Aeroelasticity and structural optimization of composite helicopter rotor blades with swept tips

This dissertation describes the development of an aeroelastic analysis capability for composite helicopter rotor blades with straight and swept tips, and its application to the simulation of helicopter vibration reduction through structural optimization. A new aeroelastic model is developed in this study which is suitable for composite rotor blades with swept tips in hover and in forward flight. The hingeless blade is modeled by beam type finite elements. A single finite element is used to model the swept tip. Arbitrary cross-sectional shape, generally anisotropic material behavior, transverse shears and out-of-plane warping are included in the blade model. The nonlinear equations of motion are derived using Hamilton's principle and based on a moderate deflection theory. Composite blade cross-sectional properties are calculated by a separate linear, two-dimensional cross section analysis. The aerodynamic loads are obtained using quasi-steady aerodynamics. This aerodynamic model is implemented in the computer code based on an implicit formulation. The trim and blade aeroelastic response are solved in a fully coupled manner. In forward flight, where the blade equations of motion are periodic, the coupled trim-aeroelastic response solution are obtained using the harmonic balance approach, and the stability of the periodic system, linearized about the time dependent equilibrium position, is determined from Floquet theory.;Numerical results illustrating the influence of composite ply orientation, tip sweep and anhedral on trim, vibratory hub loads, blade response and stability, are presented. It is found that composite ply orientation has a significant influence on blade stability. The flap-torsion coupling associated with tip sweep can induce aeroelastic instability due to frequency coalescence. This instability can be removed by appropriate ply orientation in the composite construction.;The structural optimization study is conducted by combining the aeroelastic analysis developed in this study with an optimization package to minimize the vibratory hub loads in forward flight subject to frequency and aeroelastic stability constraints, using composite ply orientations and tip sweep and anhedral as design variables. A parametric study showing the effects of tip sweep, anhedral and composite ply orientation on blade aeroelastic behavior serves as a valuable precursor in selecting the initial design for the optimization studies. However, the most appropriate combination of the design variables, for vibration reduction, can only be selected by the optimizer. Optimization results show that tip sweep has the most dominant influence on rotor vibration reduction.