Hollow structure and propeller blade

Abstract

The present invention addresses the problem of providing a production-friendly, high-strength, light-weight, and high-quality hollow structure, particularly a propeller blade, and is intended to provide a hollow structure made of resin in which a skin portion forming an outer surface and holding portions supporting the skin portion from the inside are integrated, and which has an internal space. At least one of the holding portions defines a line segment with a minimum length connecting a certain joint point and a joint surface facing a joint surface in which the joint point is included. In a cross section (referred to as "cross section 1" for convenience sake) taken across the structure in a plane including a cross section passing the center of the line segment and orthogonal to the line segment, a line ("line 1" for convenience sake) connecting the centers of the short sides of a rectangle circumscribing the cross section and having a minimum area intersects the skin portion and the holding portion at three or more points.

Description

Hollow structure and propeller blade

[0001] The present invention relates to hollow structures, in particular propeller blades.

[0002] Hollow structures are widely used in industrial fields because they can be made lightweight while maintaining high strength and rigidity. Propeller blades, in particular, are suitable for hollow structures because they require light weight and high rigidity.

[0003] Propeller blades have a wing shape and rotate around the drive shaft to push air backward, generating thrust. Reducing the weight of propeller blades contributes to increasing the aircraft's payload and range, so high-strength, high-rigidity fiber-reinforced materials are used as their structural materials.

[0004] Known propeller blade structures include a sandwich structure in which the skin layer is made of fiber-reinforced resin and the core layer is made of a porous foam core, and a structure in which a hollow portion is provided with a shear web to maintain the blade shape.

[0005] JP 2011-137386 A JP 2017-129091 A Japanese Patent No. 6066548 Specification European Patent Application Publication No. 3556544

[0006] Hollow structures are widely used in industrial fields because they can reduce the weight of a structure while maintaining high strength and rigidity. Fiber-reinforced materials are particularly suitable for use as structural materials, as they can reduce the weight of a structure while maintaining high rigidity and strength. Common methods for creating hollow structures include manufacturing and joining halved components, providing a hollow core on its outer surface and removing the core after the structure is formed, and providing a bag-shaped internal pressure application means and forcing the material into a mold with internal pressure to form the hollow structure.

[0007] When components are manufactured in half and then joined together, defects are likely to occur at the joint, and when a fiber-reinforced layer is included, discontinuities in the fibers occur, which tends to weaken the strength of the joint.

[0008] The core is necessary to support the hollow structure, but since it must be removed after molding is complete, there is not much freedom in its placement.

[0009] When using a core to create a hollow section with a complex shape, such as a long, bent shape, a shape with multiple branches, or a shape with a varying cross section, the shape of the hollow section is limited due to the need to remove the core after molding. While there is also a method of melting and removing the core after molding the material, this method takes time to remove the core and is difficult to completely remove the core material, resulting in poor productivity and weight reduction. Furthermore, providing a retaining section in the hollow section for reinforcement can sometimes effectively increase strength and rigidity while maintaining weight reduction. The benefits of having a retaining section are particularly significant for complex shapes that are prone to localized stress. However, it is difficult to precisely form the retaining section using a core, and the shape of the retaining section is limited to a shape that allows the core to be removed from the mold.

[0010] Even when using a mold and a bag-shaped internal pressure applying means, if the desired molded product has a complex shape, the bag may tear during demolding and the bag may not be able to be completely removed, and it is also difficult to accurately position the holding portion.

[0011] As the blade cross section becomes larger, a propeller blade having a hollow portion has a greater advantage in terms of weight reduction compared to a propeller blade having a porous core layer such as a foam core inside the skin layer.

[0012] One known method for manufacturing a propeller blade with a hollow section is to divide the propeller blade into a negative pressure side and a positive pressure side, mold each skin layer from a fiber-reinforced material, and then join them together, as in Patent Document 1. However, this method has drawbacks, such as weak joint strength and poor impact resistance, poor productivity due to the time-consuming joining process, and uneven joining that results in poor product quality.

[0013] Also, as described in Patent Document 2, a method is known in which a core is made from a low-melting-point material, a fiber substrate is preformed around the core, and the resulting material is molded in a mold. In this case, the molding temperature cannot be raised above the melting temperature of the core during molding, and molding takes time. If a core with a high melting temperature is used to shorten the molding time, a high temperature is required to remove the core, which reduces productivity.

[0014] Furthermore, when the unmelted core is removed by pulling it out, it is necessary to design the structure of the hollow portion so that the core can be pulled out, which places design constraints that are disadvantageous in terms of the strength and weight reduction of the propeller blade, and also reduces productivity because a pulling process is required.In order to improve the designability of the hollow portion structure, there is a method described in Patent Document 3 in which a fiber-reinforced material is preformed in a mold, and then the mold is closed, a bag is placed inside, and the bag is inflated to apply internal pressure to form the mold.However, this method lengthens the working time for preforming, resulting in lower productivity, and in addition, since two or more independent bags must be used to form a share web, position control is difficult and the product quality is inferior.

[0015] As described above, with the conventional techniques described above, it has been difficult to manufacture propeller blades that are highly productive, strong, lightweight, and have a high-quality hollow structure.

[0016] SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a propeller blade that is highly manufacturable, has high strength, is lightweight, and has a high-quality hollow structure.

[0017] Another object of the present invention is to provide a high-quality hollow structure that is highly productive, strong, lightweight, and of high quality.

[0018] The present invention employs the following means to solve the above problems. [1] A hollow structure made of resin, in which a skin portion forming an outer surface and a retaining portion supporting the skin portion from the inside are integrated and have an internal space, wherein at least one of the retaining portions is arranged so that a line segment of minimum length connecting a certain joint point between a joint surface including the joint point and an opposing joint surface is set, and in a cross section (conveniently referred to as "cross section 1") obtained by cutting the structure with a plane that passes through the midpoint of the line segment and is perpendicular to the line segment, a line (conveniently referred to as "line 1") that circumscribes the cross section and connects the midpoints of the short sides of a rectangle with the smallest area intersects the skin portion and the retaining portion at three or more points. [2] A hollow structure according to the above [1], which has a fiber-reinforced resin layer on the outer surface side of the structure. [3] A propeller blade consisting of the hollow structure according to [1] or [2], wherein in at least one of the retaining portions, a plane dividing the pressure surface side and the suction surface side (referred to as "cross section 2" for convenience) is parallel to cross section 1, and the retaining portion is arranged so that a straight line on cross section 2 passing through the blade tip and the blade root intersects the skin portion and the retaining portion at three or more points and is parallel to line 1. [4] A propeller blade according to [3], wherein at least one of the retaining portions has a bent portion bent in the chord direction. [5] A hollow structure according to [3] or [4], wherein at least one of the retaining portions has a lightening portion, and the lightening portion is provided in a manner that can be observed in a vertical cross section dividing the propeller blade into the pressure surface side and the suction surface side. [6] A propeller blade according to any of [3] to [5], wherein two or more retaining portions are provided. [7] The propeller blade according to any one of [3] to [6], which has a retaining portion having a length in the spanwise direction that is less than 30% of the propeller blade's span, and which has a length in the chordwise direction that is less than 30% of the propeller blade's chord at the center of the airfoil retaining portion in the spanwise direction. [8] The propeller blade according to any one of [3] to [7], wherein the skin portion has a thickness of 15 mm or less and 0.2 mm or more. [9] The propeller blade according to any one of [3] to [8], wherein the skin portion has a thickness of 300% or less when the thickness of the fiber-reinforced resin layer is 100%.

[10] A propeller blade according to any one of [3] to [9], wherein the skin portion has a mesh structure.

[11] A propeller blade according to any one of [3] to

[10] , wherein the first structure has a recess formed on the outer surface of the skin portion at a contact point between the skin portion and one of the retaining portions that supports the blade in the thickness direction.

[12] A propeller blade according to any one of [3] to

[11] , wherein the volume of the space within the structure is 30% or more of the volume of the propeller blade.

[13] A propeller blade according to any one of [3] to

[12] , wherein the resin constituting the structure is a resin that does not have an observed glass transition temperature of 90°C or less.

[14] A propeller blade according to any one of [3] to

[13] , wherein the structure contains a filler.

[15] A propeller blade according to any one of [3] to

[14] , wherein the fiber reinforced resin layer is made of carbon fiber.

[16] A propeller blade according to any one of claims 3 to 15, [3] to

[15] , wherein the structure is produced by a three-dimensional printing method.

[0019] According to the present invention, it is possible to provide a high-strength, lightweight, and high-quality hollow structure with excellent productivity, particularly a high-strength, lightweight, and high-quality propeller blade. When the molded article of the present invention is applied to an aircraft that uses many propeller blades, such as a UAM or drone, the lightweight propeller blades have the effect of increasing the range and payload.

[0020] Cross-sectional view of an example of a propeller blade of the present invention Cross-sectional view of another example of a propeller blade of the present invention A view to explain an example of the arrangement of airfoil retaining portions A view to explain an airfoil retaining portion having a bent portion and a curved portion in the chord direction A view to explain an airfoil retaining portion having a lightening portion, (a) being an example of a circular lightening portion, and (b) being a triangular lightening portion A view to explain a propeller blade provided with two or more airfoil retaining portions An example of an airfoil retaining portion (local airfoil retaining portion) Cross-sectional view of an example of a hollow structure of the present invention ((a) is a cross-sectional view along A-A', (b) is a cross-sectional view along the 808 plane) A view to explain a joining point

[0021] The hollow structure of the present invention is a resin structure having an internal space and an integrated skin portion that forms the outer surface and a retaining portion that supports the skin portion from the inside, wherein at least one of the retaining portions is a hollow structure in which a line segment of the shortest length is set from a certain joint point between the joint surface that includes the joint point and the joint surface that faces the joint point, and in a cross section (conveniently referred to as "cross section 1") obtained by cutting the structure with a plane that passes through the midpoint of the line segment and is perpendicular to the line segment, a line parallel to a line (conveniently referred to as "line 1") that circumscribes the cross section and connects the midpoints of the short sides of a rectangle that has the smallest area intersects the skin portion and the retaining portion at three or more points.

[0022] The hollow structure according to the present invention is characterized in that the skin portion forming the outer surface and the retaining portion supporting the skin portion from the inside are integrated together, and the arrangement of the retaining portion.

[0023] An example of a hollow structure of the present invention is shown in Figure 8. In the hollow structure of Figure 8, a retaining portion 803 is arranged to support the skin layer. A joint point 805 included in one joining surface and a joint point 806 included in the other joining surface represent the endpoints of a line segment with the shortest length connecting the joining surfaces. The retaining portion 803 is arranged so that, in a cross section 808 (corresponding to "cross section 1") obtained by cutting the structure along a plane including a cross section perpendicular to the line segment, a line 811 parallel to a line 810 (corresponding to "line 1") connecting the midpoints of the short sides of a rectangle 809 that circumscribes the cross section and has the smallest area intersects the skin portion and retaining portion at three or more points, preferably four or more points, and more preferably five or more points.

[0024] In the hollow structure of the present invention, the retaining portion is provided in the hollow portion, and the skin portion and the retaining portion are integrated. Here, "integrated" means that they are fused together without forming a physical boundary surface, and is not a structure in which different members are combined by joining with adhesive or rivets, etc. Therefore, the joint surface referred to in the present invention is a virtual surface formed by the contact of the outer surfaces of the members constituting the skin portion and the retaining portion, which is flush with the inner surface of the skin layer. The joint point is a virtual point where the retaining portion and the skin portion are in contact, and is a point provided on the joint surface 903, which is the surface of the structure where the skin portion 901 and the retaining portion 902 are integrated, minus the thickness of the surrounding skin, as shown in Figure 9 .

[0025] This structure allows weak parts to be reinforced locally, achieving high strength while keeping the overall weight of the structure light. Furthermore, by molding the retaining part and the skin part as an integral structure, the retaining part is positioned accurately relative to the skin part, resulting in a high-quality molded product with little variation.

[0026] Furthermore, the hollow structure according to the present invention has a fiber-reinforced resin layer on the outer surface side of the structure. By providing the fiber-reinforced resin layer on the outer surface of the hollow structure, the structure can be effectively reinforced and, when molding the fiber-reinforced resin layer, it is easy to preform it on the outer surface of the hollow structure, resulting in excellent moldability.

[0027] Furthermore, the hollow structure of the present invention, particularly the propeller blade, is characterized in that it has a first structure (hollow structure) made of resin with a space inside, in which a skin portion that forms the outer surface of the wing shape and a wing-shaped retaining portion that supports the skin portion from the inside are integrated, and it has a fiber-reinforced resin layer on the outer surface side of the first structure.

[0028] Furthermore, one preferred embodiment of the present invention is a propeller blade having a first structure having an internal space, in which a skin portion that forms the outer surface of the wing shape and a wing-shaped retaining portion that supports the skin portion from the inside are integrated, and a fiber-reinforced resin layer on the outer surface side of the first structure.

[0029] The present invention will be described in detail below with reference to the drawings, taking as a representative example an example in which the present invention is applied to a propeller blade.

[0030] A cross-sectional view of one embodiment of a propeller blade of the present invention cut in the chord direction is shown in Figure 1. Here, the chord is the direction connecting the leading edge and trailing edge of the propeller blade. As shown in Figure 1, the propeller blade of the present invention has a fiber-reinforced resin layer 101, a skin portion 102, and a hollow portion 104, and also has an airfoil shape retaining portion 103 integrated with the skin portion within the hollow portion.

[0031] [First structure (hollow structure)] The first structure used in the present invention is, in the case of a propeller blade, an integrated structure of a skin portion that forms the outer surface of the blade shape and a blade shape retaining portion that supports the skin portion from the inside, and also has a space inside.

[0032] The skin portion 102 and the airfoil-shape retaining portion 103 have an integral structure, and for example, the leading edge, trailing edge, pressure surface, and suction surface of the propeller blade are integrally formed. This first structure defines the general shape of the propeller blade. For this reason, the material constituting the first structure preferably does not contain continuous fibers, in order to ensure freedom of shape, and the material used preferably has a fixed composition. This enables highly accurate molding, and the propeller structure can also be highly accurate.

[0033] Furthermore, the material constituting the first structure preferably contains a filler, more preferably a filler that can be used in forming the first structure using a 3D printing method, and preferably hollow glass beads. Using hollow fillers can reduce the weight of the propeller blade. Furthermore, in the first structure, a recess is preferably formed on the outer surface of the skin portion at the joint between the skin portion and the airfoil-shaped retainer, corresponding to the airfoil-shaped retainer. A fiber-reinforced resin material can be placed in this recess to reinforce the joint. In this way, by placing the fiber-reinforced resin material from the base of the propeller blade toward the tip, the blade can be reinforced against bending loads applied to the blade. Furthermore, the fiber-reinforced resin material placed in the recess is a resin material reinforced with fibers oriented in one direction, preferably continuous fibers, which can achieve a high reinforcing effect. In particular, if the resin material is made of unidirectional CFRP using carbon fiber, it can achieve even more effective reinforcement and reduce the weight of the propeller blade. 2 shows an example of a spar cap, showing a cross-sectional view of a propeller blade cut in the chord direction with a spar cap 203 provided in a recess for forming the spar cap. The recess facilitates positioning when forming the fiber-reinforced resin material on the first structure, improving the quality of the propeller blade. Furthermore, if the depth of the recess and the thickness of the fiber-reinforced resin material are equal, no irregularities are formed on the surface when a fiber-reinforced resin layer is provided, achieving high quality. This is also advantageous in the process of providing an outer fiber-reinforced resin layer. Furthermore, when press molding is performed in a manufacturing mold, providing a fiber-reinforced resin material oriented in one direction in the recess prevents the fiber bundles from being pushed apart by the press pressure, achieving high quality.

[0034] The first structure may be provided with a spar or spars for maintaining its shape, or small ribs or ribs attached to the spar or spars, in addition to the airfoil retaining portion described below.

[0035] The resin constituting the first structure is not particularly limited, and examples thereof include thermosetting resins such as epoxy resin, unsaturated polyester resin, vinyl ester resin, phenol resin, epoxy acrylate resin, urethane acrylate resin, phenoxy resin, alkyd resin, urethane resin, maleimide resin, and cyanate resin; polyethylene terephthalate (PET) resin, polybutylene terephthalate (PBT) resin, polytrimethylene terephthalate (PTT) resin, polyethylene (PE) resin, polypropylene (PP) resin, styrene-based resin, polyoxymethylene (POM) resin, polyamide (PA) resin, polycarbonate (PC) resin, polymethylene methacrylate (PMMA) resin, polyvinyl chloride (PVC) resin, polyphenylene sulfide (PPS) resin, and polyphenylene Fluorine-based resins such as ethylene ether (PPE) resin, modified PPE resin, polyimide (PI) resin, polyamideimide (PAI) resin, polyetherimide (PEI) resin, polysulfone (PSU) resin, modified PSU resin, polyethersulfone resin, polyketone (PK) resin, polyarylene ether ketone resin (PAEK), polyarylate (PAR) resin, polyethernitrile (PEN) resin, phenolic resin, phenoxy resin, polytetrafluoroethylene resin, and further thermoplastic elastomers such as polystyrene resin, polyolefin resin, polyurethane resin, polyester resin, polyamide resin, polybutadiene resin, polyisoprene resin, and fluorine-based resin, as well as copolymers, modified products, and thermoplastic resins in which two or more of these are blended may also be used. Furthermore, examples of the polyarylene ether ketone resin (PAEK) include polyether ketone (PEK), polyether ether ketone (PEEK), polyether ether ketone ketone (PEEKK), polyether ketone ketone (PEKK), polyether ketone ether ketone ketone (PEKEKK), polyether ether ketone ether ketone (PEEKEK), polyether ether ether ketone (PEEEK), and polyether diphenyl ether ketone (PEDEK), as well as copolymers, modified products, and blends of two or more of these resins.Among these resins, it is preferable to use a resin whose glass transition temperature and melting point are not observed below 90 ° C., more preferably a resin whose glass transition temperature and melting point are not observed below 115 ° C., even more preferably a resin whose glass transition temperature and melting point are not observed below 130 ° C., and even more preferably a resin whose glass transition temperature and melting point are not observed below 180 ° C. Furthermore, among these resins, resins that can be molded by 3D printing are preferred. The glass transition temperature and melting point can be measured in accordance with JIS K 7121-1987, and measurement samples can be sampled from propeller blades.

[0036] [Skin] For example, in the case of a propeller blade, the skin has a wing-shaped outer surface and forms part of the outer shell structure of the propeller blade. Together with the fiber-reinforced resin layer (described later), it serves as the skin (outer plate) of the propeller blade and plays a role in withstanding the forces applied to the propeller blade.

[0037] The range of the thickness of the skin portion is preferably 0.2 mm or more, more preferably 0.5 mm or more, and even more preferably 1 mm or more, with the lower limit being 15 mm or less, preferably 10 mm or less, and even more preferably 5 mm or less. With such a skin portion thickness, the first structure is not deformed when a fiber-reinforced resin layer is provided on the outer surface side of the first structure, and the load applied to the propeller blade surface can be effectively supported. Furthermore, it is easy to reduce the weight of the propeller blade. The thickness of the skin portion does not need to be uniform and may vary depending on the location.

[0038] The thickness of the skin portion is determined by observing a cross section of the propeller blade cut in the chord direction, perpendicular to the blade span, with an optical microscope. Similarly, for a general hollow structure, the cross section is cut at the smallest cross section at a certain point and then observed with an optical microscope. The skin thickness is determined by measuring the minimum thickness of a skin portion of a uniform thickness, e.g., a region of small curvature on the pressure and suction surfaces of the propeller blade, excluding regions with abrupt thickness changes such as ribs and slits. A measurement point is set on the inner surface of the skin portion to be measured, measuring the minimum thickness of the region. A perpendicular line is drawn through the measurement point and perpendicularly intersects with the inner surface. The point where the perpendicular line intersects with the outer surface of the layer to be measured is set as the measurement point on the outer surface, and the length between the measurement point on the inner surface and the measurement point on the outer surface is measured. It is difficult to define the thickness of a layer in areas with large curvature, such as the leading and trailing edges of the propeller blade, or where two surfaces join, and this is not preferable.

[0039] Furthermore, when the thickness of the fiber-reinforced resin layer described below is taken as 100%, the thickness of the skin portion is preferably 300% or less, more preferably 200% or less, and even more preferably 100% or less. With this thickness of the skin portion, a sufficient weight reduction effect can be obtained.

[0040] The thickness of the skin relative to the thickness of the fiber-reinforced resin layer can be obtained by observing a cross section of the propeller blade cut in the chord direction, which is perpendicular to the blade span direction, with an optical microscope. Similarly, for general hollow structures, the thickness can be obtained by cutting the cross section at the smallest cross section at a certain point and observing the cut cross section with an optical microscope. The thickness of the skin portion relative to the thickness of the fiber-reinforced resin layer is determined by setting a measurement point so that the skin portion has the smallest thickness on the skin of a propeller blade, which is generally uniform in thickness, for example, from the areas of small curvature on the pressure and suction surfaces of the propeller blade to areas where the thickness changes abruptly, such as ribs and slits. To measure, a measurement point is set on the inner surface of the skin portion, a perpendicular line is drawn through the measurement point and perpendicularly intersects with the inner surface, and the point where the perpendicular line intersects with the outer surface of the layer being measured is set as the measurement point on the outer surface. The thickness of the skin portion obtained by measuring the length between the measurement point on the inner surface and the measurement point on the outer surface is then divided by the thickness of the fiber-reinforced resin layer, which is obtained by extending the perpendicular line toward the outer layer and measuring the length between the point on the inner surface of the fiber-reinforced resin layer where it intersects with the measurement point on the outer surface. It is difficult to define the thickness of the layer when measuring points with large curvature, such as the leading and trailing edges of the propeller blade, or where two surfaces join, and this is not preferable. In addition, a fiber-reinforced resin layer is treated as a single layer even if it is made up of layers with different fiber orientations or fiber types, and even if a foam material or honeycomb core is present between the fiber-reinforced resin layers, forming a sandwich structure, it is also treated as a single layer.

[0041] The skin may have slits or holes in some areas, which can be used to reduce the weight of the propeller blade, to prevent erosion of the propeller blade, and to insert metal members to form a conductive path during a lightning strike. The slits and holes are preferably small, and for example, the slits may be provided over the entire blade length, but the width is preferably 5 mm or less, more preferably 3 mm or less, and even more preferably 1 mm or less. The area of ​​the holes should be 78.5 mm 2 Less than 28.3mm is desirable 2 Less than 3.1 mm is more preferable. 2The following is more desirable. With such a configuration, when forming the fiber-reinforced resin layer, it is possible to perform preforming based on the shape of the first structure, and when external pressure is applied when molding the fiber-reinforced resin layer, it is possible to support the fiber-reinforced resin layer from the inside against the external pressure. In particular, in the present invention, it is preferable that the skin portion of the first structure has a mesh structure, and by controlling the opening of the mesh holes, it is possible to reduce the weight while maintaining the rigidity of the first structure. Here, the mesh structure is a region with a large number of holes, and is a structure that maintains the shape of the layer while being hollowed out.

[0042] [Hollow Portion] The hollow portion is a space provided inside the skin portion of the first structure.

[0043] The volume of the hollow portion is preferably 30% or more of the volume of the propeller blade, more preferably 40% or more, and even more preferably 50% or more. The larger the volume of the hollow portion, the greater the weight reduction effect. The volume of the propeller blade may be measured by submerging the propeller blade in liquid and measuring the excluded volume, or by 3D scanning. The volume of the hollow portion may also be measured by filling the hollow portion with liquid and measuring the volume, or by performing a CT scan.

[0044] [Airfoil-Shaped Retaining Portion] The first structure used in the present invention has an airfoil-shaped retaining portion in the hollow portion. The airfoil-shaped retaining portion is configured to prevent deformation of the propeller blade when subjected to external force, for example. This includes a thin plate structure, generally called a shear web, that connects the blade thickness direction and extends approximately in the blade length direction. Here, the blade length direction refers to the direction extending from the center of rotation of the propeller in the radial direction of rotation. The airfoil-shaped retaining portion is not limited to the form of a shear web. As illustrated in FIG. 7, it may be, for example, cylindrical, plate-shaped, or plate-shaped with a cross section. Such a structure enables localized structural reinforcement. Furthermore, a structure that maintains a hollow portion all the way to the base of the blade is preferable. For example, an airfoil-shaped retaining portion that does not extend to the base of the blade as a hollow portion and forms a closed space by the skin portion and the airfoil-shaped retaining portion is not preferable because it is difficult to fabricate using a 3D printing method.

[0045] In addition, in the present invention, it is preferable that the airfoil retaining portion is arranged so as to cast a shadow on the hollow portion when viewed from the root of the blade in the blade span direction, which in the case of a general hollow structure is viewed in the longitudinal direction of the hollow structure, and it is even more preferable that the airfoil retaining portion is located in the position of the shadow. By using such a structure, it is possible to reinforce the structure locally.

[0046] A preferred embodiment of the present invention is a propeller blade comprising a first structure having an internal space, in which a skin portion forming the outer surface of the blade shape and a blade-shaped retaining portion supporting the skin portion from the inside are integrated, and a fiber-reinforced resin layer on the outer surface side of the first structure, and in which at least one of the blade-shaped retaining portions of the first structure is arranged so that, in a cross section cut to divide the blade into a positive pressure side and a negative pressure side, a line parallel to a line passing through the blade tip and blade root intersects with the skin portion and the blade-shaped retaining portion at three or more points, preferably four or more points, and more preferably five or more points.

[0047] 3 shows a cross-sectional view (referred to as "cross-section 2" for convenience) of a propeller blade cut to separate it into a pressure surface side and a suction surface side. In the present invention, the first structure preferably has the airfoil holders arranged so that there are three or more intersections between the skin portion 301 and the airfoil holders on a line parallel to a line passing through the blade tip and the blade root (the line represented by dashed line A in FIG. 3 corresponds to this line). That is, in the same figure, dashed line B intersects with the airfoil holder 302, another airfoil holder 304, and the skin portion on the blade tip side. Furthermore, dashed line C intersects with the skin portion on the blade root side, the airfoil holder 302, and the skin portion on the blade tip side. Arranging the airfoil holders in this manner allows for a high degree of freedom in the arrangement of the airfoil holders, which is advantageous in reducing the weight of the propeller blade. Although it is difficult to arrange the airfoil-shaped retaining portion in this manner using a normal mold, it is possible to manufacture the first structure with high precision and as a single unit by, for example, 3D printing. However, if a straight line passing through the blade tip and the blade root passes through a portion other than the cross section between the blade tip and the blade root, the straight line can be replaced with a straight line parallel to the long side of a circumscribed rectangle with the smallest area relative to the cross section.

[0048] In a preferred embodiment, cross section 2 is parallel to cross section 1, and a straight line on cross section 2 that passes through the blade tip and blade root and has three or more intersections between the skin portion 301 and the blade shape holding portion is parallel to line 1.

[0049] Furthermore, it is preferable that the airfoil retaining portion has a bent portion or curved portion in the chord direction, for example, in a direction perpendicular to the longitudinal direction of the hollow structure in a general hollow structure. In both cases, the extension direction of the airfoil retaining portion 404 changes in the chord direction when viewed from the pressure surface side. The bent portion shows a change where two straight lines meet, while the curved portion shows a curved change. A corrugated plate structure with periodic curved portions may also be used. Figure 4 shows a cross-sectional view of a propeller blade cut to separate it into the pressure surface side and the suction surface side. The bent portion of the airfoil retaining structure has a bent portion 402 in the chord direction and a curved portion 403. By providing such a structure to the airfoil retaining portion, the airfoil retaining portion 404 can be optimally positioned, reducing the weight of the propeller blade. This structure can be manufactured as a single unit with high precision, for example, by 3D printing.

[0050] Furthermore, it is preferable that the airfoil retaining portion has a length of, for example, 30% or more of the blade span of the propeller blade. In the case of a general hollow structure, it has a length of 30% or more of the longitudinal length of the hollow structure. Examples of such airfoil retaining portions include a general shear web (spar). Furthermore, it is preferable that the airfoil retaining portion has a branched portion. For example, a shear web-like structure that branches into two toward the blade root or a leaf-vein-like structure that branches from the blade root toward the blade tip can effectively reinforce the propeller blade. The length of the airfoil retaining portion in the blade length direction can be measured, for example, by cutting the propeller blade in the blade length direction and measuring the length of the contact points between the airfoil retaining portion and the skin portion that appear in the cross section, from the point closest to the root and the point closest to the tip, using a ruler or calipers. Alternatively, it is also possible to measure the length of the contact points between the airfoil retaining portion and the skin portion in the blade length direction, from the point closest to the root and the point closest to the tip, using a CT scan.

[0051] Furthermore, the airfoil retaining portion is preferably provided in a manner that can be observed in a vertical cross section that divides the propeller blade into a pressure surface side and a suction surface side, and the airfoil retaining portion preferably has a length in the spanwise direction that is 30% or more of the propeller blade's span. Figure 5 shows examples of airfoil retaining portions provided with cutouts. The embodiment shown in (a) has circular cutouts, while the embodiment shown in (b) has triangular cutouts. This configuration allows for weight reduction while maintaining the reinforcing effect of the propeller blade. This structure can be produced as a single unit with high precision, for example, by 3D printing.

[0052] It is also preferable to provide two or more airfoil retaining portions. For example, the length of each airfoil retaining portion in the spanwise direction can be set independently. However, it is preferable that one of the airfoil retaining portions has a length in the spanwise direction of 30% or more of the propeller blade's span, and the other has a length in the spanwise direction of less than 30% of the propeller blade's span. Figure 6 shows a cross-sectional view of a propeller blade cut to separate it into a pressure surface side and a suction surface side, in which two or more airfoil retaining portions 602, 603 are provided. Arranging the airfoil retaining portions in this manner allows the airfoil retaining portions to effectively and efficiently support the skin portion, thereby reducing the weight of the propeller blade. This structure can be manufactured as a single unit with high precision by, for example, manufacturing the first structure using a 3D printing method.

[0053] In addition, in the present invention, the airfoil retaining portion preferably has a length in the span direction that is less than 30% of the propeller blade's span, and a length in the chord direction at the spanwise center of the airfoil retaining portion that is less than 30% of the propeller blade's chord. In the case of a general hollow structure, it is preferable to have a length that is less than 30% of the longitudinal length of the hollow structure and a length in the direction perpendicular to the longitudinal direction that is less than 30% of the propeller blade's chord (such an airfoil retaining portion is conveniently referred to as a "local retaining portion"). Such an airfoil retaining portion is shorter in the span direction than a general shear web structure and shorter in the chord direction than a general rib structure. Such a structure makes it possible to locally reinforce weak areas. Here, the "spanwise center" of the airfoil holder is the midpoint between the base-most point and the tip-most point of the airfoil holder. For example, the propeller blade may be cut in the spanwise direction and the midpoint between the base-most point and the tip-most point of the airfoil holder that appears in the cross section may be measured with a ruler or calipers. Alternatively, the base-most point and the tip-most point of the airfoil holder may be measured by a CT scan. Furthermore, the chordwise length is the length between the leading edge and the trailing edge on a plane parallel to the blade thickness direction and perpendicular to the spanwise direction at the measurement position. For example, the length between the leading edge and the trailing edge of the propeller blade may be measured using a vernier caliper or ruler. Furthermore, as a method for measuring the chordwise length of the airfoil retaining portion, for example, the propeller blade may be cut in the chordwise direction and the chordwise lengths of the leading-edge and trailing-edge points of contact between the airfoil retaining portion and the skin portion that appear in the cross section may be measured using a ruler or calipers, or the chordwise lengths of the leading-edge and trailing-edge points of contact between the airfoil retaining portion and the skin portion may be measured using a CT scan.

[0054] The shorter the length of the local airfoil retaining portion in the wing span direction, the more locally reinforced the airfoil shape is, which is preferable. For example, a length of 20% or less of the airfoil length in the wing span direction is more preferable, and a length of 10% or less is even more preferable. For general hollow structures, a length of 20% or less of the longitudinal length of the hollow structure is more preferable, and a length of 10% or less is even more preferable. Furthermore, the shorter the length of the local airfoil retaining portion in the chord direction, the more locally reinforced the airfoil shape is, which is more preferable, and a length of 20% or less of the chord direction is more preferable, and a length of 10% or less is even more preferable. The shape of the local airfoil retaining portion is not particularly limited as long as it can reinforce the airfoil structure. For example, as shown in FIG. 7, examples include structures that connect the opposing surfaces of the skin portion in a cylindrical shape 701, a flat plate shape 702, a cross-sectional plate shape 703, or a corrugated plate shape 704. Furthermore, providing such an airfoil retaining portion near the leading edge of the propeller blade can provide a more effective reinforcement effect against loads applied when an object collides with the propeller blade.

[0055] [Fiber-reinforced resin layer] The propeller blade of the present invention has a fiber-reinforced resin layer on the outer surface side of the first structure.

[0056] The fibers used in the fiber-reinforced resin layer may be discontinuous or continuous, but preferably contain at least continuous fibers from the viewpoint of mechanical properties. Moreover, it is preferable that the fiber-reinforced resin layer covers the entire outer surface of the first structure.

[0057] Examples of fiber materials include organic fibers such as aramid fibers, polyethylene fibers, and polyparaphenylene benzoxazole (PBO) fibers; inorganic fibers such as glass fibers, carbon fibers, silicon carbide fibers, alumina fibers, Tyranno fibers, basalt fibers, and ceramic fibers; metal fibers such as stainless steel fibers and steel fibers; and fibers using boron fibers, natural fibers, and modified natural fibers. Among these, carbon fibers are preferred in the present invention because they are lightweight among these reinforcing fibers and have particularly excellent properties in terms of specific strength and specific modulus, as well as excellent heat resistance and chemical resistance. Furthermore, PAN-based carbon fibers, which are easily used to obtain high-strength carbon fibers, are more preferred.

[0058] Examples of resins include thermosetting resins such as epoxy resin, unsaturated polyester resin, vinyl ester resin, phenol resin, epoxy acrylate resin, urethane acrylate resin, phenoxy resin, alkyd resin, urethane resin, maleimide resin, and cyanate resin, as well as polyethylene terephthalate (PET) resin, polybutylene terephthalate (PBT) resin, polytrimethylene terephthalate (PTT) resin, polyethylene (PE) resin, polypropylene (PP) resin, styrene-based resin, polyoxymethylene (POM) resin, polyamide (PA) resin, polycarbonate (PC) resin, polymethylene methacrylate (PMMA) resin, polyvinyl chloride (PVC) resin, polyphenylene sulfide (PPS) resin, and polyphenylene ether (P Fluorine-based resins such as PE resin, modified PPE resin, polyimide (PI) resin, polyamideimide (PAI) resin, polyetherimide (PEI) resin, polysulfone (PSU) resin, modified PSU resin, polyethersulfone resin, polyketone (PK) resin, polyarylene ether ketone resin (PAEK), polyarylate (PAR) resin, polyethernitrile (PEN) resin, phenolic resin, phenoxy resin, polytetrafluoroethylene resin, and further thermoplastic elastomers such as polystyrene resin, polyolefin resin, polyurethane resin, polyester resin, polyamide resin, polybutadiene resin, polyisoprene resin, and fluorine-based resin, copolymers thereof, modified products thereof, and thermoplastic resins in which two or more types are blended may also be used. Furthermore, examples of the polyarylene ether ketone resin (PAEK) include polyether ketone (PEK), polyether ether ketone (PEEK), polyether ether ketone ketone (PEEKK), polyether ketone ketone (PEKK), polyether ketone ether ketone ketone (PEKEKK), polyether ether ketone ether ketone (PEEKEK), polyether ether ether ketone (PEEEK), and polyether diphenyl ether ketone (PEDEK), as well as copolymers, modified products, and blends of two or more of these resins.

[0059] In the present invention, the fiber-reinforced resin layer is provided on the outer surface side of the first structure, but it may be provided over the entire outer surface of the first structure, or it may not be provided corresponding to a portion of the outer surface as long as the object of the present invention can be achieved.

[0060] [Method for manufacturing a propeller blade] The method for manufacturing a hollow structure of the present invention will be described using the method for manufacturing a propeller blade as an example. The propeller blade can be manufactured by, for example, the method described below. However, the method is not limited to this method.

[0061] The manufacturing process of the propeller blade exemplified below comprises a manufacturing process of a first structure, a process of providing a fiber-reinforced resin layer on the outer layer of the first structure, and a curing process of curing the resin in the fiber-reinforced resin layer.

[0062] While the first structure can be manufactured using a general molding method, 3D printing is preferable because it allows for the manufacture of a highly flexible blade shape-retaining structure as a single unit, and is also lightweight and highly accurate. 3D printing methods include liquid-phase photopolymerization, material jetting, binder jetting, powder bed fusion, material extrusion, and sheet lamination, but a manufacturing method that does not require support materials is preferred. For example, the SLS method eliminates the need to remove support materials present in the hollow portion after molding.

[0063] The curing process of subsequently providing a fiber-reinforced resin layer and curing the resin is not particularly limited. Examples of such a process include preforming a fiber-reinforced resin sheet, in which reinforcing fibers are impregnated with resin, on the outer surface side of the first structure, placing the preform in a mold, and curing and molding the resin by autoclave or press molding; or winding a fiber-reinforced resin layer on the outer surface side of the first structure by filament winding, placing the preform in a mold, and curing and molding the resin by oven, autoclave, or press molding. Another example is a process in which reinforcing fibers are braided into the skin layer of the first structure, and then performing RTM molding in the mold. In any of these methods, it is preferable to integrally mold and cure the pressure and suction surfaces of the propeller blade. This facilitates the production of a stronger propeller blade and reduces the weight of the propeller blade compared to molding and joining the pressure and suction surface skins separately.

[0064] When curing the resin of the fiber-reinforced resin layer, a double-sided mold consisting of an upper mold and a lower mold is used, and for example, the lower mold is provided with a negative pressure side shape of the propeller blade, and the upper mold is provided with a positive pressure side shape of the propeller blade, thereby forming the surface shape of the propeller blade. In this case, it is preferable to press the fiber-reinforced resin layer against the mold surface in order to transfer the shape of the mold to the surface of the propeller blade. It is desirable to perform molding at a temperature lower than the glass transition temperature of the resin used in the first structure. By performing curing in this manner, the shape of the first structure can be maintained during curing, and molding pressure can be applied to the fiber-reinforced resin layer provided as the outer layer.

[0065] During molding, vacuuming may be performed as necessary, and after forming the shape by molding, post-curing may be performed in an oven.

[0066] There are no particular limitations on the method for providing a spar cap on a propeller blade. For example, when a fiber-reinforced resin sheet is preformed on the outer surface side of the first structure, the fiber-reinforced resin sheet can be preformed into a recess for the spar cap provided in the first structure, thereby enabling precise control of the position of the spar cap. Furthermore, the spar cap is preferably made of a fiber-reinforced composite material, and it is desirable to use a fiber base material oriented in one direction along the blade length. This configuration allows for effective reinforcement. After this process, a fiber-reinforced resin layer can be further formed.

[0067] A metal layer may also be provided on the propeller blade as a countermeasure against erosion or lightning, for example, by forming a fiber-reinforced resin layer of the propeller blade in a recess in advance where the metal member will be placed, and then bonding the layer, or by arranging the metal member in a mold when molding the fiber-reinforced resin layer and molding it as a single unit. Furthermore, a slit or hole may be provided in the first structure to position the metal member, and a corresponding positioning structure may be provided in part of the metal member to perform positioning.

[0068] An example of a fiber-reinforced resin sheet used for the preform is a prepreg. The volume content of the reinforcing fibers preferred for the prepreg is preferably 40% or more and less than 80%, more preferably 45% or more and less than 75%, and even more preferably 50% or more and less than 70%.

[0069] The amount of reinforcing fiber contained in the prepreg is 50 g / m as the basis weight of the reinforcing fiber when made into a sheet-like material. ２ More than 1000g / m ２ If the weight is too small, voids where no reinforcing fibers exist may occur within the surface of the prepreg. ２ By setting the weight to 1000 g / m or more, it is possible to eliminate voids that can become the starting points of fracture. ２If the weight is less than 100 g / m, heat can be uniformly transferred to the inside during preheating for molding. In order to achieve both uniformity in structure and uniformity in heat transfer, the weight is more preferably 100 g / m ２ More than 600g / m ２ More preferably, it is 150 g / m or less. ２ More than 400g / m ２ The basis weight of the reinforcing fiber is measured by cutting out a 10 cm square area from the reinforcing fiber sheet, measuring its mass, and dividing it by the area. The measurement is performed 10 times on different parts of the reinforcing fiber sheet, and the average value is used as the basis weight of the reinforcing fiber.

[0070] Furthermore, a cut prepreg can also be used as the prepreg. A cut prepreg is a prepreg having cuts regularly distributed throughout its entire surface. The use of a cut prepreg makes it easier for openings and misalignment to occur at the cut insertion points, improving the extensibility of the prepreg in the reinforcing fiber direction. Furthermore, the cut insertion points open during flow during compression molding, causing the reinforcing fiber bundles to separate, thereby increasing flexibility as a prepreg and enhancing its fluidity. By configuring the prepreg to flow in this way, the reinforcing fibers reach the edges, reducing areas with excess resin, and enabling the production of a flat, lightweight component with excellent mechanical properties and appearance. From the perspective of fluidity, it is preferable to make the cuts throughout the entire thickness direction of the prepreg.

[0071] The present invention will be explained in more detail below with reference to examples, but the present invention should not be construed as being limited to the description in the examples section.

[0072] (Example 1) A rectangular parallelepiped structure having a height of 50 mm, a width of 30 mm, and a length of 210 mm, a hollow portion of which is open on one side in the longitudinal direction, and a wall thickness of 2 mm, a height of 54 mm, a width of 34 mm, and a length of 212 mm, is used as the skin portion. The midpoint of the side corresponding to the width direction of the open bottom surface of the structure is the origin, the length direction is the X axis, the width direction is the Y axis, and the height direction is the Z axis, and the XY coordinate satisfies the formula y = 10 sin (6πx / 210) [mm]. A hollow structure made of nylon 6 was produced using an SLS 3D print. The skin portion of the structure was 2 mm thick, and the retaining portion was 2 mm thick. One of the longitudinal ends of the structure is formed with a skin, and the other is not formed with a skin, leaving the hollow portion open.

[0073] Next, a prepreg F6273C-07M manufactured by Toray was preformed on the skin surface of the structure, which was then bagged with a film, evacuated, and heated at 140° C. for 2 hours to be cured.

[0074] For the structure obtained in this manner, at the joint point at the upper height of the width center portion 70 mm from the side where the hollow portion is open, the line segment with the shortest length connecting the joint surface containing the joint point and the opposing joint surface is the line segment connecting the side where the hollow portion is open and the joint point at the lower height of the width center portion 70 mm from the side where the hollow portion is open. In a cross section perpendicular to the midpoint of this line segment, a projection image perpendicular to the cross section is a rectangle measuring 34 mm x 212 mm. The rectangle circumscribing the projection image perpendicular to the cross section and having the smallest area is a line connecting the midpoints of the short sides of the 340 mm x 210 mm rectangle. The line connecting the midpoints of the short sides of the 340 mm x 210 mm rectangle at a height of 25 mm intersects the support portion at seven points and the skin portion at one point.

[0075] By adopting the above-mentioned configuration, a lightweight, high-strength, and highly accurate structure was obtained.

[0076] The hollow structure, particularly the propeller blade, of the present invention is a propeller blade with a lightweight, high-quality hollow structure that is highly manufacturable, and can be suitably used for propellers for UAM (Urban Air Mobility), UAS (Unmanned Aircraft Systems), drones, aircraft, etc.

[0077] 101, 201, 801: Fiber-reinforced resin layer 102, 202, 301, 401, 601, 802, 901: Skin portion 803, 902: Retaining portion 103, 302, 404, 602: Airfoil-shaped retaining portion 104, 804: Hollow portion 203: Spar cap 304: Another airfoil-shaped retaining portion (local airfoil-shaped retaining portion) 402: Bent portion 403: Curved portion 602: Another airfoil-shaped retaining portion (local airfoil-shaped retaining portion) 603: Another retaining portion (local retaining portion) 701: Example of cylindrical airfoil-shaped retaining portion 702: Example of flat plate-shaped airfoil-shaped retaining portion 703: Example of cross plate-shaped airfoil-shaped retaining portion 704: Example of corrugated plate-shaped airfoil-shaped retaining portion 805: One of the joining points 806: The other end point (junction point) of the line segment with the shortest length connecting the joining surface including the joining point 805 and the opposing joining surface 807: Midpoint of the line segment connecting the joining points 805 and 806 808: Cross section of the structure cut by a plane including a cross section perpendicular to the line segment connecting the joining points 805 and 806 809: Rectangle circumscribing the cross section 808 and having the smallest area 810: Straight line connecting the midpoints of the short sides of the two-point rectangle 809 811: Straight line parallel to the line 810 903: Joint surface

Claims

1. A hollow resin structure having an internal space and an integrated skin portion that forms the outer surface and a retaining portion that supports the skin portion from the inside, wherein at least one of the retaining portions is arranged so that a line segment of the shortest length connecting from a certain joint point to the joint surface that includes the joint point and the opposing joint surface passes through the midpoint of the line segment, and in a cross section (conveniently referred to as "cross section 1") obtained by cutting the structure with a plane perpendicular to the line segment, a line parallel to a line (conveniently referred to as "line 1") that circumscribes the cross section and connects the midpoints of the short sides of a rectangle that has the smallest area intersects the skin portion and the retaining portion at three or more points.

2. 2. The hollow structure according to claim 1, further comprising a fiber-reinforced resin layer on the outer surface side of the structure.

3. A propeller blade consisting of a hollow structure as described in claim 1 or 2, wherein in at least one of the retaining portions, the plane dividing the pressure surface side and the suction surface side (for convenience, referred to as "cross section 2") is parallel to cross section 1, and the retaining portion is arranged so that a straight line on cross section 2 passing through the blade tip and blade root intersects the skin portion and the retaining portion at three or more points and is parallel to line 1.

4. 4. The propeller blade according to claim 3, wherein at least one of the retaining portions has a bent portion bent in the chord direction.

5. The hollow structure according to claim 3, wherein at least one of the retaining portions has a hollowed-out portion, the hollowed-out portion being arranged in a manner that can be observed in a vertical cross section that divides the propeller blade into a pressure surface side and a suction surface side.

6. The propeller blade according to claim 3 , wherein two or more of the holding portions are provided.

7. 4. The propeller blade according to claim 3, further comprising a retaining portion having a length in the spanwise direction that is less than 30% of the span of the propeller blade, and a length in the chordwise direction that is less than 30% of the chord of the propeller blade at the center of the blade spanwise direction of the airfoil retaining portion.

8. 4. The propeller blade according to claim 3, wherein the thickness of the skin portion is 0.2 mm or more and 15 mm or less.

9. 4. The propeller blade according to claim 3, wherein the thickness of the skin portion is 300% or less when the thickness of the fiber-reinforced resin layer is 100%.

10. The propeller blade according to claim 3 , wherein the skin portion has a mesh structure.

11. 4. The propeller blade according to claim 3, wherein the first structure has a recess formed on the outer surface of the skin at a contact point between the skin and one of the retaining portions that supports the blade in the thickness direction.

12. 4. The propeller blade according to claim 3, wherein the volume of the space in the structure is 30% or more of the volume of the propeller blade.

13. 4. The propeller blade according to claim 3, wherein the resin constituting the structure is a resin whose glass transition temperature is not observed to be below 90°C.

14. The propeller blade of claim 3 , wherein the structure includes a filler.

15. 4. The propeller blade according to claim 3, wherein the fiber reinforced resin layer is made of carbon fiber.

16. 4. The propeller blade of claim 3, wherein the structure is fabricated by a three-dimensional printing method.