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Flow Analysis and Performance Assessment for Pitching Rotor Blade Airfoils

An airfoil model installed in the Transonic Wind tunnel Göttingen

Influence of the transition modelling on the prediction of the pitching moment for a dynamically pitching airfoil

In constructing a rotor blade from a set of known airfoils, a helicopter manufacturer needs a range of steady and unsteady performance data for the airfoils to be available. Using this base data, the performance of the final rotor blade can be estimated in advance for an effective optimisation of the helicopter. In a close cooperation with helicopter manufacturers, the department of helicopters performs numerical simulations and wind tunnel experiments to analyse the flow and produce performance data for pitching airfoils. In addition to the classical lift, drag and pitching moment the effects of the pressure distribution, the position of the laminar/turbulent boundary layer transition under static and dynamic flow conditions, and the static and dynamic separation behaviour all play an important role.

The flow analysis is performed for the full flight envelope of the airfoil using a full-scale airfoil model with a chord l=300mm up to a Mach number Ma=0.85 and a Reynolds number Re=5e6. Wind tunnel measurements are performed in cooperation with the Institute for Aeroelastics in the Transonic Wind Tunnel Göttingen (DNW-TWG). The design and construction of the wind tunnel models is typically performed within the DLR workshops which have experience of manufacturing high quality wind tunnel models. In an initial step classical polar data is produced by increasing the angle of attack in steps for a number of Mach number and Reynolds number combinations. The flow and performance analysis is then performed by driving the model with prescribed pitching motion to mimic the movements seen on a helicopter blade. The influence of Mach number, Reynolds number, mean angle of attack, amplitude and frequency is analysed for the industrial application of interest. In addition to unsteady pressure, force and acceleration measurements, unsteady measurements of the transition position using hot-films and infrared cameras and unsteady measurements of the flow field using high speed particle image velocimetry are used.

The flow is simulated numerically both before and after the experiments using the DLR-TAU code to improve the targeting of the experimental programme and improve and extend the analysis of the experimental results. Computations of the two-dimensional flow are performed for the midline of the tunnel for static and dynamic test cases, and the experimental situation including the complete model mounting and wind tunnel is modelled to assess the installation and interference effects. The numerical simulations are also used for “2D rotor simulations”, in which the Mach number, Reynolds number and angle of attack are dynamically varied for an airfoil in the same way as for a helicopter rotor blade section in forward flight. This allows the extension of the performance evaluation from the wind tunnel data at constant Mach number and Reynolds number to its situation on a rotor blade. Using these methods, a prediction of the aerodynamic performance of an airfoil during a rotor cycle can be made for various flight conditions and radial positions on the rotor, disregarding the effects of rotation.