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A comprehensive vibration analysis of a coupled rotor/fuselage system is carried out using detailed 3-D finite element models of the AH-1G airframe from the DAMVIBS program. Predicted vibration results are compared with operational load survey flight test data of the AH-1G helicopter. Modeling of difficult components (secondary structures, doors/panels, etc.) is essential in predicting airframe natural frequencies. Calculated 2/rev vertical vibration levels at the pilot seat show good correlation with the flight test data both in magnitude and phase, but 4/rev vibration levels show fair correlation only in magnitude. Lateral vibration results show more disagreement than vertical vibration results. Refined aerodynamics such as free wake and unsteady aerodynamics have an important role in the prediction of vibration. Main rotor pylon roll mode has a significant contribution (about 26%) on the 2/rev vibration. Accurate prediction of airframe natural frequencies up to about 40 Hz appears essential to predict vibration in airframe.

An improved aeroelastic formulation for the advanced geometry blades, involving variable sweep, droop, pretwist, and planform, is presented. The blade is modeled as a series of arbitrarily-oriented elastic segments with each segment divided into finite elements. Inter-element compatibility relations governing non-Eulerian moderate rotations of the finite elements are also presented. Fuselage dynamic interaction with the advanced geometry blades is included in the formulation. The nonlinear partial differential equations of motion are discretized in space and time using Hamilton's principle. Selective results are presented in hover and forward flight. Results indicate that sweep, and droop in particular, can have a strong influence on both the rotor aeroelastic stability and the rotorcraft aeromechanical stability. Droop can be considerably stabilizing. Sweep increases the blade torsional loads, but is not detrimental to flap and lag vibratory loads.

The structural behavior of coupled, thin-walled, composite beams of open as well as closed section was analyzed using Vlasov theory and then the results were validated by experiment. The analysis modeled the walls of beams as general composite laminates and accounted for the transverse shear deformation of the cross-section. The out-of-plane warping deformation of the cross-section was included implicitly in this formulation. In order to validate the analysis, graphite-epoxy beams of various cross-sections such as solid rectangular, I-section, single-cell rectangular and two-cell airfoil were fabricated and tested for their structural response under tip bending, torsional and extensional loads. Specialized bending-torsion and extension-torsion couplings were introduced in these beams using proper ply lay-ups. Good correlation between theoretical and experimental results was achieved. Transverse-shear-related couplings were found to influence the structural response of open- as well as closed-section beams. For blades with hygrothermally stable lay-ups, bending-transverse shear coupling increased the bending flexibility by about 50%. The in-plane-bending coupling stiffness [ B ] of the walls of the beam generally influenced the structural response of the beams quite significantly; this effect was expecially large for I-beams. The influence of constraining the warping deformation was found to be substantial on the structural response of open-section beams as compared to closed-section beams. A 630% increase in the torsional stiffness due to constrained warping was noticed for graphite-epoxy I-beams of slenderness ratio 30. The feasibility of achieving the desired levels of bending-torsion and extension-torsion couplings in two-cell rotor blades was demonstrated.

The paper outlines the nonlinear dynamic analysis of an isolated three-degree flap-lag-feather wind turbine blade under a gravity field and with shear flow. Lagrangian equations are used to derive the nonlinear equations of motion of blade for arbitrarily large angular deflections. The limit cycle analysis for forced oscillations and the determination of the principal parametric resonance of the blade due to periodic forces from the gravity field and wind shear are performed using the harmonic balance method. Results are obtained first for a two-degree flap-lag blade, then the effect of the third degree of freedom (feather) is studied. The self-excited flutter solutions are obtained for a uniform wind and with gravity forces neglected. The effects of several parameters on the blade stability are examined, including coning angle, structural damping, Lock number, and feather frequency. The limit cycle flutter solution of a typical configuration shows a substantial nonlinear softening spring behavior.

The vibration characteristics of stepped thickness plates of rectangular geometry with simply supported edges are investigated through an exact analysis. To illustrate the effect of sudden variation of thickness in the step form on the dynamic characteristics of plates, numerical calculations have been made for various combinations of thickness step, step-length ratio and side ratio. Natural frequencies for the first eight to nine modes are presented in the form of tables. Nodal lines are also plotted for a few selected cases. Several interesting features such as that of the thickness step splitting the degenerate frequencies of uniform rectangular plates into distinct ones, and the “frequency crossing” of some of the modes are observed.

The vibration of simply supported skew plates having a linear variation in thickness in one direction is considered. Approximate analysis is made by using Lagrange's equations employing the double Fourier sine series in oblique co-ordinates to represent the deflected surface. Natural frequencies are obtained for rhombic plates for several ranges of thickness variation and skew angle. The nodal patterns plotted for a few typical configurations show interesting metamorphoses with variation in thickness and skew angle.