Effect of Interblade Phase Angle and Incidence Angle on Cascade Pitching Stability

Abstract

A comprehensive test program was performed at low subsonic velocity on a linear cascade of airfoils oscillating in pitch about their midchords for incidence angles up to 10 deg, reduced frequencies up to 0.193, and over a range of interblade phase angles from σ = −60 deg to +60 deg. The test conditions represent significant changes in blade loading and dimensionless frequency, and the range of interblade phase angle includes those values usually encountered in actual turbomachines. The measured pressure time histories over the airfoil chord were used to calculate the stability parameters of the system including the unsteady pitching moment coefficient and the aerodynamic damping parameter. For the range of parameters tested it was found that the interblade phase angle is the most important parameter affecting the stability of oscillating cascaded airfoils. The system was unstable for most positive values of σ over the entire range of loading and frequency. This was similar in behavior (but not in magnitude) to the predictions of available potential flow cascade theories and suppports the observation that blade stall need not be present for torsional “stalled” flutter to occur. System stability for negative values of σ was more dependent on loading and frequency and, conformed more closely with the observed behavior of stalled flutter. Specifically, for σ < 0 deg stability increased with frequency and decreased with loading. A preliminary evaluation of the pressure time histories shows that a second harmonic behavior renders the 1.2 percent chord station ineffective in contributing to the blade damping. Under these circumstances it is surmised that the induced damping is associated mainly with the first harmonic component of the pressure response at the 6.2 percent chord station.