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
中壓第六級放氣活門
高壓第三級
放氣活門
高壓第三級放棄活門
35．只有在鈦合金得到應用后，高涵道比風扇葉片圖3-19)的設計才成為可能；用實心鍛件機械加工成葉片是常規設計。重量輕的風扇葉片是必要的，因為發動機的前部結構必須能夠承受由于風扇葉片故障引起的巨大不平衡力。為了得到一種足夠輕的實心風扇葉片，即使是用鈦合金制造的，要求短的軸向長度(即弦長)。然而，采用這種設計就要求葉展中部有支持(“阻尼器”或“阻屁凸臺”)的特殊結構來防止空氣動力不穩定性。這種設計方案的缺點是阻尼器位于超音速氣流中，壓力損失極高，使效率降低，流量減少。這種缺點已經用羅爾斯-羅伊斯公司設計的寬弦風扇葉片克服了。葉片弦長的增加提供了葉片的穩定性，阻尼凸臺也就沒有必要了。用鈦合金蒙皮加蜂窩結構的核心來制造葉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.
圖3-12 一種電子控制的放氣活門系統
An electronically operated bleed valve system.左側中壓電磁閥
(調高的放氣活門組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. 圖3-19 典型的風扇葉片類型
斷電關閉高壓放氣活門
典型的高壓放氣活門和電磁閥
高壓第三級
中壓第六級
高壓第三級空氣
通電關閉中壓放氣活門
電磁閥
線圈
通風
右
活門
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 中國航空網 www.k6050.com 航空翻譯 www.aviation.cn 本文鏈接地址：勞斯萊斯噴氣引擎－中英(17)
