The design and optimization of helicopter rotor blades is a challenging task. On the one hand the flow around the rotor blades is unsteady and many different flow conditions are encountered by the blade per revolution; on the other hand very different aerodynamic requirements are given for hover and fast forward flight.
Methods for the aerodynamic optimization of helicopter rotors are developed and applied in the helicopter department of the DLR Institute of Aerodynamics and Flow Technology. Current research focuses on the optimization of the planform shape of the rotor blade including its twist. Therefore, tried and proven methods for the aerodynamic simulation of helicopter rotor blades (see the article rotor- and complete helicopter simulation) are coupled with a numerical optimization toolbox. The goal is to obtain a rotor blade that generates high thrust for little torque. Besides the aerodynamic performance of the blade, the aero-acoustics of the blade become more and more important. Future research will include the accurate prediction of the rotor noise as well as the direct inclusion into the existing optimization framework.
The correct prediction of the helicopter rotor flow field with the help of computational fluid dynamics (CFD) is very resource intensive, especially when fluid-structure coupling is required. The current research investigates acceleration techniques for the optimization process by using surrogate models which are based on variable fidelity methods.
Surrogate models mimic the simulation through simple mathematical relations. This enables the numerical optimization to perform many evaluations of rotor blades with little resources to then select the next interesting design for the simulation. The results from the new simulations are then fed back into the surrogate model to enhance its accuracy.
The optimization process is further accelerated by combining simulation results of low and high fidelity methods. The used simulation methods for the variable fidelity approach range from the blade element theory (BET) which is enhanced with simple wake models over inviscid CFD simulations of single rotor blades up to viscous CFD (RANS) simulation including all rotor blades.
The optimization of the helicopter rotor for multiple flight conditions is achieved through a multi-objective optimization. Through the use of the Pareto optimality criterion, a selection of multiple potential configurations which improve the performance in one or multiple flight conditions at once becomes possible. This allows the design engineer to pick the best compromise for the various flight conditions.
 M. Imiela, "High-fidelity optimization framework for helicopter rotors," Aerospace Science and Technology, pp. 1-16, 2012.
 G. Wilke, "Variable Fidelity Optimization of Required Power of Rotor Blades: Investigation of Aerodynamic Models and their Application," in 38th European Rotorcraft Forum, Amsterdam, 2012.
 M. Imiela and G. Wilke, "Passive Blade Optimization and Evaluation in Off-Design Conditions,"
39th European Rotorcraft Forum, Moscow, 2013.
 G. Wilke, "Multi-Objective Optimizations in Rotor Aerodynamics using Variable Fidelity Simulations," in 38th European Rotorcraft Forum, Moscow, 2013.