全文获取类型
收费全文 | 206篇 |
免费 | 29篇 |
国内免费 | 23篇 |
专业分类
航空 | 104篇 |
航天技术 | 74篇 |
综合类 | 28篇 |
航天 | 52篇 |
出版年
2023年 | 8篇 |
2022年 | 6篇 |
2021年 | 21篇 |
2020年 | 8篇 |
2019年 | 11篇 |
2018年 | 12篇 |
2017年 | 8篇 |
2016年 | 10篇 |
2015年 | 6篇 |
2014年 | 20篇 |
2013年 | 9篇 |
2012年 | 17篇 |
2011年 | 28篇 |
2010年 | 16篇 |
2009年 | 10篇 |
2008年 | 15篇 |
2007年 | 4篇 |
2006年 | 4篇 |
2005年 | 4篇 |
2004年 | 5篇 |
2003年 | 4篇 |
2002年 | 6篇 |
2001年 | 3篇 |
1999年 | 2篇 |
1996年 | 1篇 |
1995年 | 3篇 |
1994年 | 15篇 |
1992年 | 1篇 |
1991年 | 1篇 |
排序方式: 共有258条查询结果,搜索用时 500 毫秒
141.
142.
首先简要介绍了第49届巴黎航展的基本情况,然后对欧美军用直升机的发展趋势和技术特点进行了较为深入的分析,阐述了军用直升机在海上多样化行动中的作用,最后给出了几点启示。 相似文献
143.
144.
Yang Zhang Binbin Ni Zheng Xiang Xianguo Zhang Xiaoxin Zhang Xudong Gu Song Fu Xing Cao Zhengyang Zou 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2018,61(9):2290-2300
We perform an L-shell dependent inter-satellite calibration of FengYun 3 medium energy electron measurements with POES measurements based on rough orbital conjunctions within 5?min?×?0.1?L?×?0.5 MLT. By comparing electron flux data between the U.S. Polar Orbiting Environmental Satellites (POES) and Chinese sun-synchronous satellites including FY-3B and FY-3C for a whole year of 2014, we attempt to remove less reliable data and evaluate systematic uncertainties associated with the FY-3B and FY-3C datasets, expecting to quantify the inter-satellite calibration factors for the 150–350?keV energy channel at L?=?2–7. Compared to the POES data, the FY-3B and FY-3C data generally exhibit a similar trend of electron flux variations but more or less underestimate them within a factor of 5 for the medium electron energy 150–350?keV channel. Good consistency in the flux conjunctions after the inter-calibration procedures gives us certain confidence to generalize our method to calibrate electron flux measurements from various satellite instruments. 相似文献
145.
低Re对某小型涡扇发动机性能影响 总被引:2,自引:0,他引:2
低雷诺数问题是影响高空长航时无人驾驶飞机的动力装置性能的关键因素之一.定量分析了低雷诺数效应对某小型大涵道比涡扇发动机性能的影响.当雷诺数减小到一定程度后,各部件的性能将发生改变.在发动机整机环境下,增压级和低压涡轮的进口叶弦雷诺数相对更低,受飞行高度和速度的影响也更大.在发动机共同工作条件的作用下,各部件的匹配关系将发生变化,除部件效率的降低以外,部件流通能力的衰减也引起发动机性能的降低.涡轮前温度的限制将使发动机转子转速下降,低雷诺数效应加剧,从而导致发动机性能的进一步降低,推力迅速减小,但发动机涵道比的增加可以减小因部件性能衰退引起的耗油率上升的趋势. 相似文献
146.
Frieda B. Taub Anna K. McLaskey 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
Pressure increased during net photosynthetic O2 production in the light and decreased during respiratory O2 uptake during the dark in aquatic Closed Ecological Systems (CESs) with small head gas volumes. Because most CO2 will be in the liquid phase as bicarbonate and carbonate anions, and CO2 is more soluble than O2, volumes of gaseous CO2 and gaseous O2 will not change in a compensatory manner, leading to the development of pressure. Pressure increases were greatest with nutrient rich medium with NaHCO3 as the carbon source. With more dilute media, pressure was greatest with NaHCO3, and less with cellulose or no-added carbon. Without adequate turbulence, pressure measurements lagged dissolved O2 concentrations by several hours and dark respiration would have been especially underestimated in our systems (250–1000 ml). With adequate turbulence (rotary shaker), pressure measurements and dissolved O2 concentrations generally agreed during lights on/off cycles, but O2 measurements provided more detail. At 20 °C, 29.9 times as much O2 will distribute into the gas phase as in the liquid, per unit volume, as a result of the limited solubility of O2 in water and according to Henry’s Law. Thus even a small head gas volume can contain more O2 than a larger volume of water. When both dissolved and gaseous O2 and CO2 are summed, the changes in Total O2 and CO2 are in relatively close agreement when NaHCO3 is the carbon source. These findings disprove an assumption made in some of Taub’s earlier research that aquatic CESs would remain at approximately atmospheric pressure because approximately equal molar quantities of O2 and CO2 would exchange during photosynthesis and respiration; this assumption neglected the distribution of O2 between water and gas phases. High pressures can occur when NaHCO3 is the carbon source in nutrient rich media and if head-gas volumes are small relative to the liquid volume; e.g., one “worse case” condition developed 800 mm Hg above atmospheric pressure and broke the glass container. Plastic screw cap closures are likely to leak at high pressures and should not be assumed to seal unless tested at appropriate pressures. Pressure can be reduced by having larger head-gas volumes and using less concentrated nutrient solutions. It is important that pressure changes be considered for both safety and closure, and if total O2 is used as the measure of net photosynthesis and respiration, the O2 in the gas phase must be added to the dissolved O2. 相似文献
147.
Carlo Scotto Alessandro Settimi 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
The results of this paper demonstrate that the effect of collisions on the group refraction index is small, when the ordinary ray is considered. If, however, in order to improve the performance of a system for automatic interpretation of ionograms, the information contained in ordinary and extraordinary traces is combined, the effect of collisions between the electrons and neutral molecules should be taken into account for the extraordinary ray. The magnitude of these differences is generally very small and must be compared with the resolution in the virtual vertical height of the ionosonde, resolution which is typically of the order of few kilometers. 相似文献
148.
A. Gil M.V. Alania 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
We show that the amplitudes of the 27-day variations of galactic cosmic ray (GCR) intensity, solar wind and solar activity parameters have a periodicity with duration of three to four Carrington rotation periods (3–4 CRP). We assume that the general reason for this phenomenon may be related to similar cyclicity of topological structure of the solar magnetic field lines created owing to the asymmetry of turbulent solar dynamo and solar differential rotation transforming the Sun’s poloidal magnetic field to the toroidal (α–ω effect), and vice versa. 相似文献
149.
A. Paul A. Kascheyev M. Rodriguez-Bouza K. Pathak A.A. Ferreira D. Shetti J.N. Yao 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2018,61(7):1890-1900
GNSS TEC values have been obtained from 18 stations distributed from the magnetic equator to nearly 80°N magnetic dip in the African and west-European longitude sector corresponding to the March 17–18, 2015 geomagnetic storm. Significantly depleted ionosphere have been observed at stations north of 50°N geographic on March 18, 2015 following the above storm over a longitude swath 11.9°–21°E covering the Eastern Africa and Western European longitude sector. High ROTI values were noted on March 17th at locations around 80°N magnetic dip. Two prominent peaks in PCN were noted around 09:00 UT and 14:00 UT on March 17, 2015 and around 15:00 UT on March 18, 2015. Daytime thermospheric (O/N2) ratio was markedly less on March 18th at latitudes above 60°N geographic which is suggested to be the major driver behind depleted high latitude ionosphere during the recovery phase of the storm on March 18, 2015. 相似文献
150.
张少杰 《西安航空技术高等专科学校学报》2012,(1):77-78
目的为了简单地求解有理函数P(x)/Q(x)的积分;方法对∫P(x)/(x-a)mdx∫,P(x)/(x-a)m(x-b)ndx型有理函数的积分给出一般计算公式;结果通过实例说明该公式简捷、对称,避免以往待定系数法带来的诸多不便;结论具有较强的实用性。 相似文献