Dynamic Analysis of Rotor Blades with Root Retention Design Variations

Kinematic and elastic couplings are often provided in rotor blade root retention systems to improve aeroelastic or flight stability. This paper uses the generalized coordinates approach and Lagrange Multiplier variational method to arrive at a unified formulation in which it is unnecessary to repeat the structural part of the analysis when root boundary conditions change. Such a method has advantages for evaluating design variations of blade retention systems. Two idealized rotor hub-hinge system models, with many features typical of advanced blade retention designs, are examined here. The in vacuo, free vibration characteristics of rotor blades with masselastic properties outboard of the attachment point typical of present-gener ation rotor craft are calculated, as influenced by these retention systems. Trends with variations of root retention system parameters are presented for natural frequencies, damping ratios, and the amount of blade pitch induced by root flapping or lagging motions. A well-known static instability, which could be insidious in advanced rotor retention system design, is also reviewed.