Vibration Suppression of a Helicopter Fuselage by Pendulum Absorbers : Rigid-Body Blades with Aerodynamic Excitation Force

Abstract

Currently, some kinds of helicopters use pendulum absorbers in order to reduce vibrations. Present pendulum absorbers are designed based on the antiresonance concept used 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. Therefore, the best suppression cannot be attained by using 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. Our previous study proposed a 2-degree-of-freedom (2DOF) model composed of a rotor blade and a pendulum absorber. The blade was considered as a rigid body and it was excited by giving a sinusoidal deflection at its end. The present paper proposes a 3DOF model that 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 aerodynamic 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 obtained when the natural frequency of the pendulum is a little bit different from the frequency of the external force. Various unique nonlinear characteristics, such as bifurcations, are also shown.

Review of Research on Nonlinear Rotordynamics in Japan

“…Since these pendulums oscillate with large amplitude to suppress blade vibrations, the nonlinear analysis is necessary. Nagasaka et al studied this problem with a 2DOF rigid blade model (a blade and a pendulum) (122) , a 3DOF rigid blade model (a fuselage, a blade and a pendulum) (123) and a 3DOF flexible blade model (a mount, a blade and a pendulum) (124) .…”

Damping Effect of Pendulum Tuned Mass Damper on Vibration of Two-Dimensional Rigid Body

“…9 Absorbers can be mounted on the rotor blades of a helicopter for attenuation of aerodynamically induced vibrations. 10 Wu 11 used an active rotational PVA to reduce the transverse vibration of a primary body. Varying the rotational speed of the pendulum enables the tuning of the absorber frequency, whereupon the inertial force of the revolving mass is helpful in reducing vibration of the main body.…”

Vibration Suppression of a Helicopter Using Pendulum Absorbers : First Elastic Mode of a Blade

“…In a pendulum, nonlinear characteristics become apparent as the deflection angle of a pendulum increases. Therefore, in the first study, the authors examined the use of a pendulum absorber to suppress vibrations using a rigid blade model considering its nonlinear characteristics (9), (10) . In these systems, the excitation frequencies were integer multiples of the rotational speed.…”

“…It was observed that the suppressing effect of the pendulum increased as the supporting position of the pendulum approached the tip of the blade. In the second report (10) , the effect of a pendulum absorber to suppress vibration of the fuselage was studied using a 3DOF system composed of a blade, pendulum and fuselage. Observations were then made as the blade was excited by an aerodynamic force.…”