Aerodynamic performance of the NREL Unsteady Aerodynamics Experiment (Phase IV) twisted rotor

A great deal of experimental data has been obtained from operation of the Unsteady Aerodynamics Experiment at the National Renewable Energy Laboratory (NREL) in Golden, Colorado. This paper presents the results of analyzing data taken during Phase IV (1997) of the experiment in order to determine the baseline aerodynamic performance of the Phase IV twisted rotor. Plots of normal force coefficient versus angle of attack show that the performance of the twisted rotor at low angle of attack is similar to that predicted by wind tunnel tests. However, as the angle of attack is increased beyond the static stall angle there is significant overshoot in the lift. Measurements of the flow angle by five-hole pressure probes demonstrated that the angle of attack remains relatively uniform along the span of the rotor, and that there is a significant spanwise component of the flow over the rotor. A preliminary investigation into the occurrence of dynamic stall on the rotor indicates that the frequency of dynamic stall events significantly less on the twisted rotor as compared to the Phase II untwisted blade.