Vibration Suppression of Helicopter Fuselage by Pendulum Absorbers (Excitation Forces Acting on Rigid-Body Blades)

Abstract

The vibration suppression of a helicopter is very important to ensure the comfort and the safety. In some kind of helicopters, the pendulum absorbers are adopted for the purposes to reduce vibrations. Present pendulum absorbers are designed based on the anti-resonance concept in the linear theory. However, since the vibration amplitudes of the pendulum are not small, it is considered that the nonlinearity has influence on the vibration characteristics and the best suppression cannot be attained by the linear theory. Also, the characteristics of the pendulum absorbers are not well analyzed even in the linear theory. In a helicopter, periodic forces act on the blades due to the influences of the air thrust. These periodic forces act on the blades with the frequency which is the integer multiple of the rotational speed of the rotor. In our previous report, we proposed a 2-degree-of-freedom model of the rotor blade and the pendulum absorber. The blade is considered as a rigid body and it is excited by giving a sinusoidal deflection at its end. In this paper, we proposed a 3-degree-of-freedom model which is more similar to the real helicopter since the freedom of the fuselage is added and the periodic forces are applied to the blade by air force. The vibration is analyzed considering the nonlinear characteristics. The resonance curves of rotor blades with pendulum absorbers are obtained analytically and experimentally. It is clarified that the most efficient condition is not the exact tuning to the frequency of the external force but slight detuning to it. Various unique nonlinear characteristics, such as bifurcations, are also shown.