Frequency and Vibration Problems of Rotors

Abstract

I proceeding from fixed wing aircraft to rotating wing aircraft, there are not only new and complicated aerodynamic and stability problems but also new vibration problems to be met and solved. These vibration problems have undoubtedly been a major item in retarding the development of rotorcraft. Theoretical studies of rotorcraft vibration and frequency problems, which were initiated about two years ago by the Kellett Autogiro Corporation, have culminated in this paper. This study has been confined to the frequency problems which are peculiar to hinged rotor blades. It is felt that rigid-blade rotor construction may be analyzed by substantially the same methods as are available for analyzing propellers and, therefore, no consideration has been given to this specialized type in this paper. When a rotorcraft is operating in horizontal flight, the airflow over the blades on the advancing side of the rotor is considerably greater than the airflow over the retreating side. This unsymmetrical flow of air over the blades, in combination with the non-uniform flow of air through the rotor disc, causes the blades to rise and fall during each revolution of the rotor. In addition, the blades flex in bending in the plane of flapping and oscillate about the vertical hinges with a frequency of one cycle for each revolution of the rotor.