Vibration and aeroelastic analysis of highly flexible HALE aircraft

The highly flexible HALE (High Altitude Long Endurance) aircraft analysis methodology is of interest because early studies indicated that HALE aircraft might have different vibration and aeroelastic characteristics from those of conventional aircraft. Recently the computer code Nonlinear Aeroelastic Trim And Stability of HALE Aircraft (NATASHA) was developed under NASA sponsorship. NATASHA can predict the flight dynamics and aeroelastic behavior for HALE aircraft with a flying wing configuration. Further analysis improvements for NATASHA were required to extend its capability to the ground vibration test (GVT) environment and to both GVT and aeroelastic behavior of HALE aircraft with other configurations.; First, the analysis methodology, based on geometrically exact fully intrinsic beam theory, was extended to treat other aircraft cofigurations. Conventional aircraft with flexible fuselage and tail can now be modeled by treating the aircraft as an assembly of beam elements. NATASHA is now applicable to any aircraft cofiguration that can be modeled this way. The intrinsic beam formulation, which is a fundamental structural modeling approach, is now capable of being applying to a structure consisting of multiple beams by relating the virtual displacements and rotations at points where two or more beam elements are connected to each other. Additional aspects are also considered in the analysis such as auxiliary elevator input in the horizontal tail and fuselage aerodynamics.; Second, the modeling approach was extended to treat the GVT environment for HALE aircraft, which have highly flexible wings. GVT has its main purpose to provide modal characteristics for model validation. A bungee formulation was developed by the augmented Lagrangian method and coupled to the intrinsic beam formulation for the GVT modeling. After the coupling procedure, the whole formulation cannot be fully intrinsic because the geometric constraint by bungee cords makes the system statically indeterminate. Displacement and rotation variables need to be introduced, but only at points to which bungee cords are attached.; Third, because many HALE aircraft are propeller driven, the structural modeling was extended to include an engine/nacelle/propeller system using a two-degree-of-freedom model with pitch and yaw angles. This step was undertaken to predict a dynamic instability called "whirl flutter," which can be exhibited in such HALE aircraft. It can investigate how the nacelle whirling and wing motions affect each other. For simplicity, two fundamental assumptions are made regarding the propeller aerodynamics and inertia matrix of two-bladed propeller system. The propeller airloads are evaluated by the constant approximation which uses the averaged values for one revolution per blade. Periodic side forces and hub moments are evaluated based on how they affect the trim condition determined by the constant approximation. The next assumption is for certain HALE aircraft which can use a two-bladed propeller system. The inertia matrix appears as periodic in time in the governing equations. If the periodic inertia effect is negligible, then the inertia matrix can be replaced by that of equivalent three-bladed propeller system so that the stability analysis can obviate the need for Floquet theory.; These new development have been fully integrated into the current version of NATASHA. Finally, a parametric study for representative HALE aircraft is presented to show how the current methodology can be utilized as a unified preliminary analysis tool for the vibration and aeroelastic analysis of highly flexible HALE aircraft.