Overview
The strategic goal of the MORALI project was to improve the rotor design capability, including comprehensive analysis and evaluation skills of different designs, simulation competence at various modelling levels, and automated optimisation support. While helicopter rotor design involves many different disciplines, with often conflicting demands. However, this proposal only addressed the aerodynamic problems, taking other requirements into account by properly defined constraints.
Aerodynamics of helicopters is a very demanding undertaking in itself, challenging the underlying modelling as well as high performance computing capabilities. The toolbox in this project consisted of blade element models at the lower end of the methodology spectrum as well as coupled flow-structure dynamics simulations at the high fidelity side.
The blade element model was improved by a semi-empirical modelling of dynamic stall, very important for aerodynamic performance, but also for endurance estimations. Furthermore, the wake modelling was enhanced to represent flow phenomena generated by local wake structures better at this modelling level.
CFD can benefit from the introduction of a transition model to take laminar flow at the leading edge into account. Additionally, the trim procedure was sharpened, taking advantage of automatic trim time decision and convergence acceleration.
Both are used to optimise sensibly chosen free parameters in the design process, taking advantage of a comprehensive assessment of the detailed rotor simulation results. This assessment included acoustic post-processing of the simulations and emulates the qualified analysis of an experienced engineer, in order to attenuate artificial deficiencies in the numerical procedures involved.
Finally, the developed tool chain was used for the optimisation of a specific rotor design and the results validated using trusted high-fidelity CFD simulations.
Funding
Results
Executive Summary:
The MORALI project took place from 2011-2014 and was embedded in the Cleansky Joint Technology Initiative. It answered a call for proposals resulting from requirements defined by Eurocopter Germany, now Airbus Helicopters Germany. The proposal consortium consisted of two parties: University of Stuttgart from Germany, represented by the helicopter and aeroacoustics group of the Institute for Aerodynamics and Gasd ynamics, as the coordinator and taking responsibility for aerodynamic simulations on different fidelity levels, and as the second partner MACROS Solutions from Bulgaria, taking care of advanced optimisation technology in a general framework.
The goal of the project was to establish helicopter rotor design capabilities at significantly advanced levels. Although other aspects as structural stability, mass and construction cost are finally certainly of great importance as well, the rotor as the main means of generating lift, propulsion and control is mostly aerodynamics driven, so the project concentrated on this aspect only, considering other factors only by appropriate boundary conditions. Of course, later on those other factors can be included as well, as soon as models become available and are accessible to numerical treatment.
Even on the aerodynamic side along the complexity of the problem is tremendous. About 20-30 design variables at least allow a fine-tuning of a given rotor to get optimal performance. However, “optimal performance” is difficult to define, as several flight states (hover, forward flight at different speeds, start and approach) have to be taken in to account, on terms as power requirements, loads and acoustics. Furthermore, a reliable evaluation at all degrees of freedom necessitates extremely demanding computational fluid dynamics, allowing only for a literally handful of variants to be considered in detail. Other approaches, as blade element theory or free wake simulations, are several orders of magnitude faster, but may mis predict the performance dependence on some parameters due to missing physical phenomena.
As stated, the University of Stuttgart took responsibility for the aerodynamic simulation on all modelling levels, enhancing the tools in order to improve the physical modelling (dynamic stall for blade element theory, transition prediction for CFD), to enable another method (free-wake model) or to boost performance (trim acceleration). Another task was the appropriate definition of performance – at least in view of power input – for a reliable differentiation between variants, where a new optimisation goal could be defined to drive the process. However, the exact balance between power at different speeds and acoustics is a strategic decision of industry, specific for a certain product with defined missions, and thus delegated to them, in this case Airbus Helicopters.
The optimisation procedure itself was taken care of and targeted by MACROS Solutions, who adapted their optimiser to the specific needs of MORALI. They regard the problem as high-dimensional, with various function evaluations at different reliability levels, from low (depending on the parameter, but very cheap) to high (and extremely costly). The tool can generate a new parameter set, which is then to be evaluated aerodynamically, and the result fed back to the optimiser to drive the process further. The goal here is to blend and integrate the different reliability and cost levels to create the best design with minimal computational effort.
In summary, the project was a huge success. In spite of some intermediate technical problems and political difficulties, all deliverables were deployed and all milestones reached. Indeed, the technical program was more than fulfilled. The improved tool chain is now in active use at Airbus Helicopters and supports the development of upcoming new products.