Fan Blade Flutter: Single Blade Instability or Blade to Blade Coupling?

Different types of fan blade flutter occur at the various compressor flow regimes. Sub/transonic stall flutter and two forms of supersonic started flow flutter have been studied in a straight cascade wind tunnel. Results show clearly that these three common forms of flutter can exist as single-degree-of-freedom (single-blade instabilities). Cascade effects, though at times important, are never the only flutter mechanism: flutter limits are essentially controlled by single-blade aeroelastic coefficients, though blade-to-blade coupling arising from cascade effects can modify these limits according to the mode order. Thus, contrary to widespread practice, the fundamental approach to flutter problems should lie at least as much in the study of single blade flutter as in that of unsteady cascade effects. The two should anyhow best be considered separately when searching for a better physical insight.