In reference to the dynamic stall problem of helicopter rotor blades, a semi-empirical theory is presented for predicting unsteady lift and moment on an oscillating two-dimensional airfoil, in the presence of dynamic stall, from static airfoil data. Based on the concept of equivalent angle of attack, the time delay of dynamic stall is considered and the linear potential flow theory is used for the calculation of wake vortices and their effects on the equivalent angle of attack. The theory can be applied to the case of nonsinusoidal airfoil oscillation as well as sinusoidal one since it utilizes the FOURIER expansion technique. It is shown in numerical examples that the theoretical results are in good agreement with available dynamic experimental data.
Analytical and experimental investigations to clarify the dynamical similarity rules for pitched rotor blades were carried out. A new experimental method to deduce the vibration characteristics for pitched rotor blades more precisely from those of scaled ones was developed and the results obtained in proving tests were discussed. A number of geometical similar blades whose material properties were different from each other were tested under various operating conditions and their dynamical properties were determined from the peak values of the mechanical impedances at the rotor hub center. The tests results clearly showed that the vibration characteristics for those blades were uniquely governed by the combinations of such typical similarity parameters as the pitch angle θ, the ratio of bending rigidities about the major and minor principal axes of the cross section λ=EIn/EIt and the dynamic similarity parameter Kt=mR4ΩΩ2/EIt where m is mass of the blade per unit length, R is the radius of the rotor, Ω is the revolutional speed of the rotor and EIt is the flatwise bending rigidity. It was also shown that the influences of aerodynamic forces acting on the blades due to its rotational and translational motions on the dynamical properties could be remarkably mitigated if the combinations of the LOCK number and the dynamic similarity parameter of the blade would be properly selected, that is, by using the blades whose LOCK number is as low and dynamic similarity parameter is as high as possible. Adding to these experimental study, an extension of the MYKELSTAD method to the coupled flap-lag vibration system were undertaken and its numerical results were compared with experimental ones. The correlations between them were good and the validity and usefulness of those approaches which were well suited for preliminary design were ascertained.