TABLE OF CONTENTS The aeroelasticity of rotating wings is an especially abundant source of nonlinear dynamics problems. In this branch of aeroelasticity, significant nonlinearities can be found in each of the three basic types of forces traditionally defining the subject matter (i.e., inertia, elastic, and aerodynamic). Nonlinearities arise in the inertia loadings of rotor blades because of their freedom to flex with significant amplitudes in a rotating coordinate frame in at least four modes of elastic deformation: transverse (beam) bending in two directions, axial torsion, and axial extension. The internal elastic forces in rotor blades are also rich in nonlinearities. The relatively high aspect ratios of rotor blades taken together with the inherent twist (both built-in and elastic) and the aforementioned flexing amplitudes define a variety of elastic nonlinearities.
However, perhaps of all of the nonlinearities present in rotor aeroelastic phenomena, those arising from aerodynamic sources are the least tractable. Principal sources of aerodynamic nonlinearities are the dynamic stall effects treated in the preceding chapter and those of transonic flow occurring on the advancing blades in high-forward-flight conditions. Indeed, even the unsteady aerodynamic phenomena not associated with dynamic stall and advancing blade compressibility effects are subject to nonlinearities because blade motion in the air mass is not infinitesimal.
Beyond these basic sources of nonlinearities, other sources exist as well. An important one is that afforded by the (articulated) blade lead-lag dampers. Although these dampers do provide a measure of "linear" damping, they are still essentially hydraulic devices...
