The instantaneous aerodynamic force and force center during a vibration cycle were determined from interferometrically measured pressure distributions around the leading blade of a low pressure turbine blade group vibrating in the tangential, axial or twist modes. The corresponding reduced frequencies were 0.0796, 0.1088 and 0.1312, and the inlet flow Mach number was 0.59 in all tests. The energy exchange per vibration cycle between the air flow and the leading blade of a group, and the lift and drag dynamic loops were determined for each of the three vibration modes. The periodic aerodynamic force coefficients were nearly sinusoidal for the axial and torsional modes but not for the tangential mode. At large negative flow incidence, the four-blade group tested is strongly unstable in the twist mode, weakly unstable in the axial mode, and strongly stable in the tangential mode. The experimental results can be used to investigate the validity of analytical predictions of the aerodynamic forces on a vibrating low pressure blade group.

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