A dual-model framework for quantifying air-floating fixture effects on large flexible cable-strut deployable articulated mast

Ground modal tests of Large Flexible Cable-Strut Deployable Articulated Mast (LF-CSDAM) cannot accurately reflect on-orbit dynamics due to fixture-induced perturbations from gravity compensation systems. This paper presents a novel dual-model framework that systematically quantifies the effect of an air-floating fixture on the modal characteristics of a 60m LF-CSDAM. A clamped-free reference model representing the idealized on-orbit state is established first. The holistic model that couples the LF-CSDAM with the air-floating fixture is then developed. The Global Mode Method (GMM) is employed to derive a reduced-order analytical model, enabling direct extraction of global mode shapes and natural frequencies. Comparative analysis confirms that GMM achieves superior accuracy over FEM truncation approach. A comprehensive sensitivity analysis is applied to quantify the contributions of four key fixture parameters, including added mass, support stiffness, support location and boundary stiffness. The results reveal that support stiffness is the dominant factor, contributing up to 8.5% frequency shift, while added mass produces a consistent 1.12% reduction. Based on quantified sensitivity results, model correction via dimensionless criteria reduces frequency error from 2.15% to 0.25%. This provides a validated methodology for enhancing ground-test fidelity and on-orbit prediction reliability of large deployable space structures.