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Article on the stealthiness of canards versus conventional tailed aircraft by 611 Institute (Chengdu). I'll finish up translation and post pics later.
Journal of Aeronautics and Astronautics Nov. 25 2019 Vol. 38 No.X:
Acta Aeronautica et Astronautica Sinica ISSN 1000-6893 CN 11-1929 / V
XXXXXX-1
http://hkxb.buaa.edu.cn hkxb@buaa.edu.cn
DOI: 10.7527 / S1000-6893.2019.23485
Research on the RCS Impact of Duck Wing
Guo Zhanzhi, Chen Yingwen, Ma Lianfeng
Chengdu Aircraft Design Institute, Chengdu 610091
Abstract: In this paper, the effects of canard wings of a canard layout fighter on the RCS of the aircraft are studied and analyzed in detail. The paper first analyzes the scattering mechanism of canard wings, and then uses the multilayer fast multipole (MLFMM) method to calculate the RCS of the whole model of the specific model. By comparing the RCS of the canard layout and the conventional layout, the canard wings scattering The effects of the RCS of the aircraft, including the influence of the canard wing deflection on the whole machine; In addition, the edge scattering of the canard wing and the effect on the slit scattering and the corresponding suppression measures were studied by experimental methods. The research results show that the canard layout can be applied to the layout design of high stealth aircraft after thecanard wing scattering is suppressed or eliminated, and its stealth performance is equivalent to the conventional layout. Finally, the guiding principles of canard wing stealth design are summarized.
Keywords: canard layout; RCS; MLFMM; conventional layout; stealth
Chinese picture classification number: V218 Document identification code: A Article number: 1000-6893.2019.23485
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Received date: 2017-xx-xx; Retired date: 2017-xx-xx; Accepted date: 2017-xx-xx;
Web publishing address: http://hkxb.buaa.edu.cn/CN/html/2018XXXX.html
Fund Project: National Natural Science Foundation of China (Fund Number); Aviation Science Foundation of China (Fund Number)
* Corresponding author. E-mail: hkxb@buaa.edu.cn
In the history of the development of fighters in the world, according to the characteristics of different horizontal stabilizers, common fighter layouts can be roughly divided into the following categories: conventional layouts, duck layouts, tailless layouts, three-wing layouts and flying wing layouts [1]. Among them, the duck layout has received great attention in the layout design of modern fighters.
Canard layout has a very important position in the development history of fighter layout design in China. Domestic aerodynamic layout design technicians rely entirely on independent innovation. After years of research and successful application of models, they have mastered the duck layout aerodynamic design technology. Facts have proved that the combination of duck layout and relaxation of longitudinal static stability technology can avoid the shortcomings of duck layout, give full play to its advantages in balance and increase, and help to obtain a higher supersonic balance. Conducive to focus matching and center of gravity configuration in the sub-, trans-, and supersonic full flight envelopes [2].
With the development of various electronic detection equipment, the battlefield environment is becoming increasingly complex. Stealth technology has a profound impact on the survivability and combat effectiveness of fighters. Stealth has become one of the important features necessary for a new generation of combat aircraft [3]. As the largest country leading the development of aviation technology in the world, the United States has developed advanced stealth fighters such as F-117, B-2, F-22, F-35. As we all know, stealth capability is the core technical feature of the fourth-generation fighter aircraft. According to the public information available, it can be speculated that the F-22 and F-35 have significantly improved their stealth performance compared to traditional three-generation aircraft or three-generation modified stealth aircraft. Stealth aircraft constitute a fatal advantage, and the development of stealth aircraft has become a consensus around the world. The stealth performance of fighters depends to a large extent on the design of the layout. F-22 and F-35 aircraft have adopted conventional layouts, which fully shows that Americans have mastered the stealth design technology of conventional layouts. For the domestic market, adopting a duck-type layout is a unique choice. Although adopting the duck layout can give full play to its aerodynamic advantages, the disadvantage of stealth compared to the conventional layout is obvious: the front of thecanard wing is equivalent to adding a scattering component in front of the aircraft, and The gap between the canard wing and the fuselage is directly exposed in front of the aircraft. These factors are likely to cause the stealth physical characteristics deteriorate. So, what factors need to be considered for the stealth design of the duck layout aircraft? Can its stealth ability be even better than the conventional layout?
This paper studies and analyzes the impact of canard wings on RCS for canard layout aircraft. The stealth layout design of some fighters has many factors affecting it: intake and exhaust methods, fuselage cross-sectional shape, main wing geometry parameters, tail parameters, design of the rear body coupling area, and so on. For different layout forms, these stealth design elements are the same. Compared with the conventional layout, the canard layout only adds an invisible element-canard wings. In order to study the effects of duck wings RCS, this paper first analyzes the scattering mechanism of canard wings, and then develops the stealth shape modeling of canard layout and conventional layout, and compares the differences between them by RCS simulation calculation. In order to explain the difference of duck wings more clearly, this article assumes that the stealth characteristics of canard layout and conventional layout are calculated and compared respectively under the premise that other stealth elements are the same or equivalent; then, further research is carried out through the component stealth test method. RCS suppression scheme for edge scattering and slit scattering between duck wings and body, and its suppression effect was verified.
1 Research methods
In this paper, the shape stealth characteristics of different layouts are studied by means of RCS theoretical calculations. By comparing the RCS calculation results of duck layout and conventional layout, the scattering contribution of duck wings is analyzed, and the influence of duck wings deflection on the whole machine is studied. The stealth test of the absorbing component studies the influence of edge scattering and the suppression scheme; by developing a full-size counter-slit component model, the stealth test research is conducted on the problem of the counter-slot scattering between the duck wing and the fuselage.
1.1 RCS calculation analysis method
During the aircraft design phase and the stealth optimization process, in order to quickly complete the iterative optimization of the solution and reduce the cost of stealth test verification, it is often necessary to use the RCS simulation method to estimate and analyze the scattering characteristics of the target.
The analytical method of the electromagnetic scattering problem is to obtain a strict solution to the target problem by a wave equation that satisfies strict boundary conditions. With the development of computer technology, a large number of numerical methods have been proposed for solving. These methods are generally based on Maxwell's equations.
1.1.1 High frequency calculation method
The high-frequency calculation method is an approximation method, which was widely used in the early days. It mainly includes geometric optics (GO) and physical optical (PO), which are discussed in many works. .
The geometrical optics method is a classical method to explain the scattering mechanism and energy propagation using ray management theory. The basic theory of geometric optics is to describe the reflection and refraction of electromagnetic waves on the decomposition surfaces of two different media. For discontinuities such as edges, corners, and sharp points, geometrical optics cannot be used. For complex-shaped diffusers, the calculation accuracy of geometrical optics cannot be guaranteed.
The theory of physical optics method obtains the scattering field by approximate integration of the induction field. It can solve the structure of RCS for the structures that cannot be calculated by geometric optics such as plane and single curved surface. The physical optics method completely ignores the interaction between various parts of the target based on the principle of locality of the high-frequency field, and independently determines the surface-induced current independently based on the incident field alone. Although the RCS of an ideal conductor target can be calculated quickly and efficiently, it is only suitable for conductor targets with large electrical dimensions, smooth surfaces, and weak local coupling. When the target has many edges, spikes, or local coupling scattering regions, the calculation results of the physical optics method will produce a large error.
In order to make up for the shortcomings of geometrical optics and physical optics methods, geometric theory of diffraction (GTD) and uniform theory of diffraction (UTD) have been developed successively. There are also many limitations in the application.
1.1.2 Accurate numerical calculation method
Precise numerical calculation methods are further divided into integral equation method (IEM) and differential equation method, which correspond to the integral form and differential form of Maxwell's equation, respectively.
The integral equation method includes an electric field integral equation, a magnetic field integral equation, and a mixed field integral equation. The mixed field integral equation is actually a linear combination of the electric field integral equation and the magnetic field integral equation. The mixed-field integral has the characteristics of accurate calculation of the electric field integral equation and good convergence of the magnetic field integral equation, and eliminates the problem of internal resonance.
With the development of computational electromagnetics, more mature numerical algorithms have appeared, including the method of moment (MOM) based on integral equations, and the multi-level fast multipole method (MLFMM) . For the electromagnetic scattering problem, the current industry recognized
An effective algorithm is a multi-layer fast multipole. With the rapid development of high-performance computing servers today, the multi-layer fast multipole method has been able to accurately calculate low RCS aircraft targets with electrical dimensions exceeding 1,000 wavelengths. The electrical dimensions have covered most of the problems in stealth design [5 ].
For the RCS calculation of the general stealth aircraft, the high-frequency calculation method cannot meet the RCS calculation accuracy requirements. This paper adopts the engineering calculation software based on the multi-layer fast multipole method to solve the RCS of the target.
1.2 Radar stealth test and verification
The target RCS can be obtained through theoretical calculations and experimental tests. The theoretical calculation method is more accurate for analyzing the scattering characteristics of metal shapes, but for complex shapes and targets with complex media, the calculation difficulty will increase greatly, and the calculation accuracy will be severely restricted, making the test method the main method to obtain the target's electromagnetic scattering characteristics. [6]. Therefore, the radar stealth test technology based on RCS test and imaging diagnostic test is very important.
The RCS test is divided into several types, among which the microwave darkroom test is suitable for the aircraft scale model test and the full-scale component model test. Under the premise of determining the aircraft layout plan, conducting a full-scale stealth test on a specific scattering source is helpful to accurately grasp its scattering characteristics, so as to develop a specific scattering suppression plan in order to further improve the overall stealth level of the aircraft.
The stealth design, the rational application of the absorbing coating and the absorbing structure of local details on the surface of the stealth aircraft are very important to further improve the stealth performance of the whole aircraft, but these detailed designs are not suitable for research and optimization through scale model testing. Generally, the size of the local scattering source and scattering part of the stealth aircraft is between 1 meter and several meters. If it is tested on the whole machine, on the one hand, the size is too large to implement, and the cost is high; It is possible to cover up the local RCS, and it is impossible to accurately obtain the scattering characteristics of components or detailed structures. Usually, a full-scale component model is developed to simulate local details, and a microwave darkroom is used for testing.
2 Research on the impact of duck wings on RCS
2.1 canard wing scattering mechanism
Although the canard wing is a unique component of the canard layout, its scattering mechanism is not complicated. The scattering mechanism of canard wings is shown in Figure 1.
Canard wing scattering can be reduced to three types of scattering problems:
a) Sharp point scattering: When the electromagnetic wave hits the trailing edge of the duck wing
Fig. 1 Scattering Mechanism of Canard
Diffraction occurs at corners, and surface traveling waves also form diffractions at sharp points. Sharp point scattering is a primary scattering;
b) Edge scattering: When the electromagnetic wave hits the edge of the target, the edge diffracts the incident electromagnetic wave, and the surface
Traveling waves will also cause diffraction at the edges. Edge scattering belongs to primary scattering, and edge scattering belongs to strong scattering sources. It is an issue that must be considered in stealth aircraft design to suppress its peak scattering;
c) Slot scattering: There is an inevitable butt gap between the duck wing and the fuselage that needs to meet the deflection requirements of the duck wing. The gap is relatively
long and there is a rotating shaft mechanism. The scattering mechanism is complicated and there may be multiple reflection characteristics.
Among the three types of scattering problems, sharp point scattering and edge scattering are not unique to duck wings, and are common to airfoil components, and their scattering suppression schemes are similar. Slot scattering is a unique scattering source for duck layouts. Since the duck wings are located in the front fuselage area, the gap between the duck wings and the fuselage is easily exposed to the front of the aircraft, and a targeted solution is required.
2.2 Layout Modeling Instructions
In order to carry out the RCS calculation of the layout of the whole machine, the stealth shape modeling of different layouts needs to be completed first, and the layout scheme requirements are basically feasible, otherwise the significance of research is lost. The modeling process must follow the general principles of the shape stealth design, such as the fuselage profile meets the requirements of low RCS profile design, the vertical tail is cambered at an angle, all edges are arranged according to the principle of top-down projection parallel design [7], etc. .
This article refers to the layout of the F-35 fighter, and first generates a single-engine stealth solution with a conventional layout of air intake on both sides. To
Front edge Scattering
Trailing edge Scattering
Cusp Scattering
Gap Scattering
Based on the successful design of F-35, it can guarantee that the scheme is basically established without subversive problems. Then on this basis, the flat tail was eliminated, duck wings were added to the rear of the air inlet in an appropriate position, and the sides of the body were added, and the positions of the wings and the vertical tail were moved back appropriately to form a duck layout scheme. The two layout schemes adopt the same fuselage shape, the same main wing surface and vertical tail shape, and the same shape of the side bars, in order to make the stealth elements of the angel basically the same. The RCS difference of the layout (without duck wings) makes it easier to analyze the influence of duck wings on the whole machine.
The outline model of the conventional layout plan is shown in Figure 2, and the outline model of the duck layout plan is shown in Figure 3.
The main geometric features related to the layout shape and stealth and the RCS calculation model are described as follows:
a) Due to the complexity of cavity calculation, the influence of inlet cavity cavity scattering is not considered, and a low-scattering profile surface is designed
Blocking the air inlet and the tail nozzle, without calculating the cavity scattering and the terminal nozzle scattering, and without considering the influence of the cavity scattering, the RCS level of the whole machine is lower, which is more conducive to analyzing the influence of duck wings on the RCS of the whole machine;
(A) Top View
(B) Upward view
Fig. 2 Conventional Configuration
b) The shapes of the front fuselage, middle fuselage, rear fuselage and rear strips are similar to those of the F-35 aircraft, all adopting a low RCS profile design, and the two layout calculation models share the same fuselage shape;
c) The geometry of the vertical tail is similar to the F-35. The camber of the vertical tail is 27 °, and the top-view projection of the leading edge sweep angle is 60.7 °, and the trailing edge sweep angle is 42 °. The two layouts adopt the same vertical tail shape. ;
d) The front edge of the wing adopts a medium sweep angle design. The leading edge sweep angle of the duck wing, the wing and the flat tail are 42 °, and the leading edge sweep angle of the trailing edge are 15 °. The angle on the duck wing is 8 °, and the wing has no angle on the wing;
e) Both the duck wings and the tip of the vertical tail are designed to be invisible.
2.3 RCS calculation instructions
a) RCS is solved for the two layout schemes based on the RCS accurate numerical calculation method, and the medium property of the target model is set to metal, and the application of the absorbing coating / absorbing structure is not considered;
(A) Top View
(B) Upward view
Fig. 3 Canard Configuration
b) Considering the constraints of hardware resources, the scale of the target RCS solution needs to be reduced. This paper only selects the typical frequencies of the L-band and C-band for calculation, and focuses on the RCS characteristics of the forward sector. analysis;
c) Explanation of calculation parameters: typical frequencies are 1.7GHz and 5.6GHz, pitch angle is 5 ° (corresponding to typical attack angle in cruise state), and azimuth angle is 0 ° ～ 90 °.
2.4 RCS comparison of duck layout and conventional layout
Perform RCS calculations on the above two layout models, and compare the shape RCS of the conventional layout and the duck layout to illustrate the equivalent of the duck wings. First of all, the stealth characteristics of the layout of the active surface without deflection are studied, that is, the duck wings or flat tails are in the neutral position.
A comparison of typical RCS curves in the L band is shown in Figures 4 and 5, and a comparison of the RCS curves in the C band is shown in Figures 6 and 7. Table 1 compares the statistical data of the RCS average of the front sector of the aircraft from 0 to 30 °.
Fig. 4 RCS Comparison of Two Configuration, 1.7GHz, HH polarization
Fig. 5 RCS Comparison of Two Configuration, 1.7GHz, VV polarization
Fig. 6 RCS Comparison of Two Configuration, 5.6GHz, HH polarization
Fig. 7 RCS Comparison of Two Configuration, 5.6GHz, VV polarization
Table 1 Comparison of mean data from 0 to 30 °, unit: dBsm
Table 1 0 ～ 30 ° RCS ’s mean value, dBsm
Frequency
HH polarization
conventional canard
VV polarization
conventional canard
1.7GHz
-13.26 -11.99
-14.6 -14.08
5.6GHz
-19.74 -20.42
-16.91 -15.19
According to the comparison of the above curves and average data, the RCS analysis of duck layout and conventional layout is as follows:
a) From the macro comparison of the curves, the RCS curves of the two layouts are basically the same in the large angle range of 0 to 60 °, and the RCS of the duck layout is significantly smaller in the side angle range of 75 to 90 °. This is due to the dihedral angle feature between the vertical tail and the tail of the conventional layout scheme, which is caused by strong multiple scattering. The research focus of this article is on the RCS characteristics of the front sector, and the side differences are not analyzed;
b) According to the average RCS statistics in Table 1,
In terms of segment HH polarization, the RCS level of the duck layout is 1.27dB larger than that of the conventional layout, which is mainly caused by the front duck wings. However, in the C band, the RCS of the duck layout is reduced by 0.68dB, which is Because the scattering characteristics of airfoil components are related to frequency, for the same wingtip parameters, at low frequencies and longer wavelengths, duck wing tip scattering has a more significant effect on the whole machine. As the frequency increases and the wavelength becomes smaller, the tip scattering The impact is weakened, so the results in both C-bands are comparable;
c) For VV polarization, the average level of the duck layout has increased slightly, both at low and high frequencies. The main reason is that the RCS peak at an azimuth angle of 15 ° increases. This scattering peak corresponds to the scattering at the trailing edge of the airfoil. Although the edges of each airfoil are arranged in accordance with the principle of parallelism, since the sweeping angle of the trailing edge of the duck wing is 15 °, its scattering peak is directly exposed in front of the aircraft, and the trailing edge of the wing and the trailing edge is a sweeping angle of 15 °. The fuselage has a certain occlusion relationship for its scattering peaks (see Figure 8). Therefore, the strong scattering at the trailing edge of the duck wing causes the peak scattering peak of the trailing edge of the duck-type layout to be larger than that of the conventional layout. The increase of the peak value will not significantly affect the overall RCS characteristics, and after applying the absorbing coating and edge absorbing structure, the peak edge scattering peak will be effectively suppressed (see section 2.6 "Experimental Study of Edge Scattering");
Figure 8 Schematic diagram of trailing edge normal scattering
Fig. 8 Trailing Edge Normal Scattering
d) The trailing edge of the duck wing generally adopts a sweep-back design, which is good for trimming and aerodynamic focus matching. If it can be changed to a forward-sweep design, that is, parallel to the trailing edge of the same wing, the effective shielding of the fuselage can be used. Reduce its scattering peaks
Value, and strong peaks at the trailing edge of the wing can mask it;
e) From the peak of the azimuth angle of 42 °, there is no obvious difference between the two layouts. This is because the main wing surface is the same, and the smaller peak at the leading edge of the duck wing is masked by the stronger peak at the front edge of the main wing.
2.5 RCS Effect of Canard Wing Deflection
The level of RCS without canard deflection alone is not enough to fully explain the advantages and disadvantages of duck stealth layout.Canard wings, as an important moving surface, may deflect at any time during the flight of the aircraft, and then change the stealth characteristics of the whole aircraft. Generally speaking, when performing a typical combat mission, fighters correspond to different stealth levels at different mission phases, and maintaining a high stealth state in the full flight profile is neither realistic nor significant. When the aircraft is taking off, landing, fighting at close range, or performing a large maneuver, the stealth level is not high at this time, or the RCS characteristics of the aircraft body in certain attitudes are relatively large. Although the duck wings are deflected at large angles, The entire RCS does not constitute a fatal impact.
What really needs to focus on is the cruising stage of the aircraft, at this time corresponding to the high stealth state of the aircraft, any deflection of the rudder surface should not destroy the high stealth state of the aircraft body. In general, when the canard layout fighter cruises in subsonic and supersonic cruises, the range of deflection angles of the duck wing is relatively small, about 0 to -5 °. Generally, the negative angle of the small angle is maintained. Maintain a high lift-drag ratio during cruise.
In order to explain the influence of duckwing deflection on RCS, this paper selects duckwing deflection ± 5 ° attitude and deflection 0 ° attitude for comparison of L-band calculations. Comparison of HH and VV polarized RCS curves is shown in Figures 9 and 10. Table 2 shows the average RCS data of the front sector from 0 to 30 °.
Fig. 9 RCS Comparison of Different Canard Status, 1.7GHz, HH polarization
Fig. 10 RCS Comparison of Different Canard Status, 1.7GHz, VV polarization
Table 2 RCS ’s mean value of different canard deflection angles, dBsm
Rotation angle
HH polarization
VV polarization
0 °
-11.99
-14.08
5 °
-5 °
-11.6
-12.18
-11.26
-14.56
As can be seen from the comparison of the above curves:
a) The RCS curves before and after deflection of duck wings basically match. The only obvious difference is the scattering peak of VV polarization curve at 15 ° azimuth. After deflection of duck wings, the scattering peaks are slightly wider, whether it is positive or negative. This is because the rotation axis of the duck wing and the trailing edge are not parallel, resulting in that the top projection of the rear edge of the duck wing and the top projection of the trailing edge of the wing after deflection are not completely parallel;
b) When the duck wing is deflected by 5 °, the peak of VV polarization scattering at an azimuth of 15 ° increases significantly, which causes the average level to increase by nearly 3dB. When the deflection is -5 °, the peak value decreases slightly, but the average level slightly decreases, and the scattering peak It is essentially traveling wave scattering. Traveling wave scattering is related to the polarization mode. Traveling waves occur only when there is an incident electric field component along the surface in the direction of propagation. According to the traveling wave scattering mechanism, when a conductive target is irradiated with electromagnetic waves near the grazing incidence direction, a surface current is induced to generate a surface traveling wave. If the surface traveling wave cannot be absorbed at the target discontinuity, it will cause reflection [8]. As shown in Figure 11 (a), the aircraft elevation is 5 °, and the duck wings
At 0 ° deflection, the duck wings face the incoming wave to form a grazing incidence condition. The incident electric field E generates an electric field component Ei on the surface, which excites the surface current and propagates to the shape at the trailing edge.
Strong echo reflection; when the duck wing is 5 ° forward, as shown in Figure 11 (b), the angle between the surface of the duck wing and the direction of the incoming wave becomes smaller, the electric field component Ei along the target surface becomes larger, and the surface traveling wave becomes stronger. The echo becomes stronger; when the negative deviation of the duck wing is 5 °, as shown in Figure 11 (c), the angle between the surface of the duck wing and the direction of the incoming wave becomes larger, the electric field component Ei becomes smaller, and the surface traveling wave becomes weaker, which weakens the echo scattering;
(A) The aircraft is tilted 5 ° and the duck wings are deflected 0
(B) The aircraft is tilted 5 ° and the duck wings are tilted 5 °.
(C) The aircraft is tilted 5 ° and the duck wings are deflected 5 °
Figure 11 Traveling wave scattering mechanism of duck wings with different deflection attitudes
Fig. 11 The mechanism of traveling wave scattering with different canard status
c) For HH polarization, the scattering mechanism is different from VV polarization. The electric field component is parallel to the scanning surface. There is no problem of trailing edge scattering peaks caused by surface travelling waves. The small angle deflection of duck wings will not cause the geometric characteristics of the target. Obvious changes, so before and after the deflection of the duck wing has no obvious effect on the HH polarization RCS of the whole machine, the curves are basically consistent, and the average level is equivalent.
As mentioned earlier, when a duck-type aircraft is cruising normally, its duckwing deflection is small, and it is in a negative deflection attitude, and no positive deflection will occur, so it will not damage the RCS characteristics of the whole aircraft. When the angle is not as large as ﹢ 5 °, the RCS increase caused by trailing edge scattering can be eliminated after the trailing edge absorbing structure is applied. When the duckwing deflects at a greater angle, whether it is positive or negative, its effect on The impact of the RCS of the whole aircraft will increase, but the RCS of the aircraft itself may be relatively large at this time, and from the perspective of the stealth level management of the aircraft, the aircraft is usually in a non-high stealth level at this time. In general, the deflection of duck wings will not affect the stealth of the aircraft.
Incident wave
Strong echo
Incident wave
Echo becomes stronger
E
Ei
En
E
Ei
En
Incident wave
Weak echo
Aviation science
The comparison of the above RCS calculation results shows that the stealth characteristics of the duck layout and the conventional layout are equivalent. The following tests are used to study the edge scattering of duck wings and the effect on slit scattering, and the corresponding suppression scheme.
2.6 Experimental study of edge scattering
Edge scattering features are present at the edges of aircraft wing components. Edge scattering is a kind of strong scattering source, especially after the aircraft's strong specular reflection in the radar threat area weakens, the contribution of edge scattering is very prominent. Absorbing coatings or structures can be used to suppress edge scattering. Generally, the absorbing coating is designed for high frequency, and its absorption effect at low frequency is limited, while the absorbing structure can take into account both the low frequency and high frequency absorbing performance requirements. Generally, stealth aircraft use the absorbing structure on edge components to suppress edge scattering.
In order to make the test results universal, a full-size component model as shown in Figure 12 was specially designed to simulate the leading and trailing edge characteristics of general airfoil components. The status of the full-size component stealth test includes the model's all-metal status (that is, before the reduction measures are taken) and the absorbing structure status (where the green area is the application area of the absorbing structure).
Fig. 12 Full Size Model of Edge Part
Fig. 13 RCS of Edge Part Model, 2GHz, HH polarization
The test results of the metal state are used to explain the effect of edge scattering, and the test results of the absorbing structure state are used to verify the RCS suppression effect of the edge absorbing structure on the edge scattering. The experimental study is aimed at the typical frequencies of low-frequency L-band and high-frequency X-band. The comparison of RCS curves at 2GHz is shown in Figures 13 and 14, and the comparison of RCS curves at 10GHz is shown in Figures 15 and 16.
Fig. 14 RCS of Edge Part Model, 2GHz, VV polarization
Fig. 15 RCS of Edge Part Model, 10GHz, HH polarization
Fig. 16 RCS of Edge Part Model, 10GHz, VV polarization
Table 3 0 ～ 30 ° RCS ’s mean value of edge part, dBsm
Frequency
Metal
Absorbing structure
HH VV HH VV
2GHz
-22.22 -22.08 -27.56 -27.88
10GHz
-26 -24.8 -33.61 -31.77
As can be seen from the above results:
a) In the metallic state, the scattering peaks of the edge parts are strong, and the overall average level is high (see Table 3 for the average data), which has a greater impact on high stealth aircraft on the order of 0.001 to 0.005m2;
b) After the absorption structure is applied, the scattering peak corresponding to the edge of the airfoil is greatly reduced, and the overall RCS level is significantly reduced. The X-band gain is greater, which is determined by the performance of the absorption structure. The absorption structure is at X Better absorbing performance in the band.
The above test results show that if the RCS reduction measures are not taken, the edge scattering of the wing component (duck wing component) is mainly the effect of the trailing edge scattering on the stealth of the aircraft in front. Effectively suppressed.
2.7 Experimental study on slit scattering
The previous calculation analysis focused on the shape stealth characteristics of the overall layout, and it was impossible to simulate the problem of the scattering of the gap between the duck wing and the fuselage. Aiming at the scattering of this pair of slits, this paper conducts experiments by developing a full-scale component model to study the effect on slit scattering and the corresponding RCS suppression measures.
In order to simulate the typical gap between the left and right duck wings and the fuselage, a low RCS carrier model as shown in Figure 17 was designed. A low RCS curved surface is designed to be closed smoothly around the gap area, so that the carrier itself has low scattering characteristics in the front sector. Then a full-scale metal model is developed and tested in a microwave dark room. The test target is shown in Figure 18.
Fig. 17 Gap Model
Fig. 18 The RCS Test of Full-Size Gap Model
Since this kind of effect on slit scattering is mainly at high frequencies, it has less effect on low frequencies. This test mainly tests and studies the typical frequencies of the high-frequency X-band. It is divided into the original state of the slit and the reduced state of RCS. It is used to study the effect of the original slit slit on the whole machine and to verify the suppression effect after the RCS reduction measures are taken. The comparison of the RCS curves of the two states is shown in Figure 19 and Figure 20, with a frequency of 9.41 GHz.
Fig. 19 the RCS Curve of Gap Model, 9.41GHz, HH polarization
Fig. 20 the RCS Curve of Gap Model, 9.41GHz, VV polarization
The mean statistics are shown in Table 4.
Table 4 RCS ’s mean value of Docking gap model, dBsm
Status
HH polarization VV polarization
Primitive
-20.63 -21.48
Inhibition
-38.27 -33.1
From the above results, it can be seen that before the RCS reduction measures were taken for the seams, there were strong peaks with wide angular ranges in the forward sector of the corresponding aircraft, which had a greater impact on the overall stealth of the aircraft. The test results show that the duck-wing-to-slot scattering is an important scattering source for duck-layout aircraft. From the scattering mechanism:
a) The smooth, narrow and contoured surfaces on both sides of the slit are easy to excite surface traveling waves. The geometric discontinuities formed by the duck-wing rotating shaft mechanism during traveling wave propagation will cause strong reflection if not absorbed;
b) The electromagnetic wave is incident on the inside of the opposite seam, and there are multiple reflection features between the profiles on both sides.
Such scattering problems are mainly suppressed by radar absorbing coatings with good high-frequency absorbing performance. The RCS suppression effect obtained in a reduced state is very significant, and the strong and wide peaks in the front sector are greatly reduced. Both HH polarization and VV polarization RCS averages have fallen sharply, and the average levels have fallen below -38dBsm and -33dBsm, respectively.
Therefore, by adopting reasonable RCS suppression measures, the influence of the duck-type layout on the slit scattering problem can be basically eliminated.
3 conclusions
Canard scattering is a unique problem in the design of stealth fighters with duck layout and needs attention.
The research in this paper shows that the duck wing scattering suppression scheme is not complicated. After taking measures to eliminate the influence of canard scattering, the canard layout can be applied to the layout design of high stealth aircraft, and its stealth performance is equivalent to the conventional layout.
The canard wing stealth design for canard layout fighters must follow the following principles:
1) The edge design of the canard wing is arranged according to the parallel principle of the top-down projection of the edge to reduce the number of RCS peaks, and the stronger peak of the main wing surface edge is used to mask the smaller peak of the canard wing edge;
2) The sharp point of the trailing edge of the duck wing is integrated with aerodynamic and stealth requirements to make an appropriate chamfer to reduce the sharp point scattering;
3) Apply the absorbing structure to the edges, including the leading edge, trailing edge, and wingtips, and select the performance parameters of the absorbing structure and determine the size requirements of the absorbing structure based on the stealth requirements of the aircraft for low and high frequencies;
4) Apply a wave-absorbing coating with excellent high-frequency absorption performance to the slit area between the duck wing and the fuselage to suppress the slit scattering, and develop a reasonable coating area.
References
[1] Zhang Jigao. Division of fighter aerodynamic layout [J]. Journal of Aerodynamics, 2009, 27 (5): 618.
[2] Zheng Sui, Zhan Jingxia, Cao Yuan, etc. The influence of relaxation of longitudinal static stability on the lift-drag ratio characteristics of fighter layout [J]. Journal of Nanjing University of Aeronautics and Astronautics, 2008, 40 (4): 56.
[3] Sang Jianhua. Stealth requirement promotes technological advancement of aircraft [J], International Aviation, 2015, 2: 38.
[4] [U.S.] E.F. Clart et al. Translated by Ruan Yingying, Chen Hai et al. Radar Scattering Cross Sections—Estimation, Measurement and Reduction [M].
Press, 1988: 115-120.
[5] Sang Jianhua. Stealth technology of aircraft [M]. Beijing, Aviation Industry Press, 2013: 181
[6] Xiao Zhihe, Gao Chao, Bai Yang et al. Technology and development of aircraft radar stealth test evaluation [J], Journal of Beijing University of Aeronautics and Astronautics, 2015, 41 (10): 1873
[7] Zhang Yong. "Parallel Design Principle" of Separate Parts of Radar Stealth Vehicle [J]. Stealth Technology, 2009, 3: 5.
[8] Ruan Yingkun. Radar cross section and stealth technology [M]. Beijing, National Defense Industry Press, 1997: 51.
Aviation science
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Research on the RCS of Canard
GUO Zhanzhi, CHEN Yingwen, MA Lianfeng
Chengdu Aircraft Design Institute, Chengdu 610091, China
Abstract: This paper researchs the RCS effect on canard configuration fighter of canard. First, the scattering mechanism of canard needs to be analyzed. And then, to compute the configuration's RCS of the given fighter models by multi level fast multipole method (MLFMM). And analysis the RCS effect of canard by comparing the computational results of both canard configuration and conventional configuration, including the RCS effect on fighter after the canard is rotated; In addition, researching the RCS effect on fighter of both edge scattering and gap scattering and respective inhibition measures by full--size parts stealth testing. The test results show that the RCS level of canard configuration is almost compared with conventional configuration after the scattering of canard be inhibited, canard configuration can be applied to configuration design of stealth fighter. Finally, the guiding principles of canard stealth design are obtained.
Key words: canard configuration; RCS; MLFMM; conventional configuration; stealth
https://translate.google.com/community?source=mfooter
Journal of Aeronautics and Astronautics Nov. 25 2019 Vol. 38 No.X:
Acta Aeronautica et Astronautica Sinica ISSN 1000-6893 CN 11-1929 / V
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http://hkxb.buaa.edu.cn hkxb@buaa.edu.cn
DOI: 10.7527 / S1000-6893.2019.23485
Research on the RCS Impact of Duck Wing
Guo Zhanzhi, Chen Yingwen, Ma Lianfeng
Chengdu Aircraft Design Institute, Chengdu 610091
Abstract: In this paper, the effects of canard wings of a canard layout fighter on the RCS of the aircraft are studied and analyzed in detail. The paper first analyzes the scattering mechanism of canard wings, and then uses the multilayer fast multipole (MLFMM) method to calculate the RCS of the whole model of the specific model. By comparing the RCS of the canard layout and the conventional layout, the canard wings scattering The effects of the RCS of the aircraft, including the influence of the canard wing deflection on the whole machine; In addition, the edge scattering of the canard wing and the effect on the slit scattering and the corresponding suppression measures were studied by experimental methods. The research results show that the canard layout can be applied to the layout design of high stealth aircraft after thecanard wing scattering is suppressed or eliminated, and its stealth performance is equivalent to the conventional layout. Finally, the guiding principles of canard wing stealth design are summarized.
Keywords: canard layout; RCS; MLFMM; conventional layout; stealth
Chinese picture classification number: V218 Document identification code: A Article number: 1000-6893.2019.23485
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Received date: 2017-xx-xx; Retired date: 2017-xx-xx; Accepted date: 2017-xx-xx;
Web publishing address: http://hkxb.buaa.edu.cn/CN/html/2018XXXX.html
Fund Project: National Natural Science Foundation of China (Fund Number); Aviation Science Foundation of China (Fund Number)
* Corresponding author. E-mail: hkxb@buaa.edu.cn
In the history of the development of fighters in the world, according to the characteristics of different horizontal stabilizers, common fighter layouts can be roughly divided into the following categories: conventional layouts, duck layouts, tailless layouts, three-wing layouts and flying wing layouts [1]. Among them, the duck layout has received great attention in the layout design of modern fighters.
Canard layout has a very important position in the development history of fighter layout design in China. Domestic aerodynamic layout design technicians rely entirely on independent innovation. After years of research and successful application of models, they have mastered the duck layout aerodynamic design technology. Facts have proved that the combination of duck layout and relaxation of longitudinal static stability technology can avoid the shortcomings of duck layout, give full play to its advantages in balance and increase, and help to obtain a higher supersonic balance. Conducive to focus matching and center of gravity configuration in the sub-, trans-, and supersonic full flight envelopes [2].
With the development of various electronic detection equipment, the battlefield environment is becoming increasingly complex. Stealth technology has a profound impact on the survivability and combat effectiveness of fighters. Stealth has become one of the important features necessary for a new generation of combat aircraft [3]. As the largest country leading the development of aviation technology in the world, the United States has developed advanced stealth fighters such as F-117, B-2, F-22, F-35. As we all know, stealth capability is the core technical feature of the fourth-generation fighter aircraft. According to the public information available, it can be speculated that the F-22 and F-35 have significantly improved their stealth performance compared to traditional three-generation aircraft or three-generation modified stealth aircraft. Stealth aircraft constitute a fatal advantage, and the development of stealth aircraft has become a consensus around the world. The stealth performance of fighters depends to a large extent on the design of the layout. F-22 and F-35 aircraft have adopted conventional layouts, which fully shows that Americans have mastered the stealth design technology of conventional layouts. For the domestic market, adopting a duck-type layout is a unique choice. Although adopting the duck layout can give full play to its aerodynamic advantages, the disadvantage of stealth compared to the conventional layout is obvious: the front of thecanard wing is equivalent to adding a scattering component in front of the aircraft, and The gap between the canard wing and the fuselage is directly exposed in front of the aircraft. These factors are likely to cause the stealth physical characteristics deteriorate. So, what factors need to be considered for the stealth design of the duck layout aircraft? Can its stealth ability be even better than the conventional layout?
This paper studies and analyzes the impact of canard wings on RCS for canard layout aircraft. The stealth layout design of some fighters has many factors affecting it: intake and exhaust methods, fuselage cross-sectional shape, main wing geometry parameters, tail parameters, design of the rear body coupling area, and so on. For different layout forms, these stealth design elements are the same. Compared with the conventional layout, the canard layout only adds an invisible element-canard wings. In order to study the effects of duck wings RCS, this paper first analyzes the scattering mechanism of canard wings, and then develops the stealth shape modeling of canard layout and conventional layout, and compares the differences between them by RCS simulation calculation. In order to explain the difference of duck wings more clearly, this article assumes that the stealth characteristics of canard layout and conventional layout are calculated and compared respectively under the premise that other stealth elements are the same or equivalent; then, further research is carried out through the component stealth test method. RCS suppression scheme for edge scattering and slit scattering between duck wings and body, and its suppression effect was verified.
1 Research methods
In this paper, the shape stealth characteristics of different layouts are studied by means of RCS theoretical calculations. By comparing the RCS calculation results of duck layout and conventional layout, the scattering contribution of duck wings is analyzed, and the influence of duck wings deflection on the whole machine is studied. The stealth test of the absorbing component studies the influence of edge scattering and the suppression scheme; by developing a full-size counter-slit component model, the stealth test research is conducted on the problem of the counter-slot scattering between the duck wing and the fuselage.
1.1 RCS calculation analysis method
During the aircraft design phase and the stealth optimization process, in order to quickly complete the iterative optimization of the solution and reduce the cost of stealth test verification, it is often necessary to use the RCS simulation method to estimate and analyze the scattering characteristics of the target.
The analytical method of the electromagnetic scattering problem is to obtain a strict solution to the target problem by a wave equation that satisfies strict boundary conditions. With the development of computer technology, a large number of numerical methods have been proposed for solving. These methods are generally based on Maxwell's equations.
1.1.1 High frequency calculation method
The high-frequency calculation method is an approximation method, which was widely used in the early days. It mainly includes geometric optics (GO) and physical optical (PO), which are discussed in many works. .
The geometrical optics method is a classical method to explain the scattering mechanism and energy propagation using ray management theory. The basic theory of geometric optics is to describe the reflection and refraction of electromagnetic waves on the decomposition surfaces of two different media. For discontinuities such as edges, corners, and sharp points, geometrical optics cannot be used. For complex-shaped diffusers, the calculation accuracy of geometrical optics cannot be guaranteed.
The theory of physical optics method obtains the scattering field by approximate integration of the induction field. It can solve the structure of RCS for the structures that cannot be calculated by geometric optics such as plane and single curved surface. The physical optics method completely ignores the interaction between various parts of the target based on the principle of locality of the high-frequency field, and independently determines the surface-induced current independently based on the incident field alone. Although the RCS of an ideal conductor target can be calculated quickly and efficiently, it is only suitable for conductor targets with large electrical dimensions, smooth surfaces, and weak local coupling. When the target has many edges, spikes, or local coupling scattering regions, the calculation results of the physical optics method will produce a large error.
In order to make up for the shortcomings of geometrical optics and physical optics methods, geometric theory of diffraction (GTD) and uniform theory of diffraction (UTD) have been developed successively. There are also many limitations in the application.
1.1.2 Accurate numerical calculation method
Precise numerical calculation methods are further divided into integral equation method (IEM) and differential equation method, which correspond to the integral form and differential form of Maxwell's equation, respectively.
The integral equation method includes an electric field integral equation, a magnetic field integral equation, and a mixed field integral equation. The mixed field integral equation is actually a linear combination of the electric field integral equation and the magnetic field integral equation. The mixed-field integral has the characteristics of accurate calculation of the electric field integral equation and good convergence of the magnetic field integral equation, and eliminates the problem of internal resonance.
With the development of computational electromagnetics, more mature numerical algorithms have appeared, including the method of moment (MOM) based on integral equations, and the multi-level fast multipole method (MLFMM) . For the electromagnetic scattering problem, the current industry recognized
An effective algorithm is a multi-layer fast multipole. With the rapid development of high-performance computing servers today, the multi-layer fast multipole method has been able to accurately calculate low RCS aircraft targets with electrical dimensions exceeding 1,000 wavelengths. The electrical dimensions have covered most of the problems in stealth design [5 ].
For the RCS calculation of the general stealth aircraft, the high-frequency calculation method cannot meet the RCS calculation accuracy requirements. This paper adopts the engineering calculation software based on the multi-layer fast multipole method to solve the RCS of the target.
1.2 Radar stealth test and verification
The target RCS can be obtained through theoretical calculations and experimental tests. The theoretical calculation method is more accurate for analyzing the scattering characteristics of metal shapes, but for complex shapes and targets with complex media, the calculation difficulty will increase greatly, and the calculation accuracy will be severely restricted, making the test method the main method to obtain the target's electromagnetic scattering characteristics. [6]. Therefore, the radar stealth test technology based on RCS test and imaging diagnostic test is very important.
The RCS test is divided into several types, among which the microwave darkroom test is suitable for the aircraft scale model test and the full-scale component model test. Under the premise of determining the aircraft layout plan, conducting a full-scale stealth test on a specific scattering source is helpful to accurately grasp its scattering characteristics, so as to develop a specific scattering suppression plan in order to further improve the overall stealth level of the aircraft.
The stealth design, the rational application of the absorbing coating and the absorbing structure of local details on the surface of the stealth aircraft are very important to further improve the stealth performance of the whole aircraft, but these detailed designs are not suitable for research and optimization through scale model testing. Generally, the size of the local scattering source and scattering part of the stealth aircraft is between 1 meter and several meters. If it is tested on the whole machine, on the one hand, the size is too large to implement, and the cost is high; It is possible to cover up the local RCS, and it is impossible to accurately obtain the scattering characteristics of components or detailed structures. Usually, a full-scale component model is developed to simulate local details, and a microwave darkroom is used for testing.
2 Research on the impact of duck wings on RCS
2.1 canard wing scattering mechanism
Although the canard wing is a unique component of the canard layout, its scattering mechanism is not complicated. The scattering mechanism of canard wings is shown in Figure 1.
Canard wing scattering can be reduced to three types of scattering problems:
a) Sharp point scattering: When the electromagnetic wave hits the trailing edge of the duck wing
Fig. 1 Scattering Mechanism of Canard
Diffraction occurs at corners, and surface traveling waves also form diffractions at sharp points. Sharp point scattering is a primary scattering;
b) Edge scattering: When the electromagnetic wave hits the edge of the target, the edge diffracts the incident electromagnetic wave, and the surface
Traveling waves will also cause diffraction at the edges. Edge scattering belongs to primary scattering, and edge scattering belongs to strong scattering sources. It is an issue that must be considered in stealth aircraft design to suppress its peak scattering;
c) Slot scattering: There is an inevitable butt gap between the duck wing and the fuselage that needs to meet the deflection requirements of the duck wing. The gap is relatively
long and there is a rotating shaft mechanism. The scattering mechanism is complicated and there may be multiple reflection characteristics.
Among the three types of scattering problems, sharp point scattering and edge scattering are not unique to duck wings, and are common to airfoil components, and their scattering suppression schemes are similar. Slot scattering is a unique scattering source for duck layouts. Since the duck wings are located in the front fuselage area, the gap between the duck wings and the fuselage is easily exposed to the front of the aircraft, and a targeted solution is required.
2.2 Layout Modeling Instructions
In order to carry out the RCS calculation of the layout of the whole machine, the stealth shape modeling of different layouts needs to be completed first, and the layout scheme requirements are basically feasible, otherwise the significance of research is lost. The modeling process must follow the general principles of the shape stealth design, such as the fuselage profile meets the requirements of low RCS profile design, the vertical tail is cambered at an angle, all edges are arranged according to the principle of top-down projection parallel design [7], etc. .
This article refers to the layout of the F-35 fighter, and first generates a single-engine stealth solution with a conventional layout of air intake on both sides. To
Front edge Scattering
Trailing edge Scattering
Cusp Scattering
Gap Scattering
Based on the successful design of F-35, it can guarantee that the scheme is basically established without subversive problems. Then on this basis, the flat tail was eliminated, duck wings were added to the rear of the air inlet in an appropriate position, and the sides of the body were added, and the positions of the wings and the vertical tail were moved back appropriately to form a duck layout scheme. The two layout schemes adopt the same fuselage shape, the same main wing surface and vertical tail shape, and the same shape of the side bars, in order to make the stealth elements of the angel basically the same. The RCS difference of the layout (without duck wings) makes it easier to analyze the influence of duck wings on the whole machine.
The outline model of the conventional layout plan is shown in Figure 2, and the outline model of the duck layout plan is shown in Figure 3.
The main geometric features related to the layout shape and stealth and the RCS calculation model are described as follows:
a) Due to the complexity of cavity calculation, the influence of inlet cavity cavity scattering is not considered, and a low-scattering profile surface is designed
Blocking the air inlet and the tail nozzle, without calculating the cavity scattering and the terminal nozzle scattering, and without considering the influence of the cavity scattering, the RCS level of the whole machine is lower, which is more conducive to analyzing the influence of duck wings on the RCS of the whole machine;
(A) Top View
(B) Upward view
Fig. 2 Conventional Configuration
b) The shapes of the front fuselage, middle fuselage, rear fuselage and rear strips are similar to those of the F-35 aircraft, all adopting a low RCS profile design, and the two layout calculation models share the same fuselage shape;
c) The geometry of the vertical tail is similar to the F-35. The camber of the vertical tail is 27 °, and the top-view projection of the leading edge sweep angle is 60.7 °, and the trailing edge sweep angle is 42 °. The two layouts adopt the same vertical tail shape. ;
d) The front edge of the wing adopts a medium sweep angle design. The leading edge sweep angle of the duck wing, the wing and the flat tail are 42 °, and the leading edge sweep angle of the trailing edge are 15 °. The angle on the duck wing is 8 °, and the wing has no angle on the wing;
e) Both the duck wings and the tip of the vertical tail are designed to be invisible.
2.3 RCS calculation instructions
a) RCS is solved for the two layout schemes based on the RCS accurate numerical calculation method, and the medium property of the target model is set to metal, and the application of the absorbing coating / absorbing structure is not considered;
(A) Top View
(B) Upward view
Fig. 3 Canard Configuration
b) Considering the constraints of hardware resources, the scale of the target RCS solution needs to be reduced. This paper only selects the typical frequencies of the L-band and C-band for calculation, and focuses on the RCS characteristics of the forward sector. analysis;
c) Explanation of calculation parameters: typical frequencies are 1.7GHz and 5.6GHz, pitch angle is 5 ° (corresponding to typical attack angle in cruise state), and azimuth angle is 0 ° ～ 90 °.
2.4 RCS comparison of duck layout and conventional layout
Perform RCS calculations on the above two layout models, and compare the shape RCS of the conventional layout and the duck layout to illustrate the equivalent of the duck wings. First of all, the stealth characteristics of the layout of the active surface without deflection are studied, that is, the duck wings or flat tails are in the neutral position.
A comparison of typical RCS curves in the L band is shown in Figures 4 and 5, and a comparison of the RCS curves in the C band is shown in Figures 6 and 7. Table 1 compares the statistical data of the RCS average of the front sector of the aircraft from 0 to 30 °.
Fig. 4 RCS Comparison of Two Configuration, 1.7GHz, HH polarization
Fig. 5 RCS Comparison of Two Configuration, 1.7GHz, VV polarization
Fig. 6 RCS Comparison of Two Configuration, 5.6GHz, HH polarization
Fig. 7 RCS Comparison of Two Configuration, 5.6GHz, VV polarization
Table 1 Comparison of mean data from 0 to 30 °, unit: dBsm
Table 1 0 ～ 30 ° RCS ’s mean value, dBsm
Frequency
HH polarization
conventional canard
VV polarization
conventional canard
1.7GHz
-13.26 -11.99
-14.6 -14.08
5.6GHz
-19.74 -20.42
-16.91 -15.19
According to the comparison of the above curves and average data, the RCS analysis of duck layout and conventional layout is as follows:
a) From the macro comparison of the curves, the RCS curves of the two layouts are basically the same in the large angle range of 0 to 60 °, and the RCS of the duck layout is significantly smaller in the side angle range of 75 to 90 °. This is due to the dihedral angle feature between the vertical tail and the tail of the conventional layout scheme, which is caused by strong multiple scattering. The research focus of this article is on the RCS characteristics of the front sector, and the side differences are not analyzed;
b) According to the average RCS statistics in Table 1,
In terms of segment HH polarization, the RCS level of the duck layout is 1.27dB larger than that of the conventional layout, which is mainly caused by the front duck wings. However, in the C band, the RCS of the duck layout is reduced by 0.68dB, which is Because the scattering characteristics of airfoil components are related to frequency, for the same wingtip parameters, at low frequencies and longer wavelengths, duck wing tip scattering has a more significant effect on the whole machine. As the frequency increases and the wavelength becomes smaller, the tip scattering The impact is weakened, so the results in both C-bands are comparable;
c) For VV polarization, the average level of the duck layout has increased slightly, both at low and high frequencies. The main reason is that the RCS peak at an azimuth angle of 15 ° increases. This scattering peak corresponds to the scattering at the trailing edge of the airfoil. Although the edges of each airfoil are arranged in accordance with the principle of parallelism, since the sweeping angle of the trailing edge of the duck wing is 15 °, its scattering peak is directly exposed in front of the aircraft, and the trailing edge of the wing and the trailing edge is a sweeping angle of 15 °. The fuselage has a certain occlusion relationship for its scattering peaks (see Figure 8). Therefore, the strong scattering at the trailing edge of the duck wing causes the peak scattering peak of the trailing edge of the duck-type layout to be larger than that of the conventional layout. The increase of the peak value will not significantly affect the overall RCS characteristics, and after applying the absorbing coating and edge absorbing structure, the peak edge scattering peak will be effectively suppressed (see section 2.6 "Experimental Study of Edge Scattering");
Figure 8 Schematic diagram of trailing edge normal scattering
Fig. 8 Trailing Edge Normal Scattering
d) The trailing edge of the duck wing generally adopts a sweep-back design, which is good for trimming and aerodynamic focus matching. If it can be changed to a forward-sweep design, that is, parallel to the trailing edge of the same wing, the effective shielding of the fuselage can be used. Reduce its scattering peaks
Value, and strong peaks at the trailing edge of the wing can mask it;
e) From the peak of the azimuth angle of 42 °, there is no obvious difference between the two layouts. This is because the main wing surface is the same, and the smaller peak at the leading edge of the duck wing is masked by the stronger peak at the front edge of the main wing.
2.5 RCS Effect of Canard Wing Deflection
The level of RCS without canard deflection alone is not enough to fully explain the advantages and disadvantages of duck stealth layout.Canard wings, as an important moving surface, may deflect at any time during the flight of the aircraft, and then change the stealth characteristics of the whole aircraft. Generally speaking, when performing a typical combat mission, fighters correspond to different stealth levels at different mission phases, and maintaining a high stealth state in the full flight profile is neither realistic nor significant. When the aircraft is taking off, landing, fighting at close range, or performing a large maneuver, the stealth level is not high at this time, or the RCS characteristics of the aircraft body in certain attitudes are relatively large. Although the duck wings are deflected at large angles, The entire RCS does not constitute a fatal impact.
What really needs to focus on is the cruising stage of the aircraft, at this time corresponding to the high stealth state of the aircraft, any deflection of the rudder surface should not destroy the high stealth state of the aircraft body. In general, when the canard layout fighter cruises in subsonic and supersonic cruises, the range of deflection angles of the duck wing is relatively small, about 0 to -5 °. Generally, the negative angle of the small angle is maintained. Maintain a high lift-drag ratio during cruise.
In order to explain the influence of duckwing deflection on RCS, this paper selects duckwing deflection ± 5 ° attitude and deflection 0 ° attitude for comparison of L-band calculations. Comparison of HH and VV polarized RCS curves is shown in Figures 9 and 10. Table 2 shows the average RCS data of the front sector from 0 to 30 °.
Fig. 9 RCS Comparison of Different Canard Status, 1.7GHz, HH polarization
Fig. 10 RCS Comparison of Different Canard Status, 1.7GHz, VV polarization
Table 2 RCS ’s mean value of different canard deflection angles, dBsm
Rotation angle
HH polarization
VV polarization
0 °
-11.99
-14.08
5 °
-5 °
-11.6
-12.18
-11.26
-14.56
As can be seen from the comparison of the above curves:
a) The RCS curves before and after deflection of duck wings basically match. The only obvious difference is the scattering peak of VV polarization curve at 15 ° azimuth. After deflection of duck wings, the scattering peaks are slightly wider, whether it is positive or negative. This is because the rotation axis of the duck wing and the trailing edge are not parallel, resulting in that the top projection of the rear edge of the duck wing and the top projection of the trailing edge of the wing after deflection are not completely parallel;
b) When the duck wing is deflected by 5 °, the peak of VV polarization scattering at an azimuth of 15 ° increases significantly, which causes the average level to increase by nearly 3dB. When the deflection is -5 °, the peak value decreases slightly, but the average level slightly decreases, and the scattering peak It is essentially traveling wave scattering. Traveling wave scattering is related to the polarization mode. Traveling waves occur only when there is an incident electric field component along the surface in the direction of propagation. According to the traveling wave scattering mechanism, when a conductive target is irradiated with electromagnetic waves near the grazing incidence direction, a surface current is induced to generate a surface traveling wave. If the surface traveling wave cannot be absorbed at the target discontinuity, it will cause reflection [8]. As shown in Figure 11 (a), the aircraft elevation is 5 °, and the duck wings
At 0 ° deflection, the duck wings face the incoming wave to form a grazing incidence condition. The incident electric field E generates an electric field component Ei on the surface, which excites the surface current and propagates to the shape at the trailing edge.
Strong echo reflection; when the duck wing is 5 ° forward, as shown in Figure 11 (b), the angle between the surface of the duck wing and the direction of the incoming wave becomes smaller, the electric field component Ei along the target surface becomes larger, and the surface traveling wave becomes stronger. The echo becomes stronger; when the negative deviation of the duck wing is 5 °, as shown in Figure 11 (c), the angle between the surface of the duck wing and the direction of the incoming wave becomes larger, the electric field component Ei becomes smaller, and the surface traveling wave becomes weaker, which weakens the echo scattering;
(A) The aircraft is tilted 5 ° and the duck wings are deflected 0
(B) The aircraft is tilted 5 ° and the duck wings are tilted 5 °.
(C) The aircraft is tilted 5 ° and the duck wings are deflected 5 °
Figure 11 Traveling wave scattering mechanism of duck wings with different deflection attitudes
Fig. 11 The mechanism of traveling wave scattering with different canard status
c) For HH polarization, the scattering mechanism is different from VV polarization. The electric field component is parallel to the scanning surface. There is no problem of trailing edge scattering peaks caused by surface travelling waves. The small angle deflection of duck wings will not cause the geometric characteristics of the target. Obvious changes, so before and after the deflection of the duck wing has no obvious effect on the HH polarization RCS of the whole machine, the curves are basically consistent, and the average level is equivalent.
As mentioned earlier, when a duck-type aircraft is cruising normally, its duckwing deflection is small, and it is in a negative deflection attitude, and no positive deflection will occur, so it will not damage the RCS characteristics of the whole aircraft. When the angle is not as large as ﹢ 5 °, the RCS increase caused by trailing edge scattering can be eliminated after the trailing edge absorbing structure is applied. When the duckwing deflects at a greater angle, whether it is positive or negative, its effect on The impact of the RCS of the whole aircraft will increase, but the RCS of the aircraft itself may be relatively large at this time, and from the perspective of the stealth level management of the aircraft, the aircraft is usually in a non-high stealth level at this time. In general, the deflection of duck wings will not affect the stealth of the aircraft.
Incident wave
Strong echo
Incident wave
Echo becomes stronger
E
Ei
En
E
Ei
En
Incident wave
Weak echo
Aviation science
The comparison of the above RCS calculation results shows that the stealth characteristics of the duck layout and the conventional layout are equivalent. The following tests are used to study the edge scattering of duck wings and the effect on slit scattering, and the corresponding suppression scheme.
2.6 Experimental study of edge scattering
Edge scattering features are present at the edges of aircraft wing components. Edge scattering is a kind of strong scattering source, especially after the aircraft's strong specular reflection in the radar threat area weakens, the contribution of edge scattering is very prominent. Absorbing coatings or structures can be used to suppress edge scattering. Generally, the absorbing coating is designed for high frequency, and its absorption effect at low frequency is limited, while the absorbing structure can take into account both the low frequency and high frequency absorbing performance requirements. Generally, stealth aircraft use the absorbing structure on edge components to suppress edge scattering.
In order to make the test results universal, a full-size component model as shown in Figure 12 was specially designed to simulate the leading and trailing edge characteristics of general airfoil components. The status of the full-size component stealth test includes the model's all-metal status (that is, before the reduction measures are taken) and the absorbing structure status (where the green area is the application area of the absorbing structure).
Fig. 12 Full Size Model of Edge Part
Fig. 13 RCS of Edge Part Model, 2GHz, HH polarization
The test results of the metal state are used to explain the effect of edge scattering, and the test results of the absorbing structure state are used to verify the RCS suppression effect of the edge absorbing structure on the edge scattering. The experimental study is aimed at the typical frequencies of low-frequency L-band and high-frequency X-band. The comparison of RCS curves at 2GHz is shown in Figures 13 and 14, and the comparison of RCS curves at 10GHz is shown in Figures 15 and 16.
Fig. 14 RCS of Edge Part Model, 2GHz, VV polarization
Fig. 15 RCS of Edge Part Model, 10GHz, HH polarization
Fig. 16 RCS of Edge Part Model, 10GHz, VV polarization
Table 3 0 ～ 30 ° RCS ’s mean value of edge part, dBsm
Frequency
Metal
Absorbing structure
HH VV HH VV
2GHz
-22.22 -22.08 -27.56 -27.88
10GHz
-26 -24.8 -33.61 -31.77
As can be seen from the above results:
a) In the metallic state, the scattering peaks of the edge parts are strong, and the overall average level is high (see Table 3 for the average data), which has a greater impact on high stealth aircraft on the order of 0.001 to 0.005m2;
b) After the absorption structure is applied, the scattering peak corresponding to the edge of the airfoil is greatly reduced, and the overall RCS level is significantly reduced. The X-band gain is greater, which is determined by the performance of the absorption structure. The absorption structure is at X Better absorbing performance in the band.
The above test results show that if the RCS reduction measures are not taken, the edge scattering of the wing component (duck wing component) is mainly the effect of the trailing edge scattering on the stealth of the aircraft in front. Effectively suppressed.
2.7 Experimental study on slit scattering
The previous calculation analysis focused on the shape stealth characteristics of the overall layout, and it was impossible to simulate the problem of the scattering of the gap between the duck wing and the fuselage. Aiming at the scattering of this pair of slits, this paper conducts experiments by developing a full-scale component model to study the effect on slit scattering and the corresponding RCS suppression measures.
In order to simulate the typical gap between the left and right duck wings and the fuselage, a low RCS carrier model as shown in Figure 17 was designed. A low RCS curved surface is designed to be closed smoothly around the gap area, so that the carrier itself has low scattering characteristics in the front sector. Then a full-scale metal model is developed and tested in a microwave dark room. The test target is shown in Figure 18.
Fig. 17 Gap Model
Fig. 18 The RCS Test of Full-Size Gap Model
Since this kind of effect on slit scattering is mainly at high frequencies, it has less effect on low frequencies. This test mainly tests and studies the typical frequencies of the high-frequency X-band. It is divided into the original state of the slit and the reduced state of RCS. It is used to study the effect of the original slit slit on the whole machine and to verify the suppression effect after the RCS reduction measures are taken. The comparison of the RCS curves of the two states is shown in Figure 19 and Figure 20, with a frequency of 9.41 GHz.
Fig. 19 the RCS Curve of Gap Model, 9.41GHz, HH polarization
Fig. 20 the RCS Curve of Gap Model, 9.41GHz, VV polarization
The mean statistics are shown in Table 4.
Table 4 RCS ’s mean value of Docking gap model, dBsm
Status
HH polarization VV polarization
Primitive
-20.63 -21.48
Inhibition
-38.27 -33.1
From the above results, it can be seen that before the RCS reduction measures were taken for the seams, there were strong peaks with wide angular ranges in the forward sector of the corresponding aircraft, which had a greater impact on the overall stealth of the aircraft. The test results show that the duck-wing-to-slot scattering is an important scattering source for duck-layout aircraft. From the scattering mechanism:
a) The smooth, narrow and contoured surfaces on both sides of the slit are easy to excite surface traveling waves. The geometric discontinuities formed by the duck-wing rotating shaft mechanism during traveling wave propagation will cause strong reflection if not absorbed;
b) The electromagnetic wave is incident on the inside of the opposite seam, and there are multiple reflection features between the profiles on both sides.
Such scattering problems are mainly suppressed by radar absorbing coatings with good high-frequency absorbing performance. The RCS suppression effect obtained in a reduced state is very significant, and the strong and wide peaks in the front sector are greatly reduced. Both HH polarization and VV polarization RCS averages have fallen sharply, and the average levels have fallen below -38dBsm and -33dBsm, respectively.
Therefore, by adopting reasonable RCS suppression measures, the influence of the duck-type layout on the slit scattering problem can be basically eliminated.
3 conclusions
Canard scattering is a unique problem in the design of stealth fighters with duck layout and needs attention.
The research in this paper shows that the duck wing scattering suppression scheme is not complicated. After taking measures to eliminate the influence of canard scattering, the canard layout can be applied to the layout design of high stealth aircraft, and its stealth performance is equivalent to the conventional layout.
The canard wing stealth design for canard layout fighters must follow the following principles:
1) The edge design of the canard wing is arranged according to the parallel principle of the top-down projection of the edge to reduce the number of RCS peaks, and the stronger peak of the main wing surface edge is used to mask the smaller peak of the canard wing edge;
2) The sharp point of the trailing edge of the duck wing is integrated with aerodynamic and stealth requirements to make an appropriate chamfer to reduce the sharp point scattering;
3) Apply the absorbing structure to the edges, including the leading edge, trailing edge, and wingtips, and select the performance parameters of the absorbing structure and determine the size requirements of the absorbing structure based on the stealth requirements of the aircraft for low and high frequencies;
4) Apply a wave-absorbing coating with excellent high-frequency absorption performance to the slit area between the duck wing and the fuselage to suppress the slit scattering, and develop a reasonable coating area.
References
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Aviation science
XXXXXX-3
Research on the RCS of Canard
GUO Zhanzhi, CHEN Yingwen, MA Lianfeng
Chengdu Aircraft Design Institute, Chengdu 610091, China
Abstract: This paper researchs the RCS effect on canard configuration fighter of canard. First, the scattering mechanism of canard needs to be analyzed. And then, to compute the configuration's RCS of the given fighter models by multi level fast multipole method (MLFMM). And analysis the RCS effect of canard by comparing the computational results of both canard configuration and conventional configuration, including the RCS effect on fighter after the canard is rotated; In addition, researching the RCS effect on fighter of both edge scattering and gap scattering and respective inhibition measures by full--size parts stealth testing. The test results show that the RCS level of canard configuration is almost compared with conventional configuration after the scattering of canard be inhibited, canard configuration can be applied to configuration design of stealth fighter. Finally, the guiding principles of canard stealth design are obtained.
Key words: canard configuration; RCS; MLFMM; conventional configuration; stealth
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