| This dissertation describes a study in which structural optimization techniques are used to minimize the oscillatory vertical hub shears for a helicopter in forward flight, subject to aeroelastic stability constraints, blade natural frequency placement constraints, and autorotation constraints. A special technique is used to build a sequence of approximate, inexpensive to solve approximate optimization problems, the solutions of which converge to the solution of the exact, expensive to solve optimization problem. Blade configurations with both straight and swept tips, and single- and double-cell cross sections are analyzed. The results show that the approach used in this study is very efficient, and produces improved designs with a very small number of blade aeroelastic analyses.;As part of this study, a special finite element for the modeling of a swept tip rotor blade is derived. The swept tip undergoes moderate deflections in flap, lag, and torsion. The nonlinear, partial differential equations of motion are discretized using a Galerkin finite element method. Tip sweep introduces flap-torsion and lag-axial couplings, and may lead to aeroelastic instabilities associated with frequency coalescence. When frequency coalescence does not occur, sweep is usually stabilizing. A comparison of the approximate model of a swept tip blade, using a blade with straight elastic axis and offsets of aerodynamic centers and centers of gravity, with the exact swept tip model developed in this study indicates that the approximate model can produce inaccurate results for the case of hingeless rotor blades.;A new methodology for the formulation of the aeroelastic stability and response for helicopter rotor blades is proposed, which reduces considerably the implementation effort, and is applicable to both straight and swept tip blades. The mathematical expressions for the aerodynamic loads need not be explicit algebraic functions of the blade displacement quantities. This methodology is combined with a finite element model of the blade, and a quasilinearization solution technique. As a preliminary to the analysis of swept tip blades, a study is conducted of the aeroelastic behavior of rotor blades with noncoincident elastic axis, aerodynamic centers, and centers of mass. |
