Low Aspect Ratio Axial Flow Compressors: Why and What It Means

One of the more visible changes which has occured in fans and compressors for aircraft turbine engines which have entered development since about 1970 has been a significant reduction in the aspect ratio of the blading. This has brought with it a greatly reduced engine parts count and improved ruggedness and aeroelastic stability. This paper traces the evolution of thinking concerning appropriate aspect ratios for axial flow compressors since the early years of the aircraft turbine engine. In the 1950s, moderate aspect ratios were favored for reasons of mechanical design. As mechanical design capability became more sophisticated, several attempts were made, primarily in the 1960s, to employ very high aspect ratios to reduce engine size and weight. Four of these programs are described which were largely unsuccesful for both mechanical and aerodynamic reasons. After 1970, the pendulum swung strongly in the other direction and designs of very low aspect ratio began to emerge. This has had a significant impact on compressor design systems, and a number of the ways in which design systems have been affected are discussed. Some concluding remarks are made concerning the author's opinion of trends in the near future in aerodynamic design technology.