Analyses of radar stealth characteristics of morphing rotor based on dynamic time-varying mesh method
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摘要:
为显著改善旋翼气动性能，变直径、智能扭转和变转速等变体技术被相继提出，但旋翼变体后其电磁回波可能受到潜在干扰，进而影响其雷达隐身特性。为探索这一影响机理并选用合适的变体方案以提高隐身性，首先，建立全机配平模型以获得旋翼实际工作状态，并提出一种能够动态表征旋翼变体、周期变距、挥舞和旋转等特征响应的时变电磁计算网格方法；然后，在旋翼气动性能约束下，基于弹跳射线法和一致性绕射理论计算对比3种变体旋翼的动态电磁散射特性，并采用逆合成孔径成像揭示变体对旋翼雷达隐身特性的影响机理。分析表明：旋翼直径的变化需耦合桨距的调整以保持气动性能，桨叶的运动姿态、面积等多重因素共同作用导致雷达散射截面（RCS）呈起伏式变化，当直径降低8%～10%时，其RCS将显著降低；旋翼智能扭转时，其电磁波反射方向会发生改变，但仅在少数频率下其RCS减缩明显；旋翼变转速对降低RCS的效果虽不突出，但会使微多普勒特征发生显著改变，使其具备三者中最好的反识别隐身性。因此，在保证旋翼气动性能的同时，根据电磁环境对变体方案进行择优，还能够有效提高旋翼的雷达隐身性。
Abstract:Morphing techniques such as varying diameter, active twist and varying speed were proposed for significant improvement of rotor aerodynamic performance. However, rotor echo may be potentially interfered after morphing, which in turn affected its radar stealth characteristics. In order to explore the influence mechanism and select appropriate morphing parameters for better stealth characteristics, firstly, a whole helicopter trim model was established to obtain actual state of the rotor, and a time-varying electromagnetic computing mesh method was proposed to characterize the dynamic responses of rotor morphing, cyclic pitch, flapping and rotating. Then, under the constraint of rotor aerodynamic performance, the dynamic electromagnetic scattering characteristics of three morphing rotors were calculated and compared based on the shooting and bouncing rays method and the uniform theory of diffraction, and the influence mechanism of the morphing on rotor radar stealth was revealed by inverse synthetic aperture radar imaging. The results showed that the change of rotor diameter shall be coupled with pitch adjustment to maintain the aerodynamic performance, and the combined effects of rotor attitude and area caused fluctuation of radar cross section (RCS). When the diameter was reduced by 8%—10%, the RCS could be significantly reduced. When active twist was performed on rotor, the ray reflection direction may change, but RCS reduction was obvious only at a few frequencies. The effect of varying speed on RCS reduction was not prominent, but varying speed rotor could make a significant change in micro-Doppler features and had the best anti-identification stealth characteristics among the three morphing rotors. Therefore, optimization of the morphing scheme according to the electromagnetic environment can also effectively improve the rotor radar stealth while ensuring the rotor aerodynamic performance.
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图 1 变直径、智能扭转和变转速示意图
Figure 1. Scheme of rotor variable diameter, active twist and variable speed
图 2 地轴系和体轴系变换示意图
Figure 2. Scheme of transformation from earth’s coordinate system to body-fixed coordinate system
图 3 桨毂轴系和旋翼挥舞运动示意图
Figure 3. Schemes of hub-fixed coordinate system and rotor flap motion
图 6 变体旋翼雷达隐身特性研究流程
Figure 6. Research process of radar stealth characteristics of morphing rotor
图 8 旋翼动态时变电磁计算网格
Figure 8. Dynamic time-varying electromagnetic computing mesh of rotor
图 11 变直径过程的动态RCS时频域谱
Figure 11. Time-frequency domain spectrum of dynamic RCS in process of varying diameter
图 13 变负扭过程的动态RCS时频域谱
Figure 13. Time-frequency domain spectrum of dynamic RCS in process of varying twist
图 15 变转速过程的动态RCS时频域谱
Figure 15. Time-frequency domain spectrum of dynamic RCS in process of varying speed
图 17 额定参数旋翼与变体旋翼的ISAR图像
Figure 17. ISAR images of rotor with rated parameters and the morphing rotors
表 1 计算资源和效率
Table 1. Computing resources and efficiency
参数 数值 参数 数值 计算频率/GHz 10 网格数 495000 内存占用/GB 1.83 运行核数 8 计算点数 1 计算时间/s 42 
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表 2 旋翼模型主要额定参数
Table 2. Main parameters of rotor model
参数 额定数值或说明 参数 额定数值或说明 直径/m 16.36 弦长/m 0.53 负扭角/（°） −11.7 翼型 NACA0010 转速/（rad/s） 27 桨叶片数 4
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表 3 变体旋翼RCS峰值和均值的最大降幅及其变体参数
Table 3. Maximum reduction of RCS peak and mean of morphing rotor and its corresponding morphing parameters
变体方案 3 GHz 6 GHz 10 GHz 最大降幅/dB 变体参数 最大降幅/dB 变体参数 最大降幅/dB 变体参数 变直径 RCS峰值 2.01 降10%直径 2.23 降8%直径 3.17 降9%直径 RCS均值 1.29 1.72 2.43 智能扭转 RCS峰值 RCS增大 3.18 降5°负扭角 RCS增大 RCS均值 1.52 变转速 RCS峰值 0.33 降10%转速 1.78 降10%转速 RCS增大 RCS均值 0.05 1.15
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