Stagger Angle Dependence of Inertial and Elastic Coupling in Bladed Disks

Abstract

The natural frequencies and mode shapes of nonrotating blade-disk-shaft systems have been experimentally and analytically investigated. Two mechanisms of blade motion coupling by the disk and shaft were investigated: inertial coupling by the rigid body motion of the disk on a flexible shaft; and out-of-plane elastic coupling due to disk flexure. A Ritz analysis was carried out which identifies the non-dimensional frequency and mass ratios which govern the blade-disk-shaft coupling. The mass ratios depend directly on the effective blade stagger angle. Estimates of these parameters were made for three typical rotors. Two experiments were performed which model these typical rotors. A single set of well-tuned flat blades was mounted on two different disks, one flexible and one rigid, which were in turn mounted on a flexible shaft. The blade-disk attachments were designed to allow for variations in the blade stagger angles. Experimental results show excellent agreement with simple analytical models derived by Ritz analysis. Data are reported in terms of nondimensional parameters. The results clearly show the strong dependence of the system coupling on the blade stagger angle and the blade-disk frequency and mass ratios.