Radio wave absorber, method for manufacturing a radio wave absorber, and aircraft

Abstract

The goal is to reduce the weight and cost increase that would result from using radar-absorbing materials to give aircraft stealth capabilities. [Solution] The radio wave absorber according to the embodiment is a radio wave absorber that forms the surface of an aircraft structure having a shape that repeatedly reflects radio waves to be absorbed, and comprises a first radio wave absorbing portion that forms a first region of the surface and has a first radio wave absorbing performance, and a second radio wave absorbing portion that forms a second region of the surface different from the first region and has a second radio wave absorbing performance lower than the first radio wave absorbing performance.

Description

Technical Field

[0001] Embodiments of the present invention relate to a radio wave absorber, a method for manufacturing the radio wave absorber, and an aircraft.

Background Art

[0002] In stealth aircraft typified by stealth fighters, stealth against radio waves from radar is an important aspect of the aircraft's capabilities. In order to enhance the stealth of an aircraft against radio waves from radar, it is necessary to suppress the radar cross-section (RCS). That is, if the reflected wave of the radio wave transmitted from the radar can be suppressed, it becomes difficult to detect the stealth aircraft from the radar.

[0003] Typical countermeasures for reducing RCS in stealth aircraft include shaping the surface of the stealth aircraft so that radio waves from the radar are not reflected in the direction of arrival, reflecting radio waves in a specific direction, and the use of radio wave absorbers (see, for example, Patent Document 1, Patent Document 2, and Patent Document 3).

[0004] A radio wave absorber has the property of absorbing the energy of radio waves transmitted from a radar and is also called a radio wave absorbing material. A radio wave absorber has an absorption layer that absorbs radio waves and a reflection layer of radio waves made of a metal plate or the like. A general radio wave absorber is classified into a single-layer type in which one absorption layer is stacked on the reflection layer, a two-layer type in which one absorption layer is sandwiched between a surface layer and the reflection layer, and a multi-layer type in which a plurality of absorption layers are sandwiched between the surface layer and the reflection layer (see, for example, Non-Patent Document 1).

[0005] Main radio wave reflection sites in a stealth fighter include the leading edge of wings such as the main wing, steps existing on the outer skin (panel) of the aircraft body, and ducts (intake ducts) of air intakes. Therefore, suppressing radio wave reflection at these sites leads to a reduction in RCS. For this reason, techniques for making the shape of the aircraft body and the panel difficult to reflect radio waves and techniques for providing a radio wave absorber in the intake duct are used.

[0006] Known techniques for installing radio wave absorbers in intake ducts include techniques for absorbing radio waves that enter the intake duct by covering the outer perimeter of the intake duct with a radio wave absorber so as not to obstruct the flow of incoming air, and techniques for broadening the absorption bandwidth of radio waves that enter the intake duct by configuring the intake duct with multiple types of radio wave absorbers whose center frequencies of absorption bands are different from each other (see, for example, Patent Documents 4 and 5). [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 01-260298 [Patent Document 2] Japanese Patent Publication No. 2000-031684 [Patent Document 3] Japanese Patent Publication No. 2000-031685 [Patent Document 4] Japanese Patent Application Publication No. 07-257492 [Patent Document 5] Japanese Patent Publication No. 2022-068899 [Non-patent literature]

[0008] [Non-Patent Document 1] Osamu Hashimoto, "Principles, Structure, and Applications of Radio Wave Absorbers with Expanding Applications in ETC Lanes, Wireless LAN, RF-ID, Radar, etc.," RF World, No. 7, pp. 107-108, [Retrieved September 13, 2024], Internet<URL: https: / / www.rf-world.jp / bn / RFW07 / samples / p107-108.pdf> [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] In the case of stealth aircraft, the reflective layer that makes up the radar-absorbing material is made of metal plates or carbon fiber reinforced plastics (CFRP) plates. These metal or CFRP plates often serve both as a reflective layer that reflects radar waves and as a structural component of the aircraft that bears the load. Furthermore, the thickness of the metal or CFRP plate is irrelevant to the radar-absorbing performance of the radar-absorbing material.

[0010] On the other hand, the absorption layer and surface layer of radar absorbers used in stealth aircraft utilize materials to which fillers such as carbon, ferrite, or metal powders have been added to impart dielectric and magnetic properties. While it is not always the case, typically increasing the amount of absorption layer used improves the radar absorption performance of the radar absorber. Furthermore, the absorption layer and surface layer are usually not strong enough to function as structural members that can withstand bending loads.

[0011] Therefore, conventionally, when designing radar absorbers used in stealth aircraft, the thickness of the absorption layer and surface layer is determined to satisfy the requirements for radar radar absorption performance, specifically the frequency band and attenuation of the radio waves to be absorbed. Then, the thickness and shape of the metal plate or CFRP plate are determined to meet the strength requirements.

[0012] However, the absorption and surface layers of radio wave absorbers remain heavy and expensive for materials that require weight reduction, such as those used in aircraft. In other words, the absorption and surface layers of radio wave absorbers are heavier and more expensive than metal plates and CFRP plates. As a result, the use of radio wave absorbers contributes to increased weight and cost in aircraft structures.

[0013] Therefore, the present invention aims to reduce the increase in weight and cost that occurs when using radar absorbers to impart stealth capabilities to an aircraft. [Means for solving the problem]

[0014] An embodiment of the present invention is a radio wave absorber that forms the surface of an aircraft structure having a shape that repeatedly reflects radio waves to be absorbed, and comprises a first radio wave absorbing portion that forms a first region of the surface and has a first radio wave absorbing performance, and a second radio wave absorbing portion that forms a second region of the surface different from the first region and has a second radio wave absorbing performance lower than the first radio wave absorbing performance.

[0015] Furthermore, in the aircraft according to the embodiment of the present invention, the inner surface of the intake duct is formed with the above-mentioned radio wave absorber, the first region is set on the front side of the intake duct, and the second region is set on the rear side of the intake duct.

[0016] Furthermore, the method for manufacturing a radio wave absorber according to an embodiment of the present invention comprises a design step of creating design information for a radio wave absorber that forms the surface of an aircraft structure having a shape that repeatedly reflects radio waves to be absorbed, and a manufacturing step of manufacturing the radio wave absorber based on the design information. In the design step, with constraints of ensuring the radio wave absorption performance required at each position on the surface according to the frequency of reflection of the radio waves at each position and ensuring the strength required at each position on the surface to withstand the load applied to each position, the method determines a plurality of regions on the surface having different radio wave absorption performance and the respective thicknesses of the radio wave absorption layers included in each of the plurality of regions by performing an optimization calculation that reduces the weight of the radio wave absorption layer included in the radio wave absorber. [Brief explanation of the drawing]

[0017] [Figure 1] A flowchart showing the process for manufacturing a radio wave absorber according to an embodiment of the present invention. [Figure 2] This figure shows the basic structure of each part of a single-layer radio wave absorber that can be manufactured using the manufacturing method shown in Figure 1. [Figure 3] This figure shows the basic structure of each part of a two-layer radio wave absorber that can be manufactured using the manufacturing method shown in Figure 1. [Figure 4]A diagram showing the basic structure of each part of a multilayer radio wave absorber that can be manufactured by the manufacturing method shown in FIG. 1. [Figure 5] A longitudinal sectional view showing an example of an intake duct of an aircraft manufactured by the manufacturing method shown in FIG. 1. [Figure 6] A map showing the result of radio wave reflection path analysis on the upper surface side inside the intake duct shown in FIG. 5. [Figure 7] A map showing the result of radio wave reflection path analysis on the lower surface side inside the intake duct shown in FIG. 5. [Figure 8] A cross-sectional view showing an image of an out-of-plane load that is dominant in front of the intake duct shown in FIG. 5. [Figure 9] A cross-sectional view showing an image of an in-plane load that is dominant behind the intake duct shown in FIG. 5. [Figure 10] A graph showing the result of calculating the required plate thickness for each layer of the radio wave absorber constituting the intake duct shown in FIG. 5 by dividing it into a first region on the front side and a second region on the rear side of the intake duct. [Figure 11] A table showing the total plate thickness of the CFRP layer constituting the reflection layer when the combinations of the thicknesses of the surface layer, absorption layer, and reflection layer shown in Example 1, Comparative Example 1, and Comparative Example 2 are selected in the graph of FIG. 10. [Figure 12] A graph showing an example of the analysis result of the radio wave absorption performance of the radio wave absorber 1 shown in FIG. 5.

Embodiments of the Invention

[0018] The radio wave absorber, the manufacturing method of the radio wave absorber, and the aircraft according to the embodiments of the present invention will be described with reference to the accompanying drawings.

[0019] FIG. 1 is a flowchart showing the flow of the manufacturing method of the radio wave absorber according to the embodiment of the present invention.

[0020] The radio wave absorber manufactured according to the process shown in Figure 1 is intended for use as a radio wave absorber that forms the surface of an aircraft structure having a shape that repeatedly reflects the radio waves to be absorbed. The radio wave absorber may be single-layer, double-layer, or multi-layer.

[0021] Figure 2 shows the basic structure of each part of a single-layer radio wave absorber 1 that can be manufactured using the manufacturing method shown in Figure 1. Figure 3 shows the basic structure of each part of a two-layer radio wave absorber 1 that can be manufactured using the manufacturing method shown in Figure 1. Figure 4 shows the basic structure of each part of a multi-layer radio wave absorber 1 that can be manufactured using the manufacturing method shown in Figure 1.

[0022] As shown in Figure 2, a single-layer radio wave absorber 1 has a structure in which a single-layer absorbing layer 3 is superimposed on a reflective layer 2 made of a metal plate or CFRP plate that reflects radio waves R. The absorbing layer 3 is a layer that absorbs radio waves R, and the absorbing layer 3 mainly uses a material to which dielectric properties and magnetic properties are imparted by adding fillers such as carbon, ferrite, or metal powder to an insulating material such as resin. In addition, a technology is known in which resin reinforced with SiC (silicon carbide) fibers is used as the material for the absorbing layer 3.

[0023] Therefore, when radio waves R enter the absorption layer 2, the radio waves R that pass through the absorption layer 2 while being attenuated within the absorption layer 2 are reflected by the reflection layer 2. The radio waves R reflected by the reflection layer 2 pass through the absorption layer 2 again while being attenuated within the absorption layer 2, and then exit the absorption layer 2.

[0024] As shown in Figure 3, the two-layer radio wave absorber 1 has a structure in which the absorption layer 3 is protected by a durable surface layer 4, and one absorption layer 3 is sandwiched between the surface layer 4 and the reflective layer 2. The multilayer radio wave absorber 1 has a structure in which multiple absorption layers 3 are sandwiched between the surface layer 4 and the reflective layer 2, as shown in Figure 4.

[0025] Figure 5 is a longitudinal cross-sectional view showing an example of an intake duct 11 of an aircraft 10 manufactured using the manufacturing method shown in Figure 1.

[0026] A typical example of an aircraft structure having a shape that repeatedly reflects the radio waves R to be absorbed is the intake duct 11 of the aircraft 10 shown in Figure 5. Hereafter, we will explain using the example of a case in which the inner surface of the intake duct 11 provided in the aircraft 10 is formed with a two-layer radio wave absorber 1 in which the absorption layer 3 is protected by a surface layer 4 as shown in Figure 3.

[0027] As shown in the flowchart of Figure 1, the radio wave absorber 1 is manufactured in a design step S1 and a subsequent manufacturing step S2. In the design step S1, design information for the radio wave absorber 1 is created. In the manufacturing step S2, the radio wave absorber 1 is manufactured using a known method based on the design information for the radio wave absorber 1 created in the design step S1.

[0028] The design step S1 of the radio wave absorber 1 specifically consists of substeps S11 to S13. In substep S11, a reflection path analysis of radio waves R within the intake duct 11 is performed. In substep S12, a structural strength analysis of the intake duct 11 is performed. In substep S13, based on the results of the reflection path analysis of radio waves R within the intake duct 11 and the structural strength analysis of the intake duct 11, the thicknesses of the surface layer 4, absorption layer 3, and reflection layer 2 that constitute the radio wave absorber 1 are determined to be multiple different thicknesses for each part, so as to reduce the total weight of the surface layer 4 and absorption layer 3. These analyses and calculations can be performed using analysis software installed on an electronic circuit such as a computer.

[0029] Next, we will explain in detail the substeps S11 to S13 that constitute the design step S1 of the radio wave absorber 1 shown in the flowchart of Figure 1.

[0030] In cases where radio waves R undergo complex reflections, such as within the intake duct 11 of aircraft 10, there are areas with high and low reflection frequencies for radio waves R. Therefore, by performing a reflection path analysis of radio waves R in substep S11, it is possible to identify areas with relatively high and relatively low reflection frequencies for radio waves R.

[0031] In areas where the reflection frequency of radio waves R is relatively high, high radio wave absorption performance is not required because the radio waves R are incident on the absorption layer 3 of the radio wave absorber 1 many times. Conversely, in areas where the reflection frequency of radio waves R is relatively low, high radio wave absorption performance is required because the number of times the radio waves R are incident on the absorption layer 3 of the radio wave absorber 1 is small.

[0032] Therefore, by analyzing the reflection path of radio waves R, it is possible to identify areas where the reflection frequency of radio waves R is relatively high and areas where the reflection frequency of radio waves R is relatively low, thereby identifying areas where high radio wave absorption performance is required and areas where high radio wave absorption performance is not required. In other words, the required radio wave absorption performance can be determined for each location on the inner surface of the intake duct 11.

[0033] Figure 6 is a map showing the results of the reflection path analysis of radio waves R on the upper side of the intake duct 11 shown in Figure 5, and Figure 7 is a map showing the results of the reflection path analysis of radio waves R on the lower side of the intake duct 11 shown in Figure 5.

[0034] In the analysis maps in Figures 6 and 7, darker colors indicate lower reflection frequencies of radio waves R and locations requiring higher radio wave absorption performance. According to the analysis maps in Figures 6 and 7, it can be seen that relatively high radio wave absorption performance is generally required in the front portion of the intake duct 11, while relatively low radio wave absorption performance is required in the rear portion of the intake duct 11.

[0035] The radio wave absorption performance of the radio wave absorber 1 varies depending on the thickness of the absorption layer 3. Typically, increasing the thickness of the absorption layer 3 improves the radio wave absorption performance. Therefore, the thickness of the absorption layer 3 can be reduced at the rear of the intake duct 11 where high radio wave absorption performance is not required. Thus, if the required radio wave absorption performance can be identified for each location within the intake duct 11, the required thickness of the absorption layer 3 of the radio wave absorber 1 can be determined for each location within the intake duct 11. In addition, the thickness of the surface layer 4 can also be determined for each location within the intake duct 11 according to the thickness of the absorption layer 3.

[0036] On the other hand, by performing a structural strength analysis of the intake duct 11 as shown in substep S12, the required plate thickness ranges for the surface layer 4, absorption layer 3, and reflection layer 2 of the radio wave absorber 1 that constitute the intake duct 11 can be determined for each part of the intake duct 11. The intake duct 11 of the aircraft 10 has a curved and complex shape, and the required strength and plate thickness differ depending on the part.

[0037] Figure 8 is a cross-sectional view illustrating the dominant out-of-plane load in front of the intake duct 11 shown in Figure 5, and Figure 9 is a cross-sectional view illustrating the dominant in-plane load behind the intake duct 11 shown in Figure 5.

[0038] In front of the intake duct 11, the cross-sectional shape of the intake duct 11 is asymmetrical, as shown in Figure 8. Therefore, the intake duct 11 is primarily subjected to out-of-plane bending loads due to pressure acting perpendicularly to its inner surface. In addition, the front of the intake duct 11 is at high risk of being subjected to out-of-plane bending loads due to bird impacts.

[0039] In contrast, behind the intake duct 11, there are many areas where the cross-sectional shape of the intake duct 11 is close to circular, as shown in Figure 9. Therefore, the intake duct 11 is mainly subjected to in-plane loads (hoop stresses) due to the pressure acting perpendicularly on the inner surface of the intake duct 11.

[0040] Therefore, as described above, the required strength and plate thickness of the intake duct 11 differ depending on the location. Accordingly, for the front portion of the intake duct 11 where bending loads are dominant, the plate thicknesses of the surface layer 4, absorption layer 3, and reflective layer 2 of the radio wave absorber 1 constituting the intake duct 11 can be set so as to obtain the necessary load-bearing capacity corresponding to the expected bending load. On the other hand, for the rear portion of the intake duct 11 where in-plane loads are dominant, the plate thicknesses of the surface layer 4, absorption layer 3, and reflective layer 2 of the radio wave absorber 1 constituting the intake duct 11 can be determined so as to obtain the necessary load-bearing capacity corresponding to the expected in-plane load.

[0041] Here, as the simplest method, we will explain using the example of dividing the inner surface of the intake duct 11 into two parts, a front side and a rear side, and determining the thickness of the surface layer 4, absorption layer 3, and reflection layer 2 of the radio wave absorber 1 that constitutes the intake duct 11 for the front side and the rear side of the intake duct 11, respectively. However, it is also possible to divide the inner surface of the intake duct 11 into three or more parts and determine the thickness of the surface layer 4, absorption layer 3, and reflection layer 2 of the radio wave absorber 1 for each part.

[0042] When the intake duct 11 is divided into two parts, as shown in Figure 5, the front of the intake duct 11 where out-of-plane loads are predominantly applied can be set as the first region R1, while the rear of the intake duct 11 where in-plane loads are predominantly applied can be set as the second region R2.

[0043] Figure 10 is a graph showing the results of calculating the required plate thickness for each layer of the radio wave absorber 1 that constitutes the intake duct 11, by dividing it into a first region R1 on the front side of the intake duct 11 and a second region R2 on the rear side.

[0044] In Figure 10, the horizontal axis represents the total plate thickness (mm) of the surface layer 4 and the absorption layer 3 required to provide the required load-bearing capacity of the radio wave absorber 1, and the vertical axis represents the plate thickness (mm) of the reflective layer 2 required to provide the required load-bearing capacity of the radio wave absorber 1.

[0045] Furthermore, the dashed line in Figure 10 shows the relationship between the total thickness of the surface layer 4 and the absorption layer 3 required for the first region R1 in front of the intake duct 11, which consists of a radio wave absorber 1 in which both the surface layer 4 and the absorption layer 3 are made of SiC fiber-reinforced resin, and the reflective layer 2 is made of a CFRP layer, and the thickness of the reflective layer 2. The double dashed line in Figure 10 shows the relationship between the total thickness of the surface layer 4 and the absorption layer 3 required for the second region R2 in rear of the intake duct 11, which consists of a radio wave absorber 1 in which both the surface layer 4 and the absorption layer 3 are made of SiC fiber-reinforced resin, and the reflective layer 2 is made of a CFRP layer, and the thickness of the reflective layer 2.

[0046] Furthermore, the calculations were performed assuming that the bending load required in the first region R1 in front of the intake duct 11, where out-of-plane loads are predominantly applied, is 4.0 kN, the in-plane load required in the second region R2 behind the intake duct 11, where in-plane loads are predominantly applied, is 3.0 kN / mm, the Young's modulus of the surface layer 4 and absorption layer 3, both made of SiC fiber-reinforced resin, is 35 GPa, the allowable material strain is ±5000 (μm / m), and the Young's modulus of the reflective layer 2, made of CFRP, is 55 GPa, with an allowable material strain of ±6000 (μm / m).

[0047] Since the bending stiffness of a material is expressed as EI, the product of the material's Young's modulus E and its second moment of area I, the relationship between the total thickness of the surface layer 4 and the absorption layer 3 required to provide bending load capacity and the thickness of the reflective layer 2 is curvilinear, as shown by the dashed line in Figure 10. On the other hand, since the in-plane stiffness of a material is expressed as EA, the product of the material's Young's modulus E and its cross-sectional area A, the relationship between the total thickness of the surface layer 4 and the absorption layer 3 required to provide in-plane load capacity and the thickness of the reflective layer 2 is linear, as shown by the double-dashed line in Figure 10.

[0048] Therefore, for the first region R1 in front of the intake duct 11 where out-of-plane bending loads are dominant, it is preferable to reduce the thickness of the reflective layer 2 made of CFRP while increasing the total thickness of the surface layer 4 and the absorption layer 3 to secure the second moment of area. Conversely, for the second region R2 behind the intake duct 11 where in-plane loads are dominant, it is structurally efficient to increase the thickness of the reflective layer 2 made of CFRP, which has excellent mechanical strength, while reducing the total thickness of the surface layer 4 and the absorption layer 3. In other words, it is important to secure the thickness in the first region R1 in front of the intake duct 11 where out-of-plane bending loads are dominant, while it is important to secure the strength in the second region R2 behind the intake duct 11 where in-plane loads are dominant.

[0049] Next, in substep S13, the total thickness of the surface layer 4 and the absorption layer 3, and the thickness of the reflective layer 2 can be independently optimized for the first region R1 in front of the intake duct 11 where out-of-plane bending loads are dominant, and the second region R2 behind the intake duct 11 where in-plane loads are dominant, so as to satisfy the strength requirements shown in Figure 10. In other words, the total thickness of the surface layer 4 and the absorption layer 3, and the thickness of the reflective layer 2 can be optimized so as to reduce the total weight and total cost of the entire radio wave absorber 1 forming the intake duct 11.

[0050] This optimization can utilize the results of the reflection path analysis of radio waves R in substep S11. In other words, the total thickness of the surface layer 4 and the absorption layer 3, and the thickness of the reflection layer 2 can be optimized by combining the results of the reflection path analysis of radio waves R in substep S11 and the results of the structural strength analysis of the intake duct 11 in substep S12.

[0051] Figure 11 is a table showing the total plate thickness of the CFRP layers constituting the reflective layer 2 when selecting the combination of thicknesses of the surface layer 4, absorption layer 3, and reflective layer 2 shown in Example 1, Comparative Example 1, and Comparative Example 2 in the graph of Figure 10.

[0052] The results of the reflection path analysis of radio waves R inside the intake duct 11 show that, as shown in Figures 6 and 7, relatively high radio wave absorption performance is required in the front portion of the intake duct 11, while relatively low radio wave absorption performance is required in the rear portion of the intake duct 11.

[0053] In other words, the first region R1 in front of the intake duct 11, where out-of-plane bending loads are dominant, is a region where the reflection frequency of radio waves R is relatively low and relatively high radio wave absorption performance is required. On the other hand, the second region R2 behind the intake duct 11, where in-plane loads are dominant, is a region where the reflection frequency of radio waves R is relatively high and relatively low radio wave absorption performance is required.

[0054] Therefore, as shown in Embodiment 1 in Figures 10 and 11, the total thickness of the surface layer 4 and the absorption layer 3 can be increased to 10.00 mm in the first region R1 in front of the intake duct 11, while the total thickness of the surface layer 4 and the absorption layer 3 can be decreased to 4.00 mm in the second region R2 behind the intake duct 11. In this case, in order to satisfy the strength requirements shown in Figure 10, the thickness of the CFRP layer constituting the reflective layer 2 must be 6.64 mm in the first region R1 in front of the intake duct 11, while the thickness of the CFRP layer constituting the reflective layer 2 must be 8.36 mm in the second region R2 behind the intake duct 11. In this case, the total thickness of the CFRP layers in the front and rear of the intake duct 11 will be 15.00 mm.

[0055] In contrast, as shown in Comparative Example 1 in Figures 10 and 11, if the total thickness of the surface layer 4 and the absorption layer 3 is set to the same 7.00 mm in the first region R1 in front of the intake duct 11 and the second region R2 behind the intake duct 11, then in order to satisfy the strength requirements shown in Figure 10, the thickness of the CFRP layer constituting the reflective layer 2 in the first region R1 in front of the intake duct 11 must be 9.35 mm, while the thickness of the CFRP layer constituting the reflective layer 2 in the second region R2 behind the intake duct 11 must be 6.45 mm. In this case, the total thickness of the CFRP layers in the front and rear of the intake duct 11 becomes 15.81 mm, which is larger than that of Example 1.

[0056] Furthermore, contrary to the results of the reflection path analysis of radio wave R inside the intake duct 11, as shown in Comparative Example 2, if the total plate thickness of the surface layer 4 and the absorption layer 3 is reduced to 4.00 mm in the first region R1 in front of the intake duct 11, while the total plate thickness of the surface layer 4 and the absorption layer 3 is increased to 10.00 mm in the second region R2 behind the intake duct 11, then in order to satisfy the strength requirements shown in Figure 10, the plate thickness of the CFRP layer constituting the reflection layer 2 must be 10.97 mm in the first region R1 in front of the intake duct 11, while the plate thickness of the CFRP layer constituting the reflection layer 2 must be 4.55 mm in the second region R2 behind the intake duct 11. In this case, the total plate thickness of the CFRP layers in the front and rear of the intake duct 11 becomes 15.52 mm, which is larger than that of Example 1.

[0057] As shown in the calculation example in the table in Figure 11, even if the total thickness of the surface layer 4 and the absorption layer 3 at the front and rear of the intake duct 11 is set to be the same, changing the total thickness of the surface layer 4 and the absorption layer 3 between the front and rear of the intake duct 11 changes the overall weight of the intake duct 11, and it can be seen that the case of Example 1 is lighter than the cases of Comparative Example 1 and Comparative Example 2.

[0058] However, in order to further reduce the total weight and cost of the radio wave absorber 1 that constitutes the intake duct 11, it is essential to reduce the thickness of the surface layer 4 and the absorption layer 3. Therefore, for the first region R1 in front of the intake duct 11 and the second region R2 behind the intake duct 11, the total thickness of the surface layer 4 and the absorption layer 3 can be independently reduced to the lower limit while increasing the thickness of the reflective layer 2 made of CFRP, in order to satisfy the strength requirements shown in Figure 10.

[0059] The lower limit of the total thickness of the surface layer 4 and the absorption layer 3 in the first region R1 in front of the intake duct 11 and the second region R2 behind the intake duct 11 can be set based on the required radio wave absorption performance. Specifically, in the first region R1 in front of the intake duct 11, a relatively high lower limit can be set relative to the total thickness of the surface layer 4 and the absorption layer 3 to obtain high radio wave absorption performance. On the other hand, in the second region R2 behind the intake duct 11, a relatively low lower limit can be set relative to the total thickness of the surface layer 4 and the absorption layer 3 to obtain low radio wave absorption performance.

[0060] Then, once the total thickness of the surface layer 4 and the absorption layer 3 is determined for the first region R1 in front of the intake duct 11 and the second region R2 behind the intake duct 11, the thickness of the reflective layer 2 made of CFRP can be uniquely determined for the first region R1 in front of the intake duct 11 and the second region R2 behind the intake duct 11, respectively, based on the relationships shown in Figure 10.

[0061] In other words, by optimizing calculations to reduce the weight of the surface layer 4 and absorption layer 3 contained in the radio wave absorber 1, it is possible to determine the thicknesses of the surface layer 4, absorption layer 3, and reflection layer 2 contained in each of the multiple regions R1 and R2 on the inner surface of the intake duct 11, each having different radio wave absorption performance, and the first and second regions R1 and R2 on the inner surface of the intake duct 11, respectively, as well as the thicknesses of the surface layer 4, absorption layer 3, and reflection layer 2 contained in each of the multiple regions R1 and R2, respectively, with constraints that the radio wave absorption performance required at each location on the inner surface of the intake duct 11 must be ensured according to the reflection frequency of radio waves R at each location, and the strength required at each location on the inner surface of the intake duct 11 must be ensured.

[0062] This makes it possible to design an intake duct 11 for an aircraft 10 that satisfies the radio wave absorption performance according to the reflection path analysis results of the radio waves R shown in Figures 6 and 7, and the strength requirements shown in Figure 10, while reducing the total weight and cost of the surface layer 4 and the absorption layer 3. In other words, by reducing the amount of material used in the surface layer 4 and the absorption layer 3, it is possible to lighten and reduce the cost of the intake duct 11 made up of the radio wave absorber 1.

[0063] As illustrated in Figure 5, the radio wave absorber 1 has a first radio wave absorbing section 1A that forms a first region R1 on the inner surface of the intake duct 11 and has a first radio wave absorbing performance, and a second radio wave absorbing section 1B that forms a second region R2 different from the first region R1 on the inner surface of the intake duct 11 and has a second radio wave absorbing performance lower than that of the first radio wave absorbing section.

[0064] More specifically, the first radio wave absorbing section 1A has a first reflective layer 2A, a first absorbing layer 3A, and a first surface layer 4A, while the second radio wave absorbing section 1B has a second reflective layer 2B, a second absorbing layer 3B, and a second surface layer 4B. Between the first radio wave absorbing section 1A and the second radio wave absorbing section 1B, the thicknesses of the reflective layers 2A, 2B, the absorbing layers 3A, 3B, and the surface layers 4A, 4B are determined independently of each other. Typically, the thickness of the first absorbing layer 3A included in the first radio wave absorbing section 1A is greater than the thickness of the second absorbing layer 3B included in the second radio wave absorbing section 1B.

[0065] In the example described above, the inner surface of the intake duct 11 was divided into a first region R1 and a second region R2. However, it is possible to divide it into three or more regions and determine the thickness of the reflective layer 2, absorbing layer 3, and surface layer 4 independently for each region. Also, as mentioned above, the radio wave absorber 1 may be a single-layer type as shown in Figure 2 or a multi-layer type as shown in Figure 4. Therefore, each of the multiple radio wave absorbing sections constituting the radio wave absorber 1 will contain at least one absorbing layer 3 and a reflective layer 2.

[0066] (effect) The above-described radio wave absorber 1 is designed so that the thickness of the absorption layer 3 is determined to be different between multiple regions R1 and R2, in order to reduce the amount of material used to make up the absorption layer 3 in aircraft structures where radio waves R are repeatedly reflected, such as the intake duct 11 of an aircraft 10.

[0067] Therefore, compared to conventional radio wave absorbers in which the thickness of the reflective layer, such as a CFRP layer, is adjusted to satisfy strength requirements, while the thickness of the absorbing layer remains constant regardless of the location, the radio wave absorber 1 can achieve weight reduction and cost reduction while maintaining radio wave absorption performance and strength.

[0068] Figure 12 is a graph showing an example of the analysis results of the radio wave absorption performance of the radio wave absorber 1 shown in Figure 5.

[0069] In Figure 12, the horizontal axis represents the frequency (GHz) of the radio wave R, and the vertical axis represents the median RCS (dBsm: DeciBel square meter). Furthermore, the solid lines in Figure 12 represent the radio wave absorption performance of radio wave absorber 1, where the thicknesses of the absorption layers 3A and 3B and the surface layers 4A and 4B differ for each region R1 and R2, as shown in Figure 5. The dashed-dotted line represents the radio wave absorption performance of a conventional radio wave absorber with constant thicknesses for the absorption layer and surface layer, and the double-dotted-dotted line represents the radio wave absorption performance of metal only.

[0070] As shown by the solid and dotted lines in Figure 12, it can be confirmed that the radio wave absorber 1 manufactured using the manufacturing method shown in Figure 1 has radio wave absorption performance equivalent to that of a conventional radio wave absorber in which the thickness of the absorption layer and the surface layer are constant.

[0071] (Other embodiments) Although specific embodiments have been described above, these embodiments are merely examples and do not limit the scope of the invention. The novel methods and apparatus described herein can be embodied in various other forms. Furthermore, various omissions, substitutions, and modifications can be made in the forms of methods and apparatus described herein, without departing from the spirit of the invention. The attached claims and equivalents include such various forms and modifications as being encompassed within the scope and spirit of the invention. [Explanation of Symbols]

[0072] 1. Radio wave absorber 1A First radio wave absorption section 1B Second radio wave absorption section 2 reflective layer 2A First reflective layer 2B Second Reflective Layer 3. Absorption layer 3A First absorption layer 3B Second absorption layer 4 Surface layer 4A First surface layer 4B Second surface layer 10 aircraft 11 Intake duct R radio wave R1 First Domain R2 Second Domain

Claims

1. A radio wave absorber that forms the surface of an aircraft structure having a shape that repeatedly reflects radio waves to be absorbed, A first radio wave absorbing portion forms a first region of the surface and has first radio wave absorbing performance, A second radio wave absorbing portion is formed on the surface, which is different from the first region, and has a second radio wave absorbing performance lower than the first radio wave absorbing performance. A radio wave absorber having [a certain characteristic].

2. The region where the reflection frequency of the radio waves is relatively low is designated as the first region, while the region where the reflection frequency of the radio waves is relatively high is designated as the second region. The radio wave absorber according to claim 1, wherein the thickness of the first radio wave absorbing layer included in the first radio wave absorbing section is greater than the thickness of the second radio wave absorbing layer included in the second radio wave absorbing section.

3. The region of the surface where out-of-plane loads are predominantly applied is defined as the first region, while the region of the surface where in-plane loads are predominantly applied is defined as the second region. The radio wave absorber according to claim 1, wherein the thickness of the first radio wave absorbing layer included in the first radio wave absorbing section is greater than the thickness of the second radio wave absorbing layer included in the second radio wave absorbing section.

4. An aircraft in which the inner surface of an intake duct is formed with the radio wave absorber described in any one of claims 1 to 3, the first region is set on the front side of the intake duct, and the second region is set on the rear side of the intake duct.

5. A design step to create design information for a radio wave absorber that forms the surface of an aircraft structure having a shape that repeatedly reflects radio waves to be absorbed, The process includes a manufacturing step of manufacturing the radio wave absorber based on the design information, In the aforementioned design step, A method for manufacturing a radio wave absorber, in which an optimization calculation is performed to reduce the weight of the radio wave absorbing layer contained in the radio wave absorber, thereby determining a plurality of regions on the surface having different radio wave absorption performance and the respective thicknesses of the radio wave absorbing layers contained in each of the plurality of regions, with constraints that the radio wave absorption performance at each of the respective positions required according to the frequency of reflection of the radio waves at each position on the surface be ensured and the strength at each of the respective positions required to withstand the load applied to each of the positions on the surface be ensured.

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