Multidisciplinary Adjoint-Based Design Optimization Techniques For Helicopter Rotor

Helicopter rotor design optimization is a challenging task due to the multidisciplinary nature of rotorcraft design: the helicopter operates in a highly unsteady aerodynamic environment, highly flexible, slender rotor blades highlight the importance of blade aeroelasticity, while the ever more stringent noise requirements that the helicopter must satisfy underlines the need to include aeroa- coustic considerations early in the design process.;Such a large scale problem can be efficiently solved with the use of gradient-based optimization methods. In gradient based optimization, the gradient of the objective function with respect to the design variables is needed to determine a search direction. The objective function's gradient can be computed either with the finite difference approach, the tangent or forward linearization approach and the adjoint or reverse approach. The finite difference approach is easy to implement but its cost scales with the number of design variables and can be affected by the choice of the step size used in the differentiation. The tangent or forward approach computes the exact gradient vector of the objective function by exact differentiation of the computational code, however its cost still scales linearly with the number of design variables. On the other hand, the adjoint or reverse approach computes the sensitivity vector with respect to a potentially infinite number of design variables at a cost essentially independent of the design variables, making the adjoint technique the only viable approach when the number of design variables is large. Hence, it is the adjoint approach that makes gradient based optimization techniques competitive for large scale problems characterized by a large number of design parameters, such as the current helicopter design problem.;The focus of this work is the development of a high-fidelity multidisciplinary adjoint technique that encompasses the three disciplines of aerodynamics, structural mechanics and aeroacoustics for ro- torcraft problems. Upon successful implementation and verification, the multidisciplinary adjoint method is applied to the problem of noise minimization of a flexible rotor in trimmed forward flight with no performance penalty. Optimization results highlight the potential of high-fidelity multidisciplinary design optimization for helicopter rotors.