Optimising Aspects of BERP-Like Rotors Using Frequency-Domain Methods

This work presents a framework for the optimisation of certain aspects of a BERP-like (British Experimental Rotor Programme) rotor blade in hover and forward flight so that maximum performance can be obtained from the blade. The proposed method employs a high-fidelity, efficient CFD technique that uses the Harmonic Balance method in conjunction with artificial neural networks (ANNs) as metamodels, and genetic algorithms (GAs) for optimisation. The approach has been previously demonstrated for the optimisation of blade twist in hover and the optimisation of rotor sections in forward flight, transonic aerofoils design, wing and rotor tip planforms. In this paper, a parameterisation technique was devised for the BERP-like rotor tip and its parameters were optimised for a forward flight case. A specific objective function was created using the initial CFD data and the metamodel was used for evaluating the objective function during the optimisation using the GAs. The objective function was adapted to improve forward flight performance in terms of pitching moment and torque. The obtained results suggest optima in agreement with engineering intuition but provide precise information about the shape of the final lifting surface and its performance. The main CPU cost was associated with the population of the CFD database necessary for the metamodel, especially since a full factorial method was used. The CPU time of the optimisation process itself, after the database has been obtained, is relatively insignificant. Therefore, the CPU time was reduced with the use of the Harmonic Balance method as opposed to the Time Marching method. The novelty in this paper is twofold. Optimisation methods so far have used simple aerodynamic models employing direct “calls” to the aerodynamic models within the optimisation loops. Here, the optimisation has been decoupled from the CFD data allowing the use of higher fidelity CFD methods based on Navier-Stokes CFD. This allows a more realistic approach for more complex geometries such as the BERP tip. In addition, the Harmonic Balance method has been used in the optimisation process