The effect of compressibility on unstalled torsional flutter

Abstract

The report presents the results of calculations of torsional flutter of unstalled cascade blades at zero mean deflection and subsonic Mach numbers. Three kinds of flutter are found. Subcritical flutter occurs when no acoustic waves Can propagate in the duct. The effect of increasing Mach number is highly favourable, and tends to suppress the flutter predicted by incompressible theory. Acoustic resonance flutter occurs when the blade frequency is very slightly less than the acoustic resonant frequency of the upstream and downstream ducts. It is critically dependent on this coincidence, and would give very small areas of flutter on a compressor performance map. Supercritical flutter occurs when some acoustic waves can propagate in the duct. It only occurs at Mach numbers close to unity, and is probably not of much practical importance.