Hover performance of helicopter main and tail rotors with swirl velocities

It is important to quickly predict the hover performance of main and tail rotors with sufficient precision for helicopter design. To investigate the effects of swirl velocities on the hover performance of main and tail rotors, and give a better prediction for the hover performance, a flight performance model was derived and a swirl velocity model was coupled into it. The test data of the UH-60A helicopter were used for validation. When the blade loading coefficient of the main rotor was higher than 005, the effects of the swirl velocities on the main rotor power became significant. The swirl velocities increased the profile torque of the main rotor. The increased torque required the tail rotor to produce more thrust with more power consumption. At a higher blade loading coefficient of the main rotor of 012, the swirl velocities increased the main rotor power, tail rotor power and total power by 380%, 524% and 508%, respectively. The profile power increase of the main rotor caused by the profile swirl velocity was less than that of the induced swirl velocity, but the power increase was higher at high rotor blade loadings. Considering the swirl velocities in the main rotor can improve the prediction precision of the hover performance, especially at high blade loadings      

Thermomechanical coupling effect on characteristics of oxide film during ultrasonic vibration-assisted ELID grinding ZTA ceramics

Ultrasonic vibration-assisted ELID (UVA-ELID) grinding is utilized as a novel and highly efficient processing method for hard and brittle materials such as ceramics. In this study, the UVA-ELID grinding ZTA ceramics is employed to investigate the influence of thermomechanical loading on the characteristics of oxide film. Based on the fracture mechanics of material, the model of internal stress for oxide film damage is proposed. The thermomechanical loading is composed of mechanical force and the thermal stress generating from grinding temperature. The theoretical model is established for the mechanical force, thermal stress and internal stress respectively. Then the finite element analysis method is used to simulate the theoretical model. The mechanical force and grinding temperature is measured during the actual grinding test. During the grinding process, the effect of grinding wheel speed and grinding depth on the thermomechanical force and the characteristics of oxide film is analyzed. Compared with the conventional ELID (C-ELID) grinding, the mechanical force decreased by 25.6% and 22.4% with the increase of grinding wheel speed and grinding depth respectively, and the grinding temperature declined by 10.7% and 12.8% during the UVA-ELID grinding. The thermal stress in the latter decreased by 16.3% and 20.8% respectively, and internal stress reduced by 12.3% and 15.6%. It was experimentally found that the topographies of oxide layer on the surface of the wheel and the machined surface in the latter was better than that in the former. The results indicate that the action of ultrasonic vibration establish a significant effect on the processing. Subsequently, it should be well considered for future reference when processing the ZTA ceramics.     

长期贮存条件下固体发动机药柱应力分析

固体火箭发动机在贮存过程中壳体和推进剂共同承担各种复杂随机载荷,这些载荷的作用会引起推进剂装药力学性能的变化,从而直接危及发动机工作的可靠性。论文选用随机温度载荷为计算背景,并以通用有限元软件为平台,建立了固体发动机结构模型,通过仿真得到了发动机药柱在随机温度载荷下的等效应力变化规律。     

一种未知信源数的快速DOA估计算法

针对众多性能优良的超分辨DOA估计算法大都是以预知信源数为前提,信源数估计不准可能会导致DOA估计失败这一问题,提出了一种基于协方差矩阵对角加载的超分辨DOA估计算法。该算法不需预判信源个数和进行特征值分解,且通过对协方差矩阵进行对角加载,可以平滑小快拍数时噪声特征值分散程度,因此该算法更适用于快拍数较少的情况。理论分析表明：该算法的统计估计性能接近于MUSIC（Multiple Signal Classifjcation）算法。计算机仿真结果验证了该算法的鲁棒性和可行性。     

一种满足静止气象卫星测控要求的天线设计方案

提出了满足新一代静止气象卫星测控要求的测控天线设计方案,采用测控天线斜装实现卫星变轨时天线波束覆盖。在FEKO电磁仿真软件中进行了仿真,讨论了单天线的性能及整星状态星上布局条件对测控天线的影响,并对方案进行了优化。结果表明:所选测控天线设计方案可行,满足卫星测控需要。     

Unequal-thickness billet optimization in transitional region during isothermal local loading forming of Ti-alloy rib-web component using response surface method

Avoiding the folding defect and improving the die filling capability in the transitional region are desired in isothermal local loading forming of a large-scale Ti-alloy rib-web component (LTRC). To achieve a high-precision LTRC, the folding evolution and die filling process in the transitional region were investigated by 3D finite element simulation and experiment using an equal-thickness billet (ETB). It is found that the initial volume distribution in the second-loading region can greatly affect the amount of material transferred into the first-loading region during the second-loading step, and thus lead to the folding defect. Besides, an improper initial volume distribution results in non-concurrent die filling in the cavities of ribs after the second-loading step, and then causes die underfilling. To this end, an unequal-thickness billet (UTB) was employed with the initial volume distribution optimized by the response surface method (RSM). For a certain eigenstructure, the critical value of the percentage of transferred material determined by the ETB was taken as a constraint condition for avoiding the folding defect in the UTB optimization process, and the die underfilling rate was considered as the optimization objective. Then, based on the RSM models of the percentage of transferred material and the die underfilling rate, non-folding parameter combinations and optimum die filling were achieved. Lastly, an optimized UTB was obtained and verified by the simulation and experiment.     

TMS320C6678 多级程序加载模式的实现

为了对实地、多单元部署的TMS320C6678 产品同时提供独立启动、就地调试、远程加载功能，利 用TMS320C6678 片上多种通讯接口和资源，联合片上固化的内置加载核，设计和实现了一种灵活可配置的多 级程序加载模式，满足应用需求。     

低温推进剂深度过冷加注技术研究及对运载火箭性能影响分析

介绍了运载火箭深度过冷加注技术国内外研究现状,分析了深度过冷加注对火箭性能影响。经分析显示,深度过冷加注在降低增压系统规模、减小贮箱容积需求、降低贮箱壁厚、提高发动机预冷适应性、提高推进剂整体品质及发射适应性、缩短加注发射流程等方面均有明显作用。提出了实现深度过冷加注的关键技术及主要技术方案设计。     

Uncertainties and effects on geocenter motion estimates from global GPS observations

Global positioning system (GPS) observations can be used to estimate the geocenter motion, but are subjected to large uncertainties and effects due to uneven distribution of GPS stations and high-degree aliasing errors. In this paper, uncertainties and effects on geocenter motion estimates from global GPS observations are investigated and assessed with different truncated degrees and selected GPS network distributions based on different plate motion models, including NUVEL-1A, MORVEL56 and ITRF08. Results show that the selected GPS stations have no big effects on geocenter motion estimates based on different plate motion models, while large uncertainties are found at annual and semi-annual components when using different truncated degrees. Correlations of geocenter motion estimates from selected GPS networks with GRACE and SLR are better with truncated degree 3, and higher truncated degrees will degrade geocenter estimates. Smaller RMS also shows better results with the truncated degree 3 and the NUVEL1A has the worse results because more GPS sites are eliminated. For annual signal with truncated degree 3, four GPS strategies can reduce annual amplitudes by about 29.2% in X, 5.6% in Y, and 27.9% in Z with respect to truncated degree 1. Annual phases of all GPS solutions from MORVEL56 and ITRF08 are almost close to the GRACE solution with truncated degrees from 3 to 10, while the semi-annual signals are relatively weaker for all cases.     

柔索并联式导弹自动抓取装置动力学仿真

针对导弹质量大、作业环境复杂等形成的难以自动装填问题,融合钢丝绳柔性浮动对位原理,对柔索并联式导弹自动抓取装置进行动态特性分析。基于有限段离散建模方法,对该抓取装置钢丝绳进行柔性体建模,进而建立了该抓取装置的多体动力学模型。通过仿真计算,分析了抓取装置和导弹模块,在处于不同的对位姿态时,钢丝绳柔性和结构接触耦合作用下,抓取装置下放-抓弹-吊弹过程中的运动和受力特性,验证了引入钢丝绳柔性环节的抓取装置方案,针对对位时姿态偏差的自适应调整能力,为该导弹自动装填系统设计提供了参考。