Linearization method of nonlinear aeroelastic stability for complete aircraft with high-aspect-ratio wings

A linearization method and an engineering approach for the geometric nonlinear aeroelastic stability analysis of the very flexible aircraft with high-aspect-ratio wings are established based on the little dynamic perturbation assumption. The engineering practicability of the method is validated by a complex example. For a high-altitude long-endurance unmanned aircraft, the nonlinear static deformations under straight flight and the gust loads are calculated. At the corresponding nonlinear equilibrium state, the complete aircraft is linearized dynamically and the vibration modes are calculated considering the large deformation effects. Then the unsteady aerodynamics are calculated by the double lattice method. Finally, the aeroelastic stability of the complete aircraft is analyzed. The results are compared with the traditional linear calculation. The work shows that the geometric nonlinearity induced by the large structural deformation leads to the motion coupling of the wing chordwise bending and the torsion, which changes the mode frequencies and mode shapes. This factors change the aeroelastic coupling relationship of the flexible modes leading to the decrease of the flutter speed. The traditional linear method would give not only an imprecise flutter speed but also a possible dramatic mistake on the stability. Hence, for a high-altitude long-endurance unmanned aircraft with high-aspect-ratio wings, or a similar very flexible aircraft, the geometric nonlinear aeroelastic analysis should be a necessary job in engineering practice.