' has a thickness 55' and carries, on its internal wall, a skin 56' impregnated with resin 58'. As explained previously, the projections 57 of the skin 56 extend inside the convex volume defined by the skin 56'.

[0093] [Fig. 10] represents section AA after step 28, when the expanding foam 63 has been introduced into the mold.

[0094] [Fig. 11] represents the section AA after step 29, when the expanding foam 63 has expanded until it exerts pressure first on the projections 57 of the skin 56 until it is pressed against the skin 56', then throughout the internal volume of the mold, the surplus expanded foam being evacuated through the evacuations provided in the mold.

[0095] [Fig. 12] represents the section AA after step 33, that is to say the manufactured part 70, after complete drying of the foam, polymerization of the resin to form a rigid skin 72, cooling of the part below 0 °C and removal of the mandrel 60 leaving room for a reservation, or fillet, 71 and removal of the central part 64 leaving room for another reservation 73. The quadcopter cell 70 can then receive motors at the locations 74 at the ends of the extensions 52, and an electronic module accompanied by an electric battery in the reservation 73, electric cables connecting the electronic module to the motors.

[0096] The inventor compared the average vibration amplitudes obtained at the flight controller directly in connection with three identical cells manufactured using three manufacturing processes, including the process that is the subject of the invention, during thirty-second hover flights. With a cell manufactured from uniform Kevlar® skin with an average thickness of two millimeters, the vibrations have an average amplitude approximately ten times higher than with a cell manufactured using the process illustrated in [Fig.l]. With a cell manufactured from a foam-carbon-aramid sandwich composite, with a carbon thickness of 1.2 millimeters, the vibrations have an average amplitude twice as high as with a cell manufactured using the process illustrated in [Fig.l].

[0097] The type of cell obtained by implementing the invention has several advantages: - damping of vibrations from the engines on the on-board sensors (particularly the camera), - the stiffness / weight ratio of the cell, - repairability (compared to that of a plastic cell): a simple glue on the foam and installation of a skin superimposed on the old skin is sufficient, - a single phase of marriage (polymerization) of flexible materials of different natures.

[0098] The technical objectives of the method which is the subject of the invention also include: - that the drone cell is more rigid, deforms less in order to improve the behavior of the machines in flight: (direct gain in wind resistance in flight), - the core material is fireproof, self-extinguishing, so that a battery fire does not cause a crash. Since lithium batteries are quite unstable and sensitive to temperature, this feature allows the batteries to be insulated from heat thanks to the core material trapping oxygen, - Increase cell repairability, - Allow the same part to integrate a maximum number of functions, such as structural functions, combined with electromagnetic shielding functions for example or energy circulation functions by digging trenches in the mold.

[0099] Finally, we tend to avoid any sharp edges and apply fillets with a radius of at least seven mm to sew two surfaces perpendicular to each other. The mold parts are designed to integrate all the functionalities such as cable passages or fixing inserts.

[0100] NOMENCLATURE

[0101] 20. flowchart of steps

[0102] 21 to 34. steps of an example of implementation of the method

[0103] 40. chronogram

[0104] 4L foam drying time

[0105] 42 start of foam drying time

[0106] 43. end of the foam drying time

[0107] 44. resin polymerization time

[0108] 45. start of the resin polymerization time

[0109] 46. end of the resin polymerization time

[0110] 50, 50. mold part

[0111] 51. obviously, or cavity, internal 51 which corresponds to the body of the quadricopter

[0112] 52. extension which corresponds to an arm of the quadcopter

[0113] 53. axis of symmetry of the quadcopter

[0114] 54. axis of an arm

[0115] 55, 55. thickness of the mold part

[0116] 56, 56. skin made of soft material fibers

[0117] 57. skin protrusion beyond the mold part

[0118] 58, 58. resin

[0119] 59. cylindrical recess

[0120]

[0121]

[0122]

[0123]

[0124]

[0125]

[0126]

[0127]

[0128] 60. mandrel 61. mandrel skin 62. resin impregnating the mandrel skin 63. expanding foam 64. central piece 71. reservation, or fillet, 72. rigid skin 73. reservation, or fillet, 74. position of a motor

Claims

Claims

1. Method (20) for manufacturing a rigid part (70), which comprises: - a step (26) of closing a mold (50) whose internal wall is covered with a skin (56) made of fibers of flexible material impregnated with resin (58), - before the resin hardens, a step (28) of introducing an expanding foam into the mold and a step (29) of expanding the foam, - a step (30, 31) of drying the foam, - before the complete glass transition of the foam during the drying step, a step (31) of baking the part at a polymerization temperature of the resin and - after the resin hardens, a step (34) of demolding the rigid part (70).

2. The method (20) of claim 1, wherein the resin is a slow epoxy resin having a working time of greater than or equal to 24 hours.

3. Method (20) according to one of claims 1 or 2, in which the expanding foam (63) is an epoxy foam with the addition of expanded polystyrene beads.

4. Method (20) according to one of claims 1 to 3, in which, during the step (23) of placing a skin (56) made of fibers of flexible material on the surface of several parts of a mold, the skin of the mold is made to protrude from the corresponding mold part and, during a step, the protrusion (57) of skin of a part (50) of the mold is superimposed with the skin placed on another part (50') of the mold.

5. Method (20) according to one of claims 1 to 4, which further comprises a step (24) of placing, in a part (50) of the mold, at least one mandrel (60) surrounded by a skin (61) made of fibers of flexible material impregnated with resin (62).

6. Method (20) according to claim 5, in which at least one mandrel (60) is made of a material having a high coefficient of thermal expansion, the method further comprising, after the baking step (31), a step (32) of cooling the mold (50, 50') comprising the molded part (70) to a temperature below 0°C and a step (33) of removing at least one mandrel at this temperature.

7. The method (20) of claim 6, wherein at least one mandrel

8. (60) is made of polyoxymethylene (or polyformaldehyde or polyacetal). Molded part (70) obtained by implementing the method (20) according to one of claims 1 to 7.

9. A molded part (70) according to claim 8, which constitutes a vehicle cell.

10. A molded part (70) according to claim 8, which constitutes an aircraft cell.

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