33.
Stator vanes are normally produced from steel or nickel based alloys, a prime requirement being a high fatigue strength when "notched" by ingestion
damage. Earlier designs specified aluminium alloys but because of its inferior ability to withstand damage its use has declined. Titanium may be used for stator vanes in the low pressure area but is unsuitable for the smaller stator vanes further rearwards in the compression system because of the higher pressures and temperatures encountered. Any excessive rub which may occur between rotating and static components as a result of other mechanical failures, can generate sufficient heat from friction to ignite the titanium. This in turn can lead to expensive repair costs and a possible airworthiness hazard.
34. In the design of rotor discs, drums and blades, centrifugal forces dominate and the requirement is for metal with the highest ratio of strength to density. This results in the lightest possible rotor assembly which in turn reduces the forces on the engine structure enabling a further reduction in weight to be obtained. For this reason, titanium even with its high initial cost is the preferred material and has replaced
Compressors
中压第六级放气活门
高压第三级
放气活门
高压第三级放弃活门
35．只有在钛合金得到应用后，高涵道比风扇叶片图3-19)的设计才成为可能；用实心锻件机械加工成叶片是常规设计。重量轻的风扇叶片是必要的，因为发动机的前部结构必须能够承受由于风扇叶片故障引起的巨大不平衡力。为了得到一种足够轻的实心风扇叶片，即使是用钛合金制造的，要求短的轴向长度(即弦长)。然而，采用这种设计就要求叶展中部有支持(“阻尼器”或“阻屁凸台”)的特殊结构来防止空气动力不稳定性。这种设计方案的缺点是阻尼器位于超音速气流中，压力损失极高，使效率降低，流量减少。这种缺点已经用罗尔斯-罗伊斯公司设计的宽弦风扇叶片克服了。叶片弦长的增加提供了叶片的稳定性，阻尼凸台也就没有必要了。用钛合金蒙皮加蜂窝结构的核心来制造叶片亦使其重量保持在低水平。
the steel alloys that were favoured in earlier designs.
As higher temperature titanium alloys are developed
and produced they are progressively displacing the
nickel alloys for the disc and blades at the rear of the
system.
图3-12 一种电子控制的放气活门系统
An electronically operated bleed valve system.左侧中压电磁阀
(调高的放气活门组2、4、6）
Fig. 3-18
35. The high by-pass ratio fan blade (fig. 3-19) only became a design possibility with the availability of titanium, conventional designs being machined from solid forgings. A low weight fan blade is necessary because the front structure of the engine must be able to withstand the large out of balance forces that would result from a fan blade failure. To achieve a sufficiently light solid fan blade, even with titanium, requires a short axial length (or chord). However, with this design, the special feature of a mid-span support ('snubber' or 'clapper') is required to prevent aerodynamic instability. This design concept has the disadvantage of the snubber being situated in the supersonic flow where pressure losses are greatest, resulting in inefficiency and a reduction in airflow. This disadvantage has been overcome with the introduction of the Rolls-Royce designed wide chord fan blade; stability is provided by the increased chord of the blade thus avoiding the need for snubbers. The weight is maintained at a low level by fabricating Fig. 3-19 Typical types of fan blades. 图3-19 典型的风扇叶片类型
断电关闭高压放气活门
典型的高压放气活门和电磁阀
高压第三级
中压第六级
高压第三级空气
通电关闭中压放气活门
电磁阀
线圈
通风
右
活门
左
the blade from skins of titanium incorporating a honeycomb core.
36. Centrifugal impeller material requirements are similar to those for the axial compressor rotors. Titanium is thus normally specified though aluminium may still be employed on the largest low pressure ratio designs where robust sections give adequate ingestion capability and temperatures are acceptably low. 平衡 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：劳斯莱斯喷气引擎－中英(17)
