Patent Description:
In the related art, to improve radio wave transmittance of a radome, various ideas have been made in terms of materials and structures.

As the material of the radome, a material having a low dielectric constant is generally used, and, for example, Fiber Reinforced Plastics (FRP) using a cyanate ester resin is used as a material that allows obtaining the lowest dielectric constant most (for example, see Patent Document <NUM> below).

In addition, as a structure of a radome, the radome is formed by using a multilayer structure having a sandwich structure in which both surfaces of a core member are covered with skin layers to widen a transmission frequency band (for example, see Patent Document <NUM> below). Patent Document <NUM> discloses a radome comprising a multilayer structure in which a skin layer is layered on a surface of a core member, and one skin layer on the other surface of the core member. Both skin layers are fibre reinforced.

However, the cyanate ester resin has hard and brittle properties and a weak adhesive force with the core member, and therefore the sandwich structure is difficult to be formed. For this reason, there is a problem that a yield in manufacturing the radome is deteriorated and the radome after manufacturing is easily damaged.

The present invention has been made in view of such circumstances, and an object thereof is to improve quality of a radome using a cyanate ester resin for a skin layer.

In order to achieve the above-described object, a radome according to an embodiment of the present invention may be a radome including a multilayer structure in which a plurality of skin layers are layered on a surface of a core member. The skin layer may include a first fiber reinforced plastic layer containing a cyanate ester resin and a fiber material and a second fiber reinforced plastic layer containing an epoxy resin and a fiber material. The second fiber reinforced plastic layer may be disposed at a position in contact with the surface of the core member.

In the radome according to an embodiment of the present invention, a proportion of a thickness of the second fiber reinforced plastic layer to a thickness of all of the skin layers is <NUM>% or less.

In the radome according to an embodiment of the present invention, the skin layers are disposed on both surfaces of the core member. A thickness of each of the skin layers disposed on each of the surfaces is <NUM>/<NUM> or less of a free space wavelength at a center frequency of a transmission frequency band of the radome.

In the radome according to an embodiment of the present invention, the first fiber reinforced plastic layer contains glass fiber as the fiber material.

In the radome according to an embodiment of the present invention, the first fiber reinforced plastic layer contains quartz fiber as the fiber material.

In the radome according to an embodiment of the present invention, the first fiber reinforced plastic layer contains E-glass fiber as the fiber material.

In the radome according to an embodiment of the present invention, the first fiber reinforced plastic layer contains NE-glass fiber as the fiber material.

In the radome according to an embodiment of the present invention, the core member is formed of a foamed body of an organic resin or a honeycomb core structure.

According to one embodiment of the present invention, the first fiber reinforced plastic layer containing the cyanate ester resin as a main component is used for the skin layer of the radome and the second fiber reinforced plastic layer containing the epoxy resin as a main component is disposed at the position in contact with the surface of the core member. Accordingly, the second fiber reinforced plastic layer functions as an adhesive layer that bonds the core member and the first fiber reinforced plastic layer together, which is advantageous in providing radio wave transmission performance of the radome and strength of the radome in a compatible manner.

According to one embodiment of the present invention, the proportion of the thickness of the second fiber reinforced plastic layer to the thickness of all of the skin layers (a sum of the thicknesses of the first fiber reinforced plastic layer and the thicknesses of the second fiber reinforced plastic layer) is set to <NUM>% or less. Accordingly, it is possible to suppress a decrease in radio wave transmission performance due to arrangement of the second fiber reinforced plastic layer having a higher dielectric constant than the first fiber reinforced plastic layer to a certain range.

According to one embodiment of the present invention, since the thickness of the skin layer is <NUM>/<NUM> or less of the free space wavelength at the center frequency of the transmission frequency band of the radome, the radio wave transmission performance of the radome can be satisfactorily maintained.

According to one embodiment of the present invention, since the first fiber reinforced plastic layer contains the glass fiber having the low dielectric constant as the fiber material, the dielectric constant of the fiber reinforced plastic layer can be reduced and the radio wave transmission performance of the radome can be improved.

According to one embodiment of the present invention, since the first fiber reinforced plastic layer contains the quartz fiber having the low dielectric constant as the fiber material, the dielectric constant of the fiber reinforced plastic layer can be reduced and the radio wave transmission performance of the radome can be improved.

According to one embodiment of the present invention, since the first fiber reinforced plastic layer contains the E-glass fiber as the fiber material, the radome can be produced at a relatively low cost.

According to one embodiment of the present invention, since the first fiber reinforced plastic layer contains the NE-glass fiber as the fiber material, the dielectric constant of the first fiber reinforced plastic layer can be lower compared with the case where the E-glass fiber is used and the radio wave transmission performance of the radome can be improved.

According to one embodiment of the present invention, since the core member is formed of the foamed body of the organic resin or the honeycomb core structure, the second fiber reinforced plastic layer functions as the adhesive layer that bonds the core member and the first fiber reinforced plastic layer together, which is advantageous in providing the radio wave transmission performance of the radome and the strength of the radome in a compatible manner.

Preferred embodiments of a radome according to an embodiment of the present invention will be described in detail below with reference to the accompanying drawings.

<FIG> is an appearance view of the radome according to an embodiment. <FIG> is an enlarged cross-sectional view (A-A cross-sectional view) of the radome. <FIG> omits a curvature in an extension direction of a radome <NUM>.

As illustrated in <FIG>, the radome <NUM> is formed in a substantially conical shape, and a radar or the like (not illustrated) is disposed inside the radome <NUM>, for example, disposed at an end portion of an aircraft. Note that the shape of the radome <NUM> is not limited to the substantially conical shape, and various known shapes, such as a semicircular shape and a polygonal pyramid shape, can be employed.

As illustrated in <FIG>, the radome <NUM> is formed of a multilayer structure in which a plurality of skin layers (first fiber reinforced plastic layers <NUM> and second fiber reinforced plastic layers <NUM>) are layered on surfaces of a core member <NUM>.

The core member <NUM> is formed of, for example, a foamed body of an organic resin. As the organic resin forming the core member <NUM> (foamed body), for example, polymethacrylimide (PMI), polyvinyl (PVC), rigid urethane, polyether sulfone (PES), polyimide (PI), and polyethylene terephthalate (PET) can be used.

Further, the core member <NUM> may be formed of a honeycomb core structure. As the material of the honeycomb core structure forming the core member <NUM>, for example, a resin (aramid), glass fiber, and wood (balsa) can be used.

The plurality of skin layers are formed of respective different types of fiber reinforced plastics. In the present embodiment, the plurality of skin layers include the first fiber reinforced plastic layers <NUM> containing a cyanate ester resin and a fiber material and the second fiber reinforced plastic layers <NUM> containing an epoxy resin and a fiber material.

Each of the first fiber reinforced plastic layer <NUM> and the second fiber reinforced plastic layer <NUM> is formed by layering one or a plurality of fiber reinforced plastic prepregs <NUM>, <NUM>. In the example of <FIG>, each of the first fiber reinforced plastic layer <NUM> and the second fiber reinforced plastic layer <NUM> is formed by layering the three prepregs <NUM>, <NUM>.

The multilayer structure constituting the radome <NUM> has a structure in which the first fiber reinforced plastic layer <NUM>, the second fiber reinforced plastic layer <NUM>, the core member <NUM>, the second fiber reinforced plastic layer <NUM>, and the first fiber reinforced plastic layer <NUM> are layered in this order from one surface side of the multilayer structure and is symmetrical in the thickness direction. That is, in the multilayer structure constituting the radome <NUM>, the second fiber reinforced plastic layers are disposed at positions in contact with the surfaces of the core member <NUM>.

More specifically, assume that each of the first fiber reinforced plastic layer <NUM>, the second fiber reinforced plastic layer <NUM>, and the core member <NUM> has a first surface and a second surface (a front surface and a back surface), the core member <NUM> is disposed at the center of the multilayer structure, and the respective first surfaces of the second fiber reinforced plastic layers <NUM> are disposed to be in contact with the first surfaces and the second surfaces of the core member <NUM>. The respective first surfaces of the first fiber reinforced plastic layers <NUM> are disposed to be in contact with the second surfaces of the second fiber reinforced plastic layers <NUM>, and the second surfaces of the first fiber reinforced plastic layers <NUM> form the outer surface of the radome <NUM>.

To manufacture the radome <NUM>, prepregs using an epoxy resin and a fiber material as materials to be the second fiber reinforced plastic layers <NUM> are layered on the surfaces (the front surface and the back surface) of the core member <NUM> formed in the shape of the radome <NUM> by a desired thickness and further, prepregs using a cyanate resin and a fiber material as materials to be the first fiber reinforced plastic layers <NUM> are layered on the surfaces by a desired thickness, thus forming the multilayer structure. Heat and pressure are applied to the multilayer structure for bonding and curing to form the radome <NUM>.

By providing the first fiber reinforced plastic layer <NUM> containing the cyanate ester resin, which is a low dielectric constant material, the dielectric constant of the skin layer can be lowered and transmittance of a radio wave can be improved. In addition, by disposing the second fiber reinforced plastic layer <NUM> containing the epoxy resin having relatively high flexibility at the position in contact with the core member <NUM>, the second fiber reinforced plastic layer <NUM> functions as an adhesive layer between the core member <NUM> and the first fiber reinforced plastic layer <NUM>, and strength of the radome <NUM> (adhesive strength of the first fiber reinforced plastic layer <NUM>) can be improved.

The fiber material contained in the first fiber reinforced plastic layer <NUM> or the second fiber reinforced plastic layer <NUM> is, for example, glass fiber, and quartz fiber, E-glass fiber, NE-glass fiber, or the like can be used as the glass fiber. Using the quartz fiber allows reducing the dielectric constant of the fiber reinforced plastic layer and improving radio wave transmission performance of the radome <NUM>. Further, using the E-glass fiber allows producing the radome <NUM> at a relatively low cost. Using the NE-glass fiber allows reducing the dielectric constant of the fiber reinforced plastic layer and improving the radio wave transmission performance of the radome <NUM> compared with the case of using the E-glass fiber.

Here, a proportion of the thickness of the second fiber reinforced plastic layer <NUM> to the thickness of all of the skin layers (the sum of the thicknesses of the first fiber reinforced plastic layer <NUM> and the second fiber reinforced plastic layer <NUM>) is preferably <NUM>% or less.

<FIG> is a graph showing the radio wave transmission characteristics of the radome and indicates the transmission loss of the radio wave (dB) in the vertical axis and a frequency (GHz) in the horizontal axis.

More specifically, <FIG> shows the results of measuring the transmission loss of each of the radomes <NUM> formed by changing the number of layers of prepregs containing a cyanate ester resin and quartz fiber corresponding to the first fiber reinforced plastic layer <NUM> (denoted as "cyanate layers" in the drawing) and prepregs containing an epoxy resin and quartz fiber corresponding to the second fiber reinforced plastic layers <NUM> (denoted as "epoxy layers" in the drawing).

The transmission characteristics of A: the radome <NUM> of layering five cyanate layers and one epoxy layer per one surface of the core member <NUM>, B: the radome <NUM> of layering four cyanate layers and two epoxy layers per one surface of the core member <NUM>, C: the radome <NUM> of layering three cyanate layers and three epoxy layers per one surface of the core member <NUM>, D: the radome <NUM> of layering six cyanate layers per one surface of the core member <NUM>, and E: the radome <NUM> of layering six epoxy layers per one surface of the core member <NUM> are indicated. Excluding D or E having the six layers all of which are made of the same material, similar to the one illustrated in <FIG>, the layering order of the respective layers is that the epoxy layers are layered so as to be in contact with the core member <NUM> and the cyanate layers are layered on the epoxy layers.

The core member <NUM> is <NUM> in thickness, and both of the prepregs of the epoxy and the cyanate are <NUM> in thickness.

That is, in A to E, proportions of the contained epoxy and cyanate are changed in the radomes <NUM> having the same thickness.

Comparing with the transmission losses of the respective radomes <NUM>, the transmission loss of D in which all of the skin layers are formed of the cyanate layers are the lowest, and the transmission performance of the radio wave is good. On the other hand, the transmission loss of E in which all of the skin layers are formed of the epoxy layers is the largest, and the transmission performance of the radio wave is low. In the case where the cyanate layers and the epoxy layers are provided, the transmission loss becomes low in the order of A, B, and C. The more the proportion of the epoxy layers becomes, the larger the transmission loss becomes. On the other hand, a difference in transmission loss between C in which the proportion of the epoxy layers is the largest (<NUM>%) and D in which all of the skin layers are formed of the cyanate layers is about <NUM> dB, which is sufficiently practical.

<FIG> is also a graph showing the radio wave transmission characteristics of the radome and indicates the transmission loss of the radio wave (dB) in the vertical axis and the frequency (GHz) in the horizontal axis.

<FIG> indicates the transmission characteristics of A: the radome <NUM> of layering two cyanate layers and one epoxy layer per one surface of the core member <NUM>, B: the radome <NUM> of layering three cyanate layers per one surface of the core member <NUM>, and C: the radome <NUM> of layering three epoxy layers per one surface of the core member <NUM> are indicated.

In <FIG> as well, the core member <NUM> is <NUM> in thickness, and both of the prepregs of the epoxy and the cyanate are <NUM> in thickness.

That is, in A to C, proportions of the contained epoxy and cyanate are changed in the radomes <NUM> having the same thickness.

Comparing with the transmission losses of the respective radomes <NUM>, the transmission loss of B in which all of the skin layers are formed of the cyanate layers are the lowest, and the transmission performance of the radio wave is good. In addition, the transmission loss of C in which all of the skin layers are formed of the epoxy layers is the highest, and the transmission performance of radio wave is inferior. On the other hand, while A of the two cyanate layers and one epoxy layer has the transmission loss larger than that of B, it maintains good transmission performance as compared with C.

As described above, although the adhesive force between the core member <NUM> and the skin layer is improved by providing the epoxy layer, the transmission performance of the radome <NUM> deteriorates as the proportion of the epoxy layers in the skin layers increases. Therefore, the thickness of the epoxy layers in the skin layers preferably does not exceed the thickness of the cyanate layers, which are the low dielectric constant layers. That is, the proportion of the thickness of the second fiber reinforced plastic layer <NUM>, which is the epoxy layer, to the thickness of all of the skin layers is preferably <NUM>% or less.

The thickness of the radome <NUM> is determined by the radio wave transmission performance required for the radome <NUM>. Specifically, the thickness of the skin layers of the radome <NUM> (the sum of the thickness of the first fiber reinforced plastic layers <NUM>, which are the cyanate layers, and the thickness of the second fiber reinforced plastic layers <NUM>, which are the epoxy layers) needs to be <NUM>/<NUM> or less of a free space wavelength at a center frequency of the transmission frequency band required for the radome <NUM>.

Therefore, in the present embodiment, the thickness of the skin layers (the sum of the thickness of the first fiber reinforced plastic layers <NUM>, which are the cyanate layers, and the thickness of the second fiber reinforced plastic layers <NUM>, which are the epoxy layers) is configured to be <NUM>/<NUM> or less of the free space wavelength at the center frequency of the transmission frequency band required for the radome <NUM>.

That is, the skin layers are disposed on both surfaces (the front surface and the back surface) of the core member <NUM>, and each of the thicknesses of the skin layers disposed on each of the surfaces is configured to be <NUM>/<NUM> or less of the free space wavelength at the center frequency of the transmission frequency band of the radome <NUM>.

<FIG> is a table showing peel test results of multilayer structures in which skin layers are layered on core members.

The peel test was conducted in accordance with the standard (ASTM D1781) of the American Society for Testing and Materials as follows.

First, a sample <NUM> as illustrated in <FIG> was formed.

The sample <NUM> is a multilayer structure including a core member <NUM>, a first skin layer <NUM> layered on one surface (test surface) of the core member <NUM>, and a second skin layer <NUM> layered on the other surface (non-test surface) of the core member <NUM>.

The core member <NUM> is formed of a foamed body of polyether sulfone (PES). The first skin layer <NUM> and the second skin layer <NUM> are formed of prepreg containing an epoxy resin and quartz fiber. The sample <NUM> is formed by layering the prepregs to be the skin layers <NUM>, <NUM> on the respective front surface and back surface of the core member <NUM>, and after applying heat and pressure to cure the prepregs, cutting the multilayer structure into a desired size (cutting in the width direction in the present embodiment). Since the purpose of this test is to measure the adhesive strength between the core member and the epoxy layer, layering of the cyanate layers was omitted.

The core member <NUM> is <NUM> in thickness, the first skin layer <NUM> is <NUM> in thickness, and the second skin layer <NUM> is <NUM> in thickness. The length of the core member <NUM> in the longitudinal direction (the arrow L direction in <FIG>) is <NUM>, the length in the width direction (the arrow W direction in <FIG>) is about <NUM> (with a variation of less than <NUM> as will be described later), and the second skin layer <NUM> is also formed to have the same size as the core member <NUM>. While the length of the first skin layer <NUM> in the width direction is about <NUM>, which is the same as that of the core member <NUM>, the length in the longitudinal direction is <NUM>, and surplus portions <NUM> projecting from both ends of the core member <NUM> in the longitudinal direction are formed. The surplus portion <NUM> is sandwiched by a drum portion <NUM> of a testing device <NUM> (see <FIG>) described later and is formed to peel the first skin layer <NUM> from the core member <NUM>.

As shown in <FIG>, in the present embodiment, the four samples <NUM> (samples <NUM> to <NUM>) were produced. The widths of the respective samples <NUM> were <NUM> for the sample <NUM>, <NUM> for the sample <NUM>, <NUM> for the sample <NUM>, and <NUM> for the sample <NUM> with an average value of <NUM> and a standard deviation of <NUM>.

Next, the sample <NUM> is set in the testing device, and measurement is performed after measurement parameters are set as appropriate.

<FIG> illustrates the configuration of the testing device <NUM>. The testing device <NUM> includes the drum portion <NUM> and a fixing portion <NUM>. The drum portion <NUM> and the fixing portion <NUM> are supported by a frame (not illustrated) and are disposed at positions separated by a predetermined distance at the start of measurement. The drum portion <NUM> has a cylindrical shape and simultaneously with rotation about an axis O by a driving mechanism (not illustrated), the drum portion <NUM> linearly moves so as to approach the fixing portion <NUM>. The drum portion <NUM> includes a holding plate <NUM> along the axial direction of the cylinder on the outer circumferential surface thereof. One surplus portion <NUM> of the sample <NUM> is sandwiched by the holding plate <NUM>. The fixing portion <NUM> is fixed to be immovable during measurement, and the other surplus portion <NUM> side of the sample <NUM> is sandwiched by the fixing portion <NUM>.

The drum portion <NUM> rotates with the first skin layer <NUM> sandwiched by the drum portion <NUM>, and the first skin layer <NUM> is peeled off from the core member <NUM> by movement of the drum portion <NUM> in the fixing portion <NUM> direction. At this time, the testing device <NUM> measures a load applied to the drum portion <NUM> and the like to calculate the peel strength of the sample <NUM>.

Specifically, a peel strength T is calculated using the following formula (<NUM>).

In the following formula (<NUM>), T is a peel strength (kg•cm/<NUM>), R is a torque arm length (R = R0 - Ri = <NUM>, R0: an outer flange radius where a spring steel is fixed to the drum portion <NUM>, Ri: an inner drum radius around which the first skin layer <NUM> to be peeled is wound), Fp is an average peel load (kg) of the peel length from <NUM> to <NUM> of the sample <NUM>, F0 is an average peel load (kg) of the peel length from <NUM> to <NUM> of the first skin layer <NUM>, and W is the width (mm) of the sample <NUM>. Fp is a load taking the mass of the drum portion <NUM> into account, and the load of the sample that is not bonded is acquired by F0, and F0 is subtracted from Fp to obtain a value not affected by the mass of the drum portion <NUM>.

In the present embodiment, a cross head speed (pulling speed), which is the movement velocity of the drum portion <NUM>, was set to <NUM> ± <NUM><NUM>/min.

As shown in <FIG>, the peel loads (N/m) of the samples <NUM> to <NUM> are <NUM>, <NUM>, <NUM>, and <NUM>, respectively and the average value is <NUM> (the standard deviation is <NUM>), which finds that the samples <NUM> to <NUM> have sufficient strength.

As Comparative Example, it is conceivable to conduct a peel test on a multilayer structure in which a prepreg containing a cyanate resin and quartz fiber (hereinafter referred to as a "cyanate resin prepreg") is layered on a core member. However, the test was not conducted because the multilayer structure using the cyanate resin prepreg has only an adhesive force at which the cyanate resin prepreg is peeled off by hand and an appropriate result cannot be obtained in the test using the device illustrated in <FIG>.

As described above, according to the radome <NUM> of the embodiment, the first fiber reinforced plastic layer <NUM> containing the cyanate ester resin as the main component is used for the skin layer and the second fiber reinforced plastic layer <NUM> containing the epoxy resin as the main component is disposed at the position in contact with the surface of the core member <NUM>. Accordingly, the second fiber reinforced plastic layer <NUM> functions as the adhesive layer that bonds the core member <NUM> and the first fiber reinforced plastic layer <NUM> together, which is advantageous in providing the radio wave transmission performance of the radome <NUM> and the strength of the radome in a compatible manner.

According to the radome <NUM>, the proportion of the thickness of the second fiber reinforced plastic layer <NUM> to the thickness of all of the skin layers (the sum of the thicknesses of the first fiber reinforced plastic layer <NUM> and the thicknesses of the second fiber reinforced plastic layer <NUM>) is set to <NUM>% or less. Accordingly, it is possible to suppress the decrease in radio wave transmission performance due to the arrangement of the second fiber reinforced plastic layer <NUM> having the higher dielectric constant than the first fiber reinforced plastic layer <NUM> to a certain range.

In addition, according to the radome <NUM>, since the thickness of the skin layer is <NUM>/<NUM> or less of the free space wavelength at the center frequency of the transmission frequency band of the radome <NUM>, the radio wave transmission performance of the radome <NUM> can be satisfactorily maintained.

Further, in the radome <NUM>, when the fiber material contained in the first fiber reinforced plastic layer <NUM> is quartz fiber having the low dielectric constant, the dielectric constant of the first fiber reinforced plastic layer <NUM> can be lowered to improve the radio wave transmission performance of the radome <NUM>.

In the radome <NUM>, when the fiber material contained in the first fiber reinforced plastic layer <NUM> is E-glass fiber, the radome <NUM> can be produced at a relatively low cost.

Further, in the radome <NUM>, when the fiber material contained in the first fiber reinforced plastic layer <NUM> is NE-glass fiber, the dielectric constant of the first fiber reinforced plastic layer <NUM> can be lower compared with the case where E-glass fiber is used and the radio wave transmission performance of the radome <NUM> can be improved.

Claim 1:
A radome comprising
a multilayer structure in which a plurality of skin layers are layered on a surface of a core member,
the skin layer comprising a first fiber reinforced plastic layer containing a cyanate ester resin and a fiber material and a second fiber reinforced plastic layer containing an epoxy resin and a fiber material, and
the second fiber reinforced plastic layer being disposed at a position in contact with the surface of the core member.