Determination of Airfoil and Rotor Blade Dynamic Stall Response

Measurements were made of the unsteady lift and pitching moment on an NACA 0012 airfoil model oscillated both sinusoidally and nonsinusoidally over a range of incidence angles, including a substantial penetration into stall. The sinusoidal lift and pitching moment data were reduced and tabulated as functions of the angle of attack, α, the angular velocity parameter, A = bα˙/V, and the angular acceleration parameter, B = b²α¨/V². This generalized form of the data was used to reconstruct the measured sinusoidal aerodynamic response of the model airfoil with excellent results. Additional correlations were made using nonsinusoidal pitch schedules which included periodic ramp changes in angle of attack and a typical 1/rev incidence angle change for a helicopter blade. The agreement between predicted and measured lift and moment loops was very good. Integration of the nonsinusoidal moment loops to calculate aerodynamic damping in pitch has produced good agreement between predicted and measured results. Correlation studies were also performed on the NH‐3A helicopter rotor by introducing the tabulated unsteady aerodynamic data into the Normal Modes Blade Aeroelastic Analysis. Significant improvements in correlation between predicted and measured blade response were obtained compared with previous correlations. These improvements are believed to be due to a combination of the improved unsteady data table and the use of variable inflow in the analysis. Both the shape and the amplitude of the blade root torsional moment response have been predicted with reasonable accuracy.