Abstract: A composite high-aspect-ratio wing of a high-altitude long-endurance
(HALE) aircraft was modeled with FEM by MSC/NASTRAN, and the nonlinear static
equilibrium state is calculated under design load with follower force effect,
but without load redistribution. Assuming the little vibration amplitude of the
wing around the static equilibrium state, the system is linearized and the
natural frequencies and mode shapes of the deformed structure are obtained.
Planar doublet lattice method is used to calculate unsteady aerodynamics in
frequency domain ignoring the bending effect of the deflected wing. And then,
the aeroelastic stability analysis of the system under a given load condition
is successively carried out. Comparing with the linear results, the nonlinear
displacement of the wing tip is higher. The results indicate that the critical
nonlinear flutter is of the flap/chordwise bending type because of the
chordwise bending having quite a large torsion component, with low critical
speed and slowly growing damping, which dose not appear in the linear analysis.
Furthermore, it is shown that the variation of the nonlinear flutter speed
depends on the scale of the load and on the chordwise bending frequency. The
research work indicates that, for the very flexible HALE aircraft, the
nonlinear aeroelastic stability is very important, and should be considered in
the design progress. Using present FEM software as the structure solver (e.g.
MSC/NASTRAN), and the unsteady aerodynamic code, the nonlinear aeroelastic
stability margin of a complex system other than a simple beam model can be
determined.
