SMART SPRING - AN ACTIVELY TUNABLE VIBRATION ABSORBER DESIGNED TO CONTROL AEROELASTIC RESPONSE

Most Individual Blade Control (IBC) approaches have attempted to suppress rotor vibration by actively altering the varying aerodynamic loads on the blade using techniques such as trailing edge servo-flaps or imbedded piezoelectric fibers to twist the blade. Unfortunately, successful implementation of these approaches has been hindered by electromechanical limitations of piezoelectric actuators. The Smart Spring is an unique IBC approach that is designed to suppress the rotor vibration by adaptively altering the “structural impedance” at the blade root out of phase from the time varying aerodynamic forces. It overcomes many limitations associated with the capabilities of piezoelectric actuators inherent in other IBC approaches. The details of the Smart Spring concept as a class of actively Tunable Vibration Absorbers (TVA) for the rotor blade application is presented in this paper. A proof-of-concept hardware model of the Smart Spring was fabricated and a real-time adaptive control algorithm was implemented to demonstrate the concept as an actively TVA. The initial dynamic tests using a mechanical shaker achieved significant vibration suppression at harmonic peaks as well as the broadband reduction in vibration. Further testing of the hardware was conducted in a wind tunnel using a non-rotating blade to evaluate the performance of the device in a more representative rotor blade aerodynamic environment. Shaker test and wind tunnel test results verified the capability of the Smart Spring to suppress multiple harmonic components in rotor vibration through adaptive control of the structural impedance at the blade root. * Research Associate, member AIAA. † Group Leader, senior member AIAA. ‡ Post Doctoral Fellow. § Professor, member AIAA. INTRODUCTION Significant structural vibration due to unsteady aerodynamics caused by Blade Vortex Interaction (BVI) is a notable and undesirable characteristic of helicopter flight. Vibratory hub loads are transferred throughout the helicopter structure contributing to poor ride quality for passengers and fatigue of expensive structural components. In addition to passive techniques, such as vibration absorbers or addition of mass to tune blades, active control solutions are currently being investigated to suppress rotor vibration. Active approaches promise vibration suppression in a broadband of frequencies unlike passive techniques that are typically capable of suppressing vibration over a narrow frequency range. The Individual Blade Control (IBC) approach is one of the most promising active vibration suppression methods currently under development. IBC places actuators and sensors on the blade to control each blade independently and simultaneously to suppress the vibration at the source. IBC using active material actuators for rotor vibration suppression has been implemented using two distinct actuation concepts, namely, discrete and integral. The discrete actuation concept employs actuators imbedded in the blade to control a trailing edge servo-flap. Unfortunately, a fundamental problem with this approach is the limited displacement capability of piezoelectric actuators. Robust and compact displacement amplification devices are necessary to obtain the required flap deflection under the extreme aerodynamic environment of a rotor. In the integral actuation concept, the actuator system is either embedded or bonded to the blade skin along the span to obtain a smooth continuous structural deformation. The drawback with this approach is the requirement of very high voltages to induce sufficient structural deformation from the actuators to generate the required blade twist for rotor vibration suppression.