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Aircraft Model Features and Limitations 飞机模型的特点和局限性

发布时间:2011-09-14 23:49:13
 

Aircraft Model Features and Limitations
飞机模型的特点和局限性

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北京翻译公司 www.366translation.com
The model used was an extant public domain 3,343 facet representation constructed from publicly available high and medium resolution photographic imagery of the J-20 prototype, observed in December, 2010, and January, 2011.
现阶段公共领域使用的模型,是根据2010年12月和2011年一月份对歼20原型机的观察,从公开拍摄到的中高分辨率的摄影照片进行构造的,一共有3343个面。

Two variants of the model were tested and one then employed. One model used axisymmetric exhaust nozzles fully open, and the other used axisymmetric exhaust nozzles fully closed. This was necessary to capture the specular returns from the nozzle exterior in the aft hemisphere of the aircraft, which vary strongly with nozzle position. As the nozzles open, the principal lobes of the specular returns rotate forward, and in the fully open position contribute mostly to the beam aspect RCS, where not shielded by the aft fuselage structure. Nozzle RCS from the forward and aft aspects varies weakly with nozzle position. Therefore all simulations presented are for a closed nozzle, which is the most frequent case in operational use of such aircraft, and thus of most interest. The nozzle rim includes serrations as observed on the prototype. The intent behind the use of serrations could be rim RCS reduction in the upper bands, but could also be to promote vortex generation and plume mixing to increase plume dissipation and thus reduce blackbody radiation from the plume in the near infrared bands.
两种变体模型已经测试过了,并选择的其中的一种。一个模型的轴对称喷气口完全打开,其它模型的轴对 称喷气口关闭。这对于捕捉喷口位置变化强烈的镜面回波十分必要。当喷口打开时,镜面回波的主瓣向前旋转,在完全打开的位置(不能被机尾结构阻挡)贡献了大 部分侧向RCS。在喷口位置,前向和后向的RCS变化幅度很小。因此在所有仿真中,喷口都是封闭的,这是这种飞机最常见的 ,也是最感兴趣的模拟方法。原型机的喷口边缘有锯齿。锯齿的作用是在较高波段进行边缘RCS减缩,但也能促使涡流的形成和烟流的混合,从而增加烟流耗散, 减少红外波段的黑体辐射。

 

注释:所谓黑体是物理学家为研究热辐射而定义的一种理想模型,在任 何条件下,黑体对入射的电磁波完全吸收,而不会反射或投射。但根据热力学定律,凡事有温度的物体都会自行辐射电磁波,黑体也不例外,这就是文中提到的黑体 辐射。这里的黑体应该是指机尾排气口喷出的气焰,虽说吸收了很多电磁波,由于自身温度的缘故也向外辐射电磁波,但这不属于雷达波隐身的范畴,而是红外隐身 方面。所以文章说仿真模拟时要关闭尾喷口,就是为了避免红外辐射的干扰。


The primary nose mounted radar antenna radome is assumed to be a bandpass design, emulating United States fighter designs, and was assumed to be fully opaque at all frequencies of interest. The model assumes an insignificant structural mode RCS contribution from the radar antenna face and radar bay bulkhead, consistent with a properly designed bandpass radome in its stopband region. Given the absence of any useful data on the internal configuration of the radome and antenna bay, a more elaborate model would be speculative, unavoidably. Imagery of the prototypes does not show any evidence of the radome join to the fuselage, possibly reflecting the absence of a radome on airframes built to validate aerodynamics, shaping and flight systems. In a production design the radome seam / join to the fuselage can produce significant RCS contributions if poorly implemented.
仿效美国战机的设计,机头的雷达天线罩假定为带通滤波设计,并对所有相关频率完全不透明。这个模型 假定对结构模式的RCS贡献很少,包括雷达天线面和雷达舱壁,这与设计合理的带通滤波雷达罩在抑制频带方面保持一致。由于缺少天线罩和雷达舱内部配置的有 关资料,不得已推测出来的模型可能会更精确。根据原型机的图片,没有证据显示天线罩嵌入机身,或许为了验证当机身上没有天线罩时,飞机的空气动力学、造型 和飞行系统。如果做工不细,在生产设计中,天线罩焊接/嵌入机身可能产生重大的RCS贡献。

 

注释:带通滤波,只能通过特定频率的电磁波,对其余的波段则完全屏蔽,是为了使雷达天线罩可以发射和接受自身的雷达波,而屏蔽敌方的雷达波。


The engine inlet tunnels were modelled as Perfect Electrical Absorbers (PEA; Refer Annex E). Given the absence of any useful data on the internal configuration of the inlets and tunnels, a more elaborate model would again be entirely speculative. This is consistent with an ideal S-bend inlet tunnel clad with ideal RAM on its interior walls, and the use of an ideal engine face blocker. This is an optimistic assumption given historically observed difficulties in inlet tunnel signature reduction, as in many designs the inlet tunnel cavity RCS is a dominant wideband contributor in the forward aspect.
发动机进气道的风洞被设计成完全电磁吸收。(PEA;参见附录E)由于缺少进气道和风洞内部配置的 有关资料,推测出来的模型可能会更精确。这个模型拥有一个理想的S型进气道,理想的雷达吸波材料敷设在风洞外壁,并被用于发动机表面的预锻模。在许多设计 中,进气道风洞凹腔在前向宽频带贡献了大量的RCS。而进气道风洞的信号减缩难以观察,所以这是一个乐观的假设。

 

注释:PEA,附录的解释为材料在自由空间的特性阻抗,对相关波长无限损耗(吸收)。


The exhaust tailpipe RCS contributions were also modelled as Perfect Electrical Absorbers (PEA). Given the absence of any useful data on the internal configuration of the tailpipes, a more elaborate model would be as before entirely speculative. The PEA model is consistent with an ideal  tailpipe internally clad with ideal heat resistant RAM, and the use of an ideal turbine face  and afterburner fuel spraybar blocker. This is an inherently optimistic assumption, as can be shown by employing an approximate model for an untreated tailpipe cavity, accounting for the reduction in projected nozzle area. This is detailed in Annex C.
排气管的RCS贡献也被设计成了完全电磁吸收。由于缺少排气管内部构造的有关资料,推测出来的模型 可能会更精确。这个模型是一个理想的排气管,理想的耐热雷达吸波材料敷设在排气管内部,并被用于理想的涡轮表面和再燃装置的燃料喷嘴架的预锻模。这是一个 乐观的假定,因为采用未经处理的排气腔近似模型,喷口投影的面积将会减少。详见附录C。


The cockpit canopy transparency was modelled as a Perfect Electrical Conductor (PEC; Refer Annex E), to emulate the effect of a gold or other highly conductive plating layer in the polycarbonate laminate structure.
透明的座舱盖被设计成全完导电体(PEC;参见附录E),来模拟黄金或其他高导电性的聚碳酸酯层状结构镀层。

 

注释:PEC,附录的解释为对所有相关波长而言,材料的特性阻抗为零,材料是一种理想化的导电金属。


The closed axisymmetric exhaust nozzle employs a stacked serrated trailing edge in the manner of the F-35 nozzle, reflecting photographic imagery of the prototype. As the structural shape of the gaps between nozzle petals is not known at this time, we modelled the open nozzle as simple cylinder.
闭合的轴对称喷气口采用了F35喷口的堆叠式锯齿后缘,这是原型机的摄影图像。喷口菊蕊之间缝隙的结构形状还不清楚,我们把打开的喷口设计成了简单的圆柱体。

The photographic imagery of the J-20 prototypes was not of sufficient quality to incorporate any useful detail of panel join boundaries, door boundaries, and other surface features which produce RCS contributions due to surface travelling waves coupled to the aircraft skin. Even were such detail available, there is no guarantee production aircraft would retain the prototype configuration, reducing the value of any such results.
歼20的摄影图像质量不高,没有充分地体现面板嵌入边界、门的边界和其它表面特征,它们产生表面行波与机身耦合的RCS贡献。即便有详细的资料,也不能保证成型机为了减少这些贡献而保持原型机的结构。

The position of the canards,  delta wing leading and trailing edge surfaces, and fully moving tail surfaces was set to neutral, reflecting an optimal cruise configuration at nominal supercruise altitudes and airspeeds. Large deflections by these control surfaces in flight would produce large but transient increases in specular backscatter.
鸭翼的位置、三角翼前后缘的边缘曲面和全动尾翼表面被设定为中立,这是名义上的超音速高度的空速的最佳巡航构型。在飞行状态下,操纵面的大幅度偏斜将会产生巨大且短暂的反向散射增量。

The geometrical fidelity of the model was assessed by comparison with high resolution imagery released in January, 2011, specifically by comparing the shape of the model from the same aspect as the photograph. Particular attention was paid to the fidelity of angles, especially in the chines, engine inlet exterior, planform and wing/fuselage joins, as these determine the {θ, Φ} directions of the mainlobes and sidelobes in the specular returns.
通过与2011年1月份泄露的高分辨率图像对比,特别是从相同的角度来比较模型和图像的外形来评估模型的几何精度。应当特别关注角度的精度,尤其是机脊、发动机进气口外部,俯视图和翼身融合,这些决定了镜面回波在 {θ, Φ} 方向的主瓣和副瓣。

 

注释:由于雷达波照射机身方向不同,RCS取值不同。本次模拟通过 若干角度对歼20的RCS进行分析,球面投影图代表不同的视界角{θ, Φ},上篇详细讲述过视界角,这里不再赘述。虽然每个球面图中飞机的位置不同,但在平面图中都是正视前向的,也就是说正中间的淡蓝色部分是机头方向,两边 的蓝色是机尾,周围黄色和红色是两侧。不同的颜色代表不同的RCS值,并随着蓝、绿、黄、红而逐渐增大。

 

To establish the robustness of the 3D model for physical optics modelling, we explored the statistical distribution of edge lengths [x-axis] in the facet population [y-axis]. A substantial fraction of the facets are sufficiently large to yield good accuracy through most of the frequency bands being modelled for.
要建立稳健的物理光学模型,我们就必须先弄清楚X轴上Y值的分布情况。绝大多数的Y值通过频段建模足以提供良好的精准度。

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What the Simulation Does Not Demonstrate
什么是非论证模拟

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1.The simulator at this time does not model backscatter from edge diffraction effects, although the resulting error will be mitigated by the reality that in a mature production design these RCS contributions are reduced by edge treatments;
1、模拟器不对边缘衍射效应的反向散射进行建模,尽管模拟结果的误差会比实际中低很多,但成熟的生产设计会通过边缘处理来减缩这些RCS贡献。

2.The simulator at this time does not model backscatter from surface travelling wave effects. In the forward and aft hemispheres these can be dominant scattering sources where specular contributions are low. The magnitude of these RCS contributions is reduced by edge treatments, lossy surface coatings, gap treatments, and panel serrations;

2、模拟器不对表面行波的反向散射进行建模。对于前后半球这些明显的散射源来说,镜面RCS贡献比较低。这些RCS贡献的量级可以通过边缘处理、损耗表面涂层、缝隙处理和面板锯齿来减缩。

3.The simulator at this time does not model backscatter from the AESA bay in the passband of a bandpass radome, due to the absence of any data on the intended design of same, the resulting error will be mitigated by the reality that in a mature production design much effort will be expended in suppressing passband RCS contributions;
3、由于缺少同样的设计资料,模拟器不对有源相控阵雷达的带通滤波雷达罩的通频带反向散射进行建模。尽管模拟结果的误差会比实际中低很多,但成熟的生产设计会把注意力集中在抑制通频带RCS的贡献上。

4.The simulator at this time does not model backscatter from the engine inlet tunnels or engine exhaust tailpipes, due to the absence of any data on the intended design of same. In the forward and aft hemispheres these can be dominant scattering sources where specular contributions are low. The magnitude of these RCS contributions is reduced by suppressing these RCS contributions with absorbers, and in the case of inlet tunnels, by introducing a serpentine geometry to increase the number of bounces.
4、由于缺少同样的设计资料,模拟器不对发动机进气口风洞或发动机排气管的反向散射进行建模。对于前后半球这些明显的散射源来说,镜面RCS贡献比较低。这些RCS贡献的量级可以通过吸波材料来减缩,进气道风洞则可以采用S型设计增加反弹次数。

5.The simulator at this time does not model structural mode RCS contributions from antenna and EO apertures, panel joins, panel and door gaps, fasteners and other minor contributors; although the resulting error will be mitigated by the reality that in a mature production design these RCS contributions are reduced by RCS reduction treatments.
5、模拟器不对天线和光电孔径、面板连接、面板和门的缝隙、紧固件和其他元件的结构模式的RCS贡献进行建模;尽管模拟结果的误差会比实际中低很多,但成熟的生产设计会通过RCS减缩处理来降低这些RCS贡献。

6.The PO computational algorithm performs most accurately at broadside or near normal angles of incidence, with decreasing accuracy at increasingly shallow angles of incidence, reflecting the limitions of PO modelling. The simulator does not implement the Mitzner/Ufimtsev corrections for edge currents. While a number of test runs with basic shapes showed good agreement between the PO simulation and backscatter peaks in third party test sample measurements, even at incidence angles below 10°, characteristically PO will underestimate backscatter in nulls. This limitation must be considered when assessing results for the nose and tail aspects, where most specular RCS contributions arise at very shallow angles39.
6、物理光学逻辑算法在舷侧和靠近入射角中间的位置计算较为精确,入射角度变小,精确度随之减少, 这便是物理光学模型的局限。模拟器不会采用米茨纳/乌菲姆采夫的边缘电流修正法。在经过一系列的基本外形测试之后,物理光学仿真和反向散射的峰值与第三方 实验数值高度一致,如果入射角低于10度,物理光学的反向散射趋近于零。对于机头和机尾而言,大部分的镜面RCS贡献出现在非常低的角度,所以其评估结果 具有局限性。


7.The PO computational algorithm performs best where the product of wave number and dimension ka ≥ 5, where k ≈ 2πf [Table 5.1 in (1)], yielding errors much less than 1 dB. Knott cites good agreement for cylinders as small as 1.5 wavelengths in diameter1.
7、物理光学逻辑算法在波数k≈2πf 、波数尺寸ka ≥ 5时计算结果最佳,产生的误差小于1分贝。诺特证明了圆柱体的直径为1时,与1.5倍波长相当。(这句话吃不准,实在理解不了是什么意思)


注释:ka是目标的电磁特征参数,a是目标的特征尺寸,随目标形状的不同取不同的参数。查阅了相关资料,这句话的意思是说,中低频区目标的散射场暂时没有有效的计算方式,只能采用高频区的方法来处理。波动理论尚不能计算柱体、锥体等简单形状的有限尺度目标的散射场精确解。

8.The simulator does not account for a number of environmental factors, such as air density profile at the aircraft skin boundary layer, thermal variations in absorbent material properties, and moisture precipitation. RCS contributions from these sources are negligible for the principal lobe magnitudes studied.
8、模拟器没有计算一系列环境因素,比如飞机蒙皮边界层的空气密度、吸波材料性能的温度变化和湿度。它们的RCS贡献,相较于主瓣的研究量级,几乎可以忽略不计。

In practical terms, the combination of the J-20 aircraft geometry and the use of the PO method without the Mitzner/Ufimtsev edge current corrections will yield errors at the frequencies of interest of less than 1 dB for the beam aspect and tail aspect sectors, which both have dominant specular scatterers. The nose aspect angular sector results will underestimate RCS, in part due to the absence of shallow angle specular contributions not modelled by the Mitzner/Ufimtsev edge current corrections, and by the absence of surface travelling wave backscatter contributions from surface features, gaps and trailing edges.
事实上,在侧向和尾部区域,如果相关频率低于1分贝,歼20的几何构成和没有采用米茨纳/乌菲姆采 夫的边缘电流修正法的物理光学方法将会产生误差。机头部分的角度位面的RCS将被低估,在某种程度上取决于缺少低角度的非米茨纳/乌菲姆采夫边缘电流修正 法的模型的镜面贡献,以及缺少表面特性、缝隙和机翼后缘的表面行波反向散射贡献。

 

注释:彼得·乌菲姆采夫,俄罗斯物理学家,1962年发布了名为《物理衍射理论中的边缘波行为》的论文,其中提到的有关从平面反射雷达波的理论被研制F117的美国工程师所采用。(米茨纳缺少相关资料)

In all instances, the errors arising from the limitations of the PO computation method all fall into areas where well established RCS reduction treatments using RAS, RAM or coatings would be used, thus reducing the relative magnitude of the errors in the resulting RCS result for angles other than the peak mainlobes produced by these backscatter sources.

在所有情形下,误差是由物理光学计算方法的局限性引起的,每个区域都使用吸波结构进行了RCS减缩处理,并使用了吸波材料或涂层,从而减少这些反向散射源产生的来自角度而非主瓣峰值的RCS误差的相对量级。


Importantly, even were the simulator capable of modelling shallow angle specular and non-specular RCS contributors, the PLA would not permit sufficiently detailed disclosures on the RCS reduction treatments applied to the airframe design, as a result of which reasonable assumed parameters would have to be applied instead of actual values.
更为重要的是,模拟器拥有建模低角度镜面和非镜面RCS贡献的能力,解放军不会泄露过多适用于飞机设计关于RCS减缩处理的细节,因此合理假设的参数将会取代真实数值被应用。

The latter underscores the difficulty in attempting to perform highly accurate numerical RCS modelling of foreign airframe designs, where access to high fidelity shaping data, surface feature data, and materials type and application is actively denied.
后者强调了试图建立外部机身设计的高精确数值RCS模型的困难,想要获取这些高精度的造型资料、表面特性资料、材料类型和应用是非常困难的。

 

 

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