This paper presents the results of a rotorcraft preliminary design problem, solved as a multiobjective design optimization problem. A lift-and thrust-augmented coaxial compound configuration is used to demonstrate the approach. The basic optimization problem is converted into a sequence of approximate optimization problems, in which approximate Pareto frontiers are calculated based on response surfaces, obtained from radial basis function interpolation of all the designs analyzed at every step of the sequence. The Pareto frontiers are computed using a genetic algorithm. The designs are analyzed using a high-fidelity rotorcraft analysis that includes nonlinear finite element models of the rotor blade and a free vortex wake model of the rotor inflow. The results presented indicate that 1) the preliminary design problem can be effectively solved using formal numerical design optimization techniques, which therefore can complement classical design methodologies; 2) with appropriate physics-based constraints, the design optimization can be carried out by the computer completely unattended; 3) the optimization methodology is sufficiently robust to deal with multiple local optima and other numerical difficulties; and 4) the methodology is efficient enough to allow the use of high-fidelity analyses throughout the optimization, with the use of graphical processing unit computing (Compute Unified Device Architecture/Fortran) contributing to the computational efficiency.Nomenclature F C X = objective function 2; power required in 220 kt cruise, hover power F H X = objective function 1; power required at hover, hover power N = number of design variables X = vector of design parameters x B ; y B ; z B = body-fixed axes (forward, starboard, and downward) α F = aerodynamic angle of attack of the fuselage (nose up), deg θ FW = root mounting angle of the wing (nose up), deg
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Advanced multi-rotor configurations, such as coaxial rotorcraft, present a complicated aerodynamics problem, particularly in certain flight conditions where interactions between the wakes of two or more rotors exist. These aerodynamic couplings have large impacts on the flight dynamics of the helicopter, and consequently on flight control design. High fidelity aerodynamic models exist, such as vortex methods or computational fluid dynamics methods, that can accurately describe the behavior of rotors in complicated flow fields. However, these methods are not immediately applicable to flight dynamics and control analyses, because the equations of these models are not in state-space form, i.e., in ordinary differential equation form. In the last few years, frequency domain system identification has proven to be a valuable tool for identifying low-order state-space models of inflow dynamics, both for single main rotor configurations in hover and forward flight, and also for coaxial configurations in hover, from high-fidelity vortex methods. This paper demonstrates the method on a coaxial rotor configuration in forward flight at a pitch attitude, of a type likely to be encountered in flight, where the wakes interact strongly. This paper describes in detail the methodology, and the detailed steps required to identify the complex state-space system. A free vortex wake model is describe the aerodynamic flow field. The identification is performed for a flight condition that generates a complex aerodynamic flow field, for which closed form potential flow solutions are not available, and represents a clear example of how system identification techniques can be applied to any high fidelity aerodynamic tool to obtain models of inflow dynamics in state-space form for flight dynamics and control applications.
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