Copyright© 1998, American Institute of Aeronautics and Astronautics, Inc. A98-25287 AIAA-98-2108 BENCH-TOP CHARACTERIZATION OF AN ACTIVE ROTOR BLADE FLAP SYSTEM INCORPORATING C-BLOCK ACTUATORS Joseph W Clement*, Diann Brei t , and Andrew J. Moskalik* The University of Michigan Ann Arbor, Michigan 48109-2125 Ron Barrett § Auburn University Auburn University, AL 36849-5338 Abstract This paper presents the bench-top testing of a piezoceramic C-block driven active flap system designed to suppress the vibrations of a helicopter rotor blade. The C-block actuators are curved benders designed to generate a larger force output than a straight bender, while providing deflections large enough to eliminate the need for external leveraging systems necessary with stack driven systems. The actuators power a balanced active flap designed to minimize the effect of air speed and rotor speed on flap deflection. Quasi-static experimentation at 1 Hz produced maximum angular flap deflections of 8.4° peak-to-peak. Dynamic tests were conducted over a 40 Hz frequency range demonstrating the ability to generate significant flap deflections both before and after the first natural frequency. Over the 40 Hz range, the flap deflections never dropped below 8° pp, with a first natural frequency of 27 Hz. The flap deflection reached a maximum value of 13.6° pp at 40 Hz. If the applied voltage is increased to the maximum allowable level, it is predicted that flap deflections as large as 20° pp can be achieved. Introduction One of the main problems affecting modern helicopter performance is the large vibratory loads generated as the rotors travel through an extremely complex flow *Graduate Student, Department of Mechanical Engineering and Applied Mechanics Assistant Professor, Department of Mechanical Engineering and Applied Mechanics *Ph.D. Candidate, Department of Mechanical Engineering and Applied Mechanics § Assistant Professor, Department of Aerospace Engineering Copyright © 1998 The American Institute of Aeronautics and Astronautics Inc. All rights reserved. field. The primary source of the vibrations is the differential in air speeds encountered by the retreating and advancing blades. Because the air speed is much lower over the retreating blade, an increase in the retreating blade's pitch is required to maintain lateral stability. As a result of the pitching action, a N/rev load is transmitted to the fuselage for a N-bladed balance helicopter. In addition to the physical discomfort experienced by the passengers, the vibrations create excessive levels of noise. This noise limits the use of helicopters in domestic affairs as well as military efforts. In addition, these vibrations cause premature replacement of the rotor blades, with typical rotor blade sets pricing in the five to six figure range. Currently, helicopters control the blade pitch through the use of a swashplate, driven by actuators in the non-rotating system. The difficulty of transferring mechanical power from the non-rotating to the rotating system has limited the usage of Higher Harmonic Control (HHC) or Individual Blade Control (IBC) in today's helicopters, causing a reliance on passive methods to reduce vibrations. However, the limited bandwidth of passive vibration control has stimulated much research in active vibration suppression methods that would overcome the difficulty of power transfer to the rotor system. Researchers have investigated placing actuators in the rotating frame, eliminating the heavy, complex mechanisms necessary to transform control efforts from the fixed to the rotating system, thereby increasing the effectiveness of both HHC and IBC. A number of different actuation systems utilizing smart materials have been examined for this purpose, including hybrid systems where smart materials are combined with hydraulics 1 ' 2 ' 3 . The most widely used smart material is piezoelectrics. Conventional piezoelectric actuators, such as stacks 4 ' 5 and bimorphs 6 ' 7 ' 8 , have been constructed to actuate smart flaps with promising results. However, the primary problems facing many of the current designs for active flap control have been the authority of the 2857 American Institute of Aeronautics and Astronautics