Parametric Studies on Ground Vibration Test Modeling for Highly Flexible Aircraft

Abstract

Fundamental modeling of a flexible aircraft with a flying wing configuration, attached to a system of bungee cords, was developed in previous work of the authors. Herein this work is extended to include the capability of modeling a general configuration that can be represented as a collection of nonlinear beam finite elements. This allows one to model configurations with fuselage(s) and tail(s) in terms of beam finite elements. The finite element formulation is based on the fully intrinsic beam formulation, so that multiple beams are included by enforcing appropriate continuity conditions on velocity and angular velocity at nodes where beams intersecL Geometric constraints imposed by bungee cords attached at different points along a structure modeled with these beam elements make the system statically indeterminate. This brings certain strain-displacement relations and displacement variables into the formulation. Two in-depth parametric studies on how ground-vibration testing parameters affect the modal characteristics of highly flexible aircraft are presented as follows: 1) the effect of different bungee cord locations and 2) the effect of a fuselage structure on the modal characteristics. A comparison of results from the parametric studies provides information on how the shape of a highly flexible wing affects its modal characteristics and which eigenmodes associated with fuselage motion are involved in the low-frequency structural modes. In addition, for conventional configurations with highly flexible wings, the existence of modes identified in ground-vibration testing as predominantly rigid-body motion is discussed. Moreover, when such modes do exist, the extent to which they involve structural deformation and the effect of ground-vibration testing parameters on them is studied.