Manufacturing method of radio wave absorber

Abstract

To provide a structure with radio wave absorbing performance even when it is difficult to apply radio wave absorbing paint to the inner surface of an aircraft air intake duct.SOLUTION: A manufacturing method of a radio wave absorber according to the embodiment includes the steps of molding a fiber-reinforced plastic having radio wave transparency on a molding die, applying a radio wave absorbing paint to the molded fiber-reinforced plastic while it is placed on the molding die, and placing and adhering a conductive material on the radio wave absorbing paint applied to the fiber-reinforced plastic.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] An embodiment of the present invention relates to a radio wave absorber, a method for manufacturing a radio wave absorber, an aircraft, and an air intake duct for an aircraft. [Background technology]

[0002] In order to improve the stealth performance of an aircraft against radar, it is necessary to suppress the radar cross-section (RCS). For this reason, various measures have been implemented in stealth aircraft to reduce the RCS. A typical measure is the use of radio wave absorbers (see, for example, Patent Document 1, Patent Document 2, and Patent Document 3). Radio wave absorbers are also used in technical fields other than aircraft, and are sometimes referred to as radio wave absorbing materials (see, for example, Patent Document 4 and Patent Document 5).

[0003] Radio wave absorbers are applied to the air intake ducts and external surfaces of aircraft fuselages, and serve to reduce the reflection of radio waves from radar (see, for example, Non-Patent Document 1). Common radio wave absorbers include radio wave absorbing paints and sheets made by adding additives such as magnetic materials with radio wave absorbing properties to resin (see, for example, Non-Patent Document 2). In particular, radio wave absorbers play an important role in the design of aircraft air intake ducts, which are designed to reduce RCS by applying radio wave absorbing paint to the curved tubular inner surface.

[0004] The thickness of radio wave absorbing paint is thicker than that of paint used on general aircraft, and is designed to match the wavelength of radio waves from radar. For this reason, it is necessary to control the thickness of the paint when it is applied. Known methods for controlling the thickness of radio wave absorbing paint include applying thin layers using a spray gun, and applying it with a brush to achieve a thicker film, and then machining it to achieve the desired film thickness. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 01-260298 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-031684 [Patent Document 3] Japanese Patent Application Laid-Open No. 2000-031685 [Patent Document 4] Japanese Patent Application Laid-Open No. 2007-258623 [Patent Document 5] Japanese Patent Application Laid-Open No. 2008-166834 [Non-patent literature]

[0006] [Non-Patent Document 1] RF-IR Stealth (Techniques / Benefits), [Retrieved June 6, 2023], Internet <URL: https: / / basicsaboutaerodynamicsandavionics.wordpress.com / 2016 / 03 / 04 / stealth-techniques-and-benefits / > [Non-patent document 2] Hsiu-Che Wang, et al. "Measurement of Multi-Layer Coatings of FEF Sensor with an Overview of SEAL Laboratory Activities," [Retrieved June 5, 2023], Internet <URL: https: / / depts.washington.edu / cpac / Activities / Meetings / Fall / 2010 / documents / MamishevF10_v1.pdf> Summary of the Invention [Problem to be solved by the invention]

[0007] However, the intake ducts of small aircraft, including unmanned aerial vehicles, are small in size, and there is no working space for applying the radio wave absorbing paint using a spray gun or for machining, making it difficult to apply the radio wave absorbing paint to the inner surface of the intake duct.

[0008] In addition, because radio wave absorbing paint contains metal powder and has a thick film thickness, it has the problem of being brittle and less durable than regular paint. Although radio wave absorbing paint applied to aircraft is protected by a protective layer, it is known that cracks still occur in the radio wave absorbing paint in the aircraft operating environment.

[0009] Therefore, an object of the present invention is to provide a structure with radio wave absorbing properties even when it is difficult to apply radio wave absorbing paint, such as to the inner surface of an aircraft air intake duct.

[0010] Another object of the present invention is to improve the durability of radio wave absorbing paint applied to a structure. [Means for solving the problem]

[0011] A method for manufacturing a radio wave absorber according to an embodiment of the present invention comprises the steps of: molding a fiber-reinforced plastic having radio wave permeability on a molding die; applying a radio wave absorbing paint to the molded fiber-reinforced plastic while it is placed on the molding die; and arranging and adhering a conductive material on the radio wave absorbing paint applied to the fiber-reinforced plastic.

[0012] Furthermore, a radio wave absorber according to an embodiment of the present invention comprises a conductive material, a layer of radio wave absorbing paint, and a fiber reinforced plastic having radio wave transparency, and the layer of radio wave absorbing paint is disposed between the conductive material and the fiber reinforced plastic.

[0013] Moreover, an aircraft according to an embodiment of the present invention is provided with the above-described radio wave absorber.

[0014] Moreover, an air intake duct for an aircraft according to an embodiment of the present invention is formed from the radio wave absorber described above. [Brief explanation of the drawings]

[0015] [Figure 1] 3 is a flowchart showing the flow of a method for manufacturing a radio wave absorber according to the first embodiment of the present invention. [Figure 2] 2A to 2C are diagrams for schematically explaining the flow of a method for manufacturing the radio wave absorber shown in FIG. 1. [Figure 3] 3 is a cross-sectional view of a radio wave absorber manufactured by the manufacturing method shown in FIGS. 1 and 2. FIG. [Figure 4] FIG. 2 is a diagram showing an example of an air intake duct for an aircraft formed using a radio wave absorber manufactured by the manufacturing method shown in FIG. 1. [Figure 5] FIG. 10 is a cross-sectional view showing the configuration of a conventional radio wave absorber to which radio wave absorbing paint is applied. [Figure 6] FIG. 4 is an enlarged partial cross-sectional view showing the configuration of a modified example of the radio wave absorber according to the first embodiment of the present invention. [Figure 7] FIG. 6 is an enlarged partial cross-sectional view showing the configuration of a radio wave absorber according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] A radio wave absorber, a method for manufacturing a radio wave absorber, an aircraft, and an air intake duct for an aircraft according to embodiments of the present invention will be described with reference to the accompanying drawings.

[0017] (First embodiment) (Method of manufacturing radio wave absorber) FIG. 1 is a flowchart showing the flow of a method for manufacturing a radio wave absorber according to a first embodiment of the present invention, FIG. 2 is a diagram for schematically explaining the flow of the method for manufacturing the radio wave absorber shown in FIG. 1, and FIG. 3 is a cross-sectional view of a radio wave absorber produced by the manufacturing method shown in FIGS. 1 and 2.

[0018] The manufacturing method of the radio wave absorber 1 shown in Figures 1 and 2 is a method of manufacturing the radio wave absorber 1 in which a layer of radio wave absorbing paint 3 is formed on the surface of a structure 2 made of a conductive material 2A that reflects radio waves emitted from a radar, and the layer of radio wave absorbing paint 3 is further covered with fiber reinforced plastics (FRP) 4 that has radio wave permeability, as exemplified in Figure 3.

[0019] First, in step P1 of Fig. 1, a forming die J having a shape corresponding to the shape of a structure 2 whose surface is to be endowed with the ability to absorb radio waves emitted from a radar is produced and prepared, as shown in Fig. 2(A). That is, the forming die J is produced and prepared as a jig for forming the radio wave absorber 1.

[0020] In the example shown in Fig. 2(A), the forming die J has a rod-shaped core JA so that a duct can be formed. When a core JA is used as the forming die J for the radio wave absorber 1, a core JA that can be removed after the radio wave absorber 1 is produced is used. In this case, it is possible to produce a hollow radio wave absorber 1, such as a tubular radio wave absorber 1 like a duct.

[0021] Of course, if the radio wave absorber 1 is not hollow, a plate-shaped or block-shaped lower mold for placing the material of the radio wave absorber 1 and giving a shape to the lower surface may be prepared as the molding die J. In the following, an example will be described in which a hollow radio wave absorber 1 is molded mainly using a core JA.

[0022] The core JA can be made of an easily breakable material such as plaster or sand, or it can be made of a water-soluble material or a partially reinforced flexible bladder bag.

[0023] Next, in step P2, the radio wave permeable FRP4 is molded on the surface of the molding die J, and then a demolding process is performed to release the radio wave permeable FRP4 from the molding die J. A representative example of the radio wave permeable FRP4 is glass fiber reinforced plastics (GFRP).

[0024] Various release processes have been known for some time to allow FRP4 such as GFRP to be released from the mold J. A typical method is to place a release film made of fluororesin such as polytetrafluoroethylene (PTFE) or tetrafluoroethylene-hexafluoropropylene copolymer (FEP) between the mold J and the FRP. Other known methods include applying a release material in paste form or by spraying.

[0025] Next, in step P3, as shown in Fig. 2(B), a radio wave permeable FRP4 such as GFRP is molded on the mold J after the release treatment. The radio wave permeable FRP4 plays a role in covering and protecting the layer made of the radio wave absorbing paint 3, as shown in Fig. 3. For this reason, the thickness of the radio wave permeable FRP4 is set to a thickness necessary to protect the radio wave permeable FRP4.

[0026] However, the radio wave-transmitting FRP4 also slightly reflects radio waves. Therefore, in order to absorb as much of the radio waves incident on the radio wave-transmitting FRP4 as possible with the layer made of the radio wave absorbing paint 3, it is preferable to make the thickness of the radio wave-transmitting FRP4 as thin as possible.

[0027] When the target to be absorbed is microwaves (SHF: Super High Frequency), also known as the X-band, which have a frequency of 8 to 12 GHz and a wavelength of 25 to 37 mm, taking into consideration the physical properties such as the frequency characteristics and mechanical properties of radio wave-transparent FRP4, as well as test results, it is considered appropriate to set the thickness of the radio wave-transparent FRP4 in the range of 0.1 mm to 0.5 mm.

[0028] Typical methods for molding FRP4 include laminating and molding sheet-like prepregs, in which fibers are impregnated with thermoplastic resin or uncured thermosetting resin, and the RTM (Resin Transfer Molding) method, in which fibers are laminated and then impregnated with uncured thermosetting resin, which is then cured.

[0029] When a method of laminating and molding prepregs is used as a molding method for FRP4, a molding die J is used as a lamination jig, and prepregs made of fibers such as glass fibers impregnated with resin are laminated on top of the molding die J. If the resin contained in the prepreg is an uncured thermosetting resin, the thermosetting resin can be pressurized and heated after the prepreg is laminated to mold FRP4 having a shape corresponding to the shape of the molding die J. On the other hand, if the resin contained in the prepreg is a thermoplastic resin, the thermoplastic resin can be melted by pressurizing and heating after the prepreg is laminated, and then cooled by air cooling or the like to mold FRP4 having a shape corresponding to the shape of the molding die J.

[0030] On the other hand, when the RTM method is used as the molding method for FRP4, a molding die J is used as a laminating jig, and a fiber sheet such as glass fiber is laminated on top of the molding die J. After that, uncured thermosetting resin that has been heated to a temperature sufficient to give it fluidity is impregnated into the laminated fibers, and the thermosetting resin is heated to its curing temperature, thereby molding FRP4 having a shape corresponding to the shape of the molding die J.

[0031] In either case, a heating device such as an oven or autoclave is used to heat the resin. When molding FRP4, the resin is pressurized before molding using atmospheric pressure or a separate mold.

[0032] When atmospheric pressure is used to pressurize the resin before molding, the resin before molding is sealed with a bagging sheet and the space sealed by the bagging sheet is evacuated. On the other hand, when a mold such as an upper mold is used to pressurize the resin before molding, the mold is pressed against the molding die J.

[0033] As shown in Figure 2(B), when the molding die J is a core JA, the resin is pressurized in a 360-degree direction perpendicular to the surface of the core JA before molding. Therefore, it is practical to perform bagging to use atmospheric pressure to pressurize the resin before molding.

[0034] Furthermore, if an FRP4 molding method is adopted in which prepreg sheets are laminated and then cured or re-cured, it becomes easy to adjust the thickness of the FRP4. As a specific example, the thickness of readily available GFRP is about 0.25 mm, so by laminating and molding GFRP prepregs in a one- or two-layer molding die J, it is possible to mold FRP4 of an appropriate thickness.

[0035] Next, in process P4, pre-painting treatment is carried out that is necessary to apply the radio wave absorbing paint 3 to the molded FRP4 while it is placed on the mold J. A typical example of pre-painting treatment for FRP4 is blasting, which roughens the surface of the FRP4 to improve the wettability of the radio wave absorbing paint 3. If the pre-painting treatment involves removing the peel ply that has been attached to the surface of the FRP4 in order to improve adhesion to the radio wave absorbing paint 3, then the peel ply is placed on the surface of the FRP4 before molding in process P3.

[0036] Next, in process P5, the radio wave absorbing paint 3 is applied to the FRP4 after molding and pre-painting treatment while it is placed on the forming mold J. As a result, as shown in Fig. 2(C), a layer made of the radio wave absorbing paint 3 is formed on the surface of the FRP4 that is radio wave permeable, i.e., on the surface of the FRP4 that is not in contact with the release material on the side opposite the forming mold J.

[0037] The radio wave absorbing paint 3 can be applied by a known method using a desired tool such as a spray gun, a brush, and a cutting tool so that the thickness of the coating of the radio wave absorbing paint 3 is appropriate for ensuring the ability to absorb radio waves emitted from the radar. Of course, the radio wave absorbing paint 3 may also be applied to the surface of the FRP 4 using a dedicated device that automatically applies the radio wave absorbing paint 3.

[0038] The layer made of the radio wave absorbing paint 3 may be sandwiched between layers made of a primer (undercoat paint) such as an adhesive primer whose main purpose is to improve adhesion. In other words, the primer may be applied before or after applying the radio wave absorbing paint 3. In this case, the primer and the radio wave absorbing paint 3 are applied in layers to the surface of the FRP 4.

[0039] Therefore, the radio wave permeable FRP 4 such as GFRP not only serves to cover and protect the layer made of the radio wave absorbing paint 3, but also serves as a coating base for the radio wave absorbing paint 3, which allows the layer to be easily removed from the molding die J by means of a release treatment. In particular, release materials such as release films are difficult to adhere to, and therefore the radio wave absorbing paint 3 or primer cannot be applied directly to the release material.

[0040] In contrast, if the FRP4 such as GFRP has been pre-treated for painting, it is easy to apply the radio wave absorbing paint 3 and primer. That is, for FRP4 such as GFRP, which is widely used as a material for aircraft parts, technology including pre-treatment for painting for applying the radio wave absorbing paint 3 has already been established. For this reason, the radio wave absorbing paint 3 can be applied to the FRP4 in a state where sufficient adhesion between the radio wave absorbing paint 3 and the FRP4 is ensured.

[0041] Next, in step P6, as shown in Fig. 2(D), a structure 2 made of a conductive material 2A is placed and adhered onto the layer of radio wave absorbing paint 3 applied to the surface of the radio wave permeable FRP 4. In this way, a radio wave absorber 1 is produced, which has the radio wave permeable FRP 4, the layer of radio wave absorbing paint 3, and the structure 2 made of the conductive material 2A.

[0042] The structure 2 of the radio wave absorber 1 is made of a conductive material 2A that is strong enough to bear a load. Typical conductive materials 2A that make up aircraft parts include metals such as aluminum alloys and titanium alloys, as well as carbon fiber reinforced plastics (CFRP).

[0043] 2(D), when the molding die J is a core JA, a structure 2 made of a hollow conductive material 2A is placed and adhered onto the layer of radio wave absorbing paint 3. For this reason, although it is possible to fabricate a hollow structure 2 made of metal and adhere it to the layer of radio wave absorbing paint 3 with an adhesive, it is practical and rational to mold a hollow structure 2 made of CFRP onto the layer of radio wave absorbing paint 3.

[0044] Of course, even when the structure 2 made of the conductive material 2A is solid, the structure 2 made of CFRP can be formed on the layer of the radio wave absorbing paint 3. Alternatively, a solid structure 2 made of metal can be bonded on the layer of the radio wave absorbing paint 3 with an adhesive.

[0045] Even when molding the structure 2 made of CFRP on a layer of radio wave absorbing paint 3, the CFRP can be molded using a molding method similar to the molding method for radio wave permeable FRP 4. That is, the CFRP can be molded by laminating prepregs in which carbon fibers are impregnated with resin, or by the RTM method in which carbon fibers are laminated and then the carbon fiber laminate is impregnated with resin to mold the CFRP.

[0046] When molding the CFRP structure 2 on a layer of radio wave absorbing paint 3, the CFRP structure 2 can be bonded onto the layer of radio wave absorbing paint 3 using the unhardened thermosetting resin contained in the CFRP before molding or the molten thermoplastic resin just before re-hardening as an adhesive. In this case, the bonding and molding of the CFRP are carried out simultaneously. This method of simultaneously bonding and molding the CFRP is called co-bond.

[0047] Alternatively, an adhesive may be used to improve adhesion between the CFRP and the radio wave absorbing paint 3. As a practical example, if an adhesive film is sandwiched between the CFRP and the radio wave absorbing paint 3 before molding and the CFRP is heated, it becomes possible to bond and mold the CFRP simultaneously.

[0048] Of course, the molded CFRP may be bonded onto the layer of radio wave absorbing paint 3 with an adhesive. In this case, the CFRP may be divided into a plurality of parts, and each of the components of the structure 2 made of a plurality of CFRP may be bonded onto the layer of radio wave absorbing paint 3 with an adhesive. The assembly work of dividing the structure 2 into a plurality of parts and bonding them onto the layer of radio wave absorbing paint 3 with an adhesive can also be performed when the structure 2 is made of metal. In this case, it is possible to manufacture a radio wave absorber 1 having a complex shape without generating any seams between the layer of radio wave absorbing paint 3 and the layer made of FRP 4 that is radio wave permeable.

[0049] Next, in step P7, as shown in Fig. 2(E), the molding die J is removed from the radio wave absorber 1. If the molding die J is a core JA made of an easily breakable material such as plaster or a sand mold, the radio wave absorber 1 can be demolded by destroying the core JA. Also, if the molding die J is a water-soluble core JA, the radio wave absorber 1 can be demolded by dissolving the core JA.

[0050] At this time, because a release treatment has been applied to the surface of the molding die J such as the core JA, it is possible to easily demold the radio wave absorber 1. For example, it is possible to reliably prevent problems such as parts of the broken core JA or parts of the core JA that have not completely dissolved remaining on the surface of the radio wave permeable FRP 4.

[0051] As described above, in the manufacturing method of the radio wave absorber 1 shown in Figs. 1 and 2, in the process of producing the radio wave absorber 1 having a layer made of radio wave absorbing paint 3, contrary to normal painting, a layer made of FRP4 having radio wave permeability for protecting the layer made of radio wave absorbing paint 3 is first formed using a molding die J such as a core JA, and then the radio wave absorbing paint 3 is applied to the layer made of FRP4, and further thereafter the structure 2 made of a conductive material 2A is disposed adjacent to and bonded to the layer made of radio wave absorbing paint 3.

[0052] (Radio wave absorber, aircraft and aircraft air intake duct) The radio wave absorber 1 produced by the manufacturing method shown in FIG. 1 has a structure 2 made of a conductive material 2A, a layer made of radio wave absorbing paint 3, and a layer made of radio wave permeable FRP4, as shown in FIG. 3, and is a radio wave absorber 1 in which the layer made of radio wave absorbing paint 3 is disposed between the structure 2 made of the conductive material 2A and the layer made of radio wave permeable FRP4.

[0053] In order to maximize the radio wave absorbing properties of the radio wave absorbing paint 3, it is important not only to form a layer made of the radio wave absorbing paint 3, but also to form a layer made of a conductive material 2A that reflects radio waves well on the side opposite to the side where the radio waves enter the layer made of the radio wave absorbing paint 3. This is because the radio waves that enter the radio wave absorbing paint 3 are reflected by the conductive material 2A, and are absorbed by multiple reflection within the layer made of the radio wave absorbing paint 3.

[0054] This radio wave absorber 1 can be installed in any desired location on an aircraft. That is, the radio wave absorber 1 can be used to build a stealth aircraft.

[0055] FIG. 4 is a diagram showing an example in which an air intake duct 11 of an aircraft 10 is formed using the radio wave absorber 1 manufactured by the manufacturing method shown in FIG.

[0056] A typical air intake duct 11 of a stealth fighter, which is a representative example of an aircraft 10 that is required to be equipped with a radio wave absorber 1 that absorbs radio waves R from a radar, has a curved shape so that the engine 12 is not directly visible from the front of the stealth fighter, as shown in Figure 4.

[0057] Therefore, the tubular portion including at least the inner surface of the air intake duct 11 can be formed with the radio wave absorber 1. Specifically, a structure 2 made of a conductive material 2A can be used as a structural member of the air intake duct 11, and a layer made of radio wave absorbing paint 3 can be formed on the inner surface side of the air intake duct 11 that forms the air flow surface.

[0058] In this case, even if a portion of the radio waves R from the radar that enters the air intake duct 11 is slightly reflected by the radio wave absorber 1, the radio waves will be reflected many times by the inner surface of the winding air intake duct 11 and will therefore be incident on the radio wave absorber 1 many times. As a result, even if the radio waves R transmitted from the radar are slightly reflected by the radio wave absorber 1, the radio waves R will repeatedly be incident on the radio wave absorber 1, so the reflection intensity of the radio waves R can be gradually attenuated. Of course, other parts of the aircraft 10, such as the surfaces of the main wings, can also be made up of the radio wave absorber 1.

[0059] When forming at least a part of the radio wave absorber 1, including the inner surface that becomes the airflow surface of the air intake duct 11 of the aircraft 10, a core JA having the same shape as the inner surface of the air intake duct 11 can be produced as a molding die J. Then, after molding the radio wave permeable FRP 4 that will become the airflow surface and inner surface of the air intake duct 11, radio wave absorbing paint 3 can be applied to the FRP 4. Thereafter, a part or a main part of the air intake duct 11 can be produced by arranging and adhering a structure 2 made of a conductive material 2A that ensures the strength of the air intake duct 11.

[0060] (effect) The manufacturing method of the radio wave absorber 1 described above makes it possible to ensure the working space required for applying the radio wave absorbing paint 3. That is, even in cases where there is insufficient working space, such as the inner surface of the air intake duct 11 of the aircraft 10, and it is difficult to employ the conventional radio wave absorbing paint application methods of applying multiple coats of radio wave absorbing paint with a spray gun or scraping off radio wave absorbing paint applied with a brush, it is possible to form a layer made of the radio wave absorbing paint 3 on the surface of the structure 2.

[0061] As a result, even in narrow spaces where it is impossible to insert painting tools such as spray guns or cutting tools, it is possible to impart radio wave absorbing performance by forming a layer made of the radio wave absorbing paint 3. In particular, even in tubular structures 2 with small internal dimensions or structures 2 with complex shapes, such as the curved air intake duct 11 of an aircraft 10, it is possible to form a layer made of the radio wave absorbing paint 3 by using the core JA.

[0062] Furthermore, if the shape of the structure 2 is complex, it is possible to disassemble the structure 2 into multiple parts, apply radio wave absorbing paint to each part, and then assemble the parts. However, in this case, undesirable seams that cause radio wave reflection will appear in the radio wave absorbing paint.

[0063] In contrast, in the case of the manufacturing method of the radio wave absorber 1 described above, at least the layer made of the radio wave absorbing paint 3 and the layer made of the radio wave permeable FRP 4 can be formed seamlessly. In other words, no undesirable seams that cause radio wave reflection are generated.

[0064] In addition, by arranging the FRP4, which has radio wave permeability, between the forming mold J and the layer made of the radio wave absorbing paint 3, it becomes possible to manufacture the radio wave absorber 1 by combining conventionally performed processes. That is, the process of performing a release treatment on the forming mold J to form the FRP4, the process of applying the radio wave absorbing paint 3 to the FRP4, and the process of demolding the forming mold J from the FRP4 are all processes that have been performed conventionally and for which production technology has been established. Therefore, defects are unlikely to occur and stable manufacturing of the radio wave absorber 1 is possible.

[0065] Furthermore, by protecting the surface of the layer made of radio wave absorbing paint 3 on the air flow side with radio wave permeable FRP 4, it is possible to effectively protect the layer made of radio wave absorbing paint 3 from the external environment. That is, according to the radio wave absorber 1, the manufacturing method for the radio wave absorber 1, and the aircraft 10 described above, it is possible to improve the effect of preventing cracking and peeling of the layer made of radio wave absorbing paint 3 compared to conventional methods.

[0066] FIG. 5 is an enlarged partial cross-sectional view showing the structure of a conventional radio wave absorber coated with radio wave absorbing paint.

[0067] When a conventional radio wave absorber 20 is constructed by applying radio wave absorbing paint 22 to a structure 21, the surface side of the radio wave absorbing paint 22 is protected by a top coat 23. The top coat 23 is formed by applying a resin. Furthermore, when the structure 21 is a non-conductive structural member such as GFRP, or when the structure 21 is manufactured by assembling a plurality of parts, a conductive paint 24 is applied between the structure 21 and the layer made of radio wave absorbing paint 22.

[0068] In other words, the conductive paint 24, the radio wave absorbing paint 22, and the resin for forming the top coat 23 are applied in layers to the surface side of the structure 21. Furthermore, primers 25 are applied as a base for applying the conductive paint 24, a base for applying the radio wave absorbing paint 22, and a base for applying the resin for forming the top coat 23, respectively.

[0069] However, the radio wave absorbing paint 22 contains powder of metal or the like, and the thickness of the radio wave absorbing paint 22 is much thicker than that of ordinary paint. Therefore, the radio wave absorbing paint 22 is brittle and less durable than ordinary paint. As a result, even if the radio wave absorbing paint 22 is protected with a resin top coat 23, it will crack and peel off in the harsh operating environment of an aircraft.

[0070] In contrast to this, the FRP 4 that protects the layer made of the radio wave absorbing paint 3 in the above-described radio wave absorber 1 has high strength and rigidity within the plane of the FRP 4. Therefore, compared to the conventional radio wave absorbing paint 22 protected by a resin top coat 23, it is possible to reduce the possibility of cracking or peeling occurring in the layer made of the radio wave absorbing paint 3.

[0071] (Modification of the first embodiment) FIG. 6 is an enlarged partial cross-sectional view showing the configuration of a modified example of the radio wave absorber according to the first embodiment of the present invention.

[0072] 2 and 3 show an example in which the structure 2 is made only of the conductive material 2A, but the structure 2 may also contain a non-conductive material. In that case, as exemplified in Fig. 6, the substrate 2B of the structure 2, which contains at least a portion of the non-conductive material, may be coated with a layer made of the conductive material 2A.

[0073] The layer made of conductive material 2A is formed on the layer made of radio wave absorbing paint 3 during the production of the radio wave absorber 1. For this reason, the layer made of conductive material 2A can also be formed by applying a conductive paint on the layer made of radio wave absorbing paint 3 or by adhering a metal foil with an adhesive, but since the FRP4 having radio wave permeability has already been molded using mold J as shown in Figure 2(C), it is practical and reasonable to form the layer made of conductive material 2A by molding CFRP on the layer made of radio wave absorbing paint 3.

[0074] In the example shown in Fig. 6, a primer 3A is applied to both sides of the radio wave absorbing paint 3 to improve adhesion to the radio wave absorbing paint 3. Therefore, as illustrated in Fig. 6, CFRP can be molded as a layer made of conductive material 2A indirectly on the layer made of radio wave absorbing paint 3 with the primer 3A in between, or directly on the layer made of radio wave absorbing paint 3.

[0075] The substrate 2B can be bonded onto the layer made of conductive material 2A with an adhesive 2C such as a heat-melted adhesive film. When the layer made of conductive material 2A is formed by molding CFRP, the substrate 2B can be bonded to the conductive material 2A made of CFRP using a co-bond at the same time as the CFRP is molded, or after the CFRP is molded. Alternatively, the substrate 2B can be bonded onto the layer made of conductive material 2A using a co-bond that uses a resin contained in the CFRP before molding as the adhesive.

[0076] (Second embodiment) FIG. 7 is an enlarged partial cross-sectional view showing the configuration of a radio wave absorber according to a second embodiment of the present invention.

[0077] The radio wave absorber 1A in the second embodiment shown in Fig. 7 differs from the radio wave absorber 1 in the first embodiment in that the thickness T of the radio wave permeable FRP4 is adjusted so that the radio wave permeable FRP4 has radio wave absorption properties in addition to the radio wave absorbing paint 3. The other configurations and functions of the radio wave absorber 1A in the second embodiment and the manufacturing method for the radio wave absorber 1A are not substantially different from the radio wave absorber 1 and the manufacturing method for the radio wave absorber 1 in the first embodiment, so only an enlarged partial cross-sectional view of the radio wave absorber 1A is shown, and the same or corresponding configurations are denoted by the same reference numerals and their description is omitted.

[0078] In the harsh operating environment of an aircraft, it may not be possible to reduce the thickness T of the FRP4 that protects the layer made of the radio wave absorbing paint 3. In such cases, by setting the thickness T of the FRP4 that protects the layer made of the radio wave absorbing paint 3 to a thickness that corresponds to the wavelength λ of the radio waves R that are to be absorbed, the radio wave-transparent FRP4 can be made to function as a λ / 4 type radio wave absorber.

[0079] More specifically, as shown in Figure 7, the thickness T of the radio wave-transmitting FRP4 can be set to 1 / 4 the wavelength λ of the radio wave R to be absorbed. In this case, a portion of the radio wave R that enters the FRP4 and is reflected from the back surface of the FRP4, where the refractive index is discontinuous, and another portion of the radio wave R that is reflected from the front surface of the FRP4 without entering the FRP4, cancel each other out. Therefore, if the thickness of the primer 3A can be ignored, even if the radio wave R is slightly reflected from the surface of the primer 3A, the surface of the layer made of the radio wave-absorbing paint 3, and the surface of the radio wave-transmitting FRP4, the intensity of the reflected radio wave R can be weakened.

[0080] Furthermore, even if cracks occur in the layer made of radio wave absorbing paint 3 due to deterioration over time, if there are no defects in the radio wave permeable FRP4, the intensity of the radio waves R reflected by the radio wave absorbing paint 3 can be weakened by the radio wave permeable FRP4.

[0081] (Other embodiments) Although specific embodiments have been described above, the described embodiments are merely examples and do not limit the scope of the invention. The novel methods and apparatus described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions, and modifications may be made in the forms of the methods and apparatus described herein without departing from the spirit of the invention. The appended claims and their equivalents include all such forms and modifications as fall within the scope and spirit of the invention. [Explanation of symbols]

[0082] 1. 1A Radio wave absorber 2 structure 2A conductive material 2B Base material 2C Adhesive 3. Radio wave absorbing paint 3A Primer 4. Radio wave permeable FRP 10 aircraft 11 Air intake duct 12 Engine 20 Conventional wave absorber 21 Structure 22 Radio wave absorbing paint 23 Top Coat 24 Conductive paint 25 Primer J mold JA Nakako R radio wave T Thickness of radio wave-transmitting FRP

Claims

1. A step of molding a fiber reinforced plastic having radio wave transparency on a molding die; applying a radio wave absorbing paint to the fiber reinforced plastic after molding while the fiber reinforced plastic is placed on the molding die; A step of placing and adhering a conductive material on the radio wave absorbing paint applied to the fiber reinforced plastic; A method for manufacturing a radio wave absorber having the above structure.

2. 2. The method for manufacturing a radio wave absorber according to claim 1, wherein a tubular radio wave absorber is manufactured by using a core as the molding die which is removable after the radio wave absorber is manufactured.

3. 3. The method for manufacturing a radio wave absorber according to claim 1, wherein the thickness of the fiber reinforced plastic is set to a thickness corresponding to the wavelength of the radio waves so that a part of the radio waves that is incident on the surface of the fiber reinforced plastic and then reflected by the back surface of the fiber reinforced plastic and another part of the radio waves that is reflected by the surface of the fiber reinforced plastic without being incident on the fiber reinforced plastic cancel each other out.

4. a conductive material; a layer made of radio wave absorbing paint; a fiber reinforced plastic having radio wave transparency; A radio wave absorber having A radio wave absorber in which the layer of radio wave absorbing paint is disposed between the conductive material and the fiber reinforced plastic.

5. An aircraft provided with the radio wave absorber according to claim 4.

6. 5. An air intake duct for an aircraft formed from the radio wave absorber according to claim 4.

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