Development of a Very Flexible Testbed Aircraft for the Validation of Nonlinear Aeroelastic Codes

Abstract

This dissertation presents the development, analysis, and flight testing of an unmanned aeroelastic testbed aircraft. The experimental, remotely-piloted aircraft was designed to be aeroelastically representative of very flexible aircraft. Known as X-HALE, this Experimental High-Altitude Long-Endurance aircraft exhibited geometrically nonlinear behavior and displayed specific aeroelastic characteristics that are present in full-scale HALE vehicle designs. Three X-HALE configurations were constructed and characterization of their elastic, inertia, and aerodynamic properties were used to create a nonlinear model for aeroelastic analysis. Each configuration was instrumented with a suite of sensors to allow the collection of high quality elastic and flight dynamic data. Flight tests of each configuration were performed and the presence of the desired nonlinear aeroelastic behavior was confirmed, including coupling between elastic deformation and rigid body flight dynamics and unstable, but controllable lateral oscillations. Data gathered during flight was processed for use in validation of nonlinear aeroelastic solvers. Aeroelastic simulations of select maneuvers from the flight tests of each X-HALE configuration were performed using the reduced–order nonlinear strain-based finite element framework implemented in the University of Michigan Nonlinear Aeroelastic Simulation Toolbox (UM/NAST). The correlations support validation of UM/NAST’s capability to model the coupled aeroelastic and flight dynamic behavior of highly flexible aircraft. A number of parallel studies were conducted to support the flight test simulations and UM/NAST validation effort. The NASA-Lockheed Martin X-56A was used to illustrate the process of creating a nonlinear aeroelastic model from high fidelity structural and aerodynamic data. The nonlinear model’s structural and aerodynamic response was compared to results from high fidelity solvers, and flutter predictions were performed. To expand the modeling capability of UM/NAST, updates to the aerodynamic and kinematic constraint formulation were implemented. A propeller module was developed which incorporates downwash effects into the existing 2-D unsteady aerodynamic framework. Expanded definitions for nodal displacement constraints using the Lagrange multiplier method were implemented, allowing the modeling of mixed displacement and rotation boundary conditions as well as complex inter-member connections.